ion Kinetics of High Density Polyethylene Pre-sheared in the Melt

8/3/2019 ion Kinetics of High Density Polyethylene Pre-sheared in the Melt

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Crystallisation Kinetics Of High DensityPolyethylene Pre-Sheared In The Melt

by

Marie de Chantal Barreto

A thesis submitted tothe Faculty of Graduate Studies and Research

in partial fulfillment of the requirementsfor the degree of

Department of ChemistryMcGill UniversityMontreal, Quebec, Canada

Master of Science

@Marie de Chantal BarretoFebruary 1989


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Crystallisation Kinetics Of PolyethylenePre-Sheared In The Melt

by


A thesis submitted tothe Faculty of Graduate Studies and Research

in partial ;ulfillment of the requirementsfor the degree of

Department of ChemistryMcGill UniversityMontreal, Quebec, Canada

Master of Science

@Marie de Chantal BarretoFebruary 1989


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MSc


Crystallisation Kinetics OfHigh DensityPolyethylene Pre-sheared In The Melt

Abstract

Chemistry

The study of the effect on the crystallisation kinetics of applying a simple shearflow, or pre-shearing, to a polymer sample in the melt was the scope of this project.Forthat purpose a special apparatus was deslgned and built in which a pol ymer sample wasmelted and subjected to a simple shear flow at variom remperatures. CrystallisatlOn waslater followed, in a microscope, by ~ h e change wah time in the intenslty of the Iightdepolarised by the sample.

A slowing down of the crystallisation of the high-density-polyethylene studiedwas observed. The effect of pre-shearing increased as the difference between the shearingand crystallisation temperature decreases. Shear-induced crystallisation is known to havefaster kinetics than quieseent crystallisation. However the effeet seen in this study leadsto believe that, when pre-shearing is involved, moleeular relaxation phenomena become avery important factor in the observed slowing down of the kinetics. Although, it was notactually studied, a change in crystalline structure IS also suspected to have occurred.

i


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MSc Chimie


L'tude cintique de la cristallisation du polythylne de hautedensit pr ~ a l a b l e m e n t soumis un cisaillement

RsumL'tude de la cintique de cristallisation d'un polymre pralablement

soumis un cisaillement l'tat fondu fu t entreprise. Un instrument spcial a tconu pour pOUVOIr fondre le polymre et cisailler l'chantillon diffrentestempratures. La cnstallisauon du polymre fut observe dans un microscope,entre deux polaroides placs angle droit l 'un par rappon l'autre. Lechangement, au cours de la cnstallisation, de l'intensit de la lumire transmisepar l ' c h a n u ~ l o n fut mesur.

Un ralennssement de la cnstallisauon du polythylne de haute densit futobserv. L'effet du CIsaillement augmente, alors que la diffrence entre latemprature laquelle se faIt le CISaIllement et la temprature de cristallisationdiminue. En gnral, un polymre soumis un CIsaillement pendant lacristallisatIon, cristallisera plus rapidement. Cependant, l'effet observ dans ceprojet semble indiquer que les phnomnes de relaxatIon molculaire deviennentun facteur trs important dans le ralentissement de la vitesse de cristalEsanond'un polymre pralablement cisaIll l'tat fondu. Un changement dans la':itTUcture cristallIne es t aussi envisag.


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To my husband Gerhard


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Acknowledgements

1 wish to express my sincere gratitude to my research directors Drs. G.R.Brown and M.R. Kama! for their suppon and guidance through the course of thiswork and in the prepiiI'atlon of thlS thesis.

1 am aiso grateful to the Fond FCAR for a scholarship.1 wouid like ta thank the following people for their assistance in the

course of the research work:Mr. Kluck for the design and construction of the sheanng stage,Mr. Gaulin for the mstallation of the recording umt and for the use of his

recorder throughout the course of this work,The students in the Iaboratory of Dr. Kama!, in the Department of

Cherruca! Engmeering, for the use of their DSC-7 and their assistance,DuPont Canada for donatmg the pol ymer used in this work,My friends and colleagues ln the laboratory for helping make these two

years a very pleasant ume,My famlly and my husband Gerhard for their support, encouragements

and the mterest they mamfested in my work.


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Table of Contents

Abstract . . . .Rsum 11Aknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . .... j i iTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... vList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... viList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ixList of Symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ) (

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Shear-Induced Crystallisation . . . . . . . . . . . . . ......

1.1.1 Experimental Deviees . . . . . . . . . . . . . . ..... 71.1.2 Morphology . . . . . . . . . . . . . . . . . . . . . . . . . 21.1.3 Crystallisauon Rate . . . . . . . . . . . . . . . . . . . . 131.1.4 N ucleauon Rate . . .... ..... .141.1.5 Temperature Effect .... . .... 221.1.6 Molecular Weight Effect . . . . . . . . . . . . . . . . . 231.1.7 Nucleant and Additive Effect . . . . . . . . . . . . . . 23

1.2 Pre-Shearing Effect . . . . . . . . . . . . . . . . . . . . . . . . . 241.3 Present Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . .251.4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.1 General Description of Pre-shearing Experiments . . .... 30

2.1.1 Reeording Unit . . . . . . . . . . . . . . . . . . . . . . . 33

iv


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2.1.2 Shearing Stage . . . . . . . . . . . . . . . . . . . . . . . 332.1. 3 Expenrnental Procedure . . . . . . . . . . . . . . . . . .36i. Sample Preparation for Microscope Experiments . . . . 36Ii. Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.2 Development of the Technique . . . . . . . . . . . . . . . . . . 402.2.1 Sample Description . . . . . . . . . . . . . . . . . . . . 402.2.2 Recording Umt . . . . . . . . . . . . . . . . . . . . . . . 02.2.3 Pre Shearing Expenments ................ 432.2'-+ Punticauon of the Commercial Resm . . . . . . . . . .55

2.3 DSC Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 56References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3 Results ................................... 23.1 DSC Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 623.2 N.M.R. Experiments . . . . . . . . . . . . . . . . . . . . . . . . .673.3 Pre-shearing Experiments . . . . . . . . . . . . . . . . . . . . . 67

3.3.1 Data AIialysis ........................ 73.3.2 Descnption of the Results ................. 33.3.3 Avrami Analysis . . . . . . . . . . . . . . . . . . . . . . 1

3.4 Discussion of the Results . . . . . . . . . . . . . . . . . . . . . . 863.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96

4 Contribution to Knowll'dge and Future Work ....... 974.1 Contribution to Knowleoge . . . . . . . . . . . . . . . . . . . . 74.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.3 Reference l' , 1 1 1 99

v


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-

viList of Figures

Figure 1.1 Growth Rate as a Function of Crystallisation Temperature forPoly(ethyleneterephthalate) . . . . . . . . . . . . . . . . . . 3

Figure 1.2 The Different Modes of N uc1eation . . . . . . . . . . . . . . .4Figure 1.3 Model of Spherulitic Structure . . . . . . . . . . . . . . . . . .Figure 1.4 Viscosiy Measuring System and Thennal System of the Parallel

Plate Rheometer BlIilt by Lagasse and Maxwell . . . . . . 9Figure 1.5 Views of the Rotary ParalIel Plate Shearing DevIee Built by

Ulrich and Priee . . . . . . . . . . . . . . . . . . . . . . . . . 10Figure 1.6 A Schematic Diagram of the Cor.centric C y l i n d e r ~ Built by

Fritzche and Priee . . . . . . . . . . . . . . . . . . . . . . . .11Figure 1.7 CryStallisatlOn Kinetics As Found by Lagasse and Maxwell for

a High Molecular Weight Polyethylene . . . . . . . . . . . 15Figure 1.8 Logarithmic Plots of Crystallisation Kinetics as Found by

Fritzche and Prire for Polyethylene Oxide . . . . . . . . . 16Figure 1.9 Semi-Logarithmic Plot of Crystallisation Kineucs as Found by

Sherwood, Priee and Stein for Polyethylene Oxide . . . . 17Figure 1.10 Nucleation Kineues as Found by Sherwood, Priee and Stein fo r

Polyethylene Oxide. . . . . . . . . . . . . . . . . . . .... 18Figure 1.11 Nucleation Kinetics as Found by Wolkowicz for

Poly(butene-1) . . . . . . . . . . . . . . . . . . . . . . . . . . 21Figure 1.12 The Avrami D ~ g r e e , n and the Temperature of Maximum

Crystallisation Rate, Tmax as a Funetion of the Elongation inthe Melt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6


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vii

Figure 2.1 Experimental Set-up . . . . . . . . . . . . . . . . . . . . ... 31Figure 2.2 Photographs of the Apparatus . . . . . . .. . . . . . . . . . 32Figure 2.3 Calibration Curve . . . . . . . . . . . . . . . . . . . . . . . . . 34Figure 2.4 Shearing Stage (Original Version) . . . . . . . . . . . . . . . 35Figure 2.5 Transfer Procedure as Viewed from the Bottom Block of the

Sheanng Stage . . . . . . . . . . . . . . . . . . . . . . . . . .37Figure 2.6 Crystallisation Curves . . . . . . . . . . . . . . . . . . . .... 2Figure 2.7 Photographs Showing a Change in Colour of the Spherulites

