NETWORK FORMATION IN POLYMERIC MEDIAAND SOME NETWORK PROPERTIES

3. ASHWORTH, C. H. BAMFORD and E. G. SMITH

Department of inorganic, Physical and industrial Chemistry,Donnan Laboratories, University of Liverpool, UK

ABSTRACTThe following network copolymers have been synthesized and examined:PVTCA-PMMA, PVTCA-PSt, PVBA—PMMA, PVTCA-PCP (PVTCA-polyvinyl trichloroacetate, PMMA—polymethyl methacrylate, PSt—polysty-rene, PVBA—polyvinyl bromacetate, PCP—polychloroprene). The syntheticroutes utilized the radical-forming reactions between organic halides andorganometallic derivatives [Mn2(CO)10, photochemical initiation (,. = 435.8nm) at 25°C, Mo(CO)6, thermal initiation at 80°C]; network properties wereinvestigated by dilatometry, n.m.r., torsion pendulum and electron microscopy.

The networks show two-phase behaviour. In the case of PVTCA—PMMAsystems one phase consists of an intimate mixture of PVTCA and PMMAchains, while with PVTCA—PSt phase separation into the pure componentsis more nearly complete.

It is concluded that in the PVTCA—PCP copolymers the size and separationof polychioroprene domains depend on the mean chain length of the PCPsegments and the density of crosslinking, respectively.

The two-phase structure of PVBA—PMMA copolymers may be demon-strated directly by electron microscopy without recourse to staining.

The morphology of these networks appears to be determined by the opposi-tion between thermodynamic incompatibility of the component homopolymers

and the geometrical constraints arising from the network structure.

1. INTRODUCTION

The properties and morphologies of multicomponent polymer systemsare currently of considerable interest, and block copolymers based on styreneand butadiene of the AB and ABA types have attracted particular attention1.In this paper we shall be concerned with the synthesis and properties of morecomplex copolymers, namely networks composed of blocks of differentmolecular species.

Investigation of the relations between structure and properties requiresideally substances of which the structures are fully determinable—a situationwhich, unfortunately, is never encountered with synthetic high polymers.

Anionic polymerization has been widely employed in the synthesis of ABand ABA block copolymers, since the relatively narrow (ideally, Poisson)molecular weight distribution obtainable by this technique in favourable

25

J. ASHWORTH, C. H. BAMFORD AND E. G. SMITH

circumstances allows unusually precise control of block length. However,its application is limited, being confined, with few exceptions, to hydrocarbonmonomers. On the other hand, free-radical polymerization offers choicefrom a wider range of components, but suffers from the disadvantage ofproducing broader distributions. Nevertheless, we believe that interestingcorrelations between structure and properties can be established by studiesof copolymers prepared by free-radical reactions, and we have recentlybeen exploring the possibilities of this approach.

For present purposes, a synthetic route to block and graft copolymersand networks is only of value if it satisfies the following minimal require-ments: (i) it must not produce significant quantities of homopolymers, (ii) itshould allow calculation of average block lengths and (iii) it should permitcalculation of average crosslink or graft densities. In free-radical synthesisof block and network copolymers, a radical attached to a polymer moleculeof type A must at some stage propagate with another type of monomer togenerate ultimately a block of B, or to form a junction point with a blockof B, or it must combine with a radical attached directly or remotely to apolymer chain of type B. The basic problem is therefore to produce 'attached'radicals under conditions which minimize undesirable combination andtransfer processes leading to homopolymers. Polymerizing systems can beenvisaged which approach this situation when conventional azo or peroxideinitiators are used; the crosslinking of dienes by free-radical propagationthrough their double bonds, or grafting on to preformed polymer by chaintransfer fall into this category, provided the relevant mean kinetic chainlengths are sufficiently large. Such systems are special cases, however, andthe most general free-radical syntheses of the types of polymer under dis-cussion involve direct production of radicals attached to prepolymerchains in the initiation step. Several methods of doing this have been used,e.g. homolysis of suitable linkages, such as peroxy, incorporated in the back-bones or mechanical rupture of the backbones themselves, and redoxreactions of hydroperoxide groups attached to polymer chains. Redoxreactions are particularly suitable for our purposes since they often give nounattached radicals.

