Crystallinity in Atactic Polyvinyl Chloride

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    C R Y S T A L L I N I T Y I N A T A C T I CP O L Y V I N Y L C H L O R I D E

    proefschrift

    ter verkrijging van de graad van doc to rin de tec hnische wetensch appen aan deTech nisc he Hogesch ool Delft , op gezagvan de re cto r magnif ic us ir . H. R. vanNauta Lemke, ho ogleraar in de afdelingder elektrot echniek, voor een commissi euit de senaat te verdedigen op woensdag5 januari 1972 te 14.00 uur door

    Johannes Anthonij Juijn

    scheikundig ingenieurgeboren te Vlaardingen

    1972D r u k k e r i j J . H . P a s m a n s * s - G r a v e n h a g e

    l O ^ U i j^ :i\,

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    Dit proef schrif t is goedgekeurd door de promotor

    Prof.ir. W.A.deJongen is tot stand gekomen onder direct toezicht van

    Dr.J.H.Giso lflector aan de Technische Hogeschool Delft.

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    Helma, Koos ,Beert, Bert, Dick, Ed, Eric,Ever t, Fred, Herman, Henk, J aap,Jacob, Jan, Jan, Jo ep, Kee s,Leen, Louis, Mie i, Pieter Jan,Rien, Rie s, Wi m, W ijnand,en vooralAad,hartelijk bedankt voor julliemedewerking.

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    CONTENTS

    Chapt er 1. Intr oduct io n, aim of this studyChapte r 2. Tact ic ity and crystallinity of PVC2.1. Tacticity2.2. NMR measurement of the tacticity2.3. IR measurement of the tacticity2.4. Po lymerizati on temperature and tacticity2.5. Crystallizatio n of an atact ic po lymer2.6. ConclusionsChapte r 3. Melting of cr ystallinity in PVC3.1. Introduction3.2. Equipment and measuring te chniques3.3. Materials3.4. Pr imary cr ystallinit y3.5. Seco ndary cr ystallinit y3.6. Annealing below the glass transiti on re gionand quenching3.7. ConclusionsChapt er 4. Aging of plasti cized PVC4.1. Introduction4.2. Equipment and measuring te chniques4.3. Materials4.4. Annealing of plast ic ized PVC4.5. ConclusionsChapt er 5. Chain st ruct ure5.1. Chemical and ste reo che mical str ucture of the chain5.2. Nomenclature5.2.1. Configurati on paramet er s

    5.2.2. Conformati on paramet er s5.3. Conformation5.3.1. Monomer conformations5.3.2. Pair conformati ons5.3.3. Chain co nfor mati ons5.4. Calculation of ch ain energy5.4.1. Coo rdinate syste m5.4.2. Electrostatic energy5.4.3. Steric energy5.5. Energies5.5.1. Pair energies5.5.2. Chain energies

    11111417192124252525262734384142424243435051515353535454555760616263656567

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    5.6. Chain st ructure analysi s5.6.1. NMR analysis5.6.2. IR analysis5.6.2.1. CCl-stretching vibrations5.6.2.2. Nomenclature5.6.2.3. Main abso rp ti on fr equencie s5.6.2.4. Additional absorption frequencies5.6.2.5. Assignment of absorption frequencies5.6.2.6. The isotactic straight chain conformation5.7. Conclusions

    Chapter 6. Crystalline structure6.1. Introduction6.2. Single cr ystals6.3. X-ray analysis6.4. Cryst allinity of atacti c PVC6.4.1. The ort hor hombic lattic e6.4.2. Calculation of the cr ystalline co ntent6.4.3. Nematic st ructure s6.5. The amor ph ous phase6.6. Pr imary and se condary cr ystallinit y in PVC6.7. Conclusions

    Listo fsymbolsReferencesSummarySamenvatting

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    7CHAPTER1INTRODUCTION,A IM OFTHIS STUDY

    Crystallinity in a mater ial can occ ur when a re gular spatial arr angement of the molecules of that mater ial is po ssible.For this reason mostlow molecular compounds are able to form a crystalline ph ase. For highmolecular co mpounds, ho wever, particularly polymer s, the f ormatio n ofsuch a regular spatial arrangement is much more diffi cult,

    The first problem in po lymer crystallizatio n is caused by the c hainform of the molec ules . It is r ather usual that the length of p olymermolecules is 1000 times thei r thi cknes s. The crystallizatio n of t heselong chain molecules causes pro blems very similar to those of a womanwho is trying to knit with a knot ted ball ofwool:after a short timeshe is left with an inextricable c lew, and she is r eady to cut thethread in anger. By co mpari so n, this is the moment that th e polymerhas reached its maximum amount of cr yst allinit y. The knitt ing whic hwas finished repres ents the crystalline phase and the remainder ofthe disordered ball of wool the amorp hous ph ase,

    The sec ond factor is the ease with whic h crystallization occurswhic h depends on the c hemical st ructure of the polymer and, he nce, onits molecular confo rmati on. Polyeth ylene, for example, crystallizesvery r eadily; its simple che mical s tructure enables the ch ains toassume the hi ghly regular extended zig-zag co nfor mati on. Howeve r,many other polymers crystallize wit h diffi culty or not at all. Thereare two main re asons for the non-crystalline behaviour of these p olymers:lack of tacticity and the occurrence of steric hindrance.

    The po lymer discussed in the pres ent th es is , polyvinyl chlori de orPVC, belongs to the second group, viz. the polymers that do not for mcrystals as readily as p olyet hylene. In PVC the two ste reoi somericforms of the monomer give r ise to,diffe rent conformations of the ch ain.A part of the chain whic h is syndiotactic can have a planar zig-zag

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    8

    confor matio n. In that case the chlorine atoms are placed alternatelyat the sides of the plane of the zig-zag chain. This spatial arr angement is energetically favoured over other syndiotactic conformationsand at the same time it is a very regular chain st ructure whic h cancontribute to the f ormatio n of crystalline str uctures . With an isotactic part of the chain, however, the zig-zag chain structure isnot p os sible. Her e, the chlorine atoms should all be placed at o neside of the ch ain, but this is impossible because of steric hindrance.Therefo re the isotactic chain parts have other conformations wftich weretho ught to be hardly o r not at all cr ystallizable.

    During the technical polymerization of vinyl chloride at temperaturesof 40 - 75 C, syndiotactic propagation is only slightly favoured overisotactic pro pagation. As a res ult, a commerci al PVC is atacti c, whichmeans that about 50% of the monomer sequences in the chain are syndiotactic, the remaining 50% being is otacti c. Syndiotacti c sequences arerandomly distributed along the chain and, therefore, long syndiotacticor isotactic sequences do not occ ur.

    Here we have arrived at the basic questio n of this thesis. Althoughcommercial PVC is known to be atactic, it is also well established thatits cr ystalline content amounts to about 10%. This relatively "hi gh"crystallinity is not in accord with the "low" tactici ty of the materialif o ne assumes that o nly syndiotactic PVC is crystallizable. It is theaim of this wor k to explain the oc currence of crystallinity in atacticPVC by studying the tactic ity, the chain c onformations and the cr ystalline structures in PVC.Literature data concerning measurements of the tacticity by means ofinfrared and nuclear magnetic resonance techniques are reviewed inchapter 2. The influence of the polymerizatio n temperatures on thetacticity is also considere d. The n, the attempt is made to calculateand explain the crystallinity of atactic PVC by means of Flory's theory.

    Experi mental work is pr esented in the chapters 3 and 4.The melting of crystallites of PVC was studied by means of Diffe ren-

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    tial Scanning Calorimetry. Crystallite melting effects were found inatactic commercial PVC as well as in laboratory-prepared syndiotacticPVC.In untreated virgin P VC, melting of crystals was detected attemperatures above 150 C. Annealing of PVC at temperatures above theglass transition temperature was f ound to pr oduce crystallinity. Theseeffects are described in chapter 3.

    The formation of crystalline structures by annealing has an importantconsequence for the behaviour of p lastic ized P VC co mpounds. In chapter 4the physical aging of plasticized PVC is explained as a crystallizationproce ss whic h causes the stiffness and the density of the materi al toincrease.

    The experimental work clearly shows that considerable crystallineeffe cts occur in atactic PVC. This atactic crystallinity c annot beexplained by the classi cal i nterpret ation of Flory's the ory, viz. thatonly s yndiotactic chain se gments cryst allize. The re fo re , a the oreti calstudy on the chain structure and the crystalline packing of syndiotacticas well as isotactic chain s egments was per for med.