................................... 44Figure 2.8 Second Calibration Curve of the Photoresistor . . . . . . . . 45Figure 2.9 Experimental Curves Tc=123C, T s=140C . . . . . . . . . . 46Figure 2.10 Experimental Curves T c=121C, Ts=140C . . . . . . . . . 47F i g u n ~ 2.11 Experimental Curves T c=123C, Ts=140C . . . . . . . . . 49Figure 2.12 Experimental Curves T c=123C, a)Ts=132C b)T s=135C 51Figure 2.13 Modified Shearing Stage . . . . . . . . . . . . . . . . . . . . 53Figure 2.14 Expenmemal C u r v ~ s T c=123C, T s=140C . . . . . . . . . 54Figure 2.15 Melting Endotherm for ,he Treated Sample (Previously Heated

at 180C for Ten Minutes) . . . . . . . . . . . . . . . . . . . 59Figure 2.16 Melting Endotherm for the Treated Sample (PrevlOusly Heated

at 180C for Fifteen Minutes) . . . . . . . . . . . . . . . . . 60Figure 3.1 Determination of the Equilibriurn Melting Point . . . . . . . 63Figure 3.2 l3C NMR Spectrum of the Purified Resin . . . . . . . . . . . 68Figure 3.3 l3e NMR Spectrulll of the Untreated Resin . . . . . . . . . . 69Figure 3.4 Two Consecutive Runs Without Pre-shearing a)T s =135e

b)Ts =130C . . . . . . . . . . . . . . . . . . . . . . . . . . 4


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- - - - - - ~ - - -

Figure 3.5 Experimental Curves at a Shearing Temperature of 140C. 75Figure 3.6 Experimental Curves at a Shearing Temperature of 135C. 76Figure 3.7 Exper..mental Curves at a Sheanng Temperature of 130C. 78Figure 3.8 Experimental Curves at a Shearing Temperature of 127C. 79Figure 3.9 Experimental Curves at 125C . . . . . . . . . . . . . . . . . . 0Figure 3.10 Crystallisation and A vrami Plots at a Shearing Temperature of

130C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Figure 3.11 Crystallisation and Avrami Plots at a Shearing Temperature of

127C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83Figure 3.12 Melting Endotherm of a Sample Pre-sheared at 1300e andCrystallised at 123C . . . . . . . . . . . . . . . . . . . . . . 5

vi i i


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ix

List of Tables

Table 2.1 Experimental Conditions . . . . . . . . . . . . . . . . . . . . . 39Table 2.2 Physical Properties of the Commercial Resin . . . . . . . . . . 1Table 2.3 Experimental Conditions . . . . . . . . . . . . . . . . . . . . . 56

Table 3.1 DSC Results for the First Purification . . . . . . . . . . . . . . 5Table 3.2 Comparison of the Commercial and Purified Resin . . . . . . 66Table 3.3 A vraIlli Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 85Table 3.4 DSC Results of Samples Pre-sheared at 130C and Crystallised at

123C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4


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- - - - ----------- - - -------- ---- - - -x

List of Symbols

Symbol Definition

G Growth rate of crystallitesGo ConstantN Nucleation rateNo ConstantLlF* Activation energy for transpon of the

molecules through the hquid to the liquid-crystal boundary

~ < p . Work required to fonn a surface nucleus ofcritical size

k Boltzman constantR Gas constantLill Free energy of activatIon for transpon through

the melt to the nucleus surfaceU Lateral and end interfacial free energies of the

nucleiSupercooling

~ T ' Actual supercooling in presence of stress~ T a Additional supercooling due to the presence of

stressT Melting PointmTs Shearing temperatureTc Crystallisation temperature


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xiTe

rn Quiescent equilibrium mclting pointTrnax Temperature of maximum crystallisation rateMIf Heat of fusionMIrn Heat of fusion per statistical elementMIf Theoretical heat of fusion for a 100%

crystalline sampleNs Normal stressy Shear ratell! Viscosity.: Critical elongationn Avrami degreek Rate constantEJ Activation energyA Pre-exponential factort 02 Time required to attain 20% crystallinityX Crystallinity%t Percent transmitted lightR Resistance of photoresistor


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-. 11. INTRODUCTION

Being one of the world's major plastics, polyethylene has been widelystudied either for 11S mechanical or physical propenies, as melting and itscrystallisation behaviour. In partIcular, crystallisation studies have proven to beextremely important In the field of commodlty thennosettmg plastIcs. Most of theprocessing techniques involve crystallisation of the polymer as one of the steps.Usually the resin lS first melted 10 be shaped 111 vanOllS ways and th en cooled.Such processes as injectIon molding or blow moldmg, Involve fIlling a cold moldwith the molten polymer. The obvious result is a crystallisation of the sample Inthe shape of the mold. However, the conditions for this latter stage wIll affect thecrystallisation behaviour of the polymer. Changes ln kinetics of crystalhsation orpolymer morphology with the condltions are extremely Important 111 defmmg theproperties of the final product and the prottabihty of the wholc proccss.

The crystallisauon of polymers 1S a large field of research and, inpartlcular, extensive studies have been done on the crystallisauon l'rom thequiescent melt (1-6). The kinetics of crystallisauon of polymers can be followedby momtoring the change In physical propertles of the samples wnh ume. If anabsolute property 1S observed, direct relatIon can be determmed between thelevel of crystallinity and the propeny (as the change m specifIe volume or the heat\Jf fusion of the polymer). Microscopy can also be used for kmeuc analysis. Thegr'Jwth rate of the crystalline entities is determined from photomlcrographs. Thetemperature dependence of the crystallisation rate has been shown to be bell-


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2

shaped, as shown in Figure LI.Crystallisation consists of two steps, nucleation followed by growth.

Nucleation can either be heterogeneous or homogeneous depending on whetherthe densIty fluctmmons leading ta the formation of the nucleus are due to thepresence of foreign parucles or not. Severa! steps are implicated in the nuclcationprocess. The actual fust appearance of a solid phase from the amorphous state isthe primary nucleatlon. Secondary nucleation implies the growth of a new layeron a smooth crystal surface (as shown 10 Figure 1.2). The free energy barrier forthe process is smaller than for pnmary nucleauon since the newly created surfacearea is smaller. A third step 1S teruary nucleatlon which, as can be seen in Figure1.2, defines the stan of a new row of crystal growth at an edge (8). The freeenergy barner is even lower for teniary nucleanon.

When a stable nucleus has been formed, molecular chams in theamorphous phase transfer to the crystallising front as the crystals are growing.The rate of the transport step will be dependent on the viscosity of the melt andhence on the temperature as can be seen from the Tllmbull Fisher equation:

1.1

where G is the growth rate of the crystals, Gois a constant, .. is the activationenergy for transport of the molecules through the liquid to the liquid-crystalboundary and L1


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--:ccr 2e L-x 1., /r'r'

/1

//

'r '/7 \6\\

' :0 '.:..J 'C '8 0 ~ : ; a : :0" ' ~ ~ A P ~ : : ; !. - ..JRE .,... ,

Figure 1.1Growth Rate as a Functlon of Crystallisation

Temperature for Poly(ethyleneterephthalate) (7)

3


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4

primary

secondary tertiary

Figure 1.2The Different Modes of Nucleation


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5

decreases and hence transpon of the chain molecules to the crystallisation frontbecomes easier. As the tempera'ure increases and the maximum m the curve ispassed, the crystallisauon rate is inversely propomonal to temperature. Theformatior. of a cnncal sized nucleus becomes more d1fficult and the secondexponenual m equanon 1.1 dominates the process. The tenn L'lep * is dependent onthe free-energy of fusion per molecule, the lateral Interfaclai free-energy permolecule, the interfaClal free-energy per molecule at the nucleus end and theundercoohng. Hoffman and Launtzen have derived L1


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6

Figure 1.3Model of Spherulitic Structure (7)


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phases (10). The rate of crystallisation and morphologlcal features will differfrom those found experimentally for crystallisauon from quiescent melts, andhence they WIll affect the propemes of the finished product. The research groupof M.R. Kamal has studied, among other toplCS. the morphology of Injecuonmolded polymer samples (11,12) and they have observed different morphologIeswithin a given sample. Three distinctive layers have been Idenutied: the skin, nextto the surface of the mold, the intermediate zone and le core. Only In the corecan randornly oriented spherulites he observed. In the two other layers. themorphology IS affected by the processmg condnions. Transcrystalhnay ISobserved in the skin layer while In the mtermedlate zone the spheruhtes areoriented in the direction of thermal gradIents.

The crystall isauon near to the ~ u r f a c e IS undoubtedly affected by the shearexperienced by the polymer whlle the muid bemg tlled. Shear-mducedcrystallisauon has been exammed from dlfferent pomts of Vlew. Sorne of thework found In the literature and perhaps more closely related tu the ~ c o p e of thisstudy WIll be descnbed. Studies dealing WIth crystallisanon from polymer meltswill be emphasized. rather than those Involvmg crystallisauon t'rom ~ o l u u o n .

1.1 Shear-induced crystalHsation1.1.1 Experimental Deviees

Since what was ultimately sought in this study was mformauonconceming the effect of processing conditions on the crystallisauon of polymers,the very ChOlce of an apparatus that would enable a sample to be sheared whilesimultaneously followmg its crystallisation becomes cntical. Various methodsexist for monitonng the crystallisation of prly'l1ers, elther by followmg a physlcal


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property change, such as a change in viscosity or volume (dilatometry), or bymicrosc( ~ i c observation. These different methods have also been used for studiesof shear-Induced crystallisauon. The shearing geometry must also beaccommodated WIth the monitoring techmque employed. The cases cited allinvolved the apphcauon of a constant ::.hear-rate on the sample. However, twomain cases were considered. The melt was either placed in a translational or arotation al shear field.

The research group of H. Maxwell (13,14) has built two Instruments lnwhich polymer crystalhsation was studied while the melt was subjected to atranslational shear. A schemaue diagram (14) of one of the parallel platerheometers is shown In Figure 1"+. The sample was melted, quench c()oled andsheared betwecn two glass slides while it was observed microscopically. Thetranslational speed c01Jld be vaned and hence su.::h effects as the temperature ofcrystalhsauon and the shear rate could be examined.

