Adsorption of vinyl copolymers from melts and solutions

Citation for published version (APA):Pennings, J. F. M. (1982). Adsorption of vinyl copolymers from melts and solutions. Technische HogeschoolEindhoven. https://doi.org/10.6100/IR1723

DOI:10.6100/IR1723

Document status and date:Published: 01/01/1982

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ADSORPTION OF VINYL COPOLYMERS

FROM MELTS AND SOLUTIONS

J. F. M. PENNINGS

ADSORPTION OF VINYL COPOLYMERS

FROM MELTS AND SOLUTIONS

ADSORPTION OF VINYL COPOLYMERS

FROM MELTS AND SOLUTIONS

PROEFSCHRIFT

TER VERKROGING VAN DE GRAADVAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN

AAN DE TECHNISCHE HOGESCHOOL EINDHOVEN, . OP GEZAGVAN DE RECTOR MAGNIFICUS,

PROF. IR. J. ERKELENS, VOOR EEN COMMISSIE AANGEWEZEN DOOR HET COLLEGEVAN DEKANEN

IN HET OPENBAAR TE VERDEDIGEN OP VRIJDAG 28 MEI 1982 TE 16.00 UUR

DOOR

JOHANNES FRANCISCUS MARIA PENNINGS

geboren te Boxtel

Dit proefschrift is goedgekeurd door de promotoren

Prof. Dr. D. Heikens en

Prof. Dr. C.A. Smolders

en door de copromotor

Dr. G. Frens

ADSORPI'ION OF VINYL COPOL YMERS FROM MELTS AND SOLUTIONS

Table of contents

1 INTRODUCTION 1

2. THEORY OF POL YMER ADSORPTION AND EFFECTS ON ADHE-SION . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 Polymer solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2 Statistical mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2.1 Flory-Huggins theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2.2 Dilute solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2.3 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3 Conformatloos of adsorbed polymer molecules . . . . . • . . . . . . . . . . 7 2.3.1 Randomcon molecules vs. trains, loops and tails of segments . . . . . . . 7 2.3.2 Models of adsorption of random coil molecules . . . . . . . . . . . . . . . . 8 2.3.3 Silberberg's theory . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . 8 2.3.4 Scheutjens and Fleer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.4 Adsorption, wettabllity and adhesion . . . . . . . . . . . . . . . . . . . . . . 10 2.4.1 lnterfacial energy and wettabllity . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.4.2 Adsorption theory of adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.4.3 Other theories of adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.4.4 Combined effects ........................ .- . . . . . . . . . . . 13 2.5 Adsorption from polymer melts . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3 EXPERIMENTALMETHODS ........................... 16 3.1 Introduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2 Matenals . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2.1 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2.2 Polymers . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.3 Adsorption and relaxation experiments . . . . . . . . . . . . . . . . . . . . . 17 3.4 Contact angle measurements and surface energy calculations . . . . . . . 18 3.5 X-ray Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 21 3.5.1 Principles ofthe method ............................... 21 3.5 .2 Apparatus . . . . . . . . . . . . • . . . . . . • . . . . . . . . . . . . . . . . . . . . 22 3.5.3 Interpretation ofthe XPS spectra . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.6 Surface reactlons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4 THE SURF ACE ENERGY OF COMPRESSION-MOULDED POLYMERS . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.1 Introduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 4.2 Substrates . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . 25 4.3 Polymer surface energies . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4.3.1 Changes in surface energy ............... ~ . . . . . . . . . . . . . . 26

4.3.2 Adsorption ........................................ 26 4.3.3 Temperature dependenee of adsorption . . . . . . . . . . . . . . . . . . . . . 29 4.3.4 The influence of release agents and fillers . . . . . . . . . . . . . . . . . . . . 31 4.3.5 Relation between adsorption and strengthof adhesion . . . . . . . . . . . 31 4.4 Relaxation of polymer surface energy . . . . . . . . . . . . . . . . . . . . . . 33 4.4.1 Reversal ofthe adsorption process ........................ 33 4.4.2 PVC/ Ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.4.3 PVC-P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.4.4 PE ............................................. 35 4.4.5 The activation energy of relaxation . . . . . . . . . . . . . . . . . . . . . . . . 37

5 SURFACE ANALYSIS OF COPOLYMERS AFTER COMPRESSION MOULDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5 .1 Introduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 5.2 XPS, combined with ion etching .......................... 40 5 .2.1 Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 5.2.2 Substrates ........................................ 41 5.2.3 PVC/Ac .......................................... 41 5.2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 5.3 XPS combined with surface reactions . . . . . . . . . . . . . . . . . . . . . . 44 5.3.1 PVC/Ac/Ale: hydroxyl vs. acetate groups .................... 44 5.3.2 Adsorption and relaxation of vinyl acetate groups against gold . . . . . . 46 5.3 .3 Adsorption of vinyl acetate groups on different substrates . . . . . . . . . 48 5.3.4 Discussion ........................................ 49

6 SURFACE ANAL YSIS OF COPOL YMER FILMS, CAST FROM SOLUTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.1 Introduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 6.2 Polymer films cast from solutions . . . . . . • . . . . . . . . . . . . . . . . . . 52 6.2.1 Solvent properties ........•......... : . ............... 52 6.2.2 XPS and surface reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 6.2.3 Surface energies of cast films before and after compression moulding . 54 6.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SS

7 CONCLUSIONS AND APPLICATIONS . . . . . . . . . . . . . . . . . . . . . 57 7.1 Adsorption from polymer melts and solutions . . . . . . . . . . . . . . . . . 57 7.1.1 Composition ofpolymer surfaces ......................... 57 7.1.2 Conformation of adsorbed polymer molecules ................. 58 7.2 Consequences for practical applications ofpolymers ............. 59 7.2.1 Processing parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 7 .2.2 Product properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

SUMMARY ............................................ 62 SAMENVATTING ........................................ 64 REPERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 LIST OF SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 CURRICULUM VITAE . . . . . . . . . . . . . . . . . . . . . . . • • . . . . . . . . . . . 72

1. INTRODUCTION

This work aims at obtaining infonnation about the interface between a polymer melt and a metal substrate. An investigation into adsorption phenomena at this interface can possibly lead to a better understanding of practical problems related with polymer processing. The transfer of surface details of a mould (for example pits or grooves on audio and video records), contamination ofthe mould, the smoothness of the surface and the stability of the shape of the product depend on the interactions between melt and substrate. These interactions also detennine the surface properties and therefore the frictional and adhesional behaviour of the product.

1

We wil1 start from the hypothesis that adsorption from polymer melts is analogous to adsorption from polymer solutions. Investigations of polymer adsorption have mainly been concemed with adsorption from solutions and there is an abundance of literature on the subject. Much theoretica! work has been concemed with the polymer conformation at the interface, while the expertmental work has centered on the layer thickness, the bound fraction of the polymer and on energetic effects of the solvent. The interfacial properties of polymer roelts are not so well known. The system is simpter since there is one component less, viz. the solvent, and in theory the polymer melt can be treated as a concentraled athennal solution. Experirnentally it is less accessible than the salution counterpart because of the high temperatures to be used. A unique possibility is the study of adsorption from polymer roelts to roetal surfaces by quenching the sample and removing the sub­strata. We shall discuss a thennodynamic description of polymer solutions, theories of polymer adsorption and the conneetion between adsorption and adhesion. We shall also examine how these theories can be related to J?Olymer melts.

In the present study, vinyl copolymers are melted in contact with roetal substrates. The polymer is solidified and then separated from the substrate. The polymer and substrate surfaces can then be investigated by surface analytical methods. The surface energy is determined by contact angle measurements of sessile drops of a series of polar and non-polar liquids. The surface energy and its polar character are directly related to the surface composition and to adhesion and friction properties of the surface. The surface composition of the polymer is measured by X-ray photoelectron spectroscopy (XPS). This is a non-destruclive technique that enables a quantitative chemical analysis of the surface of a solid in vacuum. XPS is used in combination with surface reactions (1) where labels are attached to polar groups in the polymer. Spectroscopie methods such as secondary ion mass speetrometry (2) and Auger electron spectroscopy (3) prove to be of little value for the examination of polymer surfaces because of polymer degradation under electron or ion radiation. lnfrared spectroscopie methods have too low surface sensitivity to be useful in adsorption experirnents where the adsorbed species are difficult to distinguish from the bulk and where the thickness of the adsorbed layer is small. In the future

2

several techniques may prove to be of value for polymer surface analysis. Low energy ion scattering ( 4) ineasures the composition of the outermost atomie layer of a solid. lnformation on chemical bonding can be obtained from Foutier transfarm reflection infrared analysis (S) or from photoacoustic spectroscopy (6). All these new techniques are virtually non-destructive.

It will be seen that polar groups in the polymer are adsorbed at the interface of a polymer melt and a metal substrate. This situation is frozen in by rapidly cooling to a temperature where the polymer segments have such a low mobility that relaxation does not occur before analysis of the interface (7). After removal of the substrate, the polymer surface then has a high (non-equilibrium) surface energy, as well as a composition that is different from the bulk.

Experiments on the ra te of relaxation to equilibrium give additional data concernins the processes at the molecular level during adsorption and desorption (8). The measured relaxation rates corroborate the idea that a non-equilibrium situation can be frozen in during solidification of the polymer. The wettability of a surface can change for quite some time after the polymer has been processed. lt is also possible to modify the physicochemical properties of the surface by surface reactions similar to those used for decaration and analysis.

A comparison of adsorption from melts with adsorption from solutions (9) indicates that the polymer in the surface region of a moulded ftlm is not in a trains and loops conformation, as it is after adsorption from solution. The conformation is on the contrary similar to the random con conformation that we expect in the bulk of a random copolymer and in the bulk of a solution. Nevertheless, a polymer melt can still be described in several respects as a salution of macromolecules in an athermal solvent, in which the majority of segments foniung the polymer chain are more soluble than the minority of polar groups. Thus it becomes energetically favourable for the polar groups to adsorb at a polar surface.

Adding fillers or release agents to the melt can influence adsorption. Adsorption of groups from the polymer at the surface of filler particles diminishes the tendency for adsorption of these groups at the substrate interface. Competitive adsorption of release agents or plasticizers lowers the net adsorption of groups from the polymer chain. This is analogous to the situation in solutions, where solvent quality and solvent adsorption interfere with polymer adsorption. On top of this, relaxation is accelerated by the presence of plasticizers.

The chemical composition of polymer surfaces is of interest because it influences adhesion, friction, wetting and optical properties. Many operations in production and fabrication involve the flow of a polymer melt. The viscoelastic behaviour of polymer roelts is relatively well known, but the behaviour at the contact surface is less well understood.

3

Adhesion and friction are relevant for contamination and wear of the mould, and to some extent for the consistency of the shape and size of the product as wen. The surface properties of the fmished product can be very critical. Adhesion to other materials, including wetting of inks or paints is often a problem. Protective polymer layers on metals have to adhere wen too. Friction of polymer productsis related to surface energy and adhesion (tack). All these properties depend on the composition of the surface and on the surface energy of the polymer. Polymer adsorption from melts is therefore of equal technical interest as polymer adsorption from solutions is for example for the stabilisation of suspensions.

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2. THEORIESOF POL YMER ADSORPTION

2.1 Polymer solutions

Adsorption of polymers on solid substrates has in the main been studied in systems where polymers adsorb from solutions. Experimentally this has entailed the study of the thickness of the adsorbed layer and of the bound fraction of the polymer. The thickness follows from ellipsometry (10), from hydrodynamic measurements (11 ), from the rotational diffusion coefficient of particles with a polymer coating (12), from adsorption isotherms and from the influence on the stability of colloids (13). The fraction of the polymer that is bound to the surface can be studled by ESR (14) or IR (15) measurements. Neutron scattering and pulsed NMR (16) reveal the density distribution within the adsorbed layer. Important experimental parameters are the strength of the interaction between the polymer and the solvent, the concentration and the molecular weight of the polymer and the chemical nature of the surface.

Studies of the conformation of adsorbed polymers have mostly been theoretica!, by the random walk approach or by Monte Carlo calculations. The literature survey in this chapter will be limited to a discussion of flexible linear homopolymer molecules.

One effect of the high molecular weight of a polymer is that it is soluble only when there are no strong interactions between the polymer molecules or when there is an equally strong interaction between polymer and solvent. Another difference com­pared with low molecular weight solutions is that, because the polymer segments are interconnected, there is always a high local concentratien of polymer segments, even in dilute solutions. This means that theoriesof dilute solutions (where a rigarous mathematica! analysis would be possible) do not apply here. A polymer salution can be considered to consist of regions of concentraled salution separated by regions of pure solvent. F or that reason statistles are needed to describe the distribution of polymer molecules as wellas to describe the distribution of segments within a polymer molecule. A quite different reason for the use of statisti· cal methods is that the properties of individual macromolecules often differ from average properties, for example because of a broad molecular weight distribution, a random composition of copolymers etc.

2.2 Statistical mechanics

2.2.1 Flory-lluggins theory

Flory (17) and Ruggins (18) calculated in how many ways n1 solvent molecules and n2 polymer molecules can be placed on a lattice. Every polymer molecule is divided into segments (not necessarily monomeric units), each having the size of onè

5

solvent molecule, so that they all are interchangeable on the lattice. The theory is valid for solutions with a uniform occupation density of the lattice. It is not valid therefore for dilute solutions, but it remains valid up to high concentrations when the polymer molecules interpenetrate. The entropy of mixing ~Sm of polymer and solvent is:

~m = k (nt In tPt + nz In cpz ), (2.1]

where k is Boltzmann's constant and cp1 and cp2 are the volume fractions of solvent and polymer respectively. The heat of mixing follows from the energy change ~e on formation of a segment-solvent contact:

[2.2}

where elj is the interaction energy between the pair i,j. Flory and Ruggins defmed a parameter x :

x= z ~e /kT, (2.3]

where z is the coordination number of the lattice. The enthalpy of mixing then is given by:

[2.4]

Two special cases are x= 0 and x= *· For x= 0, we have an athermal salution and only entropy considerations determine the state of the system. In the case of an ideal solution x is *: the macromolecules then have unperturbed dimensions because the excluded volume effects and the polymer-polymer interactions cancel. The excluded volume is the volume that cannot be occupied by a polymer segment because of the presence of other segments. Attraction between polymer segments has an opposite effect because it diminishes the average distance between them. A solvent forming anideal salution with a polymer is called a 8-solvent. This is nota good solvent for that polymer: at high molecular weights the limit of solubility is reached at x = *· The interaction between a polymer and a solvent is temperature dependent. The temperature where x=* is called the 8-point. A polymer-solvent pair is immiscible at temperatures below the 8-point ( where x> *)· The following relation between x and 8 has been derived (19):

* -x = const * ( 1 - 8 I T ) . [2.5]

For good solvents x is small and it decreases when the temperature is raised, indica­ting better solubility at higher temperatures. This does not continue indefinitely because x increases and phase separation occurs near the boiling point of the solvent (20).

6

2.2.2 Dilute solutions ( 19)

In dllute solutions coils of polymer molecules with a high local segment concentra· tion (3 to 5% in the centre of the random coll) are separated by regions of pure solvent. Because of this inhomogeneity an excluded volume theory bas to be used instead of a lattice theory. The segment density distribution of the polymer molecule and the coil size depend on the nature of the solvent. In a good solvent the coils are large to maximize the number of polymer-solvent contacts, resulting in a large excluded volume. The excluded volume is therefore related to x and it is also connected to a1

, the intramolecular interaction parameter descrihing the size of the polymer coil relative to its unperturbed dimensions. a1 is 1 in the case of an ideal solution where the attractive forces between the polymer segments are counteracted by the physical excluded volume (X= ~). The segments appear to have no volume ànd the solution behaves like an îdeal solution. The case of a1 < 1 implies that the interaction between solvent and polymer is so small that the two are immiscible. The excluded volume and the intermolecular interactloos influence the vîscosity of the polymer solution. In chapter 6 we shall use this property of polymer solutions for evaluating the quality of different solvents for some partic· ular polymers.

2.2.3 Adsorption

The entropy of mixing a polymer and a solvent is positive but small (small number of polymer molecules). The entropy of polymer adsorption is usually negative, and especially so whèn the conformation of the molecules is changed to form a flat monomolecular layer. This can happen when there is a very strong interaction between the polymer segments and the surface. For adsorption to occur there has to he an energy gain upon formation of a contact between the surface and the polymer segment. Microcalorimetry gives quantitative data on this aspect of ad­sorption (21). When the conformational entropy lossof the polymer molecule is small (for example with adsorbing proteins), the entropy gain of the desorbed solvent moleculescan be sufficient to drive the adsorption process without an energy gain.