    In chapter 5 the res ults of studies on the structure of the chainare presented. Conformations of syndiotactic and isotactic sequencesare given, A special is otactic chain conformation is introduced whichresembles the syndiotactic zig-zag ch ain structure. The energy of thisisotactic straight chain conformation is calculated and compared withthe c hain energies of the syndiotactic zig-zag ch ain and the isotactichelix, A literature survey of the infrared studies on chain confo rmations of PVC is presented and the existence of the isotactic straightchain conformation is discussed in the light of the available infor mation,Literature studies on the crystalline packing, espe cially by X-ray analysis , are reviewed in chapter 6, The pres ent work shows thatatactic c hain se gments can crystallize in a way whi ch is similar tothat described by other authors for the entirely syndiotactic chains,On the basis of this new concept the amount of crystallinity in anatactic PVC is calculated,

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    10

    Finally, a general interp ret ation of the crystalline effe cts in PVCis pre sented,

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    CHAPTER2

    TACTICITYANDCRYSTALLINITY OF PVC2.1. Tacticity

    Po lyvinyl ch lor ide , (-CH -CHC1-) , can be a ste re or egular po lymers i n c e t h e r e ar e two di f f e r e n t ar r angement s o f t h e c hl o r in e a t om in t h eCHCl gro up. The usual way of defi ning the t wo s te re or egular for ms oft h e p olyme r c ha i n, syndi o ta c t i c and i s o t a c t i c, ha s s e v e ral di s advantage s .Th i s d e f in i t i on us e s t h e p r e s e n ta t i o n s o f z ig-zag c h a in s s h o wn in f igur e2.1. Th e ca rb on a t om s l i e in t h e plane o f t h e pa p e r , b onds de s ignat e d a s

    unde r t h i s plane and t h o s e d e s ignat e d a s .^ab o ve i t .

    CI H H CI CI H H CI CI H\ / \ / \ / \ / \ //C ^ .c c c c

    \ / \c/ \ ^ \c/ \c/ C/ \ / \ / \ / \ / \ / \H H H H H H H H H H H HSYNDIOTACTICCI H CI H CI H CI H CI Hy y y y vY ^^^ c- \ c / y / \c/

    / \ / \ y\ y\ / \ / \H H H H H H H H H H H HISOTACTICCI H CI H H CI H CI CI H\ / \ / \ / \ / \ /c c c c r/ \ / \ / \ / \ / \ / \H H H H H H H H H H H HATACTIC

    Figure 2.1. Ster eore gularity of P VC.a-syndiotactic ; b-is otactic ; c-atactic

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    In the syndiot acti c configuration the c hlorine atoms are placed inalte rnating po si tio ns around the zig-zag c hain backbone. The co nfiguration of figure 2.1b, whe re the chlorine atoms are all i ndicated assituated on the same side of the backbone (which i s , in fac t , impossi blebecause of steric hindrance between contiguous chlorine atomsI)is c allediso tactic. The atactic configuration is sh own in figure 2.1c; this configuration must be regarded as a random dist ributio n of the syndiot acti cand isotactic form.

    The above presentation suggests that tacticity depends on the spatialarrangement of the carbon backbone. This is not tr ue: the c onfigurationis indepe ndent of the co nfo rmatio n.

    It is important to use the words configuration and co nformation ina pro per way. In this th esis the lUPAC rules are obeyed.Confi guratio n: The configuration of a molec ule of defined consti tution

    is the arrangement of its atoms in space wit ho ut regardto arrangements that diff er only as after rot atio n aboutone o r more single bonds.

    Confo rmati on: The c onformatio ns of a molec ule of defined co nfigurationare the various arrangements of its atoms in space thatdiffe r o nly as after ro tation about single bonds.

    However , some s tructures diff er in configurational as well as conformational re sp ec ts . In such c ases the word confor matio n will be used inthis thesis to describe the differences.

    For a bette r descr ipti on of the relation between chain structure 2)and crystallinity a more adequate def initi on of tacticity is required .To simplify the disc ussio n, the PVC chain is thought of as exclusivelyconsist ing of monomer units connecte d to each ot her in the manner sho wnin figure 2.2. This is the "head-to-tail" additio n which pre dominatesin PVC; in other wor ds, the pr esence in the ch ain of different sequencesand end groups is not considered. A discussio n o n the way of monomeradditio n during polymerizatio n is given in the first sect ion ofchapter 3.

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    MONOMER UNIT- C H - C H C l | C H , - C H C l | C H - C H C I

    DIRECTION OF PROJECTION

    Figure 2.2. Division of the PVC chain into monomer units.

    H \H -

    C

    c(F )

    / C I^ H

    C KH ^

    C

    C(L)

    -H-H

    Figure 2.3. Newman projections of the right-handed and lefthanded monomer unit.

    The monomer units have two stereo isomeric forms that are shown asNewman pro ject ions in figure 2.3. The direct ion of pro ject ion indicatedin figure 2.2. was ch os en; the CH group lies in front of the CHCl group.The met hylene carbon ato m is designated as 9 and t he c arbon atom of theCHCl group a s Q . It is apparent from figure 2.3. that the relativepositi ons of the hydrogen and the ch lorine atom differ in the two casesshown.This is the basis for the two stereo isomeric forms of the monomer.

    A monomer in the c hain is c alled right-handed (R) if the chlorineatom is situated on the right side of the hydrogen atom (clockwise,angle of ro tati on: 120 ) . A monomer unit is called left-handed (L)when the relative p osi tion of the ch lorine atom is described by acounter -clockwise rot ation of 120 . A combinatio n of two successi vemonomer unit s in the ch ain (L)(R) or (R)(L) is called syndiotactic,the combinations (L)(L) and (R)(R) are called isotactic. The wordssyndiotactic and isotactic are often used for chains or chain segments.

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    14For instance, a ch ain with monomer units (R)(L)(R)(L)(R)(L) iscalled syndiotact ic since all t he sequences are. syndio tact ic . The sameapplies to an isotactic chain consisting of monomer units(R)(R)(R)(R)(R)~o r ~ ( L )(L) (L) (L) (L) , Atactic chains have noconfigurational or der. Here a random dis tr ibuti on of isotactic andsyndiot acti c monomer sequences exis ts . The above definition sh owsthat tacticity is independent of the co nformation of the carbonbackbone.

    It is now pos sible to define the syndiotacticity or , brief ly,tacticity, a, of the mater ial as the fraction of syndiotactic sequencesin the c hain. Wh en the tacticity is about 0.5 the polymer is calledatactic; commercial grades PVC are atactic. Values of a below 0.5 donot oc cur: the re is no known polymerizatio n tech nique that yields isotactic P VC. For an entir ely syndio tacti c mate ri al the tactici ty is 1.0.

    The tacticity can be measured by means of nuclear magnetic res onanceand infrared spectroscopy. The following sections present literaturedata o n these measurements,2.2.NMRmeasurements o f t h etacticity

    Tact ic it y measure ments of PVC by nuclear magnet ic reso nance (NMR)can be pe rfo rmed on solid po lymer samples and po lymer so luti ons. Thespectra obtained with solid samples are of the 'broad-band' or 'wide-line'type . Dissolved samples give 'high-reso lution' spec tra. For themeasurements of the tacticity of PVC only the high resolution techniqueIS suitableThe spectrum of PVC consists of two main part s: a multiplet centeredat about 5.5 T , cor re spo nding to the meth ine p rot ons or a prot ons anda group of peaks around 7.9 T for the methylene or 6 pr ot ons. A n exampleof such a spec trum was taken fr om Bovey et at. and is sho wn in figure2.4.

    Wit h double resonance techniques the a part of the spectrum consistsof thre e p eaks. In undeco upled spe ctra thre e overlappi ng quintupletsgive ri se to a complex a band. The usual i nter pre tation of the a prot onresonance is in terms of triads of monomer unit s, viz. syndiotactic,

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    Table 2.1, Tacticity of PVC as a function of polymerizationtemperature as determined by NMR

    Polymerizationote mperature, C100500- 78400- 4052.5100- 15- 25- 305030- 3055- 3050- 20- 50- 7012080605- 20- 50- 78300- 25- 35- 70

    tacticity a calculated froma proton spectrum

    0.570,610,650.550.570.530.540.560.61

    0.500.540.560.60

    3 proton spectrum0.540.550.570.63

    0.540.580.570.550.590.610.610.640.680.600.680.550.650.680.610.510.530.510.560.590.630.670.510.590.540.730.80

    Reference12)Bovey et al.