In the domam of a rotation al shear field, an interesting apparatus was builtby the group of F.P. Priee (15.16), whieh still allowed for microscopieobservauons. The supereooied melt was sheared between two rotating paralle!glass plates (Figure 1.5). Although the shear rate IS not constant for this type ofgeometry, it cao be consldered constant In the small area under studymicroseoplcally (15). A second device whieh followed the crystallisation bydilatometry was built as weil (17,18). The supercooled melt was shearcd betweentwo concentric cylinders, as shown in Figure l.6. The shear rate is constant forthis geometry. Simple r 1eometers, such as a cone-plate viscometer, are also suitedfor such studies since they develop a constant shear rate and allow thecrystallisauon ta be followed by the propeny i t measures, namely, viscosity (19).

The devices aforementioned are only a few examples. However, sorne


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,..

TOPVIEWsynchronousm)torQ - - - ~ gearbox

verticalSUODort wlre

l rans'atlonstagestralng.:ige

olymer samOle betweenglaSs slldesENDVIEW

(upper sllde(statlonary)

1 r t ransducer be3m

lower slide (translatlng)

: = ' - - C ~ : ' S ! . . . " ' C air

Figure 1.4

QUENCHING UNIT

9

Viscosity Measuring System and Thennal System of the Parallel Plate RheometerBuilt by Lagasse and Maxwell (14)


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--..................... " -; .... .... .., ", . ~ - - - ~ - U : :

1 1 \ 1 " l , l' :, "-L-.-":'. ,r Il '1 ~ - - _ & - - ~ ~ ~ - T ----1 0: ;:::;.. ..,: :,' ' .. .'

l' 1 \.QJ::, l' .... ., '_ .............. o6 L .. '.! L .... \ .. - 1": .1 -: .-. 01 ,

\ ,,

" 7hreooed Nul 5 SpeceP'ore " ' ~ ' C n ng t,u' \ C/UOIV

/ cmp'eS ~ e o " n g P'e.'e / r - -~ = = = : : : : ; l b l - , " " m , ~ , , , t::=:========/=7=l=,::LU41JI============:::::=::f Weil

Bo"om 31055 1 1P o' r ,ew,ng Por' ' :> Je Scct'8eorlng

1 Bollcm Ple:e DrIVe Shaft

Figure 1,5

10

Views of the Rotary Parallel Plate Shearing Deviee Built by Ulrich and Priee (15)


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sncf' wlthcry sect

mercury pool

m / \ ~ !T. 11 i: L 11

i! Il'r1FL...J

Figure 1.6

11

0 - rings

A Sehematie Diagram of the Coneentrie Cylinders Built by Fritzehe and Priee(17,18)


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12

general conclusions cao be drawn. A constant shear rate geometry does allow foran easier analysis of results. Thermal control is very imponant. In all of thedevices described, the thickness of the polymer melt was kept small to reduceviscous heatmg effects. A relatively thin sample allows for rapid thennal responseto either cooling or heating. This feature is important since panial crystallisationof the sample pnor to the shearing motion, as the sample is being cooled, wou djeopardize any analysis of the data in view of shear-induced crystallisation. Inaddition, ~ m a l l changes in crystallisauon temperature are known to greatlyinfluence the kmetics of crystalllsation. Whenever a shear tield lS applied, endeffects should be avoided. Generally in rotational shear fields such problems areavoided. However, they do create lImitations for devices involving transiationaishearing (13.14).1.1.2 Morphology

In most of the work descnbed, crystallisation was followed with a lightpolarizing microscope. The morphology of the shear-crystallised samples couidtherefore be observed as it developed. For polyethylene oxide samples, Ulrich andPrice (15) o b ~ e r v e d , fo r both the quiescent and sheared cases , an mitial formationof prolate eilIpsOlds which were randomly oriented in the quiescent case. Whenthe sample was sheared, these entities had their semi-major axis perpendicular tothe shear direction. AIso, these entities :Ippeared to break up during shear. Smallan,gle light scattering studies of the samples lead to the conclusion that the rodl i k ~ panic1es were chain-folded lamellar sheaf-like scattering entities with t.heirlong axes perpendicular ta the shear direction. Epimlcroscopy was used toexamme the morphology of the sample at the surfaces after crystallisation wascompleted. Row-like structures were formed only near or on the surfaces, and


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13

under shear conditions only. They were assumed to fOIm along fine scratches orheterogeneities on the plates. Wide angle X-ray scattering studies also establishedthat there was a finer morphology in the sheared case than in the qUlescent case.Ulrich and Price (15) did not prove experimentally that there were any changes inorientation within the particles from the quiescent to the sheared cases.

Haas and Maxwell (13) studled the morphology obtained by shear-mducedcrystallisation of poly{butene-l). They observed that, at a high level of shear-stress, the structure of the sample 1S no longer distingUlshable, due to theextremely profuse nucleatlon. At a relauvely low shear-stress (0.537.105dynes/cm2), only spherulites were observed but at a higher shear-stress \l.02.105dynes/cm2) rows appeared in the direcoon of the flow. It was assumed that the ywere caused by distl1rbances due to the presence of sphe:-'lhtes nr impunties in themelt. As these lines increased, lamellae forrned on them and grew out radially.The who le structure can oe related to the row structure descnbed by Keller andMachin (20).

Kawai, et al. (19) observed, with an electron microscope, the structureformed in the sarnples of polyethylene sheared betwel!n i l cane and plate.Fibrillization occurred along the sheanng direction in the sheared films.1.1.3 Crystallisation Rate

The tirst corr.mon observation for shear-induced crystallisation is adefinite enhancement of the crystalhsation rate over that obtained for thequiescent case (18,14). The overall time required for crystallisauon lS greatlydecreased by shearing (13) and the melts are found (19) to crystallise even attemperatures where, under quiescent conditions, no crystallisauon '1ccurs In areasonable amOUl.t of time. The case of polyethylene, for Instance, is quite


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remarkable: the melt is found to crystallise at temperatures above 135C with ashear rate of 0.5 S-l in a cone-plate geometry_ However, in quiescent conditions,crystallisation at 134C would be considered as not occurring at all (the time scalewou Id be too long) (19).

In another paper, most dramatic changes from the quiescent case werereported to occur at temperatures close ta the qUlescent eqUllibrium meltingtemperature (13). The rate of crystallisation increased with the shear rate (14,18)(Figures 1.7 and 1.8). However, saturatIon effects were aiso o b ~ . . ! r v e d (18,21).The rate of increase in the crystallisation rate was found ta decrease with shearrate (cf. Figures 1.8 and 1.9). The effect was enhanced by the increase in stress,and chain slippage was suggested as a possible explanation for the phenomenon(18).1.1.4 Nucleation Rate

The apparatus built by EP. Price's research group lends itself very weIl tonucleation s t u d ~ e s with photographs (15,16). Such techniques also have theadvantage of a dIrect observatIon of the morphology as it develops. Their resultsshowed a great difference between qUlescent and shear-mduced crystallisauon. Anon-lmear mcrease in the number of nuclel as a function of time was observed.Thus, nucleation rates varied during the shear-induced crystallisation. Tuwardslarger shear rates the divergence between the quiescent and sheared case wasfound to be i n c r e a ~ c d , as was the apparent nucleation rate (Figure 1.10).

The authors analysed their nucleation data by the Hoffman-Lauritzenrreatment of the Turnbull-Fisher equation,


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-.

15

100O! i fil i i 1 r "1E1e T=129"Ca> T=126 oC100 o T=125 oC

uil)II )

E l1- 1Cc0+'u:: )DC

10

0.1 ' - - - - - - . : . . . - - - - - - - - - - - - - - - - ......-----......,j.01 01 10 10 100Shear Rate, sec.- I

Figure 1.7Crystallisation Kinetics as Found by Lagasse and Maxwell for a High Molecular

Weight Polyethylene (14)


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iOt..)4 a ; ,000 IZ \ t e '000 22 If ( ,

/ "/

Figure 1.8

, /1

16

/.,

Logarithmic Plots of Crystallisation Kinetics as Found by Fritzche and Priee andUlrich for Polyethylene Oxide (18)


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l

1C ." >-1 t 1 11 1J 1 1r

1 .., .,,"'p

17

PEO 6COO7"em;l' ~ s e c ' o 56 7 0l 56 7 t 4 56 7 5 4:: 56 8 10 9" 56 5 29 0.l 56 9 56 1

, " ' ;Y______________________~ " _ ~ _ < ~ / ~ y ~ ~ ___ ____ _ _ _ _ _ _ _ _ _ _30 40 50 6 J

Figure 1.9Semi-Iogarithmic Plot of Crystallisation Kinetics as Found by Sherwood, Priee

and Stein for Polyethylene Oxide (21)


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18

80hmo 53 BC /22 nc" '" -c,0 0 . tC .,

60'"

u+-L-00..'+-0 40LIVDE::lZ

20 v , .~ ~ ~ ~ !." .r--. -, .__ ____ ____ L_____ J____ ____ ____ _____ __ ____ ____ ____ _o 20 .10 60 80 ,':.'r)

T,me, min

Figure 1.10Nucleation Kinetics as Found by Sherwood, Price and Stein for Polyethylene

Oxide (21)


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ln N = (ln No - 65) - U f ( ~ T ) RTe

19

1.2

where N is the nucleation rate, No a constant, R the gas constant. Te thecrystallisation temperature, M! is the free energy of activation for transportthrough the melt to the nucleus surface, U contains laterai and end Interfacial freeenergies of the nuclei and f is a function of ~ T , the supercoohng. The authorswanted to stress here the dependence of the free energy terrn ~ < p ' (prevlOuslymentioned in the mtroduction to this Chapten on the supercooling of the system.For each shear-stress condition, plots were made of ln N as a funcuon of f ( ~ T ) . Whereas the experimental data were found to fit equauon 1.2 in the qUlescentcase, shear-mduced crystallisation data did not follow the theory. The authorsattempted to adapt equation 1.2 for the case of crystallisauon dunng shear b)altering one of the variables, ~ T , which they expected to be affected by theapplied shear-siress. The actual supercooling of the system, ~ T ' was expressed as

~ T ' = ~ T + t 1 T a 1.3where ~ T a is an additional supercooling due to the presence of stress. The stresssupercooling, .1Ta, was related to the elastic properties of the system.