Polymer adsorption is a reversible process but desorption is often very slow. When one segment of a molecule bas become attached to a surface, there are numerous other segments close to the interface. These can then be bound in turn because thereisanenergy gain, and for desorption ofthemolecule from the surface it is nee· essary that all such honds are brok en. The bound fraction is limited by the restraints of the polymer conformation and by interactions withother segments (from the same or a different macromolecule) or with solvent molecules. For this reason a slow increase of polymer adsorption is the rule: more and more segments are bound to the surface. If one segment desorbs it will be temporarlly because it cannot move away from the interface and will be bound again. In other words, adsorption is a

reversible process of the individual polymer segments, and total desorption has to be a collective process. Usually desorption can only be accelerated by using a solvent of a quality superior to that of the solvent from which the polymer had adsorbed originally. lmproving polymer solubility by increasing the temperature does not result in desorption.

2.3 Conformations of adsorbed polymer molecules

2.3.1 Random coil molecules vs. trains, loops and tails ofsegments

To study adsorption of low molecular weight matenals from solutions one has to deal with solute-surface, solute-solvent and solvent-surface interactions. Polymer adsorption is further complicated by the additional problem of an unknown con­formation ofthe polymer molecule at the surface. When Jenckel and Rumbach (22) found that the adsorbed amount of polymer per unit area of the surface would correspond to more than 10 monolayers of segments, they suggested that the molecules are adsorbed in sequences of segmentsin contact with the surface, known as 'trains'. These trains are connected by bridges of segment sequences

7

ex tending into the solution, called 'loops'. The free ends of the polymer molecules, the 'tails', also extend into the salution (figure 2.1).

Fi~re2.1.

An adsorbed polymer molecule in a conformation of trains, loops and tails. The space between the trains is occupied by trains of other polymer molecules, the space between the loops is filled by other loops and by solvent molecules. Tails ex tending into the solution often make an important contribution to the total segment density.

There is a considerable difference between the conformation of a polymer that is adsorbed in loops and trains and that of a polymer molecule in salution (the

8

random coil). The conesponding toss of entropy must be small enough to be compensated by the adsorption energy.

2.3.2 Modelsof adsorption of random coil molecules

Adsorption of unperturbed random coil molecules at a few points of contact has been suggested by Eirich et.al. (23), but most experimental data andresultsof Monte Carlo calculations cannot be explained in this way. Frisch and Simha (24) derived a quantitative theory of polymer adsorption by a statistica! mechanical treatment. The flexible polymer molecules are placed on a lattice as in the Flory­Huggins theory. The adsorbed polymer ebains have a Gaussian distri bution of end-to-end distances, as in the bulk solution. The only differences occur at the contact points with the adsorbent surface. The surface is impenetrable to the coils and it is therefore treated as a reflecting wall in the random walk calculation resulting in a new distribution of end-to-end distances.

Forsman and Rughes (25) treated the conformation of polymer chains at an inter­face in termsof independent orthogonal componentsof the radius of gyration. Their treatment is satisfactory in the case of sparsely populated surfaces and with heavily covered surfaces, i.e. when the segment density is only a function of the distance from the surface. That is the case in very dilute solutions, where adsorbed polymer molecules do not interact, or in very concentrated solutions and melts where segments from different polymer molecules are indistinguishable. A calcula· tion of entropy effects explains why the adsorbed amount of polymer is limited. The effects of molecular weight and solvent quality are discussed as wen as the strength of the polymer-surface interaction.

2.3.3 Silberberg's theory

An important alternative theory was originated by Silberberg (26). A wealcness in farmer theories was that no àttractive energy was incorporated. The shape of ihe adsorbed molecule was therefore not considered as a variabie in the treatment of the thermadynamie equilibrium. Silberberg assumes alternate sequences ofloops and adsorbed trains that are essentially independent of each other. He prediets that the sizes ofthe loops and trains are independent of molecular weight, which is in contradiction to earlier theories. In Silberberg' s theory a flexible molecule has small loops and the molecule is close to the surface if the polymer-surface interactions are favourable. The adsorbed macromolecule is then essentially two-dimensional. Con­formational restrictions increase the size of the loops and the energy required for adsorption. Por high molecular weight materials a plateau in the adsorption iso­therm is predicted.

Uke alltheoriesin this section Silberberg's theory is based upon the methods of statistical thermodynamics. The polymer ebains are represented by a sequence of

9

segments which are connected by flexihle honds. The flexibility of these bonds is a parameter of the theory. The size of the segments is determined by the statistkal properties of the real chain of monomer units, viz. the number of monomeric units, chain stiffness and regularity etc. The segments are placed on a lattice. When the concentration of polymer segmentsis sufficiently high (in a bad solvent), the previously placed segments are considered to be uniformly distributed. This results in a pure random walk conformation. The segment density distribution is constant in a layer parallel to the surface and varles perpendicularly to the surface. At low concentrations and in a good solvent a self-avoiding random walk is used. In this case a segment is placed on the lattice (in the process of calculating the random walk) taking into account the previously placed segments. This results in a distribu­tion with larger loops and smaller trains.

The end parts of the polymer chain that are connected by only one segment to the surface are called tails. Silberberg ignores these, as he assumes that they have about · the same size as the loops. The density proftie of the adsorbed layer is a step function: there is a constant density to a distance of half the average loop length at which point the density drops to the bulk solution value. This overestimates the expansion effects due to self-exclusion. Silberberg calculates partition functions for trains and loops hoth for athermal {X= 0) and theta (x = *) solvents.

Hoeve (27) points out that Silberherg's assumption of uniform-sized loops and tails is too restrictive because a diversity of sizes will exist within the adsorbed layer. Hoeve calculates the effects of this distribution. Additionally, the size has to depend on the interaction energy of polymer and substrate. As in the theory of Silberberg, a small interaction energy suffices for adsorption. Hoeve uses an expo­nential distribution of segment density ( except at very short distances) in a purely random walk calculation for the loops, with the surface acting as an adsorbing harrier. In this case self-exclusion effects are underestimated, specially with low densities in good solvents. The results are comparable to Silberherg's case of a theta solvent. Tails are ignored and it is shown that this is permitted in the limiting case of very high molecular weights. The thickness of the adsorbed layer varles with the square root of the molecular weight, in contrast to Silberberg's theory.

Roe (28) has developed a multilayer theory of polymer adsorption from solution. He employs a lattice of layers parallel to the surface. A partition function is calcu­lated for random walks on this lattice. A step of the random walk towards a layer is weighted with the expectancy of fmding an empty site in that layer. Such weighting factors were absent in Silberberg's and Hoeve's calculations. A Flory-Huggins type of statistica! thermodynamic calculation yields a segment density profile after an iteration procedure, where each proflle is used to calculate the weighting factors for the next iteration. The adsorbed amount and the surface occupancy can be calcu­lated. Details ofthe polymer conformation (contributions oftrains,loops and tails) cannot be established. The spatial distribution of chain segments is independent of the position of the segment in the chain. This is equivalent to ignoring tails.

10

2.3.4 Scheutjens and Fleer

The most recent theory of polymer adsorption is that of Scheutjens and Fleer (29, 30, 31 ). It contains no major simplifications. Tails are not ignored and are found to make an important contribution to the segment density at greater distances, which is important for the layer thickness (measured hydrodynamically or by ellipsometry) and for the stability of colloids, for example. Each taU of a chain can have a length of up to 1/3 ofthe chain length. Such tails can be visualised as a random coil attached to the surface, the surface being covered with a layer of loops and trains.

The lattice employed is identical to that used by Roe, with layers parallel to the surface. Both a cubic and a hexagonallattice are studied to determine the effects of using a lattice. There are no important differences. Weighting factors are used, resulting in a step weighted random walk. The weighting factors originate from the degree of occupancy in each layer which is estimated in the fust calculation. The results of the first calculation are inserted in the next, and so on. Polymer-solvent interactions are calculated in every layer, with an extra adsorption energy in the first layer. A matrix procedure is used for the iterative calculation with the result that all conformations are counted separately. Contributions of segments of loops, trains and tails can be evaluated as well as average densities. This theory therefore gives more detailed information on the structure of the adsorbed layer than any of the other theories.

There is no a priori assumption of a density proffie. Close to the surface the density profile is nearly exponential, but at a greater distance from it there is a considerably higher density than the one calculated from Roe's theory. In a theta solvent the adsorbed amount depends logarithmically on the molecular weight. The depen­denee is less in a better solvent. This has been experimeritally confirmed (30). In agreement with the other theories, there is a non-zero critica! adsorption energy below which no adsorption takes place. The theories all agree with respect to the point that the adsorbed amount increases and the bound fraction decreases when x increases, i.e. fora decreasing solvent quality.

2.4 Adsorption, wettability and acthesion

2.4.1 Interfacial energy and wettability

Adhesion is related to the surface energy of the phases that are in contact. The surface energy depends on adsorption and also on density variations and surface charging. The surface energy can be obtained by contact angle measurements if the two phases can be separated without damage. After separation there can be relaxa­tion of the surface composition due to desorption. Adhesion is further influenced by meehamcal properties of the contacting phases, but adsorption is a key factor for adhesion.

11

According to Gibbs' law (32) adsorption of a surface active material (surfactant) is accompanied by a decrease in the interfacial energy. In wetting phenomena, i.e. when a liquid drop is in contact with asolid substrate, there are three relevant inter­faces at each of which adsorption may occur. The net effect of adsorption on a contact angle can be positive, negative or zero (33). At equilibrium the total free energy and not just the interfacial free energy has to be minima!. The interfacial tension ( the partlal derivative of the total free energy of the system to the area of the interface) and the interfacial energy per unit area ( the difference of the total free energy and the free energiesof the bulk phases) are unequal when adsorption occurs, but the changes in both quantities are the same. Also, Johnson (34) has proved that Young's equation:

'Ygy = 'YSL + 'YLV cos 8 [2.6]

and the defmition of the work of adhesion WA:

[2.7]

remain valid in systems where adsorption occurs. These equations therefore give the correct equilibrium conditions (33). 'Y AB is the interfacial energy between the phases A and B, and 8 is the contact angle at the three-phase line (section 3.4).

2.4.2 Adsorption theory of adhesion

Adsorption of polymers is partly determined by the forces between the polymer molecules and the substrate. Adhesion is caused by the work that is required to overcome the attractive molecular farces across an interface. A relationship must therefore exist between polymer adsorption and polymer adhesion.

The type of functional groups in the polymer chain and at the adsorbing surface, and the shape of the molecule are important for both adsorption and adhesion, because these factors determine the structure of the first polymer layer. Adsorption and adhesion experiments, however, usually have some conditlans that are largely different. Adsorption experiments mostly pertain to systems including a solvent, in adhesion experiments solvents are usually absent. In the case of a polymer melt both adhesion and adsorption can in principle be measured for the same system after solidification.ln this work, however, we shall only regard adhesive forces in a qualitative way.

In the adsorption theory of adhesion the van der Waals ( or dispersion) forces are the souree of adhesion. These farces are always present between two particles of any size or composition. Other interactlans (for example electtic farces) may supplement the dispersion forces, but in many cases they are of minor importance or even absent. When an initia! contact is formed by dispersion farces, relaxation

12

of the interfacial energy may occur by molecular changes in the interface region, i.e. adsorption. This increases the work that is necessary to separate the phases.

Adsorption at the interface can affect the supermolecular structure of a polymerie material, especially with crystalline polymers. The number and size of spherulites can be increased or decreased by the presence of asolid surface. This has an effect over a great distance ( sametimes more than 100 p.). The mechanica! properties of a surface layer alter, as do the measured adhesion forces (35, 36).

In the literature hardly any data can be found which campare adsorption with the thermadynamie work of adhesion. Adhesion is usually characterized by the separa­tion force in a non-equilibrium state and not by the thermadynamie equilibrium magnitude of the adhesion farces (37). Amongst others, elastic de formation forces have to be taken into account. Deformation farces cannot by themselves be considered as a souree ofadhesion. They may (and will) increase the workof separation only if some other force resists debonding. Further, it is often argued that failure of an adhesive bond is almast always partly cohesional (i.e. the separa­tion occurs in one or bothof the bulk materials) and not totally interfacial.

2.4.3 Other theories of adhesion

Electrastatic interactlans may be present in adhering systems as can be measured by the surface charge after separation (38, 39). Krupp (40) has shown that for polymers these farces are of comparable strength to van der Waals farces in the case of a sphere attached to a wall. The electrastatic forces are relatively unimportant in a plane-to-plane geometry (as in compression moulding) because the van der Waals forces between two planes fall offless rapidly with the distance than between a sphere and a plane.

Mechanica! interactlans can be a souree of adhesion by interlocking or keying ( 41 ). We have also found that rough metal surfaces with the same composition and the samesurface energy as smooth surfaces adhere more strongly toa polymer film than do smooth metal surfaces. Smooth metal surfaces have been used for all experimentsin chapters 4 to 6. An indication of smoothness is the lack of hysteresis of the contact angles. Hysteresis is the difference between an advancing and a re­ceding contact angle (42). lt is less than 5° on clean smooth surfaces (see section 3.4).

Adhesion of two phases can be caused by diffusion of one phase into the other, or both phases into each other ( 43). This is not important for metals well below their me1ting point, but it can happen with teflon substrates. Our XPS results point in this direction (section 5 .3). Another possible explanation of the presence of teflon is transfer by friction between a teflon substrate and a compression-moulded polymer. Metals are nottransferred to polymer surfaces during compression

moulding, but the reverse occasionally happens, probably because of cohesional failure.

13

Diffusion of low molecular weight compounds to the interface ( exudation) can be the origin of a weak boundary layer. A weak boundary layer can also be formed by oxide layers, adsorbed water or other impurities. Bikerman (44) even suggests that clean surfaces do not exist and that two matenals cannot be brought into contact without a layer of impurities at the interface. The strength of a joint always depends on the thickness and structure of a layer that is not part of one of the contacting phases. It is possible to take some precautions against weak boundary layers. In chapter 3 some polymers are listed that do not contain plasticizers or release agents. Residual monomer is absent since these polymers are synthesized in a dispersion reaction. Another precaution is the use of gold substrates, because gold does notforma weak oxide layer.

In Andrews' elastic deformation theory (45, 46), theelastic deformation farces are not additional to the thermadynamie work of adhesion, but the work of adhesion is multiplied by a factor that indicates the extent of non-equilibrium during breaking. The result is the adhesive failure energy. By measuring theelastic deformation and the energy stared in deformation it is possible to link the work of adhesion with the force and rate ofseparation of a bond. This theory has not yet been fully developed, but it promises to be the fust one to give a quantitative prediction of adhesion strength.

2.4.4 Combined effects

Obviously there is not one single model that can explain all the systems and experi­ments where two phases are in contact. In most practical cases several interactions wi11 work tagether to forma bond. Van der Waals farces are always active, and polar farces wi11 aften be present. They can be enhanced by adsorption, i.e. molecular rearrangements where strongly interacting species are positioned at the interface. In a few special cases it is even possible for primary chemical bands to be created across an interface, but chemisorption can be considered to be a special case of adsorption.

With the adsorption theory the influence of the environment ( especially water) on adhesion can conveniently be characterised by way of the work of adhesion.ln the expression for the work of adhesion (equation 2.7) termscan be incorporated that account for wetting of the new surfaces formed during debonding. A practical problem is that accurate measurements of mechanica! properties and deformations are necessary for an experimentallink with adhesion strength.

14

2.5 Adsorption from polymer melts

A polymer melt can be considered as a concentrated salution of polymer molecules. Individual molecules have random coU conformations. In small regions chains tend to be allgoed parallel or in lamellar structures and thus form nuclei of crystallites. Parts of one molecule can beloog to different lamellae. The transition ofa melt to a largely amorphous solidisnota sharp one, thermodynarnically or structurally. Kanig ( 4 7) shows that this is the result of a braad distribution of lamellar thick­nesses and to a smaller extent of conformational entropy changes on melting. Pechhold (48) proposes aso-called meander model of amorphous polymers. Bundies of polymer ebains have less free energy than interpenetrating coils. The molecules within the bundies have a random walk distribution. The bundies are folded on a cubic lattice. The overall geometry of one specific molecule has only short-range orderand the molecules interpenetrate because the bundies do so. The difference from a random walk distribution of segments is quite small. Pechhold's theory enables the calculation of molecular parameters in polymer melts.