    9)Cavalli et al.

    13)Bargon et al.

    14)Enomoto et al.

    Satoh'^^

    Tincher'^^

    Talamini et al.

    Germar et al.

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    17

    a e -

    -K)0 - 5 0 o 5 0POLYMERIZATION TEMPERATURE C

    100

    Figure 2.5. Tacticity of PVC as a furxtion of polymerization temperaturemeasuredby.NMR.Black po ints: estimated from the B part o f the NMR spect rumOpen points : estimated from the a part of the NMR spectrumV Bovey et oL.

    Cavalli et aX 9)OCTOnDa

    a nd * Bargon et alEnomoto et al.Satoh'5)and Tinch erTalamini et al.Germar et at.

    13)

    2.3. IRmeasurements o f t h etacticityCalculat i on s o f t h e t a c t i c i ty o f PVC f r om t h e inf ra r ed s p e c t ral

    data have be e n p e r f o rm ed wi t h t h e a id o f t h e i n f o r mat i on f r om two18")r e g i o n s o f t h e s p e c t r u m. G e r m a r et al. us ed t h e r e g i on o f t h e CHs c i s s o r s t o c al c ulat e t h e t a c t i c i t y f r o m t h e r a t i o o f t h e o p t i c a ldensi ti es at 1428 and 1434 cm . In the r e gio n fro m 600 to 700 cm

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    18

    t h e c a r b o n- t o - c h l o r i n e s t r e t c h i n g v i br a t i o n s a bs o r b . Th i s p a r t o f t h espectrum is quite complex and an unambiguous assignment of the absorpt i on bands c anno t b e made: a s t r ong ove rlap o f t h e bands in t e r f e r e sw i t h t h i s a s s ignment. A d e ta il ed d i s cu s s i o n o f t h e band as s ignmentwill be given in chapter 5.

    M o s t a ut h o r s o b t ai n t h e t a c t i c i t y f r o m t h e r at i o o f t wo a b s o r p t i o np eaks but t h e r e sult i ng ta c t i c i ty value s a r e v e ry unc e r ta in. Th e i rdata are shown in table 2.2. and are plotted in figure 2.6. alongwi t h t h e calculat ed l in e s f r om f igur e 2.7.

    OBp

    -100 - 5 0 0 5 0POLYMERIZATION TEMPER ATURE . C

    100

    18)Figure 2.6. Tacti city of PVC as a function of polymeri zatio n t emperature as

    measured by IR.O Germar et al.

    ,19)20)

    ,22)D GermarA Stokr et al.> Poh l and Hummel'

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    19

    Table 2.2. Tacticity of PVC as a function oftemperature as measured by IR

    Polymerizationtemper ature, C

    300- 25- 35- 70

    300- 25- 35- 7055505025250- 25- 35- 40- 404723

    - 10- 30- 63

    Tacticit y a

    0.540.590.690.720.800.550.600.680.690.750.550.550.560.550.560.560.570.580.590.590.570.590.630.660.70

    polymerization

    Reference

    181Germar et al.

    19)Germar

    Stokr et al. ^^

    Poh l and Hummel^''^^^

    2.4. Polymerization temperature a ndtacticityThe most important factor which influences the tacticity is the

    polymerizatio n te mper ature. The reacti on rate co nstants, k and k.for syndiotactic and isotactic propagation, respectively, are af . F . . 23,24)function of temperature

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    20

    From reaction rate theory we have:

    k^ = k exp(-E /RT) (2.1)k^ = k? exp(-E^/RT) (2.2)

    k khence: r ^ = exp (AE/RT) (2.3)"i k?1Her e AE = E. - E (2.4)i sis the difference between the activation energies for isotactic andsyndiotactic pr op agation. The tacticit y a can be e xpresse d as

    S 1

    This equation combined wit h equation (2.3) leads to

    (k / k?)exp(AE/RT)a = -^ -i (2,6)1 +(k / k?)exp(AE/RT)s 1The tactic ity as calculated fro m equation (2,6) is plotted in

    figure 2,7, for thre e values of A E, combined wit h three values of theratio of the frequency factors k /k., These values were ch ose n in sucha way t hat the liter ature data f rom IR and NMR measurements fall betweenthe calculated lines (cf. fi gure s 2.5. and 2.6.).The differ ence betweenthe acti vatio n energies for isot actic and syndiot acti c pr opagatio n, A E,is thus found to be AE = (0.45 +_0.15)kcal/mole. It may therefore beconcluded that the syndiotactic propagation is indeed only slightlyfavoured over isotactic propagation.

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    21Th e t a c t i c i ty o f a c o mme r c i al atac t i c PVC p r o duc ed by p o lyme r i zat i on

    at 40 to 75 C is about 0.55. This value will be used when calculatingt h e c ry s tallini ty o f a ta c t i c PVC.

    POLYMERIZATION TEMPERATURE , C

    Figure 2.7. Tacti city of PVC as a f unction of polymeri zatio n temper ature.Calculation according to equation (2.6).A: AE = 0.60 kcal/mole, k/k? = 0.60s 1B: AE = 0.45 kcal/mole, k/k? = 0.65s 1C: AE = 0.30 kcal/mole , k/k? = 0.70

    2.5. Crystallization of anatacti c polymerTh e c h em i cal s t ru c tu r e o f t h e PVC c h a in is ra t h e r r e gular. H ead-t o -

    head and tail-to-tail connections hardly occur and the degree ofbranch ing is low. (The che mical st ruct ure of PVC is disc usse d inc ha p t e r 5.) St e r e o c h e m i cally, h o w ev e r , t h e s t r u c tu r e i s v e ry i r r e gular.A s m en t i o n ed b e f o r e, a c omme r c ial PVC ha s a ta c t i c i ty o f about 0.55.Th i s means t ha t suc h p o lyme r s a r e almo s t a ta c t i c . T h e i r r e gular i t y o ft h e i r s t e r e o c h e m i c a l s t r u c t ur e h a s i t s c o n s e q ue n c e s f o r t h e c r y s t a llizat i o n o f t h e mat e r i al.

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    Several investigators have found that materials of high tacticityshow high p erc entages of crystallinity. This has led to the idea thatonly the syndiotacti c parts in the PVC chains are able to form cr ystallites.Consequently, many authors consider PVC as a copolymer consistingof a syndiot acti c, crystallizable part and an is ot acti c, non-cr ystall-izable part. On the basis of this assumption it is possible to predictthe amount of crystallinity of an atactic PVC. Below, it will beexamined whether this theoretical calculation gives the correct valueof the crystallinity.

    For the crystallization of partly crystallizable copolymers a25)theo ry was evolved by Flory . On the rmodynamic grounds he deri vedthat the size of crystallites should have a minimum length. Ther ef or e,the parts of the chains contributing to the crystallites should alsohave that minimum length which is called the minimum sequence length,min

    It is pos sible to calculate the f raction of total polymer in syndiotactic sequences of length N from the tacticity o. According to24)Fordham

    N 2 NF*" J. = N(l - a) a (2.7)syndic ^For a commercial atactic PVC with a = 0,55 the sequence distributionis pre sented in figure 2,8, The fraction of the po lymer with a syndiotactic sequence length equal to or larger than N has been derived fromthis distr ibuti on. This enables calculatio n of the amount of cr ystallinity that can oc cur in a po lymer if all s equences larger than .mincontribute to the crystalline phase (figure 2,9). However, as statedby Flory, it is impossible that all sequences of length larger than5 . contribute to the crystallizati on. This implies that figure 2.9.repr esents the maximum amount of the c rystalline fraction of thepolymer as a functio n of the minimum sequence length.

    With a value of the minimum sequence length, B, . = 12, in agre e-.,, m mment with values for oth er po lymers , the maximum amount of cr ystallinity of an atactic PVC (a = 0.55) is calculated to be 0.45% (c/. figure2.9). It is a well-es tablishe d fact that even an atactic PVC cr ystall-

    4

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    F i g u r e 2 . 8 .

    F i g u r e 2 . 9 .