A theory had been presented previollsly, by Krigbaum and Roe (22),which described the stress-supercooling of a cross-linked system as

1.4


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20

where T; is the quiescent eqUllibrium melting point. N s is the normal stressimposed on the system and L1Hm is the heat of fusion per statistical element.

It was suggested that the numerous ehain entanglements in thepolyethylene oxide system confmn that l t has a cross-linked nature. A eomparisonbetween the prediction of the Krigbaum and Roe treatmenl and the experimentalvalues for the stress supercoohng showed disagreement. A careful exammatlon ofthe elasuc nature of the polyethylene oXlde system studied by Ulnch and Priee.would glve better Insight into the ruvergences of the experimental stress~ u p e r c o o h n g s and the predICtiOnS of Krigbaum and Roe. The srress supercoohngaccordmg ta equatlon 1.3 IS Independent of the crystallisauon temperature butrather depends on the stress apphed. lndeed, thiS rreatment predicts at equalstresses, the same mutual alignment of chains. due to the elasuc nature of themelt, independently of the temperature. However, the nuclealIon process isdirectly related to temperature. since as the crystallisa tion temperature 10creases alarger enlIcal sized nucleus is needed for crystallisatlOn. Hellce, as was foundexperimentally, the nucleatlon ratt ::!caeases \VIth an increase In temperature, at aglven level of stress. AIso, stress-relaxatIon urnes decrease with an mcrease 111temperature and therefore each stausucal segment resldes for a shoner ume In afavourable confIguration. Any adequate theoretical descnption of the nucleauonprocess involved 10 the shear-1Oduced crystallisatlOn must 10clude the influence oftemperature on the relaxauon of entangled chams.

The ~ a m e experimental apparatus was used later by Wolkowicz withpoly(butene-l) (23). The expenmental results were in agreement with thoseobtained prevlOusly by Ulrich and Pnce tcf. Figure 1.11). The main contnbutionof shear was thought to be 1I1 asslsting pnmary nucleauon by causing thealignment of chains, which offers a nucleating surface fo r the formation of stable


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21

11 1.5 sec ,Tc 9 ~ c C 8 s e ~ ,

1gJ.1,1

1

1

'16T

1

!?: ,E i!!u. 4 ...Q, , 46 s':?::: .z JJ

1.1 1.

1 / 1 .:1, sec"'f . ..' 1 .// /'/ . /' ,? 1.1 1 - 1 1 10 2 6 10 14 18 22 :6Tlme mIn

Figure 1.11Nucleation Kinetics as Found by Wolkowicz For Poly(butene-l) (23)


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22nuclei. This alignment of the chains with the onset of steady shear would alsocause a reducnon in the entropy of the system. Hence, the increase in nucleationrate was accounted for by the decrease in the change in entropy of the proc:esswith shear. Wolkowicz also found that the growth rates of the crystalline entitiesin the sheared and quiescent cases were nearly identical. Therefore, he attributedthe increase ln crystallisation rate with shearing mainly to the increase in primarynucleation.1.1.5 Temperature Effect

The temperature is known to be of primary importance for crystallisationand its effect on shear-induced crystallisation has been observed by the sameresearchers. Two mam cases were observed depending on whether a low or arather high shear stress (0.4. lO5_20. 105 dynes/cm2) was applied.

Under low shear stresses, or quiescent conditions, a negative temperaturecoefficient of the crystallisation rate was observed (13). This im,cates that thekinetics of crystallisation is nucleanon controlled. Ulrich and Price (16) foundthat the nucleation rate also has a negative temperature coefficient due te thereduced probability of forming a critical sized nucleus with an increase intcmperature.

At higher shear stresses (0.4.lO5-20.105 dynes/cm2 for HMW poly(butene-1)), Maxwell et al. (13) observed the opposite temperature effeet: thecrystallisation rate has a positive temperature coefficient. They attributed this tonon-equilibrium effects rather than ta a control of the kinetics by the transportprocess since under these conditions there is insufficient time for the moleeules toreach an equilibrium confonnation before the start of the crystallisation.


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23

1.1.6 Molecular Weight Effect

At given shear stresses, Haas and Maxwell (13) observed an increase incrystallisation rate with a deerease in mclecular weight for polyethylene and.poly(butene-l) which they attributed ta an mcreased moblhty of the molecuks. Athigh shear stresses the increased mobility was thought te provide a fasterorientation while at low shelr stresses it influenced the transport process.However, Lagasse and Maxwell (14) observed, for polyethylene, a somewhatcontradictory effect in that the higher molecular weight matenal had a lowerindueol1 time, which implies a faster erystaIlisatlOn. They defined the mductionshear strain as the strain to which the melt can be sheared before crysta!lisauonoccurs and observed Ils variation with shear rate. Furthermore, they found that thecritical shear rate above whkh the mducuon shear strmn was constant, mcreasedwith the decrease of the molecular welght at a given temperature. This criticaIvalue aIso varied with temperature SInce at a given molecular weight the erit.ealshear rate inereased \\itt !emperature. The authors suggested that, according tothese results, this critical condmon was due to a molecular relaxatIon process. Intheir discussion they arrl'/ed at the conclusIOn that, for thIS llon-Newtomansystem, the shear-aecelerated crystallisation IS due to elastic chain extensionduring the start-up of shear flow.1.1.7 Nucleant and Additive Effect

To verify the theory that the enhanced crystallisation observed when shearis applied is due to localized molecular extension near heterogeneous particles,Lagasse and Max.well (14) carried out sorne experirnents with carbon blackadditives or nucleants. Within the range of shear rates (0.02 S-1 to 30 S-I), no effecton the shear-aeeelerated crystallisation rate was observed, which contradIcted the


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24

theory. Sorne other observations lead them to the conclusion that the phenomenonrathf'r reflected "elastic cham extension due to the entanglement couplingsbetween macromolecules".

1.2 Pre-shearing effectAlthough in plastics processings, polymers are subjected to stress while

crystallising, they are also subjected to harsh conditions at higher ternperatures.Then. melt memory plays an Imponant role and the experimental conditions inthe melt at the higher temperatures wIll affect the crystallisauon process at thelower temperature. Lagasse and Maxwell (14) referred ta it as "pre-sheanng" andstated its imponance for a more general understanding of shear-inducedcrystallisation. They proposed t h p studyof the crystallisation of a melt which wassheared pnor to quenching ta the crystallisation temperature at which it wouldstill be sheared. These conditions would more closely approximate processingconditions. Such a study of the pre-shearing effect would allow a verification oftheir model for shear-accelerated crystallisation. They suggested that the numberof entanglements hl the polyr!1er melt would be reduced by the inducedorientation. Hence, a decrease in the shear-induced crystallisation would beexpected as opposed to the case without pre-shearing, since elastic chainextensions at entanglements would be reduced.

V.G.Baranov et al. (24) studied the crystallisation of cross-linkedpolychloroprene films (150-200 j.lm thick) previously elongated in the melt.Elongation ratios of 1 to 5 were used at a temperature 27 above the equilibriummelting point. Their results showed an increase in the crystallisation rate as theextent of molecular orientation was increased. Their curves of l/'to2' the rime


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25required to attain 20% crystallinity, as a function of Tc were all beU shaped.Interestingly, they aiso observed a critical elongation " (1.5) above which thereis a marked change in the rdation between the A v ramI exponent n or thetemperature of maximum crystallisation rate Tmax and the elongation (cf. Figure1.12). An analysis of their results showed that bath the nucleation and growthstages were affected by the previous orientation inflicted on the melt.

1.3 Present StndyPrevious studies indicate that an important characteristic of shear-induced

crystallisation is the role of molecular mobility, elastic or relaxatIon propertles ofthe polymer in the understanding of the process. Experiments involving shearingof the melt prior to quenching it ta the crystalhsation temperature could offeradditional insight concerning the mfluence of molecular mobility or relaxation onthe kinetics of shear-induced crystallisation. RelaxatIon from the previous stressis expected ta occur simultaneously WIth crystallisation and therefore could affectthe process.

Another type of experiment whkh would more closely examine theparticular effect of a previous shear in the melt, would consist of observingcrystallisation under quiescent conditions after previous shearing. In this case,molecular relaxation from the orientation previously inflicted on the melt, couldbe seen as driving the chain molecules away from the favourable conformationobtained by the previous shear.

The purpose of the present study is to examine the crystallisatton of apolymer melt which has previously been sheared, either at or abave thecrystallisation temperature. No shear is to be applied during crystallisation. The


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26

-J-r2 'ma::,7,1J,J( , f

f 0

- -2,J11 .., l j J

Figure 1.12The Avrami Degree, n and the Temperature ofMaximum Crystallisation Rate,

Tmax as a Function of the Elongation in the Melt, for Cross-LinkedPolyethylene(24)


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,

27

crystallisation kinetics are to be microscopically observed by following thechange in the amount of light depolarised by the sarnple. Such a study requiresthe construction of a special apparatus, as described in the following chapter.