In a homogeneaus polymer melt intermolecular and intramolecular interactloos are the same. Such a melt is therefore an athermal salution (x = 0). In random co­polymers where one type of monomede unit is in excess, the solvent quality is determined by the co-unit that is present in the greatest quantity. The other co­monomers can then he in a more or less favourable environment (x + 0), depending on the compatibility of the monomers. In block copolymers the mutual insolubility of polymers usually leads to miero-pbase separation where the size of the phase regions is determined by the size of the blocks. The geometrical distribution of the phases is determined by the relative amounts of monomede units ( 49, 50). With block copolymers the phase with the lowest surface energy tends to form the surface layer.

SU herberg has given a model for adsorption of homopolymers from the melt (51 ). The treatment is closer to the case of an isolated molecule than that of adsorption from a dilute solution. The first segment of an adsorbed molecule is assumed to be in the frrst layer. This is equivalent to the assumption that adsorbed molecules have only one taU. This is necessary to make Silberberg's calculations possible but it is probably not too restrictive. A quasi-lattice model is adopted where all sites are 9ccupied by polymer segments. The lattice has layers parallel to the surface. Partition functions are calculated for loops, tails, trains and non·adsorbed macro­molecules. The excluded volume has been ignored as it is known to have no effect on the coil conformation distribution in the melt. One result of SUbetherg's calcu­lations is that the structure of the melt near the surface is not affected by the magnitude of the energetic interactloos with the surface. Other implications of this theory are not yet clear. The assumptions that equilibrium is reached and that all lattice sites are filled cannot be fulfllled in reality. The importsnee of the non-valid· ity of some of the assumptions still bas to be evaluated.

15

Scheutjens and Fleer (30} have shown that their theory can be extended to very concentrated solutions and melts. In that case polymer molecules adsorb as random coils. In concentrated systems the adsorbed amount depends on the square root of the molecular weight, in contrast with dilute solutions where there is a logarlthmic dependence. The dependenee of several properties on the concentration bas been calculated. lt appears that long chains adsorb preferentially in dilute solutions, while the reverse is true in concentrated solutions. In dilute solutions the adsorp· tion distrlbution is determined by the loss of translational entropy of small chains. The conformational entropy loss of long chains predominates in concentrated solutions. These results have also been confirmed experimentally.

16

3. EXPERIMENT AL METHOOS

3.1 Introduetion

This chapter describes the experimental methods used for the investigation of the properties of polymer surfaces (7). Infonnation wi11 be given coneerDing the method and the materials used for compression moulding. Contact angle measure­ments are described together with the theory used to calculate surface energies from contact angles. Polymer surfaces are analysed by X-ray Photoelectron Spectro­scopy (XPS). A description of this technique is given including a discussion of its interpretation. Finally we will discuss chemical reactions to modify the surface in order to further analyse polymer surfaces (1).

3.2 Materials

3.2.1 Substrates

The substrates used for compression moulding are:

Au Gold foil, cleaned with alcohol before use. Ni Polished electroplated nickel, degreased for 24 hours in hexane vapour. Al Aluminum, made optically flat by diamond turning, degreased for 24

hours in hexane vapour. Cr Electroplated mirrorlike chromium layer on a nickel substrate. PTFE Teflon plate, made optically flat by diamond turning, cleaned with alcohol.

3.2.2 Polymers

We use the following polymers:

PVC/Ac VYHH, a random copolymer of vinyl chloride (VC, 87% by weight) and vinyl acetate (V Ac, 13%), Mn = 22000 (Union Carbide Chemieals Company).

PVC/ Ac/ Ale VAGH, a random copolymer of vinyl chloride (90%), vinyl acetate ( 4%) and vinyl alcohol (V Ale, 6%), Mn = 19000 (Union Carbide Chemieals Company).

PVC/ Ac(2) A random copolymer of vinyl chloride (94 weight %) and vinyl

PVC-P

PVCC PE-l

PE-2 PTFE

acetate. Polyvinylchloride (PVC), containing plasticizers (P) and stabilizers, granulate. Rhenoflex, chlorinated PVC with 31% additional chlorine substition. Lupolen DSK 1812, low density polyethylene (0918 g/cm3 ), melt­flow index 0.2, granulate (BASF). Polyethylene foil, thickness 0.4 mm. Teflon plate, polytetrafluoroethylene, thickness 2 mm.

17

Unless it is specified differently, these polymers are in powder form and do not contain plasticizers or release agents. All polymers are dried thoroughly and stared in vacuum over silica gel befare use. This is necessary to prevent degradation of the polymer on heating.

3.3 Adsorption and relaxation experiments

For compression mou1ding the polymer powder or granulate is placed between two flat substrates, positioned between the platensof a simple hydraulic press (fig. 3.1). The assembly is heated to a temperature (T p) usually above the melting point (Tm) of the polymer. The pressure is then brought to 5 to 8 MPa, resulting in a slight flow of polymer between the substrates. After 2 to 3 minutes the press platens are caoled to room .temperature (25 K/min). The compression-mou1ded ftlms have a thickness ofO.l to 0.3 mm. The substrate and polymer samples are small compared to the press platens, which keeps the temperature gradients in the sample negligibly small.

Figure3.1. Gompression moulding. The polymer rnass is placed between two substrates in a hydraulic press. The blocks above and below can be heated or cooled. The atmosphere is nitrogen, dried over a molecular sieve and silicagel. .

To prevent oxidation of the polymer, the melting and pressing is done in a dry nitrogen atmosphere (flow system) in which the polymer powder or granu1ate is flushed for several hours in order to remove oxygen and water. Small deviations in the pressure and pressing times do not influence the resu1ts, within the range of pressures and times mentioned above. On the other hand, adhesion depends strongly on temperature and on oxidation (if not prevented).

In order to prevent deformation of the polymer samples, the polymer film and mou1d are separatedunder water, which penetrates between the polymer and the substrate and effectively reduces the work of adhesion (52). The polymer ftlms are then dried in vacuum over silica gel. The surfaces are investigated with contact angle measurements and XPS (fig. 3.2). Separation under water does not influence the contact angles measured on the polymer surfaces. During a relaxation period

18

the polymer films are stared in vacuum over silica gel and at controlled temperatures ( ± 1 K). The relaxation temperatures are varled between room temperature and approximately 30 K below the melting point ofthe polymer.

H20 l No force

t777777772l Contact ""' .... :...._~ angles / XPS

, ... i ... ,, .... ,, ... , .... , ,~, ,,...., ,.., ,.,.., .... , j":"! l"':j ,-r,,~,,

(?ZZZZZZZZJ

Figure3.2. Release. The polymer and aubstrates are releasedunder water to prevent deforma­tion. The surfaces so formed are investigated with contact angle measurements and XPS.

Salution-cast films are prepared from solutions with a polymer concentration between 5 and 8 weight %. A layer of the viseaus salution is spread over a metal substrate and the solvent is allowed to evaporate. The polymer films are dried in vacuum at room temperature untll a constant weight has been reached. The polymer films retaln less than 1% of the solvent and no traces of solvents are found with XPS or with a quadrupale mass spectrometer attached to the XPS vacuum system. The fdms are separated from the substrates in the same way as with com· pression-moulded films. Same experiments involve salution-cast layers which are backed with a second metal substrate and then compression moulded. The interface with the origi.nal substrate can then be studled to determine if a salution-cast film has an energetically more favourable surface structure than a directly compression­moulded film.

3.4 Contact angle measurements and surface energy caleulations

Contact angles are measured directly from a sessile drop (32), using a Ramé·Hart

19

goniometer. The measurements are done in a dust free, thermostated atmosphere at 293 K with a relative humidity of 50%. The liquid drops have volumes of 1-5 ~tl. As the effect of gravity is negligible they form spherical caps on homogeneous surfaces. The size of the drops has no influence on the contact angles. An average of at least four readings is taken; reproducibility is usually within 1°. The surface tensions of the liquids are checked by means of the Du Nouy ring method (32) and are found to be in good agreement with literature values (table 3.1).

TABLE3.1

Surface tension of liquids

'YL (mN/m) 1i (mN/m) ref. Liquid exp. lit. lit.

water 72.0 72.8 21.8 55,56 glycerin 66.2 64.0 34.0 55,56 formamide 57.3 58.2 39.0, 32.3 56,57 methylene iodide 51.5 50.8 48.5 57 ethylene glycol 48.1 48.3,47.7 29.3 55,56,57 a-Br-naphthalene 45.0 44.6 44.6 55,57

The workof adhesion of a liquid on asolid is given by (see list of symbols):

[3.1]

where Young's equation (32) (fig. 3.3) has been substituted for 'Ygy- 'Ysv The spreading pressure 1re = 'Yg- bgy (53) is negligible in systems where 'YLV >rsv· It accounts for the influence of the vapour phase on the contact angle. This is important only when there is adsorption from the vapour phase at the solid surface. We found that the degree of saturation of the vapour phase has no influence on the contact angles measured on the polymer surfaces.

For the calculation of solid surface energies, we assume that the dispersion (van der Waals) and polar contributions to the work of adhesion are the only contributions, and that they can be treated separately:

[3.2]

Fowkes' geometrie mean equation (54) is used for the dispersion contribution:

w1=2J;R. [3.3]

20

Vapour

Uquid

'Vsv 'VsL

Solid

Figure3.3. Young's equation. An equilibrium of forces on the circumference of a sessile liquid drop gives: 'Y SV = 'Y SL + 'Y LV cos ().

For the polar contribution we compare Kaelble's approach (SS, 57) with that of Wu (58). Kaelble uses a geometrie mean for the polar as wen as the dispersion con· tribution to the work of adhesion. Wu proposes an. inverse harmonie or reciprocal mean for the polar forces acting across the interface:

[3.4]

Ex.perimentally we have found that our data can best be fitted to Wu's equation for the polar contribution to surface energies and to the work of adhesion. Since the polar contribution is usually small compared to the dispersion contribution, the use of other equations (58) yields only slightly different resUlts and these differences do not qualitatively affect the interpretation. Neglect of the polar contribution does not permit the fit of experimental data to equations such as Zisman's (59) and Fowkes' (54). Zisman calculates surface energiesof organic matenals by adding contributions of the chemica! groups that constitute the surface. This is not correct when the bulk of the material is affected, for example when surface groups have rnigrated from the bulk.

If we combine equations 3.1 to 3.4 there remain six variables to be determined ( 1r e is neglected): 'YLV• rÏ, 'Yt. d~' 'Y~ and cos(). 'YLV and cos() are determined experi· mentally, while values for 'YL are available from the literature (table 3.1). rt is taken to be the difference between 'YL and rÏ. implying that besides polar and dispersion forces, there are no other forces actingacross the interface, like hydrogen-bonding forces. lf we measure 'YLV and cos () fora series of six liquids (table 3.1) on the same homogeneaus surface (polymer or metal), we have six non-linear equations (one for each liquid) with two variables to be calculated:

21

'Y~ and 'Y~. This problem is solved by a least sum-of-squares method. 'Yg is assumed to be the sum of 'Y~ and 'Yg, as with the liquids. The solid surface energy may have a total error of approximate1y 1 mN/m originating from the inaccuracy of the measurement of the contact angles.

3.5 X-ray Photoelectron Spectroscopy

3.5.1 Principles ofthe method

Quantitative chemical analysis of very thin surface layers (1 - 10 nm) can be done by XPS (X-ray Photoelectron Spectroscopy), also named ESCA (Electron Spec­troscopy for Chemical Analysis ). The method has been described extensively, (60) and only a few important features are mentioned here (fig. 3.4).

M

Figure 3.4. A sample (S) is irradiated with photons from an X-ray gun (X). The excited elec­trans are retarded and focused, by an electron lens system ( R), at the entrance slit of an electron analyser (A). The analyzer separates the electrans according to their energy, a[ter which they are counted with the aid of an electron multiplier (M).

lnan ultrahigh vacuum system the sample is irradiated with monochromatic X-rays. Some electrous are excited and leave the atoms to which they were bound. Only electrans from atoms in the outermost layer of a solid leave the surface without loss of energy. These electrous have a kinetic energy equal to the difference of the X-ray pboton energy and the binding energy of the electron in the atom. Electrans that lose energy on their way out of the solid have a diffuse distribution of energies.

The electrans are spearated according to their energies and counted. The non· scattered electrans appear as discrete peaks in the spectrum while the scattered electransforma background level (fig. 3.5). The chemica! composition of a surface layer can be determined quantitatively, with an accuracy of 5 - 10%. The sample depth which is analysed (3X) is approximately 10 nm.

22

i counts

Figure 3.5.

Clauger

178 254 503 716 846 967 1046 EleVl-

XPS spectrum of PVC/Ac after compression moulding against Au. counts: arbitrary scale, E: kinetic energy in eV, X-rays: Mg Ka.

3.5.2 Apparatus

XPS spectra are recorded with a Leybold-Heraeus LHS 10 spectrometer, equipped with a hemispherical analyser, which is operated with a transmission energy of 100 eV (61) or with a retardation factor B=3. The analyseris positioned perpendie­war to the horizontal sample surface. We use an Al Ka X-ray souree (1486.6 eV) at 10 kV and 30 mA, at an angle of 60° to the analyser. With these X-ray doses the degradation of the polymers during the experiments is negligible. An X-ray teeat­ment of 1 hr has no intluence on peak positions and peak areas. After the samples have been removed from the vacuum they gradually deteriorate in air.

A depth profile of the surface layer can be determined by removing layers from the surface by means of Ar ion etching. The ion beam is scanned over the surface, the acceleration voltage is 1.2 kV and the pressure is w-6 mbar (analysis is carried out at 10-9 to w-s mbar). At low ion currents surface layers are removed at a rate of 0.1 to l nm/min. Using this method, XPS spectra can be taken at newly created surfaces at different distances from the original surface. Disadvantages are the lack ofknowledge of exact sputter rates, preferentlal sputtering of specific atoms and severe damage of the polymer samples.

Insomeofthepreliminaryexperiments (figures3.5 andS.l) MgK.a radiation was used (1253.7 eV), with a cylindrical mirror analyser (Physical Electronics Industries).

3.5.3 Interpretation ofthe XPS spectra

The XPS spectra obtained are fitted with Gaussian curves on a linear background to delermine peak areas with a least sum-of-squares procedure. In order to calculate

23

relative amounts of the different elements Scofield's cross-sections (62) have been used, corrected with the spectrometer transmission function and an angle-dependent factor (63). For the inelastic mean free path (À) of the electrans and the kinetic energy dependenee of À we use values reported by Seah and Dench (64). For organicmaterials(density 1.2 g/cm3

) and kinetic energies (KE) greater than 150 eV, we can write:

À=0.09yKE, [3.5]

where À is in nm and KEisin eV. The À valuesfrom equation 3.5 are in good agreement with our own experiments *) on thin organic layers on different sub­strates. For calculations on samples where the amount (c) of element A varles with the depth (x), we assume an exponential decay in intensity (1):

.. IA= const. fcA (x). exp (-x/ÀA) dx.

0 [3.6]

Polymers are electrical insuiators and therefore accuroulate charges during analysis; this superimposes a charge shift ( ± 1 e V) of the peaks in the spectrum on the chemica! shift. The charge shift is quite stabie and does not interfere with the analysis.

Some Auger peaks are visible in the spectra, but these are not used for further analysis. The most useful peaks correspond to bindingenergiesof 100 to 600 eV, which represent the inner (core) electrans for elements with low atomie number. The quantitative analysis of the polymers is basedon the CIS, OlS, F 18 and Cl2 p peaks.

3.6 Surface reactions (fJ&. 3.6)

The purpose of the reactions described in this section is to provide a specific label of hydroxyl or acetate groups at the surface with heptafluoro-butanoic acid chloride. The amount of fluorine measured with XPS can then be attributed solely to hydroxyl or acetate groups at the original surface. Carbon and oxygen in the sample could also originate from other chemica! groups. Furthermore the surface reactions are more surface specific than XPS analysis with its sample depth of approximately 10 nm. Simllar surface reactions have been used in combination with fluorescence spectroscopy ( 66).

*) Measurements of substrate and overlayer signals of barium stearate mono- and multi-layers on glass, indium oxide and aluminum oxide indicate that À = 2.S to 3.S nm for electrans with kinetic energiesof 900 to 1400 eV. The same values for À give consistent results for measurements of silanes, siloxanes, viologens andf.erfiuoracids on platinum, PMMA, aluminum oxide and silicon oxide. See also ref. 6 .

24

f~-CH3

Figure3.6. Surface reactions.