We have utilized the radical-forming reactions between organic halidesand complexes of metals in low oxidation states2 in block copolymer andnetwork syntheses36, and summarize some of our results in the presentpaper.

2. NETWORK SYNTHESIS AND CHARACTERIZATIONThe chemistry of the radical-generating processes has been described

elsewhere2 and will not be reproduced in detail here. A wide variety of organo-metallic derivatives has been studied, but most of the preparative work hasbeen carried out with molybdenum carbonyl Mo(CO)6, which shows con-venient activity at 80°C, and manganese carbonyl Mn2(CO)10, which is aneffective photoinitiator (). = 435.8 nm) at 25°C. Radical formation is a redoxprocess, following, for example, equations la, b, which, however, do notrepresent reaction mechanisms.

Mo°+Cl—R-*M&—Cl+R (a)(1)

Mn2(CO)10 + Cl—R Mn'(CO)5Cl + R + Mn2(CO)10 (b)26

NETWORK SYNTHESIS AND PROPERTIES

Further oxidation of Mo' up to Mov may occur, with additional radicalformation7. When R is part of a polymer chain, initiation leads directly andentirely to attached radicals. Thus, if the reaction is carried out in thepresence of a polymerizable monomer M, block formation occurs, and ifthe termination reaction in the polymerization of M involves radical com-bination, a crosslink between two polymer chains is formed, as shown in 2.

Mo(CO)6 ______

(2)

CC12M MC! 2CClearly, if the prepolymer chains carry a number of halide groups, networkformation is possible3, while if the halide groups are situated only at the endsof prepolymer molecules the product is a block copolymer. The prepolymerchains will carry branches as well as crosslinks if the termination reactionoccurs partly by disproportionation. Addition of a chain transfer agent, or amonomeric halide such as ethyl trichloracetate, will lead to the formation ofunattached radicals, and hence ultimately to branches on the prepolymer.Some homopolymer of M will also be produced in these circumstances.

This type of synthesis is versatile; the only restriction on the prepolymeris that it should be soluble and carry the required halide groups, and M maybe any monomer polymerizable by free-radicals.

The initial rate of initiation 1 may be derived from observed rates ofpolymerization of the vinyl monomer under comparable conditions, exceptthat a chemically equivalent monomeric halide is substituted for the pre-polymer. This procedure eliminates effects of variation in the terminationcoefficient arising from gelation or attachment of growing chains to pre-polymer molecules. There is considerable evidence6 that 1 does not dependon the nature of M. Mean crosslink lengths may be calculated from I, [M]and the familiar kinetic parameter k,,k k When the conditions are such thatk is not constant (e.g. at high crosslink density) it is simplest to estimatethe mean crosslink length from the conversion of M and the total number ofinitiating radicals generated. The crosslink density is calculable from thegel-time of the system and the reaction time, since at the gel point the cross-link density corresponds to one crosslinked unit per weight average pre-polymer chain. In simple cases, branch: crosslink ratios may be obtainedfrom a knowledge of the relative importance of disproportionation andcombination in the termination reaction (i.e. ktC/ktd); when chain transfer issignificant, or when unattached radicals are generated the calculation is morecumbersome, although conventional. Allowance must be made for initiatorconsumption during reaction, if this is significant. The relevant procedure hasbeen described in earlier papers4' 5; unfortunately, available data do notalways allow corrections for consumption to be made with certainty, so thatuncertainties in some network parameters are inevitable at present (cf. Table 1).

27

J. ASHWORTH, C. H. BAMFORD AND E. G. SMITH

3. PROPERTIES OF NETWORKS

Most of our work has been carried out with polyvinyl trichioracetate(PVTCA) as prepolymer; other prepolymers studied include polyvinylbromacetate (PVBA), the polycarbonate (I) and cellulose acetate containinga proportion of trichioracetate groups.