    23

    0.140.12

    oOlOozz S11.0.08zOt30 .06

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    24izes to s ome extent; a crystalline content of about 10% is quit e nor mal.This would c orr esp ond to a minimum sequence length of 6 which is anunusually low value for the cr ystallization o f a polymer. The c ryst allizatio n of such sho rt c hain segments res ults in very small crystalliteswith large defect s at the borders of the cr ystalline region. However,there are many indic ati ons, both from the pr ese nt experimental wor kand from literature, that the crystalline structure is much betterdeveloped. Ther efo re it is concluded that the crystallinity of atacticPVC cannot be explained satis facto rily by assuming th at only the syndiotactic part of the polymer crystallizes.

    2.6. Conclusions1. The pr opo rti on of syndiotactic sequences in the PVC chain increases

    with decreasing poljmierization temperature.2.The tacticity of PVC can be measured by IR and NMR techniques but

    the accuracy of these measurements is poor .3. The observed crystallinity of atactic PVC cannot be explained

    satisfactori ly by assuming that only the syndiot acti c part o f thepolymer cr yst allizes .

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    25

    CHAPTER3

    MELTINGOFCRYSTALLINITY IN PVC3.1. Introduction

    The crystalline melting phenomena descri bed in this chapter wer estudied by means of Dif fe re ntial Scanning Calorimet ry (DSC).In thistype of calorimeter the temperature of a sample and its reference israised linearly and the differe nce in heat flows required for thesimultaneous temperature increase in measured. The reco rder signal givesthermograms in which the glass transition appears as an increase of theheat flux difference whilst melting effects are recorded as endothermicpeaks.

    Almos t similar ther mograms are obtained wit h Differential ThermalAnalysis (DTA).However, in DTA the temperature difference betweensample and refe rence is measured as the temperature of both is raisedby a constant heat flow. This temperature differe nce must be convertedinto a heat flux whe n, for instance, a heat of fusion has to be calculated from the measurements . In this re sp ec t, DSC has an advantageover DTA: the h eat flux is measured directly which makes DSC mor esuitable than DTA for quantitative measurements.

    3.2Equipmentandmeasuring tec hniquesThe calorimetri c studies wer e made with a Perkin Elmer Diff ere ntialScanning Calor ime t er , type DSC-IB, usi ng poljnner samples of 10 - 30 mg.

    Since the caloric effects associated with melting of crystallites inPVC are small, a high h eating rate and a high sensitivity were require d.This was attained wit h a scan speed of 32C/mincombined with a he atflux range of 4 meal/sec . At such heating rates the c alorimeter has acert ain inerti a and as a result the measured temperatures are somewhattoo high. In order to compensate for this effect to some extent theusual calibration procedure of the apparatus was modified by employinga scan speed of 32C/mininstead of 8 C/min. However, there is still

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    26anothe r source of inaccuracy because the calibratio ns are per formedwith metals as calibratio n s ample s, e.g. indium and tin. Thes e metalshave a higher heat conductivity than the polymer samples and it isther efor e pos sible that the measured temperatures are s omewhat toohigh.Ac cor ding to Illers this deviation is 1 - 2 C; a cor rec tio nfor this relatively small erro r has not been applied in the pres entwork. The cooling rate between successive scans was fixed at 32C/minexcept for experiments wher e the effe cts on the po lymer of s low coolingor quenching were tested.

    With a few samples, which were also run in the c alor imete r, densitymeasurements were performed in Davenport gradient columns filled witheither aqueous zinc chloride or pot assi um iodide solutio ns. Wh enpo tassi um io dide was used some sodium th io sulfate (5 g/1) was added toprevent air oxidation of the iodide.

    3.3. MaterialsThe following commercial PVC grades and copolymers of vinyl chloride

    with vinyl acetate were studied:

    1. Corvic D 65/6, suspension PVC, ICI2.Solvic 239, suspension PVC, Solvay3. Vest olit E 7001, emulsion PVC, Chemisc he Wer ke Hls4.Breon 121, emulsion PVC, Britis h Ge on5. Lucovyl RB 8010, bulk PVC, Pch iney - St. Gobain6. Vinylite VYDR 3, cop olymer , 4% vinyl ace tat e. Unio n Carbide7.Solvic 513 P A, c op olymer , 13% vinyl ac et ate , Solvay8. Vinnol VE 5/65 P, copolymer, 5% vinyl acetate,l-.'ackerChemie9. Vinnol E 10/65 P, co po lymer, 10% vinyl ace tat e, Wacker Chemie

    10.Vinnol E 15/45,c op olymer, 15% vinyl acet ate, Wacker Chemie11.Vinnol VH 13/50,co po lymer, 13% vinyl acet ate, Wacker Chemie12.Vinnol H40/60,co polymer , 40% vinyl acet ate, Wacker Chemie13.Rhodopas AXBM II, co po lymer, 17% vinyl ac et ate , Rhone - Poulenc

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    27

    In additi on, partly syndiot acti c PVC's were pro duced in thelaboratory by susp ensio n po lymeri zati on at -30 C and -15 C. Asrecommended by Konis hi and Nambu a wate r/met hanol mixture (1:1)was used as the continuous phase and the catalyst system lauroylper oxide/ferro us caproate (both in a concentration of 0.3 mo l% onmonomer) was employed. The reactio n was stopped after five ho urs byadding oxalic acid or styrene and the polymer purified bydissolution in cycloh exanone and s ubsequent pr ec ip it ation in met hanol.Since the product obtained in this manner still contained someimpurities this procedure was repeated with tetrahydrofuran as thesolvent. Finally, the poljrmer was filtered and washed with methanoland dried for 24 hours at 40 C in a vacuum oven.

    All the compounds tested were stabilized by adding 3 - 5 %w dibasiclead p hos phi te.

    3.4 . Pr imary c rys ta l l i n i t yTwo kinds of cryst alline melting e ff ect s wer e found. Pri mary crys

    tallinity whi ch was dete cte d in virgin PVC is descri bed in thisparagraph. Secondary crystallinity induced by annealing the polymeris discussed in the next section.

    An example of the rmograms obtained with a commerc ial vir gin PVCpowder is given in figure 3.1. The samples wer e tested as rece ivedfrom the supplier s.

    The the rmograms show a melt region whi ch starts at about 150 C.The initial melting temperature does not vary significantly among thecommerc ial samples te st ed, excep t for Solvic 239 in whic h meltingstarts at about 170 C. Invari ably, the melti ng range e xtends into thedeco mpos iti on range. Wh en a virgin PVC powder is heated to a te mperature somewhere in the melting range a permanent change is pr oducedin the th ermogram: in the subsequent scan the endoth ermic meltingeffect co mmences at the highest temperature that was reached in thefirst r un (cf. figure 3.1.),whi ch means that no rec rystallizatio noccurs on cooling. In this way, the crystallinity can be removed stepby step by successive heat treatments. Similarly, when a PVC compound

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    28

    lyi

    J I 1 1 I I50 0 50 100 150 20 0^ T E M PE R AT U R E, C

    Figure 3.1. Thermograms of commercial PVC; melting of pri mary crystallinity.first run (A)

    ^ ^ ^ second run (B).. . third run (C)

    Table 3.1. Melting t empe rature and melting energy of coTmnercialPVC's.

    Polymer grade (seesecti on 3, mate ri als)

    Corvic D 65/6Solvic 239Vest olit e E 7001Breon 121Lucovyl RB8010

    Vinylit e VYDR 3 (4% vinyl ace tate )Other co polymers (> 5% vinyl ace tate )

    Initial meltingtemperature, C

    150170150150150

    150no melting

    Melting energyin the temperaturerange 150 - 200C,cal/g

    1.2not measured

    1.01.01.2

    small0

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    29is pre pared by milling and p re ss ing, the t hermogram clearly sh ows thehighest temperature reached during processing by a steep onset of theendothermic region (thermogram B in figure 3.1.).Copoljnners of vinyl chloride with vinyl acetate behave differently.The cop olymer co ntaining 4% vinyl acetate (Vinylite VYDR 3) shows asmall melt region wher eas in the thermograms of cop olymers with ahigher vinyl acetate content no melt effe ct at all can be detec ted.

    The crystalline melting effects are sometimes obscured to someextent by anothe r ph enomenon, esp eci ally when measurements areperformed o n PVC powder. Whe n the PVC melts the heat contact betweenthe sample ho lder and the sample itself improves . As a consequencea s maller heat flux is required temporarily to raise the sample te mperature linearly. This change causes a pse udo-exothe rmic effec t in thethermogram almost at the same temperature where the endothermic meltingusually s tarts. To pre vent this there should be no ch ange in the h eatcontact during the scanning of the thermogram. This can be attainedby applying a pr es sure of about 1000 atm whe n filling the aluminiumsample pans whic h are used in the Per kin Elmer apparatus,

    The magnitude of the melt region was estimated by calculating theamount of energy required to melt the cr ystals pres ent in the polymerin the range between the initial melting temperature and 200 C, Theresults for a number of commercial samples are listed in table 3,1.