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28

1.4 References

1. M.R. Kamal and E. Chu, Polym. Eng. ScL, 23,27 (1983).2. G. Turturro, Ph.D. Thesis, MeGill University (1981).3. M.A. Kennedy, Ph.D. Thesls, McGill University (1988).4. D. Turnbull and J.c. Fisher, lChem.Phys., 17,71 (1949).5. lI . Lauritzen and J.D. Hoffman, J. Res. Nat. Bur. Stand., 64A, 73 (1959)6. 1.D. Hoffman and J.1. Lauritzen, J. Res. Nat. Bur. Stand., 65A, 297 (1961)7. L.H. Sperling, Introduction to PhY'Heal Polymer Science, Wiley Interseience,

19868. B. Wunderlieh, "Maerornoleeular PhySlCS", vo1.2, Academic Press, 19769. L. Mandelkern, Crystallisauon of Polymers, MeGraw-Hill, New York, 196410. F.B. Cheikh Larbi, M.F. Malone, H.H. Winter, J.L. Halary, M.H. Leviet andL. Monnene, Maerornol., 21,3532 (1988)11. D.M. Kaylon, M.Eng. TheSlS, McGill University (1977)12. F.H. Moy, Ph.D. Thesis, MeGill Universi ty (1980)13. T.W. Haas and B. Maxwell, Polym. Eng. ScL, 9,225 (1969)14. R.R. Lagasse and B. Maxwell, Polym. Eng. Sei., 16, 189 (1976)15. R.D. Ulrieh and F.P. Priee, J. Appl. Polym. Sei., 20, 1077 (1976)16. R.D. Ulneh and F.P. Pnee, 1. Appl. Polym. Sei., 20, 1095 (1976)17. A.K. Fritzehe andF.P. Priee, Polym. Eng. Sei., 14,401 (1974)18. A.K. Fritzehe, EP. Priee and R.D. Ulrich, Polym. Eng. Sei., 16,182 (1976)19. T. KaWa!, T. Matsumato, M. Kato and H. Maeda, Kolloid-Z., 222, 1 (1967)20. A. Keller and MJ. Machm, 1. Maeromol. Sci.(Phys.), 81,41 (1967)21. C.H. Sherwood, F.P. Pnce and R.S. Stein, J. Polyrn. Sei.: Polym. Symp., 63,77 (1978)22. W.R. Krigbaurn. RJ. Roe, 1. Polym Sci.A, 2,4391 (1964)23. M.D. Wolkowiez, 1. Polym. Sci.:Polym.Symp., 63,365 (1978)


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2924. V.G. Baranov, G.T. Ovanesov, K.A. Gasparyan, Yu.K. Kalyan and S.Ya.Frenkel, Dokl. Akad. Nauk SSSR, 217, 119 (1974)


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"l'"

(

30

2. EXPERIMENTAL

A special apparatus was designed and built to study the effeet on thecrystallisation kinetics of pre-shearing the polymer in the melt state. The mostimportant aspect of this expenmental set-up was the ability to both melt and applya simple shear flow to polymer samples. The crystallisatlOn kinenes wereobserved by followmg the llltenSlty of depolarised light with time, using anoptical microscope.

A descnption of the apparatus, with special emphasis on the shearingstage, follows. The design and the development of the new technique will bedescribed along with the expenments earned out for the completion of this work.

2.1 General Description of Pre-shearing ExperimentsThe schematic diagram (Figure 2.1) and the photographs in Figure 2.2

show the apparatus, WhlCh consisted of a shearing stage, a microscope equippedwith a hot stage and a recorder. Polymer samples could be melted and shearedbetween two glass slides wnh the sheanng stage built for that purpose. Thesamples couId then be transferred qUlckly from the shearing stage ta themicroscope hot stage (Mettler Model FP52) set at the crystallisauon temperature.CrystallisatlOn was observed between ross-polaroids In a Wild LeItz microscpe(Model HM-LUX) and was monitored by recording the intensity of lightdepolarised bl' the sample as a function of time.


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.,,,..

-

Recorder

MicroscopeCaIne ra

o Photore:si : s tor- - -T" - - - - - -r-__1 'Ana lyze

Objec t ive

--7 Po l a r i z e r

o Ligh t Sou r c eShear ing Stage

Figure 2.1Experimental ~ e t - u p

31

Var iac


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National Libraryof CanadaCanadian Theses Serv ice

NOTICE

THE QUALITY OF THIS MICROFICHEIS HEAVILY DEPENDENT Ul-ON THEQUALITY OF THE THESIS SUBMITTEDFOR MICROFILMING.UNFORTUNATELY THE COLOUREDILLUSTRATIONS OP THIS THESISCAN ONLY YIELD DIFFERENT TONESOF GREY.

B i b l i o t h ~ q u e nat ionaledu Canada

Service des t h ~ s e s canadiennes

AVIS

LA QUALITE DE CETTE MICROFICHEDEPEND GRANDEMENT DE LA QUALITE DE LATHESE SOUMISE AU MICROFILMAGE.

MALHEUREUSEMENT, LES DIFFERENTESILLUSTRATIONS EN COULEURS DE CETTETHESE NE PEUVENT DONNER QUE DESTEINTES DE GRIS.


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da2!iJSi ZZ

-

- Figure 2.2Photographs Of The Apparatus

32


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33

2.1.1 Recording Unit

The intensity of the depolarised light was measured with a photoresistor(Clairex CL605L with CdS as the active materia!) placed in one of the eyepiecesof the microscope. The second eyepiece was blocked to minimize contributionsfrom stray light and to allow for a more constant light input to the microscope.The changes m the resistance of the photoresistor were monitored with a Bausch& Lomb V.O.M.5 recorder, using a 140 kil resistor in parallel to gJt the readingson scale.

The photoresistor was calibrated wah a series of neutral density filterswhose transmissions were measured with a Hewlett Packard photodiode a.,-ayspectrophotometer (Model 8452A). The neutral density filters were placedimmedlately ubove the light source of the microscope so that the whole opticalpath in the ffilcroscope was kept the same as during the experiments, except forthe absence of the sample. A typical calibratlon curve is shown in Figure 2.3.2.1.2 Shearing Stage

A dedicated stage was designed and built for use in the pre-shearingexperiments. Originally the stage was to be adapted to the microscope; however,in consIderation of the difficulties encountered, the presheru experiments wereperformed wnh a stagr. mdependent from the microscope.

The shearing stage IS shown schematically in Figure 2.4. The polymersample, which was placed between two ffilcroscope glass slides, could be meltedand sheared in the stage .md later transferred to the microscope hot stage forobservation of the crystallisation process. The stage consisted of two parts: afixed bottom block WhlCh initially contamed two heaters and a thermocouple, anda movable top block whose motion was manually driven by a micrometer. To


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34

40

-.......

..c 30CJ)-..J""0IDE 20f)cru

10

55 105 155 255Resistance (kO)

Figure 2.3Calibraon Curve For The Recording Unit


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{

Spring LoadedScrews

M i c r o s c o p e ~ - - ~ 4 - - - - - - - - - - - - - - - ~ + - - - - l Slide t - - . . ; : . + - - - - - - - - - 4 - + - - - - - . ~ F = ~ ~ ~ ~ ~ ~ Sample-- o

Heaters

2 cm

Figure 2.4Shearing Stage (Original Version)

35


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......

36

minimize temperature variations, the blocks were constructed of brass with highthermal conductivity. The temperature of the stage was controlled by adjustingthe power to the heaters using a vanable transformer. According to thethermocouple readings, temperature control of approximately a.soc could beobtained.

The pol ymer sample was contained between two microscope slides whichfit into the bottom block. The bottom black had a slot with a depth thatcorresponded to the thickness of the bottom slide, thereby keeping it statIonary,while the top slide moved along WIth the top block, thereby sheanng the pol ymersample. The displacement of the top slide was controlled by the rotation of themicrometer and at a rate corresponding tG an approxlmate velocity as requiredaccording to shear rate calculanons. Pressure was apphed to the top slide by abrass plate in contact with two spnng loaded screws, as shawn in Figure 2.4. Themovement of the top slide was hence ensured to be paralIel to the bottom slidewhile the thickness of the sample was mamtamed essentially constant dunng thepre-shearing by thin alummum foil spacers between the glass slides.

At the campletion of the pre-sheanng, the sample was transferred to themicroscope he(ltmg stage, as shown in Figure 2.5. This transfer step was ratherdifficult and caused consIderable experimental difficulnes. First of all it had to bedone quickly and carefully to mimmize temperature fluctuatIons during transfer toleast affect the crystallisation. Secondly, it was impossible to transfer certainsamples for which the slides fi t too snugly ih.0 the slots.2.1.3. Experimental Procedurei. Sample Preparation For Microscope Experiments

When preparing a sample for microscopie observation in crystallisation


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3. t r a n s ~ e r r e d to

micr( scope

37

2.pushed outf __stage1. pushedsideways

Figure 2.5

\

Transfer Procedure As Viewed FrOla The BottomBlock Of The Shearing Stage


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38

experiments, care was taken in the cleaping of the slides ta minimizeheterogeneous nucleation. The edges of the microscope slides were frrst ground tafit snugly (by Iength) into the siot of the shearing stage. They were ultrasonicatedin deionised water and immediately dried by placing the slides over a clean hotsurface of aluminurn foil. The thickness of the two slides without sample wasmeasured.