Figure 1. Dilatometric data for networks IV, V, VI (Table 1) showing the presence of two glass-transition temperatures (except for VI). 0 represent experimental data: represent experimentaldata for network V after holding the specimen at the indicated temperatures for several hours.

Curves below 7 and above 7;2 calculated from equation 3.

28

(I)

Methyl methacrylatd (MMA), styrene (St) and chioroprene (CP) have beenused as vinyl monomers.

3.1 PVTCA-PMMA and PVTCA—PSt networks

3.1.1 Dilatornetric studies8Nine networks were prepared by the methods outlined. After polymeriza-

tion, excess monomer was removed under vacuum for 15 h, approximately.

5

4

3

E

2Q

0

0

TV

20 40 60 80 100 120

T, °C

NETWORK SYNTHESIS AND PROPERTIES

Polymers were annealed at 100°C in the dilatometer bulbs in high vacuum,before the confining liquid, mercury, was added. Differential dilatometry wasused, to eliminate errors arising from temperature fluctuations in the thermo-stat. Some preparative details and structural characteristics of the networksare given in Table 1. Dilatometric observations showed the existence of the

Table 1. Network preparation and structures (PVTCA—PMMA, PVTCA—PSt systems)

Specimen Vinyl monomerMean no. of crosslinkedunits per weight average

prepolymer chainP,, crosslinks Branch:

crosslink ratio

I MMA 3 4560—4740 4II MMA 9 5060—5440 4

III MMA 16—18 6900—8000 4IV MMA 1 11820 10V MMA 35 12700 10

VI MMA 7 16760 10VII St 1 2880 o

VIII St 3 4750 —0IX St 6 5800 0

1—Ill, Vu—tX polymerized at 25CC; Mn2(CO)10 as photoinitiator (A = 435.8 nm); IV—Vt polymerized at 80C; thermalinitiation by Mo(C0)6; P(PVTCA) 2 6430.

two glass transitions (temperatures 1, Ta), except for network VI, whichcontained only 3% w/w of PVTCA. Figure 1 illustrates some typical results,as well as those with network VI. Under our conditions, therefore, all thenetworks, with the possible exception of VI, exist as two-phase systems.

Analysis of the s1opes in volume/temperature diagrams such as Figure 1provides results of interest. We define S1, S3 as the slopes below IJ andabove c2 respectively, and S2 as that in the intermediate region. If thecoefficients of cubical expansion are additive we have:

Si = GgppVgpp + OCgpmI'pm + XHgAI"Hg (3)S3 = ippVipp + 1pmT11pm + Hg'1"Hg

in which the ccs are the coefficients of cubical expansion and the Vs the volumesof the components at the appropriate temperature, and in the appropriatestates, which are denoted by g (below glass transition) and 1 (above glasstransition), respectively. Subscripts pp, pm refer to PVTCA and PMMA,respectively and LWHg is the difference between the volumes of mercury inthe two dilatometers. Values of S1 calculated from equations 3, with the aidof the measured coefficients of expansion, agree very closely with thoseobserved. This is illustrated in Figure 1, in which lines with the calculatedslopes are superimposed on the experimental points. In the temperatureregion just above 2 the calculated values of S3 also appear to be consistentwith the experimental points, although there is some divergence above 115°C,especially with polymers relatively rich in PYTCA. This is attributable tothermal decomposition of the prepolymer, which has been shown in inde-pendent experiments to set in above 115°C.

Values of S2 have been calculated from equation 4, analogous to 3, and arecompared with the observed slopes in Table 2.

29

J. ASHWORTH, C. H. BAMFORD AND E. G. SMITH

S2 G(ippVipp + agpmVgpm + tXHgL\Vig (4)

It will be seen that for the PVTCA—PMMA networks the observed slopesare appreciably higher than those calculated (except for network VI). Theassumption of independent behaviour of the two network components istherefore invalid in this region. We believe this implies that the two phasesdo not consist of the pure components, and that the phase relatively rich inPVTCA (phase 1) also contains PMMA which partakes in the transition at7. We assume then that phase 1 contains all the PYTCA together with afraction a of the PMMA in the network, while phase 2 consists of pure PMMA.This assumption leads to equation 5.