    From the above data it is co ncluded that the differe nces betwee nthe vario us P VC grades teste d ar e small. Susp ensi on, emulsi on and bulkPVC all give the same re sult. The cop olymers sho w a smaller eff ect ornone at all; apparently, the introductio n of vinyl acetate diminishe sor prevents crystallizatio n,

    Thermograms of the laboratory-prepared, partly syndiotactic PVCpolymerized at -30 C are shown in figure 3,2, Her e, melting starts at180 C. The e ndoth ermic eff ect is larger than in commercial po lymers .However, the polymer was purified by dissolution and subsequent precipitation. It will be shown below that this procedure causes thecrystallinity of the po lymer to diminish. Never th eles s, larger

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    30

    melting effects were found for this polymer than with commercialPVC's,which confirms that its crystallinity is hi gher.

    < 2

    50TEMPERATURE, C

    100 150

    Figure 3.2. Thermograms of PVC polymerized at -30 C; melting of primary crystallinity.^^^^^^ first run ^ second run

    The heat of fusion o f the pr imary crystallinity was found as follows.Whe n s amples from a PVC plate are heated up in the c alorimeter to 200Cthe crystallinity decreases. The heat which is required for the meltingwas obtained from the the rmograms. The decrease in crystallinity wascalculated from density measurements on parallel samples. The resultsof the density measurements are shown in table 3.2.

    3 29)The densi ti es of entir ely syndiotactic (1.44 g/cm ) and entir ely3 30) 3amorphous PVC (1.41 g/cm ) differ by 0.03 g/cm . The data of table33.2.show that the density diminishes by0.0026g/cm whe n he ating fro m165 to 200 C, which means that the crystalline content diminishes byabout 9%. The thermograms reveal that the corresponding melting energyamounts to 1.2 cal/g (table 3.1.).On this basis , a heat of fusion ofappr oximately 13 cal/g or 850 cal/mole of monomer unit is fo und.

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    31

    Table 3.2. Densities of commercial PVC samplesdifferent heat treatments

    before and after1 1

    Thermal treatmentMilled and pre ss ed at 165 CHeated at 180CHeated at 200C

    Density in g/cm1.43351.43251.4309

    To c ompare the above re sult with lite rature data, values of the(initial) melti ng temperature and the heat of fusion were collecte dand listed in table 3.3. The results of the present work agree satisfactori ly with those of Kockott , Anagnos to poulos et al. and35)Nakajima et al, ,but they differ considerably with the results ofother authors using thermal analysis as measuring method. A low meltingtemperature was r epo rte d by Mich el ; hi s low value of 114 C can beeasily explained, since the experimental work was done with a mixtureof PVC and plasticizer which was dry-blended at room temperature. Insuch a mixture the p lasti cizer depr ess es the melting temper ature. In37)the thermograms of Lebedev et al. the autho rs indicate a melti ngof crystalline structures starting at 200 C. However, the comparis onof the thermogram of their initial spe cimen (A) with that of thesample which they cooled slowly after pr eh eating to 210 C (B) sho wsan endother mic eff ect whi ch starts at 150 C and extends into thedecomposition range (figure 3,3,).The peak at 200 C may be due todecomposition, since appreciable degradation of the material startsat that temperature wh en the heating rate is but 4 C/min. The peak at120 C cannot be explained.

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    32

    Table 3.3. Melting temperature and heat of fusion of PVC1

    MeltingtemperatureC

    1 150220273212

    155220265> 300174

    285310225200(150)114

    150180

    Heat offusioncal/mole

    --2700

    -

    -660

    785785--

    750*)

    Polymerspecification

    a = 0.5a = 0.8a = 1.0copolymerwith 2-5%vinylacetateT = 125CTP1= 45CT P | = - 1 0 CTP]= -80CpolcommercialT = -15CT P ^ = -75CpolcommercialcommercialcommercialPVCplasticizedcommercialT = - 30Cpol

    Measuring-method

    X-ray

    flow ofplasticized PVC

    flow ofplasticized PVC

    polymer-diluentpolymer-diluentDTADTADTA

    DSC

    Reference

    Kockott^') 1

    Walter^^)

    Reding et al.^^^

    Anagnastopouloset al.^^^35)Nakajima et al.

    Clark36)Lebedev et al.^^^Michel 30)

    39)Juijn et at.

    *) In this thesi s the value of the heat of fusion was re calculatedto be 850 cal/mole.

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    33

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    34slowly added to this solution whilst stirring vigorously. The powderobtained in this way contains crystallinity (see f igure 3.4.).On theot her hand, entire ly amor ph ous PVC is obtained by 'quick' pr ec ip it ation,when the PVC solutio n is added to the met hanol. In that case noendothermic effects are found in the thermogram.3.5. Secondary crystallinity

    We have seen that recrystallization of the primary crystallinityis impossible. However, annealing at a temperature above the glasstransition temperature does produce some crystalline order inpreheated poljraier. This follows from figure 3.5., in whi ch th er mogramsare sho wn of a commer cial PVC powder whic h was annealed at 100 C.An e ndoth ermic region is formed adjacent to the annealing temper ature .The maximum of this endothe rmic peak is located at Tmax

    The original melting range (primary crystallinity) is never entirelyres tor ed, not even after pro longed annealing. The ther mograms of figure3.5. show that the annealing effect is eliminated by subsequent heating;it does not reappear unless the polymer is again annealed.

    Figure 3.5. Thermograms of conmercial PVC; annealing at 100 C.^^^ ^^^ first run^ ^ ^ ^ second run

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    36The magnitude of the annealing effe ct depends on the extent to which

    the pri mary crystallinity h as been melted by prior heating. The effectbecomes larger when the amount of primary crystallinity left afte r preheating decreases .An untreated virgin powder does not show any effectat all (see figure 3.7.).

    The annealing effect increases in intensity when annealing is prolonged. This is s hown in figure 3.8, Mor eo ver , figure 3.9. sh ows thatthe newly formed melting range then shifts to a higher temperature.These results indicate that the secondary crystalline order inducedby annealing becomes more extensive and increasingly perfect atlonger annealing times.

    Secondary crystallinity is formed in all kinds of commercial PVCand also in syndiotactic PVC. However, the copolymers containing vinylacetate behave differently. In figure 3.10. the annealing effect forvarious copolymers is compared with that of the homopolymer (Solvic239). There is a gradual decrease of the annealing eff ect when th evinyl acetate content of the c opolymer incre ases; at 40% vinyl acetatesecondary crystallinity is not formed at all. This is in contrast withthe influence of comonomers on the formation of primary crystallinity;as was sho wn previo usly a small vinyl ace tate co ntent (5%) enti re lyprevents the formation of primary crystallinity.

    u 0.6uLi. .J2 0.4_ J

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    37

    _L _L10 100ANNEALING TIME, MIN .

    1,000 10,000

    Figure 3.9. Influence of the annealing ti me on the melti ng temp er ature of theinduced st ruct ures (annealing at 100 C, pr eh eat ing te mper ature 165 C ) .

    u 06

    VINYL ACETATE CO NTE NTIN CO POL YM ER, /,Figure 3.10. Influence of vinyl acetate content in copolymer on the magnitude of

    the annealing eff ec t (ann ealing for 1 hr at 100 C, pr eh eat ingtemperature 200C).