.llT ~OH

n-hexane

Esterification of alcohol groups is carried out in a 2% solution of heptafluoro­butanoic acid chloride in n-hexane ( dried over molecular sieve 4A), for 2 hours at room temperature. Acetate groups must first be hydrolysed in 0.1 N NaOH for 2 hours at 315 K, after which the polymer films are dried thoroughly. Using the hydrolysis reaction in combination with the esterification both vinyl alcohol and vinyl acetate groups are labelled. Using the esterification without the hydrolysis reaction, one obtains a labelling of the vinyl alcohol groups, but not of the vinyl acetate groups at the surface.

These reactions are limited to the surface region, because the polymers do not dissolve or swell in water or in n-hexane. This is confirmed by XPS analysis: if 1 nm is etched away with argon ions, no fluorine can be detected anymore. lf acetone is added to the NaOH solution more fluorine can be attached to the surface as a result of swelling of the polymer. The heptafluoro-butanoic acid chloride doesn't react with vinyl acetate groups if no prior hydrolysis has occured. The inability of reaction between an ester and heptafluoro-butanoic acid chloride is confirmed by means of gas chromatography, combined with infrared detection, on mixtures of butyl acetate and heptafluoro-butanoic acid chloride in n-hexane. Reaction products containing fluorine can not be detected.

4. THE SURF ACE ENERGY OF COMPRESSION-MOULDED POLYMERS

4.1 Introduetion

In this chapter we wil1 discuss the surface energy of polymer fûms made by com­pression moulding, i.e. from the melt. Fîrst of all we discuss the substrates used

25

for compression moulding. Then we show that the polymer surface energy depends on the circumstances of compression moulding, especially on the kind of substrate and on the temperature. In all cases the surface energy of a polymer is increased after contact with a metal substrate. Some parameters that influence the adsorptîon process are temperature, release agents and fillers. We also consider the adhesion strength of the polymer to the metal in relation to adsorption. Equally important to adsorption is the relax.ation process that occurs after removal of the metal sub­strate. The temperature dependenee of relax.ation times tells us something about the mechanism of desorption, and therefore presumably about adsorption too.

4.2 Subskates

The experimental surface energies of the substrates which we have used for com­pression moutding experiments are listed in table 4.1. It is a well known observatîon that liquid drops on cleaned metal surfaces have contact angles unequal to zero, contrary to what might be expected on high energy surfaces. Some authors suggest that metal surfaces are never completely clean (67) and/or oxidized. Further, it must be doubted whether the spreading pressure can be disregarded for these surfaces (68). Certainly, the surface energies calculated for the cleaned metals are much lower than expected from cohesive energy densitîes or surface tensions of molten metals (69). Nevertheless, the values in table 4.1 are reproducible. They a1so prove to be indicatîve of their behaviour with respect to molten polymers.

TABLE4.1

Surface energies of subskates obtained by contact angle measurements

cleaned after pressing Substrate 'YM(mN/m) p "') 'YM (mN/m) p

Au 43 .02 43 .02 Ni 37 .04 30-45 -Al 33 .20 >35 -Cr 28 - <40 -

I I PTFE 19 .00 - -

*) p is the polar fraction of the surface energy.

26

The surface energy of gold after pressing against polymer melts remains the same. This indicates that there is no contamination by polymer residues. The nickel surface does contain polymer remnants after the contact. The amounts depend on the polymer used and on the temperature during compression moulding. The nickel substrates are permanently contaminated after repeated cleaning and re-use. The results on the teflon plates are in agreement with literature data. To minimize contamination, substrates for XPS samples are cleaned samples that have not previously been used for pressing.

4.3 Polymer surface energies

4. 3.1 Changes in surface energy

The results of surface energy calculations for the polymers are summarized in table 4.2. The first column lists equilibrium values of surface energies. 'Y~ has been calculated from Zisman's table {59) on surface energies resulting from constituent groups of the polymer. Por PVCC, Kaelble's data {70) on polyvinylidene chloride are averaged with those on PVC. It is also possible todetermine 'Y~ experimentally. In order to measure contact angles of powders, measurements are made by the sessile drop method on liquid-saturated pressed pellets of which the porosity is known {71). Results scatter several mN/m, in contrast with measurements on flat surfaces. Polymer foils {PE-2, PTFE) can be measured directly, the results agree with literature data.

The most striking result in table 4.2 is that the surface energy of a polymer after melting and cooling in contact with a metal surface, 'Yp. is unequal to the equilib­rium value 'Y~· 'Yp depends on the type of substrate against which the polymer has been pressedandon the highest temperature employed during moulding. Substrates with high surface energies ('YM· table 4.1) or high temperatures (TB) result in high values of 'Yp· These conditions also enhance the adhesion strength ~ 72).

When after separation from the substrate the polymer films are stored at room temperature in air or vacuum and the surface energy is measured after a month, we fmd that 'Yp has decreased to 'YpR (table 4.2). The relaxation is not always com­plete, but it looks as if 'Yp tends to 'Y~ in the course of time. Relaxation does not occur as long as the polymer remains sandwiched between the metals at room temperature. Insection 4.4 it will beseen that the relaxation proceeds faster at higher temperatures.

4.3.2 Adsorption

Combining the results from the preceding paragraph with the XPS results (section 5.2) we find that adsorption ofpolar groups to the substrate surface has occurred during melting. There is an equilibrium situation at T P' which is frozen in by rapid

27

TABLE4.2

Surface energies of polymers

aft er before moulding after moulding relaxation

Polymer 'Y~(mN/m) substrate T p Tm 'Yp p 'YpR

lit. exp. (K) (K) (mN/m) (mN/m)

PVC/Ac 39 41 Au 403 393 49 .12 42 Ni 403 393 46.5 .11 -PTFE 403 393 42 .10 41

PVC/Ac/Ale 39 42 Au 383 393 44 .03 -Au 403 393 48 .06 45 Ni 403 393 47 .06 41

PVC/Ac(2) 39 - Au 413 403 47 .14 -Ni 413 403 43 .13 Cr 413 403 42 - -

PVCC 39 a) - Au 428 423 48 .19 -Ni 428 423 46 .17 -

PE-l 31 Au 428 398 46 .10 36 Au 358 398 37 .06 -Al 428 398 42 .01 34 Al 358 398 35 .03 -

PE-2 31 31 b) Al 398 383 38.5 .07 31

PTFE 19 19 c) Ni 423 600 25 .12 19 Al 423 600 23 .19 19

a)p=.07 b)p=.03 c)p=.OO

cooling, after which measurements are made. When the substrate is removed, the system goes slowly toa new equilibrium situation: no adsorption of acetate groups at the polymer surface.

The increase of 'Yp upon moulding can be explained as follows. At the polymer melt­metal interface there is a region where the polymer concentration, the density or the conformation will change due to adsorption. The driving force for these changes is a decrease of the polymer-substrate interfacial energy 'YpM· On rapid cooling of

28

the assembly this situation remains unchanged: the adsorption depends on the energy of interaction between substrate and polar segments. Removal of the substrate disturbs the equilibrium, but there is not enough time for complete relaxation (a matter of weeks at 295 K and of days at 365 K). This results in a non-equilibrium surface of relatively high energy 'Yp. which then slowly relaxes towards the equilibrium surface with a minimum energy 'Y~ = 'Y~· A high value of 'YM corresponds to a high initial value of 'YPM and to a strong driving force for adsorption at the interface. This leads to a situation further away from equilibrium (after the substrate has been removed), with a higher non-equilibrium energy value 'Yp·

The theoretica! surface energy of a roetal is much larger (1000 - 2000 mN/m) than the surface energy of a polymer. The polymer-metal interfacial energy is then approximately equal to the roetal surface energy (53, 54). This suggests a direct relation between adsorption and substrate surface energy. The experimental surface energy of metals as determined by contact angles is of the same order of magnitude as the surface energy of the polymers ( tables 4.1 and 4.2). The polymer-metal inter­facial energy then depends on relative contributtons of polar and dispersion forces. Section 4.3.3 shows that it is still possible to relate the experimental roetal surface energy to adsorption of polar groups in the polymer melt.

TABLE4.3.

Surface energiesof (partly) crystalline polymers

Polymer 'Y~ (mN/m)

PE 31

PTFE 19

FEP-Teflon 17

Kel-F 31

Nylon 6-6 46

substrate T p

(K)

Au 443 Ni 443 PTFE 443

Au 673

Au 589 Al

Au 503

Au 553

Tm 'Yp ref.

(K) (mN/m)

383 50 36 383 45.5 75 383 35 36

600 45 70,73

563 46 74 563 29 70

483 46 75

518 57 75

Changes in the surface energy of a polymer, depending on substrates, have been noticed before with (partly) crystalline homopolymers like polyethylene and fluoropolymers(7, 36, 73, 74, 75); see table 4.3. Forthese polymers an

29

explanation was sought in an increase of density and crystallinity in the interfacial region (35, 74). These observations may be explained by assuming that there is adsorption ofpolar groups (present by oxidation ofthe surface layer, as we have determined by SIMS experiments on polyethylene). Adsorption can subsequently lead to nucleation of a transcrystalline layer with a thickness of several microns. In the present case of vinyl copolymers with low crystallinity, adsorption reveals itself in a concentration gradient near the surface rather than in a density gradient.

4.3.3 TemperafUre dependenee of adsorption

The adsorption from the melt, or at least the change in surface energy of the polymer, is found todependon the melt temperature. One wonders ifthis phenomenon is thermadynamie in nature or that it depends on the rate of adsorption, which could then depend on temperature via the melt viscosity or the mobility of polymer segments.

(A:;J I ,'+2

,+1 I

I 1 I

I I

I

.5

.2

.1

2-

Figure4.1. ( !; )

Isotherms giving the relation between !1-ypl-fj,and 'YMh~(log scales). The symbols and numbers { 1 - 5) are related to different values ofT piT m {Figure 4.2).

ar 3

.2

.8 1.0 Tp1Tm­

Figure4.2.

1.

a as a tunetion of temperature.

To analyse this temperature dependenee of fl'Yp (= 'Yp- 'Y~ on the compression moulding temperature Tp we have (fig. 4.1) plotted fl-rphp as a function of 'YMh~ for different polymers and for different (maximum) compression moulding

30

temperatures ( tables 4.2 and 4.3). Por different polymers the temperatures Tg can be related to the melting points (Tm) of the polymers. The surface energies 'Yp. 'YM and the temperature Tm are thermodynamic equilibrium quantities.

In figure 4.1 points, representing different polymers, with the same value ofTp/Tm appear to lie on a straight line. The sets of points with different T .JT m values are represented by different parallellines. The slope of these lines {in lhe logarlthmic plot of figure 4.1) is 1.85 with a standard deviation of0.05. Hence it is found emperically that:

!J.'Y 'Y 1.85 _P=a{_M_) , (4.1] 'Yp 'Yp

where a is depending on Tp/Tm only. Since this observation involves a number of different polymers it seems safe to assume the !J.'Yp depends only on TpfTm and not on T p and Tm separately, and also that, since tJ.'Yp/'Yp is a function of 'YMI'Y~ and not of the separate quantities 'YM and 'Yp. the change tJ.-yp is caused by the same mechanisms for all the polymer-metal combiflations investigated.

Apart from a certain minimum time, necessary to reach equilibrium, time does not seem to play a role, except when oxidation of the polymer occurs. a is plotted against temperature in figure 4.2. There is a sharp increase of a with TpfTm at the melting point of the polymer. Mobility of the side groups of the polymer chain is possible at all temperatures above the glass transition temperature Cf gfT m ""'0.6), but this is clearly not enough to permit adsorption.

The relation between tJ.-yp. 'Y~ and 'YM cannot be derived theoretically. In view of the uncertainties about 'YM (section 4.2) this is not surprising. Equation 4.1 is useful only for the study of the temperature dependenee of adsorption.

Qualitatively, there is the following explanation. The first groups of a polymer molecule can adsorb at the metal surface without a change of conformation of the molecule. The energy gain is tJ.e for each adsorbing group. As the number of ad­sorbed groups increases, part of the adsorption energy is needed to compensate for lossof entropy. Adsorption stops when there is nonet energy gain for new adsorbing groups. lf the number of adsorbed groups is n, the total energy gain will be * n tJ.e. This is analogous to the cases of building up an electtic double layer or charging of a condenser. The system is now frozen in and the polymer and the metal are separated. The energy needed to break n bondsis n tJ.e. The metal surface has not been changed, but the polymer surface still contains extra free energy (* n tJ.e) as a remalnder of adsorption. This is measurable as an increase of the

31

surface energy Ä'Yp· Ä'Yp depends on ternperature, but .Ae doesn't *). Therefore n depends on the temperature: the number of adsorbed groups is limited by the energy that is needed to change the conformation of polymer molecules. The sharp increase of Ä'Y at Tm shows that molecules in a melt are more easily deformed than in the solid state. After remaval of the substrate the molecules will return to their equilibrium conformation (energy loss Y.t n Ae vs. entropy gain), but this is a slow process because ofthe low mobility in the solid polymer.

4.3.4 The influence of release agents and fillers

The change of the surface energy on melting against a substrate is found to be much smaller when the vinyl copolymers contain release agents and plasticizers, even at higher temperatures (table 4.4). We explain this as being due to competitive ad­sorption of low molecular weight components to the metal surface (76, 77) of which exudation may be considered to be an extreme case (78). This can lead to low adhesion strength by the formation of a weak boundary layer. Some of the release agents contammate the substrates, as can be measured with contact angles, resulting in low surface energies (30 - 40 mN/m). The action of the release agents depends on the specific surface area of the polymer material (granu­lated, powder or f!.lm) and, with block copolymers, on the distribution of the blocks in the polymer chain (7).

Carbon black in rigid PVC greatly reduces adhesion to the mould surface. This also happens with Aerosil (silica particles, 200 m2 /g) in vinyl copolymers or PVC. We can explain this if we assume the flller to be an active substrata with a large surface area for adsorption, thus effectively neutralising the adsorptive groups that can adhere to the mould. When we dehydrate the Aerosil surface (24 hours at 1200 K) to make it hydrophobic, the effect largely disappears. The effect offillers on the glass transition temperature (76), melt viscosity, molecular weight, mechanica! strength etc. may complicate or obscure this behaviour (79). Particularly in the presence of competing low molecular weight compounds, the mechanism may be reversed, so that overall adhesion increases when a filler is added.

4.3.5 Relation between adsorption and strength of adhesion

On the basis of the observations made when the polymers were released from the moulds, the polymers can be arranged in a series of decreasing strength of their adhesion toa metal substrate:

PVC/ Ac/ Ale> PVC/ Ac> PVCC >> PVC-P > PE >PVC/ Ac-P >> PTFE.

*) I::J.e is only slightly dependent on temperature, in the same way that x depends on temperature. This dependenee is probably not discontinuous at the melting point of the polymer.

32

TABLE4.4.

Surface energies of technically used polymers

'Y~ sub- Tp Tm .

Polymer 'Yp p

(mN/m) strate (K) (K) (mN/m)

PVC/Ac-P granulate 39 Au 423 413 46 .15 Ni 423 413 44 .20 Cr 423 413 42 .16

PVC/Ac-P powder .39 Au 433 413 41 .07 Ni 433 413 42 .05

PVC/Ac-Pa) foU 39 Au 433 413 42 .06

PVC-P granulate 39 Au 463 423 46 .15 Ni 463 423 44 .14

PVC/PV Ac/PE powder 33 Au 423 408 39 .02 b) Ni 423 408 36 .03

a) Astralon N b) Vestolit

The order in the series is independent of the substrate and the temperature. If adhesion is the result of adsorption we find the following strength of adhesion of adsorptive groups:

hydroxyl> acetate > chlorine > hydrogen > fluorine.

Polymers with low molecular weight additives show low adhesion strength because of exudation of these smaller molecules, which can prevent adsorption of polymer segments and evenforma weak interphase.

A simllar series can be constructed for the substrates, for which there is a correla· tion between adhesion, adsorption and '>'M• even though 'YM is only a value indicating the strength of interaction with some polar liquids.

In view of these results it is not surprising that most techniques for adhesion impravement (apart from chemica! bonding) have two aims: the prevention of the formation of a weak boundary layer, and the introduetion ofpolar groups into the surface (80, 81, 82). For example, a polymer sample treated in an oxygen plasma has a high surface energy and polarity as wen as strong adhesive tendencies.

33

4.4 Relaxation of polymer surface energy

4. 4.1 Reversal of the adsorption process

The surface of a solid plastic material, formed by compression moulding of a thermoplast in contact with a roetal substrate, bas a high surface energy in excess of the equilibrium value. After removal of the roetal substrate, the surface energy of the polymer relax es to a new equilibrium value. The surface energy and its time dependenee àre of practical interest because of the adhesion properties of the plastic after processing, for example in gluing, lacquering or metallization. There is also a theoretical interest because of the insight it provides into the underlying mechanism.