S2 = clN,V1 + aoipmVipm + (1 a) gpnYgpm + cLHgAVig (5)

Values of a may be calculated from equation 5 by equating S2 with the observedslope and are shown in Table 2. The composition of phase 1 may readily bededuced from a and the overall composition of the network; we present inTable 2 values of H the weight of PMMA associated with unit weight ofPVTCA in phase 1. It appears that this phase contains large proportions ofPMMA, especially when the content of PMMA in the network is high. Ifphase 2 is not pure PMMA as assumed, but contains also some PVTCAwhich does not take part in the transition at 7, our calculations give under-estimates of W.

The values of 7 in networks I—V are perceptibly higher than for purePVTCA (Table 2), and therefore support the view that phase 1 containsPMMA. On the other hand, 2 is always close to the value for PMMA, aswould be expected if phase 2 consists wholly of this polymer.

A blend of PVTCA and PMMA (52:48 w/w) behaves differently from thenetworks. It shows two transitions located at temperatures indistinguishablefrom the 1s of the pure components, and the observed slope S2 agrees withthat calculated from equation 4 to within four per cent. Separation of thecomponents is therefore much more nearly complete in this case.

The PVTCA—PSt networks VII- IX also show two glass transitions. As

Table 2. Network properties (PVTCA—PMMA, PVTCA—PSt systems)

Sample104S2 (obs.

mlK1)calc. a W

PVTCA innetwork(% w/w)

7 C 2 °C

I 3.75 2.64 0326 1.2 21.5 74 97.5II 5.23 4.66 0.15 1.7 8 71 102

III 3.83 3.57 0.071 2.1 3.5 69 101IV 4.78 2.34 0.87 2.8 21 74 99V 5.06 3.42 0.37 4.4 7.5 71 102

VI 3.41 3.51 — — 3 — 103VII 3.03 2.89 0.57 0.17 77 55 97

VIII 3.64 3.52 0.083 0.12 40.5 60 97IX 4.00 3.85 0.061 0.22 21.5 60 100

PVTCA 59PSt 100PMMA 103

30

NETWORK SYNTHESIS AND PROPERTIES

with the other networks, the observed slopes S1, S3 agree with those calcu-lated from equation 3 with the appropriate parameters for polystyrene(gps' T etc.). Values of S2 similarly estimated from equation 4 are again

5.0

1.0-Ua

0.5

0.1(a)

-150 -100 -50 0 50 100 150

Figure 2. Dependence of spin—lattice relaxation time T1 on temperature at 45 MHz. (a) PYTCAhomopolymer, 0PMMA homopolymer; (b) 0 PMMA homopolymer, PMMA/PVTCAnetwork (88% w/w PMMA), LII PMMA/PVTCA network (83% w/w PMMA), •

PMMA/PVTCA network (50% w/w PMMA).

31

Temperature, °C

0.4

0.3

Ua

0.2

0.120 60 100 140 180

Temperature7°C

J. ASHWORTH, C. H. BAMFORD AND E. G. SMITH

higher than those observed (Table 2), but the differences are much smallerthan with the PVTCA—PMMA networks. Table 2 presents the derived valuesof ci and W, which are also relatively low, showing that only small amounts ofpolystyrene are incorporated into phase 1. In agreement with this, we findthat TI, T12 for the networks are close to the transition temperatures of thepure components.

3.1.2 N.rn.r. studiesPulsed n.m.r. experiments on the PVTCA—PMMA system have been

carried out to measure the relaxation times T1 (spin—lattice) and T2 (spin—spin). Figures 2 (a, b) show the dependence of T1 on temperature for PVTCA,PMMA and networks of PVTCA—PMMA. The rapid decrease in T1 forPYTCA at temperatures exceeding 60°C, approximately (Figure 2a) isassociated with the glass transition region, and demonstrates the sensitivityof the n.m.r. techniques to changes in molecular motion. At much highertemperatures, T1 would be expected to pass through a minimum as thecharacteristic frequency of the motion passes through the experimental n.m.r.frequency (cf. Slichter9). A similar onset of chain motion occurs with thePMMA sample (Figure 2a) when the temperature reaches 100° to 110°C; inthis case, however, the overall behaviour of T1 is more complicated, andrelaxation below this temperature region is mainly determined by a spindiffusion process through the rotating methyl groups.