    T he p r o c e d u r e f o r t h e e s t i m a t i o n of t h e h e a t o f f u s i o n o f t h es e c o n d a ry c r y s t a l l i n i t y i s s i m i l a r t o t h a t a p p l i e d t o t h e p r i m a r yc r y s t a l l i n i t y d e s c r i b e d i n t h e p r e v i o u s s e c t i o n . A g a i n , t h e d e n s i t yd i f f e r e n c e a nd th e t h e r m a l e f f e c t w e r e m e a s u re d w i t h t h e sa me s a m p l e s

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    38

    The results of the measurements on samples of commercial PVC beforeand after annealing are listed in table 3.4. The density values areplotted as a function of the annealing effect in figure 3.11. A densitydifference of0.022g/cm is measured when t he annealing ef fe ct amountsto 0.5 cal/g. From the se data a heat of fusio n of about 400 cal/moleis c alculated assuming that a mate ri al having 100% secondary crystallinity h as the same densit y as a mate rial of 100% pr imary cr ystallinity.3.6. Annealing below t h eglass transit ion temperature andquenching

    In literature reports many authors mention crystalline phenomenaoccuring in or just above the glass transit ion re gion. The e ffec tsaris e whe n a sample of PVC is annealed at a te mper ature just below theglass transition region and when a sample is quenched from a hightempe rature (approximately 200 C) to a low temper ature ' ' ' .In figure 3.12. thes e effe cts are sho wn. Curve A rep rese nts a normalDSC thermogram. Curve B shows an endothermic effect for an annealedsample;similar thermograms are found when a sample is slowly cooledfrom a temperature above the glass transit ion ' . The exoth ermiceffect in curve C is caused by quenching.

    Both these thermal effects have been ascribed to crystalline40,41,42) ^ ,. , , , - Jph enomena: the annealing peak has been attr ibuted to themelting of structurally ordered material and the exothermic quenchpeak was interpreted as crystallization. The explanations must beincorrect since the two effects can be obtained with various kinds ofpo lymers , whet her they are cr ystalline, semi-crystalline or amorph ous.PVC, co polymers of vinyl chlor ide and vinyl ace tat e, polyvinyl ace tate44) 45)

    , po lyst yrene all behave in this manner . Alth ough po lystyrenecontains neither primary nor secondary crystallinity and thus iscompletely amorphous, it nevertheless shows the glass transition effectsdiscussed h er e. 40The c orre ct e xplanation for these effec ts is enthalpy relaxatio n '44-49) . .40)

    . Figure 3.13. (accor ding to Wi lski ) sho ws the enthalpy-temperature relationship. After quenching the enthalpy of the polymer

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    39

    Table 3.4. Densit ie s of commercial PVC samples befo re and after 1annealing

    Thermal treatment

    Preheated at 180C

    Preheated at 180Cannealed at 100 C for:

    Annealing time,min

    0

    510153060120240900

    Annealing effect,cal/g

    0

    0.050.140.150.200.250,340,440,52

    Density,g/cm

    1.4334

    1.43371.43391.43401.43431.43491.43481.43551.4357

    H 0 6

    z2z _jz

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    41

    is too high and theref ore subsequent slow heating gives a drop inenthalpy in the glass transiti on re gion which is observed as anexother mic peak in a DSC the rmogram. Conver se ly, annealing in the glassregion or slow cooling of a sample causes the e nthalpy to reach alow level. In that case a high heating rate produces an endothermiceffect in the ther mogram. Such enthalpy curves wer e measured by 45)Sharonov and Volkensh te in for po lyst yrene. It was thus sho wn thatthese so-called crystalline effects are actually relaxation phenoma.3.7. Conclusions1. Two types of cr ystallinity can exist in PVC:

    a. Pri mary crystallinity of relatively hi gh o rder , which occ urs invirgin powder.

    b. Secondary cr ystallinity h aving a lower degree of or der, whic hresults from annealing when the primary crystallinity has whollyor partly been melted.

    2.The thermal effects in or just above the glass transition regionwhich are caused by annealing below the glass transition temperatureand quenching are re laxati on ph enomena.

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    42

    CHAPTER4AGINGOFPLASTICIZEDPVC4.1. Introduction

    Object s made of plasti cized PVC st iff en in the co urse of ti me. Sofar, this ph enomenon has not bee n explained satisf acto rily and onlyfew data have bee n publishe d on the aging of tec hnically and economically important plast ic ized co mpounds containing PVC. A RAPRA-rep or tattr ibutes the aging to segregatio n of polymer and plasti cizer. Asimilar"st if fening is observed when aging PVC in solutio n ' andsoft PVC gels ' . I n these cases the for matio n of supramolecularstructures is held responsible for the aging.

    When PVC is plasticized the glass transition temperature decreases.A compound with about 25 % w of plasti cizer has a glass transiti ontemperature below room temperature. Since annealing of rigid PVC abovethe glass transition temperature causes secondary crystallinity it islogical to assume that crystallization p roc eeds to some e xtent inplasti cized compounds during s tor age at roo m t emperature. Below, thehypoth esi s that crystallizatio n causes the aging of these materi alsis examined expe ri mentally.4.2. Equipmentandmeasuring techniques

    The calorimetric studies and density measurements were carried outas descri bed in chapter 3, sec tio n 2. Moduli of elasti city weremeasured on an Instron Universal Testing Instrument, floor model(TT-CM).P olymer samples with a cross sec tion of 1 x 0.2 cm and alength between the clamps of 8 cm were tested at a strain rate of0.5 cm/min. The modulus of elast ic ity at 0.5% relative strain wascalculated from the stress-strain diagram.

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    43

    4.3. MaterialsThe following commercial pol}miers were studied in plasticized

    compounds:

    1. Corvic D 65/6, suspension PVC, ICI2.Solvic 239, suspension PVC, Solvay3. Vesto lit E 7001, emulsion P VC, Chemisc he We rke Hls4. Carina 16 F, emulsi on PVC, Shell5. Vinylite VYDR 3, co po lymer, 4% vinyl acet ate. Union Carbide6. Solvic 513 PA , co po lymer, 13% vinyl ace tat e, Solvay7.Haloflex 243, chlori nated po lyet hylene containing 43% ch lor ine , ICI8. -15 C p olymerizate, se e chapter 3, secti on 3

    The following p lastici zers were used in the compounds:

    1. Scadoplast 8 P, di-2-aet hylhexyl ph talate (DOP), Scado-Archer-Daniels2. Tricresylphosphate (TCP),U.C.E.3. Scadoplast 8 A, di-octyl adipate (DOA), Scado-Archer-Daniels4. Scadoplast W 2, polyest er from ph talic acid, Scado-Arch er-Daniels5. Scadoplast RA 5, polyester from adipic acid, Scado-Archer-Daniels6. Cer eclor S 52, chlori nated paraffine containing 52 % w c hlo ri ne, ICI

    All c ompounds wer e stabilized wit h 2 % w dibutyl tin dilaurate and0.5 % w zinc s te arate.4.4. Annealingo fplasticizedPVC

    Thermograms were recorded on samples of plasticized PVC compoundscontaining 30 parts by weight of DOP per 100 parts of PVC. The testsamples had been st ored at room temperature for varying per iods oftime.An example of the results is shown in figure 4.1.; secondarycrystallinity is clearly sho wn by the th er mograms.

    The glass transiti on temperature is si tuated below room te mpe rature.This means that storage at roo m temperature is , for this mate ri al,

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    44equivalent t o annealing it above t h e glass t r ansi t i on. The sameph enomenon obse rved when annealing rigid PVC is t hus f ound he r e f o rt h e plast i c ized c omp ounds: s e c ondary cryst allinity i s f o rmed inincr e asing amounts as t h e t i me o f s t o rage i s longer. A ls o , t h e melt ingrang e t h e n s h i f t s t o h i g h e r t e m p e r a tu r e. Con c om i t antly, t h e modulu sof elasti c i t y o f t h e mater ial i ncr ease s (se e f igure 4.2.) and th esame applie s t o t h e densi ty o f t h e c o mpound (figure 4.3.).

    For all th e p olymer s s t udied -including th e low t e mpe r aturep o lym e r i za t e, t h e c o p o lym e r s and t h e c h l o r i na t ed p o ly e t h yl en e-similar r e sults wer e obtai ned. This i s illust rated by an extensivec o m par i s o n o f t h e s t i f f e n ing o f var i o u s c o m p o unds a t t h e e nd o f t h i ss e c t i o n .

    The f o rmatio n o f s e c ondary cryst allinity in plasti c i zed PVC is ar e v e r s i bl e p r o c e s s . R e h e a t i ng t o 80 C caus e s t h e c r y s t allin e o r de r t od i s a p p e a r; s i mul t ean ou sly, t h e m odulus o f e las t i c i t y d e c r e a s e s t o t h evalue w h i c h was f o und r i g h t a f t e r p r o c e s s i ng o f t h e c o m p o und. T h ep h e n om ena s t udi ed h e r e h ave s o m e t i m e s b e e n a s c r i b ed t o l o s s ofplasti c i zer by evapo rat i o n, but t h i s i s dis p r o ved by ch emical analysis .