TABLE4.5.

Compression moutding parameters

Polymer Tp 'Yp 'Y~ (K) (mN/m) (mN/m)

PVC/Ac 403 49.0 ± 0.5 39 PVC-P 463 42.3 ± 0.2 39 PE 408 36.0 ± 0.5 31

The compression moulding parameters for three polymers used in relaxation experi­ments are üsted in table 4.5, which includes the temperature during moulding (T p) and the surface energy immediately after removal of the substrate ('Yp). The equilibrium value of the polymer surface energy ('y~ is from Zisman's table (59).

4.4.2 PVC/Ac

The time dependenee ofthe excess surface energy ofthis polymer, after removal of the substrate, is depicted in figure 4.3 for several storage temperatures. Initially the surface energy drops with an exponential time dependenee because of relaxation. Later on, the surface energy rises again, especially at high temperatures. This increase of the surface energy bas nothing to do with the relaxation process. lt is caused by polymer degradation, whièh becomes manifest in discoloration and embrittlement of the polymer films after long periods at elevated temperatures. The curves in figure 4.3 can be fitled to the data points according to:

I:J.'Y (t) = I:J.'Y (0) .e-kt + A.t, [4.2]

where k and A are temperature-dependent parameters, repcesenting the rates of relaxation and degradation respectively. We assume that the increase ofthe surface energy, resulting from the degradation of the polymer, is linearly time dependent.

34

10 PVC/Ac

i a E -z 6 E

>-<! 4

2

0 0 40 80 120 160

t ( hr) _____...

Figure4.3. Exceas surface energy vs. time tor PVC/Ac tor several temperatures.

Curve fitting with a different time dependenee of the degradation part in equation 4.2 does not seriously affect the calculated values ofk, because these are mainly determined by the initia! parts of the curves. After prolonged periods, when relaxation is nearly completed and the degradation slows down, the surface energy values tend to stabilize. The values ofk and A (found by a least sum-of-squares method) are listed in table 4.6. In an Arrhenius plot (log k vs. 1/T) ofthe rate of relaxation (figure 4.4) we find a straight line, from which an activation energy of 43 ± 2 kJ/mol for the relaxation processis determined for PVC/ Ac.

4.4.3 PVOP

The surface energy of this polymer shows a very fast relaxation to the equilibrium value. In the short experimental times, no degradation of the polymer can be de­tected. The excess surface energy is an exponentlal function of time only, as can be seen from figure 4.5. The measured relaxation rates are given in figure 4.4. The activa ti on- energy of relaxation of this polymer is 20 ± 3 kJ /mol.

The compounds oflow molecular weight in this polymer do not influence the contact angle measurements by exudation to the surface; cleaning the surface with ethanol has no effect on the contact angles measured after the ethanol film has evaporated.

Table4.6

Rates of relaxation and degradation of PVC/ Ac

TR k A (K) (h-1) (mN/m h- 1 )

363 .237 ± .009 .147 ± .002 348 .123 ± .007 .092 ± .003 323 .044 ± .003 .027 ± .002 308 .020 ± .002 .016 ± .006 295 .0084 ± .0004 -

5

l I 2 L. .t:. PVC-P .::t: 0.1

5

2

0.01

PVC/ Ac 5

2.7 3.1 3.5 1 o31T ! ", )____..

Figure4.4. Rate ofrelaxation vs. reciprocal temperature for PVC/Ac and PVC-P. o: data from section 5.3.

4.4.4 PE

35

With polyethylene and other partly crystalline polymers an increase of the density and crystallinity of the surface has been found after compression moulding against

36

f 5

E - 2 z E ;> <l

.5

0

Figure4.5.

2 4 t (hr )____.,.

PVC-P

295K

6

Excess surface energy vs. time {or PVC-P for three temperatures.

metal substrates (74). The change of density results in an increase of the surface energy, as with the vinyl copolymers.

From XPS measurements, combined with Ar+ ion etching, we find that PE contains a small amount of oxygen (0/C = 0.01) distributed homogeneously throughout a layer that is much thicker than the sample depth of XPS. This is confirmed by a SIMS experiment, where material sputtered away from the sample surface is analysed by mass spectrometry, giving a thickness of approximately 100 nm for the oxygen-containing layer. If oxygen is mainly present in the form of polar C=O groups, these may have adsorbed to the polymer-metal interface in the melt, in the same way as the vinyl acetate groups adsorbed in the vinyl copolymers. Insome way these adsorbed segments have nucleated the crystalline region. Since gold is not a catalyst for oxidation ofpolyethylene, in contrast with e.g. copper (83), any oxidation occurring during compression moutding should be homogeneaus through­out the material and would not specifically affect the polymer surface.

The relaxation experiments for PE are not influenced by further oxidation or de­gradation, as relaxation at high temperatures is practically complete within 4 hours . .:1-y(t) for this polymer fits the exponentlal term of equation 4.2 ( tigure 4.6). The Arrhenius plot of the rate of relaxation in tigure 4.7 shows two linear parts with an intersection at 325 K. At higher temperatures the activation energy of relaxation is 33 ± 3 kJ/mol, at lower temperatures it is 130.± 10 kJ/mol. A Differentlal Scanning Calorimetry experiment shows a peak at the melting point, with a tail extending to 325 K (figure 4.8). This indicates that crystallites disintegrate above 325 Kandit explains the decrease of the activation energy in the region from 325 to 380 K.

37

t 5 313K E -z

2 363K E PE

;>-1 <l

0 2 4 6 t (hr)___.

t 5 E 303K

z E 2

323K ;>- PE <l

1 0 10 20

tI hr) ___..

t 5 E 0 295K -z E 2

~ 313K PE

0 100 200

t [hr)___.

Figure4.6. Excess surface energy vs. time {or PE in a range of temperatures.

4.4.5 The activation energy o{relaxation

Activation energies of molecular motions in polymers have been measured in NMR (84, 85, 86), dielectric (87), ultrasonic (88), mechanica! (89) and other experiments. The relaxations are usually divided into three groups (90): - At relatively low temperatures the local mode relaxations are found, with acti­

vation energiesof 4 12 kJ/mol, which are related to vibrations and rotations of polymer groups not betonging to the main chain. This type of relaxation plays no role in the present experiments as all measurements are perfonned above room temperature, where side group movements are unrestricted. This type of motion does not result in the displacement of large sections of molecules.

- At higher activation energies, 12- 65 kJ/mol, we find the regime of the {j-re­laxations, which are connected with kink movements and the mobility of chain segments in amorphous polymers.

38

J 0.1

0.01

0.001 PE

2.7 3.1 3.5 103/T ( K-1) ___,.

Figure4.7. Rate ofrela:xatlon vs. reciprocal temperafUre for PE.

- At still higher energies there are the a-rela.xations and crystalline relaxations, correlated with coupled dislocations and the mobility of chain segments near crystallite surfaces.

The activatien energies found from the surface energy relaxation rates fall in a range connected with movements of molecular chain segmtmts ~relaxations ). When a gradient (of composition or density) exists, these movementscan result in rearrangements or displacements of molecular segments, as is the case in desorption after removal of a substrate. This suggests that diffusion of segments to or from the interface is the mechanism of adsorption and desorption causing the surface energy changes which are described in this chapter.

For PVC/ Ac we fmd an activation energy for surface energy relaxation of 43 kJ/mol, which lies in between the expertmental values of 40 kJ/mol and 63 kJ/mol for the

i (/) -c: ::J

>. r.. c r.. -:ö c.. c 0 1-<J

Figure4.8.

380 340 ••-- T(K)

PE

300

Differential Scanning Calorimetry experiment on PE.

39

/3-relaxations of pure PV Ac and PVC respectively (84, 85, 87, 91). The copolymer, which consistsof 10% vinyl acetate units, has an activation energy close to that of PVAc. This low value is consistent with the irregular structure of a random co­polymer, which prevents close pacldng ofthe chains. The copolymer also has a low glass transition temperature. We may conclude that the surface energy relaxation is connected with segment mobility and that it is the result of desorption of the polar acetate groups from the polymer surface (section 5.3).

The plasticized polymer PVC-P shows a significantly lower activation energy (20 kJ/mol) than PVC (/3-relaxation 63 kJ/mol). This can be explained by the effect of plasticizers, by which the mobility of the polymer ebains is enhanced, giving a greater free volume and a lower glass transition temperature. Thus the activation energy for the diffusion of segments away from the surface drops and the rate of relaxation increases.

With crystalline polymers the nomendature and explanation of the relaxations differs from the above. For PE we fmd an activation energy of 130 kJ/mol at temperatures below 325 K, which is reasonably consistent with the /3-relaxation activation energy of 140 kJ/mol (89). This is the primary dispersion due to the amorphous region in polyethylene and is connected with segmental motions. Above 325 K the activation energy is lower. As the crystalline fraction of the polymer diminishes, the polymer ebains have more freedom to move and rearrange them­selves to lower the polymer surface energy.

40

S. SURFACE ANALYSIS OF COPOL YMERS AFTER COMPRESSION MOULDING

S .l Introduetion

This chapter is divided into two sections, each with its own introduetion and discussion, since we are dealing with two different approaches for the analysis of the compression-moulded samples by XPS. The first section describes the analysis of copolymer surfaces with a combination of XPS and ion etching of the surface. The interpretation of XPS as described he re is used in section 5.2 only. In all other experiments we have used the more general calculation metbod described in section 3.5.3. The XPS results are difficult to explain quantitatively, even when ion etching is introduced to analyse the composition as a function of depth. In the second section XPS is combined with a surface decaration technique. In that case a surface reaction of hydroxyl and acetate groups with heptafluoro-butanoic acid chloride precedes the XPS analysis. After the reaction the amount of fluorine is measured with XPS. The concentrations of hydroxyl and acetate groups at the surface are determined separately. Results are then compared with the results from contact angle measurements for the same surfaces.

5.2 XPS combined with ion etching

5.2.1 Interpretatkm

After analysis of a compression-moulded polymer a surface layer is removed by ion etching up to a depth x, so that a new surface is obtained which can be analysed with XPS. The observed composition of the sampled layer starting at depth x is indicated by fobs (x). This is not the actual composition at x but the exponentially weighted average of a (new) surface layer. It is assumed to be related to the actual composition, fact (x), as follows (80):

.. f fact (x) . exp((x

x s)/À) ds

fobs(x)= --------.. f exp((x- s)/À) ds

x

[5.1]

where À is the mean free path of the electrans in the sample and s is the distance of the sampled region to the original surface. In the calculations in this section (5 .2) À is assumed to be constant within the kinetic energy range of C1 s and 0 1 s electrons. The value used for À in this sectionis 3 nm (64). From equation 5.1 it can be derived that (92):

fact (x) = fobs (x) d fobs (x)

dx [5.2]

41

This technique can be used for the analysis of the surfaces of both the metal sub­strates and of the polymers after compression moulding .

5.2.2 ~ubstrates

An XPS experiment on cleaned gold foil (before use), reveals a slight contamination with carbon. There is an unresolved C1 s peak in the spectrum, which disappears partly after Ar+ ion etching (10 minutes). As the gold contains less than 10 ppm of carbon, this means that during etching part of the carbon is not removed but scattered into the gold. Altematively, the contamination originates from the vacuum system. After pressing gold foil against PVC/Ac (403 K) small patches of polymer are left on the surface. Oxygen and chlorine and a double C1 s peak are present. After an etching period of 4 minutes oxygen and chlorine have disappeared, the amount of carbon has decreased and shows a single peak again. The amount of polymer is too small to be detected by contact angle measurements.

5.2.3 PVC/Ac

PVC/ Ac is melted against Au at 403 K. The surface of the resulting polymer fûm is analysed with XPS. The results are summarized in table 5 .1. A representative spectrum ("first spectrum" in table 5.1) is shown in figure 3.5. There is no sign of metal contamination of the surface; this is also true of all further XPS experiments on polymer surfaces.

TABLE 5.1

XPS results on PVC/Ac, binding energies

elS Ou Cl2p *)

energy half- energy half- energy half-width width width

Experiment (eV) (eV) (eV) (eV) (eV) (eV)

first spectrum 294.1 1.81 541.0 1.87 209.4 1.94 1 minute 500 V At etch 294.1 1.88 540.8 1.90 209.5 2.01 1 minute 1 kV Ar+ etch 293.7 1.95 540.2 1.83 208.6 2.07 3 minutes.l kV Ar+ etch 293.6 1.95 540.0 1.89 208.7 2.10 8 minutes 1 kV Ar+ etch 293.3 1.92 540.0 1.97 208.2 2.08

13 minutes 1 kV Ar+ etch 292.8 1.94 540.1 2.03 208.2 2.20

*) not well-resolved in 2p1 12 and 2p3/l

42

For quantitative analysis the areasof the 0 1 s and C18 peaks can be used(table 5.2). As the etch rate (y) is not exactly known, fact (x) is calculated forsome representa­tive values, viz. y = 0.2, 0.3 and 0.5 nm/min respectively. Results are given in flgure 5.1. For y > 0.5 nm/min, fact (x) 5!!! fobs {x) since the second term in equation 5.2 is then relatively small. For y < 0.2 nm/min fact {0) would have to be negative to explain the value of fobs (0). The limiting value of fact (x) at great depth is not found equal to the bulk composition, probably because Scofield's cross sections (62) do not apply exactly here. It may be necessary to use an internat standard. Furthermore À is not totally independent of the kinetic energy of the electrons as is assumed in this calculation.

TABLES.2

XPS results on PVC/ Ac, quantitative analysis

1 kV etch time 0/C ratio

u > a.. c:: 0 .5 ti 0 ... -(I)

0 E

F'zgure5.1

(minutes) ( from peak areas)

0 .190 1 .165 3 .135 8 .122

13 .110

bulk

3 6 depth x(nm)---+

mole%VC

76.6 80.2 84.3 86.2 87.6

Mole fraction of vinyl chloride in PVC/Ac after compresston moulding against Au, as a lUnetion of distance to the surface, calculated for various values ofy, the un­known etch rate. 1 to 3: fact (x) for y is 0.2, 0.3 and 0.5 nmfmin respectively.

43

Oxidation of the polymer, if it occurred at all, cannot explain the profiles in tigure 5 .1. The small N 1 8 peak ( from nitrogen adsorbed on the polymer powder before melting) which is visible in the first spectrum (see tigure 3.5), is still of the same magnitude after 13 minutes of etching. Wherever nitrogen is present, oxygen (if present) has access too, and oxidation should be homogeneous.

The presence of some carbon on the metal surface, which could be transferred to the polymer, does not invalidate the conclusions drawn from the depth profillng. The 0/C ratio decreases with depth (table 5.2), but a significant amount of non­indigenous carbon on the surface would have lowered the 0/C ratio there. Thus the profiles in tigure 5.1 could only underestimate the difference between the bulk and the surface composition.

counts

t

295 285 BE (eVl---

Figure5.2. XPS spectra of PVC/Ac, C1 s peak. BE: binding energy in eV. a: at the surface, b: after Ar+ ion etching (3 min).

The composition vs. depth proflle found above is confmned by the chemica! shift of the C1 8 peak(s). The spectra are not very well resolved. Therefore all C1 8 modes cannot be seen separately. A spectrum of the surface shows a peak at 287 eV (C-O) with a shoulder at 285 eV (C-Cl); see tigure 5.2 (peak a). After argon ion etching there is a peak at 285 e V with a shoulder at 28 7 e V; see figure 5 .2 (peak b ). In this case the binding energy values have not been corrected for the charge shift, but the 0 18 peak for both spectra is found at the same spot, indicating that the charge shift in both cases was the same.

44

Both the composition vs. depth profile and the shift of the C1 s peaks indicate that the surface has been enriched with acetate groups. It follows that adsorption of vinyl acetate groups has taken place to the polymer-substrate interface during melting. The depth of the reorganised layer is less than 10 nm and the surface contains approximately 50% vinyl acetate. Such a shallow layer could not be detected by infrared analysis (81), for example.

A distance of 10 nm is not larger than the average radius of gyration of a polymer coil. Let us consider the non-crystalline polymers to be a solution of random coils in a medium of similar molecules. If one polymer chain (M = 22000) is a spherical random coil ofwhich the segments occupy between 1 and 10% ofthe volume that it pervades (19), the diameter would be 17 or 8 nm respectively. Arearrangement within such molecules (with on the average 300 vinyl chloride monomede units and 30 vinyl acetate units), appears to be quite feasible and forms an acceptable explanation for the composition vs. depth profdes. In a melt the polymer con­formation can be changed by adsorption, but not from a random coil to a trains and loops conformation (section 6.3). The segmentsof an adsorbed polymer molecule are on the average closer to the interface in the case of trains and loops adsorption than is possible in an unperturbed random coil.