Corresponding increases in T2 for PVTCA and PMMA occur at 60°Cand 110°C, respectively.

The variation ofT1 with temperature for the networks (Figure 2b) resemblesthat for pure PMMA, but the onset of mobility occurs at temperaturesappreciably lower than that for MMA homopolymer. Since PYTCA protonsmake a negligible contribution to the n.m.r. signals, and hence to the mea-sured T1 in these networks, it follows that the decrease in T1 arises from theonset of motion in the PMMA component. Thus the presence of PVTCAhas induced motion in some PMMA chains at temperatures below the 7; ofpolymethyl methacrylate. Clearly this conclusion is in accord with dilato-metric evidence indicating the existence of a composite phase 1 with a rela-tively low 7;. Figure 2b shows that the turning points in T1 occur at succes-sively higher temperatures in the network with increasing PMMA content.This also is consistent with the dilatometric results since, according toTable 2, the weight of PMMA in phase 1 of a given type of network (e.g. I,II, III) decreases with increasing PMMA content.

3.1.3 Mechanical propertiesA few determinations are available of the glass transition temperatures

derived from torsion pendulum measurements of mechanical loss tangents.The technique is not suitable for measuring both 7; values in these systems.Two PVTCA-PMMA networks and one PVTCA-PSt network wereprepared, with structures summarized in Table 3. The glass-transition tem-peratures are shown in Table 3, together with values for the homopolymersand blends of the latter. 7; is higher for the PVTCA—PMMA networksthan for the blend ; the latter has a 7;identicalwith that of PYTCA. Accordingto Table 3, the glass transition of the network containing 7% w/w PVTCA

32

NETWORK SYNTHESIS AND PROPERTIES

Table 3. Properties of networks and blends (PVTCA—PMMA, PVTCA—PSt systems): glasstransition temperatures from torsion pendulum observations

PYTCA Mean no. of P Branch:

Samplein sample,

, w/wcrosslinked

.units per

weight-averagePYTCA chain

crosslinks crosslink.ratioT C

PVTCA 100 — — — 60(0.35 Hz)PMMA 0 — — — 105 (0.25 Hz)PVTCA—PMMA network 7 11 2850 4 76(0.54Hz)PVTCA—PMMA network 34 3 1690 4 73 (0.28 Hz)PVTCA—PMMA blend 33 — — — 60(0.20 Hz)PVTCA—PSt network 25 8 1400 0 60 (0.20 Hz)PVTCA—PSt blend 26 — — 60 (0.20 Hz)

is 16°C higher than the value for the blend. On the other hand, PVTCA—PStnetworks and blends of the two homopolymers have similar values of 7,(60°C). These findings therefore reinforce the view that in our PYTCA--PMMA networks phase 1 contains PMMA in substantial amounts whereasin the PVTCA—PSt systems separation of the components is more nearlycomplete.

3.2 Networks containing polyvinyl bromacetate

Electron microscopy is capable of providing much direct informationabout the morphology of copolymers. Attainment of adequate contrast oftennecessitates use of a selective staining technique to create suitable variationsof electron density in the specimen. Attachment of heavy atoms such asosmium and silver may be employed; thus most morphological studies ofcopolymers containing polydienes have involved the osmium tetroxidefixation treatment first used for these systems by Kato1 O•

The networks we have considered so far do not respond to attemptedstaining by osmium tetroxide, since they do not contain suitable acceptorsites. Although it might be possible to introduce these by performing chemicalreactions on the copolymers we feel that such treatments could alter thestructures and introduce other complications. Preparation of a prepolymercontaining bromine such as polyvinyl bromacetate therefore appeared tobe of interest; the high reactivity of the bromine makes PVBA suitable forincorporation into networks by the technique described in §2 and it seemedlikely that domains of PYBA would be directly visible in the electron micro-scope by virtue of the high electron density associated with the bromineatoms.