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    45

    7 8 910STORAGE TIME, HRS

    Figure 4.2. Influence o f s tor age time on the modulus of elastic ity for plastic izedPVC co ntaining 30 part s of DOP on 100 par ts of P VC. Stor age andmeasurement of modulus at room temperatMre.

    1.2905r-

    12900JJ

    1.2895O

    280 300 320 340 360"-MO DU LU S OF ELASTICITY AT 1'/.STRAIN,K G F / C M ^

    Figure 4.3. Relation between modulus of elastici ty and densit y of plastic ized PVCcontaining 27.6% DOP. Increase o f modulus of elastic ity and densi tyobtained by sto rage at roo m temper ature (Acco rding to Gis olf ) .

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    46Anothe r theory based on the segregatio n of plasti cizer and PVC isincompatible with the fact that co mpounds wit h all kinds of plast ic izer,including polymeric plasticizers, show aging. Even the chlorinatedpo lyet hylene, without plast ici zer, shows an increasing sti ffness inthe co urse of ti me. The increase in the moduli of the plastic izedmaterial should therefore be ascribed to the formation of secondarycrystallinity. This is in line with the theory of several authors, viz.that the rubberelastic behaviour of plasticized PVC may be attributedto physical cro sslinking caused by crystallites ' . It also explainsthat the aging pro ces s is re versi ble: upon heating the annealingstructures are melted.

    Aging of p lasti cized P VC at room temperature is similar to agingof p ure PVC at a te mperature above the glass transiti on region. Wh enPVC is annealed at 100 C and the modulus of elast ic ity is measured atthat same temperature an increase in stiffness is observed. This isshown in figure 4.4. which closely resembles the aging of plasticizedPVC presented in figure 4.2.

    10 100STORAGE TI M E, HRS

    ^ 0 0 0 10 000

    Figure 4.4. Influence of storage time on the modulus of elasticity of unplasticizedPVC (Solvic 239).Storage and measurement of modulus at 100 C.

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    The i ncrease of the modulus of elasti city was deter mined for alarge varie ty of plasticized compo unds. From p lots like that offigure 4.2. the moduli of elasti cit y at 1 ho ur and 1000 ho urs weretaken (E and E _,re spe cti vely).The ratio of these values ,E _/E , is a measure of the r ate of aging of the c ompound. Theexperimental results are listed in table 4.1.

    The rate of aging, E /E , is plotted as a function of themodulus of elasticity, E , in figure 4.5. The aging is acceleratedby lowering the stif fness of the co mpound. It is considered significantthat all the c ompounds tested fit in the same curve: it means thatcompounds containing the partly syndiotactic poljrmer, the copolymersand even the chlorinated polyethylene most probably produce the sametype of crystallinity as the commercial PVC compounds.

    The above findings lead to a general conclusion which is importantin applications, viz. that it will be very difficult to prevent theaging of plasticized compounds. The structures formed upon aging havesuch a low degree of order that the i ntroduction of defects in thechain does not help to prevent their being for med. Changing the typeof plasti cizer is equally with out ef fe ct .

    2.5 I -

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    Table4.1,Agingo f

    Polymer

    CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6CorvicD 65/6

    plasticized PVC,Measurementofmodulus elasticity

    Plasticizer

    Scadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PTricresylphosphateScadoplast8AScadoplastW 2ScadoplastW 2ScadoplastW 2ScadoplastRA5CereclorS52

    Amounto fplasticizer,parts/100partsof P VC3030303030303030302630303033403030

    Presstemperature

    C165165165165165165165165185165165165165165165165165

    J;kgf/cm

    1720180018201850190019301940210017503600935054057003700110076008650

    ^10002kgf/cm

    2500290026502800286026402800276027505500105001270730050001850925011100

    ^10Q0''^1'ratecfaging

    1,451,601.451.501.501.351.451.301.551.501.102.351.301.351.701.351.30

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    Table4.1.continued

    Polymer

    Solvic239VestolitE 7001Carina 16FVinylite VYDR3Vinylite VYDR3Vinylite VYDR3Solvic513 PASolvic513 PAHaloflex243Haloflex243-|5CpolymerizateSolvic239Solvic239Solvic239Solvic239Solvic239Solvic239

    Plasticizer

    Scadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8P-Scadoplast8PScadoplast8FScadoplast8PScadoplast8PScadoplast8PScadoplast8PScadoplast8P

    Amountofplasticizer,parts/100partsof PVC30303030302030205-3001020304050

    Presstemperature

    C165165165165140150110100165165165160160160160160160

    kgf/cm

    17201500142016001780105003807100145076002400220002250011300176024086

    ^1000^kgf/cm

    2860235022202700280013800780970029501010035502440025600132002580500162

    ^IOOQ/^I'rateo faging

    1.651.551.551.701.551.302.051.352.051.301.501.101.151.151.452.101.90

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    4.5. Conclusions1. Seco ndary crystallinity is f ormed in plast ic ized PVC during s tor age

    at room temperature.2. The formation of such structures accounts for the s tiffening of

    plastici zed compounds in the course of ti me.3. Aging oc curs with vario us kinds of p last ic izer.4. The aging proc ess is independent of the s yndiotactic ity of the PVC

    and also occ urs in copolymers of vinyl ch lori de with vinyl acetateand even in a highly chlorinated polyethylene.

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    CHAPTER5CHAIN STRUCTURE

    5.1. Chemical andstereo chemical structureo f t h echainIn the previous chapters it has been pointed out that atactic PVC

    crystallizes to some e xtent. Since crystalline order in a polymeri cmaterial requires strict regularity of the chains, the questionarises how and to what extent even atactic PVC can still be packedin a regular crystal latti ce . Chain r egularity is deter mined by itsche mical st ructure (chemical impuri ti es , branch ing, head-to-tailstr ucture) and its ste reoc hemical st ructure (configuration, co nformation).These subjects will be discussed in this ch apte r.

    Chemical impurities may be incorporated in the chain duringpolymeri zatio n. Many of these are attached to the chainends,e.g.as initiator groups. However, there is a high level of chain transferduring the po lymerizatio n of vinyl c hlor ide. Thi s means that a largefraction of the molecules is st arted through transfer reactions and,consequently, many molec ules do not contain initiator groups . Thiswas shown by Razuvayev et al. , who found only 0.19 to 0.40initiator groups pe r molecule. The molecules which were startedthrough transfer reacti on c ontain olefinic end groups with almostthe same size as the monomer unit. Hence, the effect of chemicalimpuritie s on the regularity of the c hain is likely to be of minorimportance.

    Another factor causing ir regular chain structure is branch ing.The measurement of the degree of branchi ng is very laborio us: thepolyvinyl chloride is first reduced by lithium aluminium hydride topolyethylene and then the content of methyl groups of this po lyeth ylene is est imated by infrared analysis ' . The metho d does notreveal the length of the branch es : each branch, whethe r short or long,is counted as o ne side c hain. At bes t, the results are semi-quentative.The degree of branching is usually given as the methyl content of the

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    52

    reduced PVC (CH /lOOCH ) . For co mmer cial PVC's values of 0.2^^\ lessthan 0.5^^\ 0.4 to ].\^^\ 1.6^^^ and 1.5 to 1,8^^ have bee n reported. From t hese figures we co nclude that branchi ng is not the majorfactor that dete rmines the cr ystallinity of P VC.

    The third phenomenon which contributes to the chemical structureo2 the chain is the manner in which monomer is added during the growthof the chain. The monomer unit in the ch ain consists of two groups ,viz. the CHCl group and the CH group. These gro ups can be called thehead and th e tail of the monomer, res pec tively. Thus,the propagationstep in the formation of PVC can yield a head-to-tail, a head-to-headand a tail-to-tail addition. It was shown by Marvel et al. thathead-to-tail addition of the monomer occurs far more readily thanhead-to-head or tail-to-tail addition. Fierz-David and Zollingeresti mated the pro por tio n of irre gular monomer additi ons to be lessthan 1.5%. From this low figure it is concluded that head-to-headand tail-to-tail str uctures are not likely to pr event the cryst allizatio n of P VC.

    Since the content of chemical i mpuri ti es , degree of branchi ng andthe amount of irre gular monomer additi on are low, the s tructure ofthe c hain is determined almost exclusively by ste reo che mical factor s.For a vinyl polymer like PVC two factors should be considered:1. The ste reor egularit y of the chain (configuration)2.The spatial arrangement of the chain backbone (conformation).