The sample was prepared by melting a small amount of the polymerbetween the two slides which hadjust previously been rendered "dust-free" with a"dust-off' spray. Two thm (12 Ilm) alurrunum spacers were placed on euher sideof the sample ta ensure a minimum thickness for the sample on application ofpressure. The thickness of the sample was determined by measuring the thicknessof the two slides and sample. After the experiment, the thickness of the two slidesand sample was measured again to de termine any changes in the thlckness of thesample due ta the applied pressure. The range of sample thicknesses was from 15to 451lm.ii. Procedure

Each experiment consisted of two separate procedures, usmg sunHarconditions, but only the second involved pre-shearing. In this manner thecrystallisation curve, obtained after shearing could be compared to the first whichhad no pre-shearing.In each experiment, the sample was tirst melted at 1800C in the shearing

stage for ten minutes, conditions determined for this polymer by E. Chu (1) taerase thermal history. The stage temperature was then lowered ta the desired~ ' h e a r i n g temperature, and the sample was either sheared or simply allowed tastand for an equivalent time. The shearing motion \Vas tirned and the


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39

displacement was measured with the micrometer. At this point, the sample wasquickly transferred, according to the procedure described previously (cf. Figure2.5), to the microscope heating stage maintained at the shearing temperature. Thesample was maintained at the shearing temperature in the microscope hot stagefor one minute, sufficient time to allow for proper placement of the sarnple,preparation of the recorder and generally ensurlng that all the experiments weresubjected to similar conditions. The temperature of the hot stage was thenlowered to the crystallisation temperature and the crystallisation was monitored.

The vanous conditions used are listed in Table 2.1. For all experiments,the shear rate was maintained between 3 and 5 SI. Following each experiment,the thickness of the two slides and sample was measured. Hence, the shear ratescould be detennined by di viding the velocity of the shearing motion by thethickness of the samples.

CrystallisationTemperatureTc ,oC

123121123123123

Table 2.1Experimental Conditions

ShearingTemperatureTs ,oC

140140135132130


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40

Considerable care was taken so that the sample was placed at the sameposition under the microscope objective for both expenments, although there wasobviously sorne uncert31my or margin of error in thIS placement. ThIS precautionwas taken since, for nuCroSCOplC studies, a very small area of a no t completelyhomogeneous sample is observed and therefore shghtly displacing the sample canchange the area under study (much more than If a larger area were observed) andhence its behaviour. In the pre-sheanng experiments, care was taken to aVOldmeasurements at the sheared edges of the samples, thus mmlmIzing end effects.

2.2 Development of the technique2.2.1 Sample Description

A commerclally available Injection molding grade high-densltypolyethylene resin (ScIair 2908, kmdly donated by DuPont Canada, Ltd) was usedfor the expenments. The physical propenies of the pol ymer, as reponed by thesupplier, are hsted In Table 2.2.2.2.2 Recording Unit

Prelimmary crystallisation experiments were done using the microscopehot stage without any pre-shearing to determme the charactenstics of therecording unt. The chan recordings of reslstance as a functlOn of crystalhsauontime ail showed a peak In otherwlse normal sigmoidal curves. as shown in Figure2.6. Since such a peak on slgmoidal crystall1satlon curves had not been reponedpreviously, i l seemed likely that This behaviour represented an artlfact of thedetection unit. A repon of simllar behavlOur was later found in the 1 teraturereponed in a Japanese patent (2).


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41

Table 2.2Physical Properties of the Commercial Resin( 1)

Mw' g/mole 7.45 x 104Mn' g/mole 2.23 x 104MJMn 3.33Density, g/cm3 0.962Melt Index, g/1O min 7.40Melting Range, oC 113-146Average Speclfic HeatC Solid, cal/g.oC 0.607c: Melt, cal/g.oC 0.587A verage Thermal Conductivity 8.24 x 104k Solid, cal/cm,oC.sk Melt, caVcm,c's 6.25 x 104A verage Thermal Diffusitya Solid, cm'2/s 18.9 x 104a Melt , cm'2/s 12.9 x 104Power Law Index, n. 0.822Llli/R, K-I 2167.4A, g/cm,sn-2 139.90

* llt = AEXP ( ~ I R T ) . "f-lwhere llt is in poise, y is in S-I, and T is in K


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80

.........

"- 60>-+->cru+->(f) 40>-C-U

20

-------------

. ,

1

120

.. ,../

.. _ . - .. -. . - . . ---

--.- . . TcTc- - - - . Tc

240Time

Figure 2.6

360(sec)

= 120C= 117C= 115C

480

Crystallisation Cuzves

42

600


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43

To investigate this behaviour, coloured photographs were taken during thecrystallisation process to see whether there was anything which could -::ause thephotoresistor to respond in that way. Since the photoresistor used in theseexperiments is known to have a respon!:.e curve which closely matches the humaneye, with highest sensnivity at 550 nm, a change m colour in the sarnple duringcrystallisation could cause such an anomalous response from the phOloreslstor. Asshown in the accompanying photgraphs (Figure 2.7), a shght change in colouroccurred at the ume corresponding ta the peaks m the chart recording.

To correct this probiem a rnonochromauc filter, wnh a band pass in thewavelength reglOn of 560 10 610 nrn, as determmed with a Hewlett Packard diodearray spectrophotometer (Model 8452A), was placed in the light pathway Justbefore the photoresistor. The chan recordings thereafter no longer showed theanomalous peaks and essenually slgmOldal curves were obtained.

A second calibration of the photoreslstor needed to be done due te thepresence of the rnonochromauc filter m the optical path. A typical calibrationcurve, shown in Figure 2.8, was analysed by a polynomml fa. The equationobtained for the curve was u ~ c d to wnte a baslc program te conven the reslstancerecordings at vanous times mto 1l1tenslty of depolarised light.2.2.3 Pre-shearing Experiments

Pre-sheanng experiments were undertaken followmg the procedurepreviously described. A certain familiarity with the new stage and technique hadto be acquired frrst at the cost of several trial experiments. The results of thesepreliminary experiments with successful transfer proved to be rrreproducible atsimilar condiuons, as shown for example m Flgures 2.9 and 2.10. For sornesamples nn effect on crystallisauon kinetIcs was observed with pre-fnearing. For


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-- -- - - - --------------------

Figure 2.7Photographs Showing A ChangeIn Colour Of The Spherulites

44


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45

(

4...---+->..c01-.1""0 3ru-+->

ECf)cro' - 2--( ---

40 80 120 160 200Resistance (kO)

Figure 2.8Second Calibration Curve of the Photoresistor


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46

50

x 40 --------------.-..c / '0 ') 1-1 30 1/""0ru

20ECf) no pre-shearingcra -1L 10 - - - - pre-shearing, 4.0 s1 -

050

x40

..c0 ') -----------_ ......-1 30 -""0ru

20ECf) no pre-shearingcra pre-shearing, 3.6 -1L 10 - - - - s1 -300 600 900 1200 1500

Time secFigure 2.9


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47

5 0 ~ - - ~ - - - - ~ - - ~ ~ - - ~ - - - - ~ - - ~

..c(J )40

30-0Q)

20E(j)cruL lOr -

05040

..c(J )--.J 30-0Q)-+0,) 20E(f)cruL 10r -

- .", ,- ------ --------

--- no pre-shearingpre-shearing, 4.8 S-1

--------

no pre-shearingpre-shearing, 5.3 -1- - -- s

100 200 300 400 500 600T i m ( ~ secFigure 2.10

Experimental Curves Tc= 21C, Ts= 140C


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48

others, when an effect could be seen, the magnitude showed extreme differencesfrom one sample to another. In fact, in sorne cases, an increased rate, rather thanthe more frequent decrease in rate was seen. Although no firm conclusions couldbe drawn from these expenments, the effect, if there was one, appeared to be toosmall to be properly detected.

Experimental conditions were changed in an attempt to increase the effectof pre-sheanng on the crystallisation kinetics. To thls end, the shear rate and thesheanng temperature were vaned.

At first shear rates were increased in an attempt to mcrease the effect onthe crystallisauon kinetics. In the lite rature (3,4), it has becn reported that forshear-induced crystallisanon an increase ln shear rates mcreases the rate ofcrystallisanon. However, 10 the present case with pre-shear, an Increase 10 shearrate from values around 4 S-1 to 23.5 S-I, had no effect on the observed rate, asshown in Figure 2.11.

Upon examination of the sample slides for the higher shear rate, l t wasapparent that the top slide probably "skipped" over the sample without slgnificantshear. Consequently, a slower motion was tried, and hence a lower shear rate, sothat the sample mIght more easily follow the motIon of the top shde. This onceagain proved to be unsuccessful and slrrular irreproduclble results were recordedfor a senes of experiments.

At this pOInt, a change in the sheanng temperature was attempted_ It iswell known that as the undercooling of polymers, i.e., the difference between themelt and crystallisation temperature, decreases the melt memory mcreases. Inaddition, the melt visCOSIty varies inversely with temperature. A higher VlSCOSlty,at a lower temperature, is expected to slow down molecular motion and henceincrease the effect of shearing on the melt. Based on this concept, the sheanng


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50

temperature VIas lowered in an attempt to increase the effeet of pre-shearing onthe crystallisation rate. The experiments with shearing temperatures of 135, 132and 130C were no more ~ u c c e s s f u l than the previous ones, as is shawn in Figure2.12, for example.

Subsequentiy, experiments were earried out at a higher sheanngtemperature (150C), hoping in that way ta ease the sheanng procedure Wlth thedecrease in the viseosity of the sample at a higher temperature. However. thisattempt was also as unsuecessful as the previous ones.

The variations in the experimental shear rates and shearilig temperaturesdid not result in an unequivocal answer regardmg the effeet uf pre-sheanng on thecrystallisation kinees. An inerease in the crystallisauon rate was expeeted, sinceit has been reponed in the hterature, as discussed in the introduetory chapter, thatshear-induced e r y s t a l l i : " ~ t i o n shows a faster rate than in the quieseent case. Suchbehaviour was not observed consistently. lndeed, more often the opposite effectwas seen. These observation') were quite confusing and 1Jore attempts were madeto ob tain reproducible results and an increase in the effect so that lt eould beproperly detected. The slowing down of the erystallisauon, which seeme to bethe more general result, was thought to be improbable and l t was expected thatthis would not be seen when the experiments wOllld be performcu ln such a waythat a reproducible and defmite effeet could be observed.