5.2.4 Discussion

Ion etching results in severe damage to the polymer structure and sometimes also in charging of the surface. The sputter rates are not accurately known and are different for different species. For example, barium from barium stearate multi­layers deposited on aluminum, is still found on aluminum after the aluminum oxide has been completely etched away. For these reasons we have developed a new method for the further quantitative analysis of polymer surfaces with XPS. This method consists in binding groups in the polymer surfaces with compounds that have easily identifiable peaks in the XPS spectrum. By choosing solvents which do not penetrate into the polymer such a reaction can be limited to the surface region. Therefore the information obtained is even more surface specific than XPS itself. Ion etching can be used to show the absence of marker reagents in the bulk of the polymer.

5.3 XPS combined with surface reactions

5.3.1 PVC/Ac/Ale: hydroxyl vs. acetate groups

After compression moutding against gold substrates at 408 K the surface composi­tion of PVC/ Ac/ Ale is determined by performing the acid chloride coupling reaction (section 3.6) withand without previous hydrolysis. In the first case both vinyl acetate and vinyl alcohol groups react, in the second case only vinyl alcohol groups react. From the fluorine: carbon ratio as measured with XPS we calculate

45

(using equation 3.6) the fraction ofthe surface that is covered with a fluorine containing layer. This is the fraction of the surface that originally contained vinyl acetate and/or vinyl alcohol groups. We have assumed that the thickness of the fluorine containing layer is 0.5 run, which seems to be a reasonable value in view of the structure of the molecules. If we take a smaller thickness, or assume that the surface reactions are incomplete, even larger fractions of vinyl acetate and vinyl alcohol groups in the surface are found. There is, however, not enough oxygen in the original surface to justify such assumptions.

TABLE5.3

Composition of PVC/ Ac/Ale, compression moulded against gold.

Volume fractions: VAle V Ac

Surface after compression moutding 0.28 0.048 Surface after 18 hours relaxation at 343 K 0.23 0.045 Bulk 0.06 0.06

Table 5.3 gives the results for samples on which the coupling reaction was performed immediately after compression moulding and for different samples, which were decorated after (incomplete) relaxation during 18 hours at 343 K. In both cases it is found that the surface is enriched in vinyl alcohol groups but that the vinyl acetate concentration is approxlmately equal to the bulk concentration. Upon relaxation the vinyl alcohol concentration decreases. This agrees with a model for adsorption, in which after removal of the substrate the polymer relaxes to an equilibrium situation with fewer polar groups at the surface. The fractions listed in table 5.3 are volume fractions, which are approxlmately equal to weight fractions, because the different constituents of the copolymer have similar densities.

From the surface composition and from the oxygen content of the subsurface layer we can calculate the concentration as a function of depth. Experiments with argon ion etching indicate that the composition of the surface region varles smoothly as a function of depth, without a depletion region. Let us assume that the volume fraction of vinyl alcohol groups decreases exponentially from the surface to the bulk value and let us take a constant value for the volume fraction of vinyl acetate groups:

c(VAlc)=0.06 +0.22 exp(-x/d), c(V Ac) = 0.04, [5.3]

where x is the distance to the surface and d is a measure of the ex tent of the ad­sorption layer. At the surface (x= 0) the fraction of vinyl alcohol groups is 0.28, in the bulk (x= oo) this is 0.06. From equations 5.3 we calculate the concentration

46

of oxygen as a function of the depth x, using bulk values for the densities of the polymer fractions. Equation 3.6 gives the expected signa} intensity. From the experimental oxygen:carbon ratio (0.097) for PVC/ Ac/ Ale measured with XPS after compression moulding, it is found that dis 3.1 ± 0.3 nm. The total amount of vinyl alcohol groups adsorbed in the surface and subsurface layer appears to be 10-9 mole/cm2 • In the first layer near the surface there are 2•10-10 mole/cm2

vinyl alcohol groups. Since 24 hours have elapsed between the release after com­pression moulding and the XPS experiment (at room temperature) some relaxation may already have occurred.

5.3.2 Adsorption and relaxation of vinyl acetate groups against gold

After compression moulding PVC/ Ac against Au at 408 K, sections of the polymer sheet are stored in vacuum at 343 K during different periods and at 293 K for the rest of the time period up to a total of 48 hours. In this way the surface reactions on the different samples can all be performed in one batch. Since relaxation at 293 Kis much slower than at 343 K (section 4.4) polymer surfaces in different stages of relaxation can be obtained. From the fluorine:carbon ratlos it is found that the average thickness of the fluorine containing layer is 0.2 ± 0.1 nm. This value is smaller than was assumed in the previous paragraph. However, higher values would correspond to such low surface fractions of vinyl acetate groups that they are incompatible with the amount of oxygen which was measured directly with XPS. An explanation for the low value may be a surface structure that does not make all groups equally attainable, i.e. an incomplete reaction. For further calcula­tions we have nevertheless used a value of 0.2 nm for the average thickness of the fluorine containing layer.

TABLES.4

Relaxation of PVC/ Ac compression moulded against gold

· Time at 343 K Time at 293 K F/C Surface fraction V Ac

(h) (h) fmeas

0 48 0.015 0.25 2.5 45.5 0.012 0.19 7 41 0.007 0.11

23 25 0.004 0.07 47 1 0.010 ! (0.15)

Table 5.4 shows that after remaval of the substrate the amount of vinyl acetate groups in the surface decreases with time, as expected. After longer relaxation times, when the degradation of the polymer begins to interfere (these polymers

47

do not contain stabilizers), we fmd an increase in the amount ofheptafluoro­butanoic ester that can be attached to the surface. An experiment in which the hydralysis step is skipped shows that this increase is not caused by acetate groups. On a fresh sample of PVC/ Ac fluorine cannot be attached, but after 24 hours at 363 K we find that the fluorine:carbon ratio is 0.012 after reaction with heptafluoro-butanoic acid chloride. This shows that degradation or oxidation of the polymer creates groups at the surface that can react in the same way as hydroxyl groups. Similar observations were made in the study of the surface energy, which decreases through relaxation, but later increases through degradation of the polymer (section 4.4).

The fraction of the surface covered with the heptafluoro-butanoic ester layer (fmeas) as a function of the timet of relaxation at 343 K, can be written as:

fmeas(t) = feq + f0 * exp (-k2 t) * exp (-k1 (48-t)) +A* t, [5.4]

using an exponentlal time dependenee for the relaxation process. The first term, feq' is the equilibrium value of the surface concentration of vinyl acetate groups. feq is not necessarily equal to the bulk composition. The second term shows the increase of the surface concentration immediately after compression moutding, f0

, diminished through relaxation in two steps: during a time t at 343 K with rate constant k2 , and duringa time 48-t at 293 K with rate constant k1 • The last term in equation 5.4 contains the rate of degradation A. Using for k1 the rate of relaxation as determined from surface energies (k1 = 0.008 h-1

) the other parameters can be calcutated by a least sum-of-squares fit of the data in table 5 .4 to equation 5.4. The rate of relaxation at 343 Kis 0.13 ± 0.01 h-1 , which compares well with the rate determined earlier from surface energies: k2 is 0.11 ± 0.01 h- 1 •

These rates indicate that whole polymer segments are reoriented. The surface fraction of vinyl acetate groups after complete relaxation feq tums out to be zero, indicating complete desorption of vinyl acetate groups or adsorption of vinyl chloride groups. Forthermore we fmd that the surface fraction f

0 of vinyl

acetate groups immediately after compression moutding is 0.36, and that the rate of degradation A is 0.004 h-1 •

The oxygen:carbon ratio for PVC/ Ac immediately after compression moutding is 0.11. From this we find that the thickness d ofthe adsorbed layer is 3.8 ± 0.3 nm ( calcutated as for PVC/ Ac/ Ale but now for vinyl acetate groups, see above ). This gives a total amount of 1.4 * 1 o-9 mole/cm2 vinyl acetate groups adsorbed directly after compression moutding and -0.6•10-9 mole/cm2 for the equilibrium situation. At t = 0 there are 3•10- 10 mole/cm2 vinyl acetate groups in the first layer near the interface (fig. 5.3).

The choice of 0.2 nm for the average thickness of the fluorine containing layer affects the calculated surface fractions directly, but the adsorbed amounts are mainly determined from the oxygen contents of the subsurface region. The

48

1

1 u ~ VC

c .e .... r "' 1Cf9 mol I cm2

u e -Ql ö ~

V Ac 0

0.3nm 15 nm

--•Depth Figure5.3. Adsorption of vinyl acetate groups in PVC/Ac. In a region near the surface, the concentranon ofvinyl acetate groups (V Ac) is higheraftercompression moulding against Au, than in the bulk (0.1). Directly at the surface, approximately 40% of the macromolecu/ar units are vinyl acetate units. This is also the fraction of the surface that can be covered with a thin layer (0.2-0.5 nm for different polymers) of heptafluoro-butanoic ester groups. The tot al adsorbed amount is the shaded region, the amount in the jirst layer can be calculated from molar volumes and the mole fraction of vinyl acetate immediately at the surface.

cal.culated ra te of relaxation at 343 K is independent of this choice, as can be seen from equation 5.4, where all fmeas val.ues will be equally affected without changing the time dependence.

5.3.3 Adsorption of vinyl acetate groups on different substrates

For PVC/ Ac pressed against different substrates we ex peet a positive correlation between the surface energy ('y) of the substrate and the number of adsorbed acetate groups (4.3.2). However, table 5.5 shows that the smallest amounts of fluorine are found on polymers pressed against substrates with the highest surface energies. In the case ofthe PTFE substrate this discrepancy is easlly explained by transfer of PTFE, before any surface reaction has been performed. This can be shown by the fluorine:carbon ratio measured on the PVC/Ac surface after compression moulding against PTFE (F/C = 0.29). No traces of the metal substrates are found on the polymer surfaces.

The measurements on PVC/ Ac, compression moulded against Ni or Al, indicate that oxidation of a surface layer has taken place (in contrast with gold substrates). This

49

oxidation is confmed toa very thin layer, but it is exactly this layer that is analysed by means of surface reactions and XPS (1 ).

TABLES.S

PVC/ Ac pressed. against various substrates

Substrates Au Ni Al PTFE

F/C with hydralysis 0.010 0.011 0.021 0.19 *) F/C without hydrolysis 0 - 0.012 -0/C before surface reactions 0.13 0.16 0.13 0.07

'Y polymer (mN/m) 51 48 46 38 'Y substrate (mN/m) 43 37 33 19

*) transfer of PTFE from the substrate, see text.

5.3.4 Discussion

Decaration of molecular units at the surface of a copolymer yields infonnation on the composition vs. depth profile of vinyl copolymers after compression moulding and subsequent relaxation. It is assumed that the fraction of the surface covered with the heptafluoro-butanoic ester confonns with the original fraction of vinyl acetate or vinyl alcohol groups, while the vinyl chloride groups remain unchanged. The composition of the polymer as measured with XPS gives information on a larger depth scale ('Y = 2.5 3.5 nm, giving a sample depth of""' 10 nm) than the reactions (less than 1 nm). The combination of XPS and surface reactions indicates how the composition changes as a function of the distance from the interface. As the average thickness of the fluorine containing layer is not know accurately, there is some uncertainty in the quantitative interpretation. Nevertheless, for sets of measurements on one type of surface a good agreement is found with earlier results (chapter 4). The observations on the rates of surface relaxation are certainly valid, while absolute values of surface coverage are correct to within 20%.

For PVC/ Ac/ Ale it may be concluded that the active groups in adsorption are the vinyl alcohol groups. The thickness of the adsorption layer is approximately 3 nm, which is well within the diroenslons of the polymer molecules, as expected (section 5.2.3).

The results of the experiments on relaxation of PVC/ Ac are similar to those of the measurements with contact angles ( section 4.4). The ra te of relaxation at 343 K as determined by these two methods is the same within experimental error. We may

50

conetude that the surface energy changes are indeed caused by adsorption and desorption of the vinyl acetate groups. Degradation of the polymer during storage for long perlods at high temperatures produces some compHcations. The adsorption layer has sub molecular thickness, as in the case of PVC/ Ac/ Ale.

6. SURF ACE ANAL YSIS OF COPOL YMER FILMS, CAST FROM SOLUTIONS

6.1 Introduetion

51

In chapter 1 we have stated the hypothesis that the adsorption of polymer molecules from a melt is comparable to the adsorption of a (concentrated) solution of the polymer. In chapter 2 we have discussed the theory of polymer adsorption from solutions. Having described the adsorption phenomena from the melt we shall now turn our attention to the adsorption behaviour from solutions of the same polymers that were used in chapters 4 and 5. Por this we shall use the same tech­niques for surface analysis (chapter 3) which were applied to compression-moulded fûms, on polymer films cast from different solutions. The resulting data are the volurne fractions of vinyl acetate groups in the surface layers and the surface energies of the fûms.

POLYMER

SOLVENT )J

SUBSTRATE

Figure 6.1. Interactions between polymer, substrate and solventand the parameters or experi­ments relevant to these interactions.

We choose solvents in a range of different qualities to examine the effects on the outer surface after drying of the fllms and removal of the substrate. In the previous chapters we had to assume that the structure at the interface which existed during compression.moulding was frozen in by cooling the samples. In the experiments with cast fûms we hope that the influence of solvents on the structure and ad­sorption of macromolecules at the interface remains measurable after the solvent has evaporated from the cast ftlm. These hypotheses are both supported by experi­ments (7, 9, 93). The environment of a polymer molecule determines if and how it adsorbs: a good solvent will diminish adsorption, and a bad solvent will enhance it. Moreover, polar solvent molecules may adsorb at the metal surface in competition with adsorbing polar groups of the polymer and thus decrease the number of polymer-metal contacts (figure 6.1).

52

The experiments with cast f!.lms provide the opportunity to compare the results from moulded and from cast films. To do this one might descrlbe a homopolymer melt as a salution of macromolecular segments in a medium of similar molecules. In a copolymer we can then have slightly different interactions between different segments, so that the melt forms a medium of different solvent quallty for the different segments of the molecule.

6.2 Polymer films cast from solutions

6.2.1 Solvent properties

In table 6.1 we have listed the measured surface compositions oflayers cast from solutions together with some of the characterlstics of the solvents and solutions. At metal surfaces polar molecules adsorb more strongly than non-polar molecules. Hence the dipole moment p. of the solvent molecules (94) can be considered as a measure of the tendency of the solvent to adsorb.

The interaction between polymer and solvent is best described by the Flory-Huggins parameter x or the intramolecular expansion factor a.1 (19). In anideal solvent X= 1/2 and al = 1, because the random coil conformation of the polymer molecules has unperturbed dimensions in this case. In a good solvent a.1 > 1 : the end-to-end distance of the polymer random coil increases with improved solvent quality. This also increases the intrinsic viscosity [11] of a polymer solution, which is therefore an indication of solvent quality. The intrinsic viscoslty, or limitins viscosity number, is given by:

11/118 - 1 [11] = lim --

c~o c [6.1]

where 11 is the viscosity of a solution with polymer concentration c, and 'lls is the viscosity of the pure solvent (95). The intrinsic viscosity is obtained by measuring the viscosity of solutions with different concentrations in a Couette (concentrlc cylinder) viscosimeter and extrapolating to zero concentration as in equation 6.1. In a good solvent the polymer random coil is large in order to maximize the number of polymer-solvent contacts. The large coils result in a high intrinsic viscosity. The intrinsic viscosity of the PVC/ Ac solutions is listed in table 6.1. For a calculation of al from these data the molecular weight dependenee of ['11] must be known, but for PVC/ Ac solutions these data have not been obtained.

Another clue to solvent-polymer compatibility is the interaction radius R of Hansen's solubility spheres (96, 97, 98). Ris calculated from the solubility para­meters li of the polymer-solvent pair. a is the root of the cohesive energy density {99). Hansen distinguishes betweèn contributtons from dispersion interactions, polar interactions and hydrogen-bond interactions, see table 6.2:

53

R2 = 4(40d)2 + (.MiJ2 + (àc5pl2 '

where àö = ö(solvent)- ö(polymer).

TABLE6.1

[6.2]

PVC/Ac adsorption from solutions, properties of solvents and solutions

IJ. [??] R2 Volume fraction V Ac *) in the surface layer

solvent (D) (ml/g) (cal/cc) Al Au

cyclohexanone 2.9 53 2.1 0.22 0.25 methyl ethyl ketone 2.8 39 1.9 0.28 0.24 ethyl acetate 1.8 41 4.1 0.38 0.28 n-butyl acetate 1.8 40 4.3 0.37 0.19 chloroform 1.0 23 6.1 0.47 0.23

*) R2 is the square ofHansen's interaction radius, see text.