In preliminary experiments carried out to investigate this aspect PVBAwas crosslinked by PMMA; the gel time was 28 mm, and specimens wereprepared for examination after a reaction time of 10 mm. These materials,which contained 60 w/w PMMA, therefore had a rather low crosslink

33

P.A.C.—30/1—C

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J. ASHWORTH, C. H. BAMFORD AND E. G. SMITH

molecules. The crosslink density in this polymer is low (Table 4) and thematerial must contain much unreacted PVTCA. Indeed, it is easy to showfrom the data in Table 3 that the probability of a given PYTCA unit reactingduring the network preparation is only 2 x io-, approximately; conse-quently the majority of the copolymer molecules are H-shaped, with onlyminor amounts of more complex structures. We propose that the domains areformed by agglomeration of the PCP crosslinks of the polymer, withexclusion of most of the attached PVTCA, which therefore assumes the formof external branches and loops. The result is a PCP particle with attachedPVTCA chains which confer stability when the particle is dispersed in aPVTCA matrix (consisting mainly of the unreacted polymer). From Figure 7we see that the average domain diameter is of the order 150 nm. This issimilar in magnitude to the unperturbed mean end-to-end distance for apolychloroprene molecule with F = 2290, so that most molecules couldhave their terminal groups on the surface of a domain, in agreement with ourproposed model. A schematic drawing of the morphology is presented inFigure 9a.

(a)

Figure 9. (a) Schematic representation of network I (Table 4). (b) Schematic representation ofnetwork II (Table 4. X, Y represent PCP, PVTCA chains trapped in the 'wrong' phases).

PCP chains; PVTCA chains.

The situation is naturally more complicated with network II (Table 4), inwhich the crosslink density is much higher. Figure 8 suggests that thedomain size is not very different (perhaps a little smaller), but the inter-domain separation is greatly reduced. We believe that the basic domainstructure is similar to that in Figure 9a, but that the domains must now belinked together, predominantly by PVTCA (see Figure 9b), but probably bysome polychioroprene in addition (X, Figure 9b). Geometrical constraintsmay necessitate the trapping of some PVTCA in PCP domains (Y, Figure 9b).

If these views are correct, it follows that the domain size in these systemsshould be mainly determined by the mean crosslink length, while the inter-domain separation should depend on the density of crosslinking.

Figure 10 shows the structure of a film cast from a blend of the homo-polymers; it resembles electron micrographs of many blends of incompatiblepolymers1 2 Comparison of Figures 7, 8 and 10 shows that only in the blendare large PCP domains (2000 nm) in evidence. Growth of a domain in

38

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REFERENCES1 See e.g. P. R. Lewis and C. Price, Polymer, Lond. 12, 258 (1971).2 C. H. Bamford, European Polymer Journal-Supplement, 1969, p 1 and references quoted

therein.C. H. Bamford, R. W. Dyson and G. C. Eastmond, J. Polyin. Sd. (C), 16, 2425 (1967).C. H. Bamford, R. W. Dyson, G. C. Eastmond and D. Whittle, Polymer, Lond. 10, 759 (1969).C. H. Bamford, R. W. Dyson and D. Whittle, Polymer, Lond. 10, 771 (1969).

6 C. H. Bamford, R. W. Dyson and G. C. Eastmond, Polymer, Lond. 10, 885 (1969).C. H. Bamford, 0. C. £astmond and F. J. T. Fildes—in course of publication.C. H. Bamford, 0. C. Eastmond and D. Whittle, Polymer, Lond. 12, 247 (1971).W. P. Slichter, J. Polym. Sd. (C), No. 14, 33 (1966).'° K. J. Kato, Electron Microscopy, 14, (3), 219 (1965); Polymer Letters, 4, 35 (1966).J. Ashworth, C. H. Bamford and E. 0. Smith—in course of publication.

12 M. Matsuo, C. Nozaki and Y. Iyo, Polym. Engng Sd. 9, No. 3, 197 (1969).

40