In view of the previous observations and the indications that the top slidecould have "skipped" over the polymer sample without properly shearing n, anattempt was made to ease the shearing motion by improvmg the adherenee of thepolymer ta the slie surface. Ta this end, the mIcroscope slides were silanised byreaction Wlth dichlorodimethylsibllt!. In general, puur adhesion b.;tween thepolymer and the surface ot the silanized slides resulted in little, if any, shear.


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51' 50x

40..cen - - - - - - ---1 ------30 -""0 //ru /

20 /E /(f) no pre-shearingcru pre-shearing, 3.8 -1L 10 - - - - s1-

050 r

1

40..c - - - - - - -" ."" . - -en--1 30""0ru

20E(f) no pre-shearingcru -1L 10 - - - - pre-shearing, 3-5 s1-

o0 120 240 360 480 600 720 840 960Time sec

Experimental Curves T c=123C, a)Ts=132C


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52

Consequently, the results obtamed with these slides did no t show any of thecxpected improvements and no definite conclusions could be drawn.

Since irreproduclbility In crystallisanon results is often related ta thethermal history of the samples, or a poor thermal controL such factors wereexamined in more detail. Several experiments were done. wahout pre-shearing,using various conditions to erase the thermal history. To thIS end. the sample washeated at qmte harsh corldmons, I80aC for thirty mmutes. This resulted ln twosuccesslvely simllar crystallisation rates. Pre-sheanng expenments were funherattempted, u ~ i n g these harsh condmons. However. no lmprovernents 10 the r e ~ u l t s could be seen and decomposnion, although not detectable by eye, was ~ m p e c t e d .

A detailed eXanllnatlOn of the c o n ~ r r u c t l o n detaIls of the stage suggestedthJt the thermal control could be greatly lrnproved. The fIrst deSIgn proved tohave sorne flaws ln that one of the heaters was located dIrectly under the sampleposition. Hence, large temperature fluctuatIons could occur ln the ~ a m p l e whenever the temperature of the stage was lowered or lncreased. Thesefluctuations would probably be undetected by the thermocouple whlch was placedmidway between the two heaters to measure an average tempe rature of the stage.Consequently appropnate changes were made to the stage, a" shawn !n Figure2.13, to avoid large thermal fluctuauons and obtam a more accurate reading of thesample temperature. Although these changes to the stage dld not resuit ln fullyreproducible results, as seen In Figure 2.14, ~ u b s e q u e n t i y t h ~ measuredcrystallisation rates were consIstently decreased by pre-sheanng.The remaining irreproduclbility ln the results


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'

Spring LoadedScrews

MicroscopeSlide

Sample1

1iUo

Thermocouple oHeater

Figure 2.13Modified Shearing Stage

53

2cm


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.' ,.. 50- x40-t->..c(J )

-.-J 30"TIID+-l-t-> 20E(f )ccoL 101-

05040

-t->..c(J)-.-J 30"TIru-t->-t-> 20E(f)ccuL 101-

------, /,.-

/'; '

,//

,/, /

, /, /


- - - - - - -


240 480 720 960Time secFigure 2.14

Expenmental Curves T =123C, T =140Cc 5

54

1200


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55

expenments. Hence, a punficaon of the resin was made, in the manner describedin the following section, in an attempt to have a more homogeneous sarnple.2.2.4 Purification of the Commercial Resin

The polyethylene resm was purified by filtenng a solution of the polymerand later precIpuaung the polymer from the solution. In the first puriticatIon177.3 mg of the resin was dissolved in 300 ml (0.059% solution) of toluene(Specrrograde), prevlOusly flushed WIth mtrogen. DissolutIon was obtained bysumng and heanng between 105C Jnd 107C under mrrogen A vacuumfiltratIon al' the hot ~ o l u t i o n was done USIng a coarse biter. Excess ~ o l v e n t wasremoved by rotary evaporanon and the remaming solutIon was poured mto morethan twIce ItS volume of methanol to precIpUate the pol ymer. The preCIpItate wasleft to settIe and later tiltered off. The powder thus obtained was left covered todry at room temperature for two days and later the polymer was kept In adessicator.

Crystallisauan ~ x p e n m e n t s , perforrned on this sample WIth the DSC gavereproducIble results. mamly inductIon nmes and heats of crystalhsauon, at theconditions used. The melung endotherms showed reproduclble transItiontemperatures as shown In the Results sec non. Funht!r details are discussed in theDSC seCllon, at the end of the chapter.

A larger quantity at polymer was reqUlred for the crystallisanonexpenments. Hence, a second punficanon was camed out m the s ~ u n e way WIth a0.086% solution. The precipltated polymer was left to dry twenty hours In avacuum oven at roorn temperature. DSe results obtamed WIth thIs secondpurificatIon were 10 agreement WIth those obtained for the sarnple from the firstpurification (cf. the Results section). 13e NMR spectra were recorded for the


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.

56

purified polymer and commercial resin in deuterated toluene at 105C with aVarian XL200 NMR spectrometer. sigmficant difference could be seenbetween the two spectra. Hence, whatever was filtered from the hot solution ofpolymer could not be detected by l3C NMR. Also rhese spectra gave no slgn ofegradauon in the purified sample. AIl the pre-sheanng crystallisationexperiments were perfonned at the condItions listed in Table 2.3 with the pol ymerprecIpitated ln the ~ e c o n d punficauon.

Table 2.3Experimental CondItions

Crystallisation Sheanng Shear RateTemperature Temperaturey,SloC T oCc' s '

123 140 4123 135 3-5,1.5,10,17123 130 3-5123 127 ..,J125 125 4

2.3 DSC ExperimentsDifferential scanning calorimetry (DSC) expenments had prevlOusly been

under-taken ta detennine the equilibrium melting pomt of the untreated resm. Thecalorimeter used was a Model DSC 2C manufacrured by Perkin Elmer. Fiveisothennal crystallisauon expenments with different samples at tve differenttemperatures: J15, 117, 120, 123, and 125C, were done. The commercial resmwas avallable in pellets and hence for calorimetry expenments, a compressed film


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( 57had to be prepared. Several pellets were compressed in a press at 160C under apressure of approxlmately 2.1.107 N/m2 for 30 seconds. The film obtained was eutand samples for the OSC were prepared by welghmg several pleces of therolymer film mto an alummum DSe pan, later sealed.

CalibratIon of the DSe was done with two standards, indium and tin, andan accuracy of 2 K cou Id be expected In the results (5). AlI the experimentsmvolved heatmg the sample at a sufficiently high temperature to erase thermalhlstory ( ISOaC for ten mmutes). The ternperature of the sample was reduced to thecrystallisauon temperature and the l ~ o t h c r m a l crystalhsauon was followed. Thesample \Vas taken out of the Instrument and left at room temperature for tenmmutes This ensured that, for the different samples, the meltmg runs werestarted at the same c o n d l t i o n ~ . Subsequently, the meltmg endotherm of the samplewas recorded using a heanng ~ c a n from 40C to 170C at a rate of five degreesper ~ i r l u t e . The sample compartment In the DSC was conunuously flushed withnitrogen dunng the expenments.

Differenual scanmng calonmetry experunents were also under-taken withthe punfied resm, at a crystalhsauon temperature ot InOC to venfy that theconditions used for meltIng the polymer would erase thermal history and also Ifcrystallisauon expenments were reproducible for dlffcrent samples. The imtialexpenments were carried out usmg the polymer obtained from the firstpurificauon. A first sample was used to test WhICh condItions would aase thethermal hlStory of the polymer. In previous stuilles WIth thIS polymer by E. Chu(1 ) the polymer was melted at 180C for ten minutes pnor to crystallisanonexpenments. Identlcal condItions were used on the polymer obtamed from thefirst punficanon. A second run was done on the same sample, but this orne thepol ymer was melted at 180C for flfteen mmutes. Results were compared to the


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58

frrst try and showed no significam differeaces as can be seen in Figures 2.15 and2.16, respectively. Hence, melting the polyethylene sample ut ISOac for tenminutes erases the thennal history of the sample. Three other samples were aisoused and no significant difference was observed 10 the crystallisauon and meltingresults on comparison WIth the first sample. When the second punficauon wasdone, one sample was taken and examined by DSe. The results \Vere once againin agreement with the previous ones, ~ h o w m g that the sample is reproduclblewhich is often not the case \VIth commercially available reSHIS. For aIl thesamples studied on the DSC, no sign of degradauon \Vas observed under theconditions used.


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5.. 1 HOPE11 lIT. 5 36 mg

,uew

uuJ l 2.51-.J'.J:L

S C I I ~ RATE, 1l' 111\--.\1

' J t ~ 1- Tt 1.1 '"-A.l 'h"''''1! ". . j'

~ - - - - - - ~ - - - - - - - = ~ = = ~ - - - - - - - ~ ~ I - - - - - - - - -

TEMPERt\ TURE (1-\) ,e

Figure 2.15Melting Endotherm for the Treated Sample

(Previously Heated at I800e for Ten Minutes)

59


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~ . T f - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

ADli)

ULU~ 2.511--l


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61

References

l.M.R. Kama! and E. Chu, Polym. Eng. Sei., 23,27 (1983)2.K. Tanaka, K. Tomura, K. Baba and Y. Mochizuki, Japan. Kokai JP53/93096(78/93096) , 15 Aug.1978, 4pp.3.R.D. Ulrich and F.P. Priee, J. Appl. Polym. Sei., 20, 1095, (1976)4.R.R. Lagasse and B. Maxwell, Polym. Eng. Sei., 16, 189, (1976)5.Insrrucllons, Model DSC-2 Differentiai Seanmng Calorimeter, Perkin ElmerCorporation, 1980, p.3-14.