Ris therefore the distance between polymer and solvent in the {) d• öp, {)h space. It is a semi-empirical indication of solvent quality. A small value indicafes a good solvent for PVC/ Ac. The data for PVC/ Ac can be calculated by the summation of contributions from molecular groups (96, 1 00).

TABLE6.2

Solubility parameters of solvents and polymer

solubility parameters (cal/cc)*

solvent/polymer I {)d {)p öh ö

cyclohexanone 8.65 4.1 2.5 9.88 methyl ethyl ketone 7.77 4.4 2.5 9.27 ethyl acetate 7.44 2.6 4.5 9.10 n-butyl acetate 7.67 1.8 3.1 8.46 chloroform 8.65 1.5 2.8 9.21

PVC/Ac I 8.12 3.7 3.4 9.53 I

Table 6.1 is organised in decreasing order of dipole moment of the solvents, i.e. decreasing adsorbability. The most polar solvents give solutions with the highest intrinsic viscosity, indicating that these are the best solvents for PVC/Ac. The result is that decreasing solvent adsorption and decreasing polymer solubility cooperate to

54

increase the adsorption of polar polymer segments, going from the top to the bottorn of the list.

6.2.2 XPS and surface reactions

When XPS and surface reactions are used to determine the fraction of the surface layer consisting of vinyl acetate groups we obtain the values listed in table 6.1, for aluminum and gold substrates respectively. The buJk value of the concentration of vinyl acetate groups is 0.10. Examining adsorption against the aluminum substrate it is found that the number of vinyl acetate groups is smaller for the more polar solvents, which are also the best solvents for the polymer. With gold we fmd fewer adsorbed vinyl acetate groups than with the aluminum substrate. Differences between solvents are much smaller for adsorption on a gold substrate. The gold substrate should show a stronger interaction with both polymer and solvent, because of its higher surface energy. Our interpretation of these observations is that in all cases the solvent molecules adsorb more strongly on the gold substrate than the vinyl acetate fraction ofthe copolymer does. The resulting adsorption ofvinyl acetate groups on the gold substrate from solutions is indeed in all cases less than from the melt (f

0 is 0.36, see section 5.3.2). On Al substrates equally low values are observed

with the most polar solvents.

From table 6.1, especially from the data for the aluminum substrate, we see that both polarity and solvent power affect adsorption. Cyclohexanone and methyl ethyl ketone have the same dipole moment, but from the best solvent (cyclohexanone, [11] = 53 ml/g) we fmd the lowest amount of vinyl acetate groups at the surface. Methyl ethyl ketone and ethyl acetate have the samesolvent power, but the less polar ethyl acetate favours adsorption of vinyl acetate groups. From ethyl acetate and n-butyl acetate, with the samesolvent power and polarity, we find identical surface compositions.

6.2.3 Surface energiesof cast films before and after compression moulding

A PVC/ Ac film cast from a chloroform solution or a methyl ethyl ketone solution on an Al substrate has a surface energy of 50 mN/m. The surface energy of the samematerial compression moulded from powder against Al at 423 Kis 47 mN/m and that of the fully relaxed film is 39 mN/m. The solution-cast film can be com­pression moulded at 423 K against its original substrate without releasingit first from that substrate or even against a clean Al substrate after it has been released. The time of compression moutding is 3 minutes. In both cases the surface energy after release from the compression mould is 4 7 mN/m. This shows that relaxation of solution-cast ftlms is possible by compression moulding.

XPS shows that PVC/ Ac powders and solution-cast PVC/ Ac films result in films with identical surface compositions after compression moulding. This proves that

ss

in contact with the substrate an equilibrium is reached that is independent of the initial conditions. Therefore the situations in roelts and solutions differ because of energetic, not kinetic effects. We should realize that polymer molecules are least mobile inthemeltand therefore the melt would be further away from equilibrium if the resulting adsorption were kinetically determined. On the contrary we find that the presence of a solvent during the casting results in higher surface energies which relax on melting against the substrate.

Campression moulding and salution casting experiments have been performed at different temperatures, but the temperature has only a small effect on the ad­sorption energy (section 4.3.3) and therefore on the adsorption equilibrium. In a solution the polymer molecules are mobile enough to permit adsorption, even in a trains and loops conformation. The higher temperature in a melt is necessary to

· make the polymer molecules mobile enough to reach equilibrium. In a melt the equilibrium is not a trains and loops conformation for energetic reasoos (30). Therefore interfaces of roetal substrates with a salution-cast film or a compression­moulded fllm both are in equilibrium, but these equilibrium states are different. A solution-cast fllm shows relaxation during compression moulding. In that case there is primarily a change of structure and nota change of composition ofthe surface region.

6.3 Discussion

The surface energy after release from Al or Au substrates is lower in the case of a moulded film than in the case of a cast film. At the same time we find experi­mentally that on a clean Au substrate there are more vinyl acetate groups adsorbed from a melt than from a solution. The adsorbed amount on Al substrates depends on the solvent. In most cases it is also lower than from a melt.

Summing up we have the following picture of the difference of adsorption from roelts and solutions. The surface energy of the compression-moulded film directly after release from the substrate is 47 mN/m. It is significant that this value can be approached from a lower value (39 mN/m on compression moulding of bulk powder) as well as from the higher value of SO mN/m for a cast fûm which is melted afterwards. In both cases an equilibrium adsorption is reached. On melting of bulk powder the number of vinyl acetate groups at the interface increases. The surface concentration of vinyl acetate groups is limited by entanglements and co· polymer structure. In a solution the solvent-polymer interaction and the com­petitive adsorption of the solvent both result in a smaller number of vinyl acetate groups that can make interfacial contacts, but the smaller polymer concentration enables adsorption in trains and loops (30).

After evaporation of the solvent and removal of the substrate, a non-equilibrium conformation of trains and loops in the solid polymer ftlms remains. These fllms have higher surface energies than simHar compression-moulded fûms, even though

56

the latter contain more polar (vinyl acetate) groups *), because intheir trains and loops conformation the cast films are further removed from equilibrium. At elevated temperatures, against a substrate or against air, the polymer molecules return to the random coll conformations, which constitute the equilibrium situation in these circumstances.

*) PVC/Ac fllms compression moulded against Au or Al have simBar surface compositions. We will assume that XPS results for Au substrates are valid for Al substrates.

57

7. CONCLUSIONS AND APPLICATIONS

7.1 Adsorption from polymer melts and solutions

Z 1.1 Composition of polymer surfaces

In this thesis two types of experiments have been used successfully to examine polymer surfaces. The surface energy of a polymer can be determined from the contact angle of sessile drops. The calculated surface energy is directly related to the adsorption, wettability, friction and adhesion ofthe polymer. All of these properties are important for applications of polymers. The second type of experi­ment, XPS combined with surface decoration, yields information on the composition of the surface layers.

The surface energy of a polymer after compression moulding depends not only on the composition of the polymer, but also on the surface energy of the substrate and on the highest temperature reached during moulding. There is adsorption of polar groups from melts of copolymers of vinyl chloride, vinyl acetate and vinyl alcohol to a metal substrate. In the case of polyethylene or a fluoropolymer crystalline regions are formed at the interface with the metal substrate. Adsorption is accom­panied by a decrease of the interfacial energy between the metal surface and the polymer. There is a high degree of adsorption when the polymer is processed above its melting point. This îndicates that motions of side groups of the polymer molecules can not explain the process, but that displacements of large parts of the polymer chain are required.

The situation during adsorption in the melt is frozen in by solidification of the polymer. After remaval of the substrate relaxation occurs. The surface energy falls to its equilibrium value. This is a slow process compared to adsorption because of the lower temperature and the lower mobility in the solidas compared to the melt. The dependenee of relaxation on time and temperature shows that large parts of polymer molecules are displaced. Plasticizers increase the rate and lower the activation energy of the relaxation process. Plasticizers and release agents can also adsorb at the interface, thus preventing adsorption of polar groups from the polymer. Solid ftller particles with a large specific surface area can provide a competing substrate for adsorption, both of polymer groups and of low molecular weight compounds present in the polymer.

X-ray Photoelectron Spectroscopy has been used in combination with argon ion etching and surface reactions to elucidate the molecular details of polymer ad­sorption in the case of vinyl copolymers. The fraction of polar groups at the surface after compression moulding against gold or aluminum is approximately 0.4 against 0.1 in the bulk of the polymer. The surface concentration of polar groups decreases to zero after relaxation. The decrease of the surface value to the bulk value is

58

smooth. A depletion region has not been found with the use of argon ion etching. The thickness ofthe region (3•d, see equation 5.3) in which the concentration of polar groups falls off, is ofthe order of 10 nrn. The total adsorbed amount of polar groups in this region is w-9 mol/cm2

, of which 20 to 30% is in the first layer which is in contact with the substrate. Vinyl alcohol groups are preferentially adsorbed compared to vinyl acetate groups, but vinyl chloride groups are displaced by both.

Apart from compression moulding, polymer ftlms can also be made by casting from a solution. A solvent acts on polymer adsorption in two distinct ways. First it is a competitor for the polymer segments at the adsorbent surface. In the second place, a solvent of good quality will tend to keep the polymer molecules in solution, whereas polymer adsorption is enhanced when the solvent is poor. The conforma­tion of adsorbed polymer molecules can be deduced by comparingsolution casting and compression moutding experiments. In a polymer melt, which can be considered as a concentrated solution, the conformation of the polymer molecules changes only slightly upon adsorption: the molecules retain their random coil shapes. In solutions there is an adsorption of trains of segments, connected by loops or bridges of segments. The ends of the polymer molecules form tails ex tending into the solution. The surface of a polymer f1lm cast from salution contains fewer polar groups then that of a compression-moulded f1lm.

7.1.2 Conformation of adsorbed polymer molecules

In a melt the mobility of polymer chains is less than in solutions and there are more entanglements. The surface structure will be similar to the original random coil conformation, as in the bulk of a polymer melt or salution (10 1 ). In the melt the concentration of the polymer and of its polar gioups is high. Relatively small changes in the conformatloos of the polymer molecules are necessary and sufficient for adsorption. Only the most mobile groups of each macromolecule move to the interface, for example by kink movementsof the polymer chain. Every molecule in the surface region contributes a few polar groups to the adsorption layer. The large number of adsorbed polar groups can be explained by the large degree of inter­penetratien of the molecules ( one molecule accupies less than 3% of the volume within its radius of gyration, see ref. 19). Scheutjens and Fleer (30) have predicted random coil adsorption in concentrated polymer solutions and polymer melts, in contrast with trains and loops adsorption in dilute solutions.

In polymer adsorption from solutions the random coil conformation is transformed into a trains, loops and tails conformation (figure 2.1) (31, 102, 103). At the same time polar groups are brought to the surface. These groups will not desorb when the polymer is melted in contact with the metal substrate, because the temperature has only a small effect on the adsorption energy and the quality of a solvent (section

59

2.2.3). Furthennore, the number of polar groups at the interface is in most cases higher in a polymer melt than in solution. Nevertheless the surface energy decreases upon melting the salution-cast film. (In previous experiments with compression­moulded fdms this always indicated a smaller number of polar groups at the surface ). Relaxation by melting of a solution-cast fdm must therefore be caused by confonnational changes, presumably by polymer molecules retuming from a trains and loops conformation to a structure that approaches the random con confonna­tion. As we have discussed before (section 4.3), a high surface energy after release from the substrate is related to strong adsorption before release. In the case of salution-cast fdms, adsorption of macromolecular segments in the fonn of trains is more pronounced than adsorption of specific polar groups.

The surface energy after release from the substrate is lower in the case of a moulded film than in the case of a cast fdm. At the sametime we find experimentally that on a clean substrate there are more vinyl acetate groups adsorbed from a melt than from a solution. The situation in the melt can be described as if the copolymer acts as a better solvent to the copolymer molecule as a whole than it does to the vinyl acetate groups. These groups do not nonnally fonn a separate phase in the random copolymer, but once a layer of vinyl acetate groups has fonned in the interface region of the melt, the high concentration of vinyl acetate groups at the interface constitutes a more favourable environment for these groups than the bulk of the co­polymer (which consists mainly of vinyl chloride units). This attracts more vinyl acetate groups to the interface and explains why the concentratien decreases over a rather large distance from the surface to the bulk value, without a depletion region. On the other hand, the solvents in table 6.1 are equally good solvents for both vinyl chloride and vinyl acetate groups, resulting in normal polymer adsorp­tion in trains and loops without a strong preferenee for one type of monomede unit. The slight preferenee for vinyl acetate groups as opposed to vinyl chloride groups (not so strong as in the melt) is now only the result of interactions with the metal substrate.

7.2 Consequences for practical applications of polymers

7.2.1 Processing parameters

Thennoplasts are often processed by compression moulding or extrusion, where a polymer melt is in contact with a metal substrate. Friction or adhesion during the process can cause problems by disturbing flow pattems or by increasing the force necessary to make the melt flow. Purthermare adhesion is often heterogeneous, starting in a more or less arbitrary region that tends to grow because of the restricted flow in the neighbourhood of that region. A mould may be incompletely or asymmetrically filled ( density variations teading to double refraction), teaving stresses in the product that can distort it when it is removed from the mould. The extruder or mould may also be damaged by too much friction or they may be

60

contaminated by adhesion. Important properties of the products (size, surface details, mechanical tolerances) may change in time with the age of a mould.

The decrease in interfacial energy caused by adsorption represents an extra amount of work needed for the separation of two phases. Adsorption from a polymer melt to a metal substrate therefore increases the adhesion between the two phases if the adsorbed species are attached to the polymer molecules. On the other hand a layer of low molecular weight compounds at the interface will usually decrease adhesion because such a weak boundary layer cannot sustain shear forces. Shear forces are always present after temperature changes at a polymer-metal interface because of the widely different linear expansion coefficients of polymers and metals.

Adsorption and adhesion properties are determined by the polymer composition. Certain polar groups (acetate, hydroxyl) increase adhesion while groups containing fluorine deercase it. The choice of a particular polymer is made according to the desired properties of the product. There is usually little flexibility left to consider the surface properties of the melt during processing. Intermediate products may require different surface properties, for further processing, than the ftnished product. This will necessitate surface modifications (section 7.2.2).

Campression moulding parameters that influence adhesion are the temperature and the substrate material. The temperature is determined by the required viscosity of the melt and the thermostability of the polymer. The substrate material is often chosen for mechanical and economie reasons. It is possible to give the substrate a surface treatment (oxidation, etching, applYing an adhesive layer) but the effect often vanishes quickly. The instability of the polymer and the development of a weak boundary layer (sections 2.4.3 and 3.3) depend on the presence of oxygen and water in the environment. In continuous processes like extrusion a stationary situation develops. In this case it is easier to keep water and oxygen out of the system than it is in compression moulding experiments.

The best way to prevent adhesion during or after processing is the use of release agents. These can either be regularly applied to the mould or mixed with the polymer. The release agents form a layer at the interface. If contamination of the mould is a serious problem, internal release agents must be used, i.e. copolymers containing blocks of monomer units with low adhesive tendencies. Plasticizers can also act as release agents. Adsorption and adhesion are influenced by solid fdlers in the polymer but these mixtures form complicated systems with new mechanical and chemica! properties. If the strength of adhesion is changed, this is usually a side-effect.

The choice of a solvent and its rate of removal determine the strength of adhesion of solution cast products. Adhesion will bestrongest when the solvent is poor.

61

7.2.2 Product properties

Relax.ation of the surface energy ( desorption) starts when the polymer-metal contact is disrupted after moulding or extrusion. The surface properties of the product change until equilibrium is reached. This process can be accelerated by raising the temperature, but at high temperatures it may be accompanied by degradation of the polymer, with possible changes of the surface energy and related properties. These are important if the polymer has to be metallized, painted or glued, or if it is applied in a situation where the frictional properties of its surface are relevant.

Wettability of a surface can be changed and adhesion to a surface can be improved by removing a weak boundary layer or by preventing its formation. Cleaning is difficult because one surface contaminant is usually replaced by another, viz. a solvent. Release agents, stabilizers and plasticizers are often necessary for pro­cessing, but they can sometimes be used in smaller quantities or in combination with a solid filler. Adsorption on filler particles can be controlled by surface treat· ments of the particles, keeping in mind that the mechanical strength of the com­posite requires that the polymer adheres to them. The change in elasticity and strength of the polymer by addition of a filler also changes the frictional and adhesional behaviour, apart from effects resulting from adsorption.