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62

3. RESULTS

This Chapter describes the results obtained using purified polyethylene.Furth enn ore the ted'niques for shearing the samples incorporated therefinements descnbed in the prevlous Chapter. The steps involved in the dataanalysis will be descnbed m the follow1Og sectIon, as weil as the descripnon ofthe results of the pre-shearing expenments and of the OSC and NMRobservatIons.

3.1 DSC ExperimentsThe equilibrium melting point of the untreated polymer was detennined

by differenual scanning calorimetry experiments. A single melting endothemlwas obtained under ai l conditions. The melting pomts of five dlfferent sampl/.!scrystallised at fIve dlfferent temperatures (l15C, 117C, 120uC, 123C:, 1 : : ~ 5 C ) were deterrruned from the melting endothenns, obtamed by the metho descnbedpreviously 10 Section 2.2. These results are plotted as a functlOn of thecrystallisauon temperature in Figure 3.1. The melting temperature, Tm' was takenas the onset of the melung peak. The 1OtersectIon between the expenmental curveand the line Tm =Tc' is the equilibrium melting pomt T: of the pol ymer, when themelting temperature is the same as the crystallisauon temperature, TL (1). The!inear least squares analysis of the expenmental data resulted in the followmgequation


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U0CDL::JrDLCD0...ECD1-(J)cCD2:::

63

133 - - -0- Experimental PointsTm = Tc

///

131/

//

129

127

1 15 119 123 127 131 135Crystallisation Temperature oC

Figure 3.1Determination of the Equilibrium

Melting Point


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.

643.1

Variances of 4.10.4 in the slope and of 5.8 in the intercept were obtained trom thelinear least squares analysis. However, expenmental errors of 2C were obtainedfor the melting transitions. The eqmlibrium melting point of the commercial resin,determined from equation 3.1 by applying the conditior that Tm = Tc' is

The powder obtained from the [Ifst purificatlon was also analysed byDSe. One sample. previously heated at I800 e for ten rrunutes, was crystallisedthree successive times at 123e and the three succeSSIve melting endotherms werealso recorded. The reproducibility of the results is shown in To.ble 3.1. Asurprisingly large deviation is seen for the inductIon time of the third run.possibly due to sorne de gradation upon heating and crystallising the same samplethree consecutive times.

Similar experirnents were camed Out usmg the powder obtained from thesecond purification. However, three dlfferent samples were used this ume. Theresults are listed in Table 3.2 along with the results obtained for the untreatedresin. The induction times and crystallinities of the treated polymer were veryreproducible. The crystallinity, X. was determmed from the experimental heats

x= MIOf 3.2

of fus:on where L l l i ~ is the theoretical heat of fusion for a 100 % crystalline


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65

Table 3.1DSC Results for the First Purification

Run Crystallisation T Heat of FusionInduction Time ( O ~ r(sec) onset J/gram

32 129.4 2052 34 129.6 2003 41 129.4 199


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Resin

commercialcommercialcommercial

punfiedpuri.fiedpurified

Table 3.2Comparison of the Commercial and Purified Resin

CrystallisationInduction Timesec

264649

333030

Crystallinity%X

798180

696971

T0:

66

128.8129.4129.5

129.5129.2129.4


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67

sample ( L l H ~ =293.08 J/g (2. The results obtained with the second purificationagree with thase from the [ifst purification (as seen In Table 3.1). A compansonwith the commercIal resm shows a decrease In the crystallinity and inductIon umeof the crystallisation WIth punfication of the pol ymer. The purification step mighthave involved the wppression of a compound WhlCh could have had a diluenteffect (3) on the polyethylene thereby sloWIng down the crystallisanon rate of theuntreated resm.

3.2 ~ M R ExperimentsDe NMR spectra of the untreated polymer and the powder obtained from

the second punficauon were obtained as mentioned In the Experimental sectIon.Both spectra showed the expected peak for polyethylene, at 30.29 ppm fo r thepurified sample and at 30.34 ppm for the commercIal reslIl (cf. Figures 3.2 and3.3). The chemical ~ h l f t s glven are relative ta TMS. The posItion of the peaksagree wlth the hterature value (4) for the methylene peak. ReferencIng in theseexpenments was achleved usmg the deuterated toluene solvent peaks. Only thesolvent and the methylene p e a k ~ were present ln bath spectra and no differencecould be seen by l3e NMR between the commerCIal and purified polymer. Noslgn of sample degradation was detected in these spectra.

3.3 Pre-shearing Experiments3.2.1 Data Analysis

For the pre-shearing experiments, the crystallisation was followed byrecording the intensity of depolarized light as described previously. The


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solvent-tfZ Z U ,

11

,J:1,fiIl1

-CH-2

solvem

1 (

- - - - - - - - - - - - - ~ ~ t ~ '."T""'t" '- - ..... - ....... .- . . ..--.. ..... ..-.. .. -

00 80 &0 ') uu .

Figure 3.213C NMR Specnum of the Purified Resin

68


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69

-CH-2

solventf"''-'

r - -

1 !t' l solvent~ l . ..--- I ~ - -;-(-*-- - ~ I - ' - ~ - : : ____ . - ~ i - , ~ " l I I o / j ~ _ . _ : _ - - " - ". 'Jo 80 FO 40 ;10 ) ;;1j;)W

Figure 3.3Be NMR Specrrum of the Untreated Resin


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70

recordings were convened from readings of reslstance to percemage oftransmitted light, and ulumately ta percentage crystallinity, as a function of rime.The data analysls was aU handled by a personal computer and programs werewritten usmg the Quick BaSIC software for that purpose.

Prior ta the expenments, the mtensity of the light ~ o u r c e was momtored,with the photareslstor, over a penod of thmy five mmutes ta establish that thelight intenstty was constant over the reqUlred ume. No fluctuatIons ln the mtcnsnycould be detected and hence the hght source output \Vas consldered essenuallyconstant over the ume of th'_ expenments.

The photoreslStor recording umt had been cahbrated \Vith netral densltyfilters of known transmission. The calIbration curve obtruned for percenttransmitted light as a function of reslstance showed the muaI cxponenual decayexpected from the response curve of a photoreslsror (5). 'PJe ploc ot the naruralloganthm of the percent transmitted hght as a funcuon ot r e s l ~ t a n c e (cf. Figure2.6) was analysed for a polynomIal fit ta obtam

1n(%1:) = 4.2132 - 4.6304.10-2 x R +4 0154.10'4 X R2- 2.9263.10-6 X R 3 + 1.2567.10 Hx R4- 2.2836.10- 11 X R5 3.3

where %1: represents the percent transmitted 1ight and R the reslstance of thephotoreslstor. Equation 3.3 was used ta wnte a program that wou Id calculate theamount L. f light transmnted by the samples from the reslstance readings and storethe data in files for funher calculatJons. From these data fIles plots of the changein transmitted 1ight as a functIon of time dunng crystallisauon cou Id be obtruned.


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72

could not be obtained from the photographs. Bowever, the y proved useful lnshowing that the average final Slze of the crystalline ennues, assumed to bespherulites as mdicated by the presence of Maltese crosses, \Vas mdeed smallerthan the thickness of the samples. Hence thlS same charactenstlc (thIckness of thesamples) which impedes quantitatIve analysis of the photographs, allows for theuse of equanon 3.4 to deterrmne the crystallimties of the samples.

The expenments camed out for thlS work Included two runs for eachsample, the tirst wnhout and the second wIth pre-shearing. The value of %'t(oo)for the sample was taken as the higher level of transmItted hght obtamed from thetwo runs for the purpose of companson. As was mennoned prevlOusly,depolarised light intensity is dependent no t only on the ~ a m p l e but also on ItSthickness. For any given sample, vanations in the thickness between the fIrst "nopre-sheanng" run and the second "pre-sheanng" ru n were al ways less than lOJ.1.m.The change in the thickness of the samples could have occurred due ta thesheanng action smce a small quantity of sample was displaced. Furthermore, theobserved intensity of depolarised hght showed no direct correlatIon with themagnitude of the variations found in the thlckness of the samples. Bence, acompansoll, for one sample, between the two runs fonmng one experiment ispossible. A program m Basic was written ta conven the data files obtamed fromthe frrst program to fractlon of crystal linity as a functIon of tIme. These were onceagain stored in data flles which were plotted later. The DSe expenments showedthat, fo r the treated polyethylene crystallised at 123C, a 70% crystalline samplewas obtained. Bence, instead of arbitrarily settmg the final cryslallinity level to100%, as would be implied by equatlon 3.4, a maximum final level of 70%crystallinity wag used.


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73

3.2.2 Description of the Results

For each condmon hsted in Table 2.2, two rJns were perfonned persample (run number one wlthout pre-sheanng, and run number two with preshearing) and plotted on the same graph for comparison. For each condition, twoplots were obtained per sample: one WhiCh descnbed the change In depolansedlight as a funclIon of time and the otller WhiCh showed the change In fraction ofcrystallinity WIth time. For most samples crystallisation was observed at 123OC.Using sheanng-stage temperatures of 140, 135 and 130De, a t'mt expenment wasattempted WhiCh consisred of performing two consecutIve runs wnhout pre-sheanng ta e s t a b h ~ h reproducibility. Two of the three graphs obtaIned are shownin Figure 3.4. Tc a good approxImatiOn the inductIon nilles are identical as IS theultimate degree of crystallinity. However, it is interestIng ta observe that a slightincrease in the finallevel of transmltted light is seen with the second run.

Expenments perf

Documents

ion Kinetics of High Density Polyethylene Pre-sheared in the Melt