A large number of surface treatments exist which can be used to prevent or increase adhesion. Chemical modifications can be used to lower the friction coefficient of the surface, for example by attaching fluorine compounds. This is possible by reactions with specific groups present in the polymer. With radiation-induced grafting a polymethylacrylate layer can be formed on fluorcarbon polymers to permit adhesive bonding (104). The introduetion ofpolar groups by oxidation (in an oxygen plasma, by chemical etching, in an electtic discharge etc., see section 4.3) is widely used to enhance the strength of adhesion to other materials. In all these cases there is a strong conneetion between the surface energy of the polymer and the strengthof the bond to the polymer.

62

SUMMARY

The purpose of the work described in this thesis was to find a description of polymer adsorption at the interface of a metal and a polymer melt during com­pression moulding. Chapter 1 gives an introduetion to the subject of polymer adsorption. Adsorption is important for processcontrolas well as for adhesion and friction properties of the product. A polymer melt can be treated as a concentrated solution. The thermodynamics of polymer solutions are relevant because of the influence of the solvent on polymer adsorption from solutions. This subject is discussed in chapter 2, together with the possible conformations of adsorbed polymer molecules and the relation of adsorption phenomena with adhesion.

Polymer adsorption from melts is studied by compression moutding of a melt in contact with a metal substrate. The polymer is solidified and the situation in the melt is frozen in. The substrate is then removed in order to analyse the surface. Contact angle measurements are used for the examination of the polymer surface energy. To this end the wettabllity of the surface is measured for a number of polar and apolar liquids. The calculation of the surface energy from these data is described in chapter 3. The chemica! composition of the surface is determined by X-ray Photoelectron Spectroscopy (XPS). With this technique a surface layer of 10 nm thickness of asolid can be analysed in vacuum. Surface reactions are used as an auxiliary to this method. Fluoro-butanoic acid chloride is coupled to existing reactive groups. The amount of fluorine is then measured with XPS, and it is a measure for the surface concentration of these groups. Chapter 3 describes these surface analytical methods and their interpretation.

The results of the compression moutding experiments ( chapters 4 and 5) prove that polar groups of a copolymer melt adsorb at the interface with a metal substrate. Relaxation ( desorption) starts after removal of the metal and is accompanied by a decrease in the surface energy. The temperature dependenee of the rate of relaxa­tion indicates that groups ofpolymer segments are displaced by adsorption and desorption. The amount adsorbed depends on the nature of the metal and on the highest temperature reached during compression moulding. Plasticizers and release agents can adsorb at the interface, thus decreasing polymer adsorption. Such a layer of low molecular weight compounds diminishes the adhesion of the polymer to the substrate and at the same time may contaminate the substrate. Fillers in the polymer can provide a competitive substrate for adsorption, thereby decreasing adsorption at the metal substrate.

The quality of the solvent determines polymer adsorption from solutions. This is investigated (chapter 6) by casting polymer films on metal substrates. The surface is examined, by the same methods as those used for the compression-moulded films, after removal of the solventand the substrate. There is only very little

63

adsorption of polar groups from a good polar solvent, in contrast to adsorption from a poor solvent. The conformation of a polymer molecule changes upon ad­sorption from a solution. Groups of consecutive polymer segments are in contact with the substrate. These groups (trains) are connected by loops ofpolymer segments extending into the solution. In contrast to this the polymer molecules retain their random coil conformation after adsorption from a melt, as is concluded from the surface energy and the composition of the outer layers of solidified polymers.

The conclusions from chapters 4 to 6 have been gathered together in chapter 7, along with possible ways of influencing adsorption and therefore adhesion and friction phenomena. The most important parameters for this purpose are the polarity of the polymer surface and the presence of low molecular weight compounds.

64

SAMENVATTING

Het in dit proefschrift beschreven werk had tot doel het verzamelen van gegevens over polymeeradsorptie aan het grensvlak tussen een metaal en een polymeersmelt gedurende persen of extruderen. Deze polymeeradsorptie is zowel van belang voor procesheersing als voor hechtings- en wrijvingseigenschappen van het produkt (hoofdstuk 1). Een polymeersmelt kan beschreven worden als een geconcentreerde oplossing. De thermodynamica van polymeeroplossingen speelt daarom een belang­rijke rol. De interacties tussen het polymeer en het oplosmiddel hebben een grote invloed op adsorptie uit oplossingen. De gevolgen van adsorptie voor de confor­matie van polymeermoleculen en het verband tussen adsorptie en adhesie worden beschreven in hoofdstuk 2.

Polymeeradsorptie wordt bestudeerd door een smelt in contact met een metaal­substraat af te koelen. Daarbij wordt de toestand bereikt in de smelt ingevroren. Daarna wordt het substraat verwijderd en het zo ontstane oppervlak geanalyseerd. Voor het onderzoek van de oppervlakte-energie van polymeren worden contact­hoekmetingen gebruikt. Hierbij wordt de mate van bevochtiging gemeten van een aantal polaire en niet-polaire vloeistoffen op het oppervlak. In hoofdstuk 3 wordt beschreven hoe dan de oppervlakte-energie van het polymeer kan worden berekend. De chemische samenstelling van het oppervlak wordt bepaald door middel van Röntgen-photoelectronen-spectroscopie (XPS). Hiermee is het mogelijk de buiten­ste 10 nm van een vast oppervlak in vacuum te bestuderen. Als hulpmiddel bij deze methode wordt een oppervlaktereactie gebruikt waarbij een fluor-bevattende ver­binding aan bestaande reactieve groepen verankerd wordt. De hoeveelheid fluor wordt dan met XPS gemeten. Deze oppervlakc-analysemethoden en hun interpre­tatie zijn beschreven in hoofdstuk 3.

De resultaten van metingen aan geperste polymeerlagen {hoofdstukken 4 en 5) wijzen uit dat de meest polaire groepen van een copolymeer adsorberen aan het grensvlak met een metaal. Na verwijdering van het metaal vindt desarptie plaats, wat gepaard gaat met een afname van de oppervlakte-energie (relaxatie). Uit de tem­peratuurafhankelijkheid van de relaxatiesnelheid volgt dat adsorptie en desarptie samenhangen met verplaatsingen van groepen van polymeersegmenten. De mate van adsorptie hangt af van de aard van het metaal en van de hoogste temperatuur tijdens het contact tussen metaal en polymeersmelt. Weekmakers en lossingsmiddelen kun­nen ook adsorberen aan het grensvlak. Door deze concurrentie wordt de polymeer­adsorptie tegengegaan. Zo'n laag van laagmoleculaire verbindingen vermindert de adhesie van het polymeer aan het substraat..Bovendien kunnen restanten voor vervuiling van het substraat zorgen. Vulstoffen in het polymeer kunnen een concur­rerend substraat voor adsorptie vormen, waardoor adsorptie van het polymeer aan het metaal negatief wordt beinvloed. Er is dus zowel een concurrentie tussen het substraat en het vulmiddel als tussen het polymeer en het lossingsmiddeL

65

Bij polymeeradsorptie uit oplossingen is het oplosmiddel een belangrijke variabele. De invloed daarvan kan onderzocht worden (hoofdstuk 6) door polymeerlagen te gieten op metaalsubstraten. Na verwijdering van het oplosmiddel en substraat kan het oppervlak onderzocht worden met dezelfde methoden als gebruikt zijn voor de gesmolten lagen. Uit een goed, polair oplosmiddel vindt weinig adsorptie van polaire groepen plaats, uit een slecht oplosmiddel juist veel. Bij adsorptie van polymeren uit oplossingen verandert de conformatie van het polymeermolecuuL Groepen van op­eenvolgende polymeersegmenten zijn in contact met het substraat. Deze groepen zijn verbonden door lussen van polymeersegmenten die in de oplossing steken. Bij adsorptie uit de smelt is dit anders. Uit de oppervlakte-energie en uit de samenstel­ling van de buitenste lagen van de vast geworden polymeren kan worden geconclu­deerd dat de polymeermoleculen dan de random-kluwenvorm behouden.

In hoofdstuk 7 worden de conclusies uit de verschillende hoofdstukken nog eens verzameld, samen met mogelijkheden om het adsorptieproces en de daarmee samen­hangende adhesie- en wrijvingsverschijnselen te beïnvloeden. De belangrijkste para­meters daarbij zijn de polariteit van het polymeeroppervlak en de aanwezigheid van laagmoleculaire verbindingen.

66

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70

UST OF SYMBOLS

A rate of degradation (h- 1 or mN/m h- 1)

c composition d thickness of adsorbed layer (nm) fact actual composition feq equilibrium value of surface composition fmeas fraction of the surface covered with perfluoro-butanoic acid chloride fobs observed composition f0 change of the surface composition at t = 0 I A signal intensity related to element A KE kinetic energy of electrans (eV) k relaxation rate (h- 1

)

Mn number average molecular weight (g/mole) p po lar fraction of surface energy = 'Y 8 P I'Y 8 R radius ofHansen's interaction sphere (cal/cc)~ s integration parameter Tg glass transition temperature (K) Tm polymer melting temperature (K) Tp compression moulding temperature (K) T R relaxation temperature (K) t time(h) WA work of adhesion (mN/m) x elistance to the (original) surface (nm) y rate of ion etching (nm/min) a temperature coefficient, defmed in equation 4.1 a 1 intramolecular expansion factor 'Y surface or interfacial energy (mN/m) 'YL liquid surface tension (mN/m) 'YM substrate surface energy (mN/m) 'Yp polymer surface energy (mN/m) 'Yg solid surface energy (mN/m) 'YLV liquid-vapour interfacial energy (mN/m) 'YpM polymer-substrate interfacial energy (mN/m) 'YSL solid-liquid interfacial energy (mN/m) 'Ygy solid-vapour interfacial energy (mN/m) ~'Yp increase of polymer surface energy (mN/m) 6 solubllity parameter (cal/cc)* od dispersion contribution to the solubility parameter (cal/cc)~ oh hydragen-bond contribution to the solubllity parameter (cal/cc)* op polar contribution to the solubility parameter (cal/cc)* 11 viscosity of a solution ( cP) 11

8 viscosity of the solvent ( cP)

(11] intrinsic viscosity (ml/g)

71

À mean free pathof electronsin the solid (nm) IJ. dipole moment (D) 1Te spreading pressure (mN/m) 6 contact angle x Flory-Huggins interaction parameter

Superscripts:

d van der Waals (dispersion) contribution p polar contribution R after relaxation o equilibrium value

72

CURRICULUM VITAE

De schrijver van dit proefschrift werd op 30 december 1951 geboren te Boxtel, waar hij in 1969 het einddiploma H.B.S.-B behaalde. In hetzelfde jaar begon hij met zijn studie Scheikundige Technologie aan de Technische Hogeschool Eindhoven. In februari 1975 studeerde hij af (met lot) bij prof. dr. H.M. Buck.

Vanaf maart 1975 is de auteur werkzaam als wetenschappelijk medewerker bij het Philips Natuurlcundig Laboratorium te Eindhoven. Het in dit proefschrift beschre­ven werk werd verricht in de groep Grensvlakchemie onder leiding van dr. G. Frens.

J.F.M. Pennings

Stellingen bij het proefschrift:

"ADSORPTION OF VINYL COPOL YMERS FROM MELTS AND SOLUTIONS"

28 mei 1

1. Uit verdunde oplossingen adsorberen polymeren in een "trains" en "loops" conformatie, in geconcentreerde oplossingen en in een polymeersmelt behou­den de polymeermoleculen hun random kluwen conformatie.

- J.M.H.M.Scheutjens and G.J.Fleer, in: The effect of polymers on dispersion properties, by T.F. Tadros (1981)

- dit proefschrift

2. Adhesie aan polymeeroppervlakken kan verbeterd worden door het aanbrengen van polaire groepen in het oppervlak door middel van adsorptie.

3. Infraroodtechnieken zijn niet genoeg oppervlaktegevoelig om de afwezigheid van geoxideerd materiaal te bewijzen na adhesieverbeterende oppervlaktebehan­delingen.

- R.H. Hansen and H. Schonhorn, J.Pol.Sci. 4B, 203 (1966)

4. Randhoeken van polaire vloeistoffen, gemeten op polymeeroppervlakken zijn het best in overeenstemming met Fowkes' formule voor de van der Waals inter­acties en met Wu's inverse harmonische formule voor de polaire interacties.

- dit proefschrift

5. De oppervlakte-energie van een materiaal wordt niet alleen bepaald door de samenstelling en de dichtheid van de buitenste laag, als deze afwijkt van de bulksamenstelling. Entropie speelt ook een rol.

- W.A. Zisman, Adv.Chem.Series 43, 1 (1964) - dit proefschrift

6. Voor een bepaling van oppervlakteruwheid met randhoekmetingen moeten heen· en teruggaande vloeistoffronten vergeleken worden, vooral bij poreuze samples.

- Z.Kessaissia, E.Papirer and J.Donnet, J.Coll.Interface Sci. 82, 526 (1981)

7. De regelmatigheid van oppervlakteruwheid is niet bewezen met de opmerking dat verhogingen en verdiepingen van dezelfde grootte.orde zijn.

- Z.Kessaissia, E.Papirer and J.Donnet, J.Coll.Interface Sci. 82, 526 (1981)

8. De onnauwkeurigheid van de plaatsbepaling (0.5 eV) en de oppervlaktebepa­ling (5%) van pieken in XPS spectra ontkracht de verdeling van een piek in een groot aantal subpieken en het ontlenen daaruit van gegevens over de aanwezig­heid en de hoeveelheid van functionele groepen in het sample.

- D.T.Clark, W.J.Feast, W.K.R.Musgrave and I.Ritchie, J.Pol.Sci., Pol.Chem.Ed. 13, 857 (1975)

9. De chemische structuur van organische verbindingen aan oppervlakken kan met XPS bepaald worden in combinatie met andere gegevens, zoals specifieke reac­ties met functionele groepen.

10. Clark's extrapolatie, naar de toekomst, van de vrije weglengte van electronen berust op een selectie van beschikbare gegevens en is, tenminste voor organische materialen, niet juist.

- D.T.Clark, H.R.Thomas and D.Shuttleworth, J.Pol.Sci., Pol. Lett.Ed. 16,465 (1978)

11. De bepaling van de vrije weglengte van electronen met behulp van hoekafhanke­lijkr XPS metingen levert systematisch lagere waarden dan analyses van multi­laag systemen.

- D.T.Clark, H.R.Thomas and D. Shuttleworth, J.Pol.Sci., Pol. Lett.Ed. 16,465 (1978)

- M.F.Ebel, J.El.Spec.Rel.Phen. 14,287 (1978) - R.F.Roberts, D.L.Allara, C.A.Pryde, D.N.E.Buchanan and

N.D.Hobbins, Surface and Interface Analysis 2, 5 (1980) - M.D.Seah and W.A.Dench, Surface and Interface Analysis 1,

2 (1979)

12. De invloed van vulmiddelen op de glasovergangstemperatuur van vinylchloride­vinylacetaatcopolymeren is afhankelijk van het percentage vinylacetaat omdat er bij een grotere hoeveelheid vinylacetaat meer beweeglijkheid te verliezen is.

- G.J.Howard and R.A.Shanks, J.Appl.Pol.Sci. 26,3099 (1981)

13. Adhesie van alumina deeltjes aan glas wordt voornamelijk veroorzaakt door van der Waals krachten.

- P.M.A.Bozorg and G.E.Klinzing, Can.J.Chem.Eng. 57,655 (1979)

14. Het bewijs van een scherpe overgang van desorptie van water naar desorptie van azijnzuur uit boeluniet kan niet worden geleverd door een punt in een gewichts· verliesdiagram.

L.Abrams and M.J.D.Low, I&EC Prod.Res.Dev. 8, 38 (1969)

15. Bijde vergelijking van energieën van verschillende configuraties van vijf-geco­ordineerde fosforverbindingen moet rekening worden gehouden met afwijkin­gen van de geïdealiseerde trigonaal bipyramidale of vierkante pyramidale geo­metrie.

- R.R.Holmes, J.Am.Chem.Soc. 100,433 (1978)

16. Adhesieverbeterende silanen vormen geen monolaag door reacties met oxidische oppervlakken, maar adsorberen na polymerisatie, in de vorm van een dikkere laag.

- W.D.Bascom, Macromolecules S, 792 (1972)

17. De adhesie van ijs aan rubber of plastics wordt onder andere beïnvloed door de afmetingen en de samenhang van de ijskristallen.

- W.Landy andA.Freiberger,J.Coll.lnterfaceSci. 25,231 (1967) A.D.Roberts, preprints International Conference on Adhesion and Adhesives, Durham (1980)