    In this invest igatio n, the use of atomic models p roved to be avery s uitable means o f s tudying the s te re oc hemical st ructure of PVC.New Courteauld A tomic Models were used, which have dimensionsco rre sp onding to the following values of atomic dist ance s: d = 1.54 A ,d = 1.07 A and d, = 1.76 A. In a Table of Selected Bond Lengths,fillpublished by the Chemical Societ y , average values of the se bondlengths are listed as follows:d^^ = (1.537 +0.005)A,d ^ = (1.096 +0.005)Aand d^^^ = (1.767 +0.005)A.Besides, all valence angles in the PVC chain are very closeto the valence angle of tetrahedral carbon. Since this also holdsfor the Courteauld models they are regarded as adequately realistic.

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    5.2. NomenclatureFor a unique description of the chain structure the configuration

    and conformation paramete rs have to be defined fir st. Ther e is nosystem whi ch is generally used in lit er ature , but this is not aserio us problem because it is quite easy to translate the symbolsof one nomenclature into the symbols of any other system. Ournomenclature can be illustrated by means of the Newman projectionof the chain (see chapter 2.1.).If two atoms have the same projectionthey are distinguished by indicating the atom lying in front by apoint and the rear atom by a cir cle. As direc tion of projection theC-C bond in the monomer was ch os en, with the CH group lying in fro ntof the CHCl group, (see figure 2.3.)5.2.1. Configuration parameters

    A m ono me r u n i t i n t h e c h a i n c a n b e r i g h t - h a n d e d (R ) o r l e f t - h a n d e d(L ) a s d e f i n e d i n c h a p t e r 2 . 1 . H e r e , f i g u r e 2 . 3 . i s r e p r o d u c e d a g a i nto show th e Newman p r o j e c t i o n s o f th e (R) and (L) monomer u n i t .

    Figu re 2. 3 . Newman pr oj ec tio ns of the rig ht-h an de d and left-h an de d monomer un it .

    5.2.2. Conformation parametersA carbon-to-carbon bond in the chain can be trans, gauche-right

    or gauche-left . The Newman pro ject ions of figure 5.1. are used todefine the symbols T, G and G,.

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    H HH H

    H HC Cc(TR)

    CC

    (G^R)Clc(6^R)H

    Figure 5.2. Newman projections of the six monomer conformations.

    5.3.2.Pair conformationsThe firs t step in building the PVC chain from atomic models is to

    assemble the monomer unit of which six different conformations exist(see above ).These monomer units are then combined into pair s. Onpaper every conformational form of both monomer units can be combinedwith any ot he r form of the monomer in three differ entways:T, Gand G,. We are thus led to expect 6 x 6 x 3 = 108 different pairconformations.

    However, steric hindrance markedly reduces the number of pairco nfor mati ons. The pair conformatio ns which are excluded can bedivided i nto two groups. The first group consists of pairs whichcannot be constructed because of internal steric hi ndrance . A goodexample of this class is the all-trans isot acti c pair (TR)T(TR) or(TL)T(TL).Here the chlorine atoms are co ntiguous and the construction of this pair with atomic models is impossible. The second group

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    56

    is e xcluded because of external steric hi ndrance . Here the pair assuch is with out steric hi ndrance , but it is not possi ble to attacha third monomer, whet her in front of or behind the pair.

    1 Table 5.1. Confo rmati ons of pairs of monomer unit s. 1(All mirror image

    Classification

    transwitho ut steric hindrance

    transwith some steric hi ndrance

    gauchewitho ut steric hindrance

    gauchewith some steric hi ndrance

    conformations have

    Syndiotactic

    (TR)T(TL)(G^R)Gj(G^L)

    (TR)G^(TL)(G R)G (G L)r r r(G^R)T(G^L)

    (TR)G^(G^L)(G R)T(TL)(G^R)T(TL)

    (TR)G^(G^L)(TR)T(G^L)(G^R)G^(TL)(G^R)T(G^L)

    been omit te d)

    Isotactic

    (TR)G^(TR)(G^R)T(G^R)

    (TR)G (TR)(G R)G (G R)r r r(G^R)T(G^R)(G^R)G^(G^R)(TR)T(G R)(G R)T(G R)(G^R)G^(TR)

    (TR)G^(G^R)(TR)T(G^R)(TR)Gj(GjR)(G^R)G^(TR)(G^R)T(G^R)

    Table 5.1. lists the pairs of monomer units with no or only alimited amount of steric hindrance. Every pair that can be constructed from New Courteauld Atomic Models without breaking bonds is mentioned in this tabel. The pairs are classified as fo llows:

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    1. A pair can be syndiot acti c or iso tactic. As was stated in chapter 2,combinations (R)(L) and (L)(R) are syndiotactic, (R)(R) and (L)(L)isotactic.

    2.A pair c an be trans or gauche. In the pre vious sec tio n a monomerwas defined as trans when the two vicinal C-C bonds around thecentral C-C bond of the monomer are parallel. The same applies tothe pair. Wh en the C-C bonds coming into the pair and that goingout of the pair are parallel the pair is called trans. Gauche pairscan be defined similarly.

    3. Some ster ic hindrance was allowed in constructi ng the p air s. As aconsequence, there are classes of pairs with and without some sterichindrance.

    5.3.3. Chain conformationsNow that all pos sible pair conformations are known the third step

    in building the chain can be made: combining pairs of monomer unit s.This is not always po ss ible. If a pair A-B is combined wit h a pairC-D, forming a part of the c hain A-B-C-D, we h ave to ch eck that thepair B-C is allowed. Thi s tec hnique yields a large number of ch ainconformations. Here, the basic question of this thesis (chapter 1)should be remembered, viz. to explain the occ urre nce of crystallinityin atactic PVC. Crystallization of a polymer requires straight chainconformations of the molecules and th ere for e, only th ese conformatio nswill be studied below. 29)

    PVC crystallinity was fir st descr ibed by Natta and Corradiniwhen re porting the results of their X-ray measurements on atactic PVC.They concluded that PVC crystallizes in an or th ot hombic latti ce andconsidered this to be the crystal form of pure syndiot acti c chainswith a planar zig-zag conformation. In the second chapter it was shownthat in atactic PVC a crystalline content of about 10% cannot be for medfrom the syndiotactic chain segments only. Obviously, other straightchain conformations must exist whic h can crystallize together with thesyndiotactic chain segments in the orthorhombic lattice described byNatta and Corradini,

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    There is some additional experimental evidence for the importanceof s traight chain conformatio ns in po lymers . Measurements of thedensity of polymer materials show that the difference in densitybetween the 100% cr ystalline and 100% amor ph ous forms of a po lymer30)is often small. For PVC Lebedev et al. found a density of 1.443 . 3g/cm for the crystalline phase and 1.41 g/cm for the amor ph ousphase. 64)Robert son , who observed similar small diff ere nces between thedensities of the crystalline and amorphous phases of many polymers,argues that even in the amorphous phase the polymer chains should bemore or less str aight and aligned in parallel. This is the secondreason why p arti cular atte ntio n must be given to straight chainconformations.

    Const ructi on wit h gauche pairs gives bulky chains or comparitivelywide h elice s. More over , such chain co nformations do not rese mble theall-trans syndiotactic chain, thus making co-crystallization impossible.However, combination of trans pairs results in straight chains orslender he lices . The following part o f this chapter contains theresults of our studies on these straight chain conformations of PVC.

    The syndiot actic parts of the ch ain pos e no pro blem. All liter aturereports indicate that these segments assume the very regular all-transco nformati on. The same was found with the atomic models. The zig-zagconfor mation has no steric hindrance and, ther ef or e, has a low chainenergy. The conformation is:

    (TR)T(TL)T(TR)T(TL)T(TR) etc.Natta and Corradini conclude that crystallites are essentially builtup from the above chain segments.

    For the isotactic parts of the chain two possible conformationswere found in the pr es ent work. The first one is the well-known helixconformation. This chain conformation has no steric hindrance;co nsequently, the chain energy is low. The conformatio n is as follows:

    (TR)G,(TR)G^(TR)G,(TR)G-(TR) eta. or the mirr or image:(TL)G^(TL)G^(TL)G^(TL)G^(TL) etc.

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    The second po ss ible is otacti c c onformation is a straight chain composedof two different pair conformations. One of these, (TR)G^(TR), is thesame as was used for the h elix c onfo rmatio n. The o th er , (TR)G (TR), hassome steric hindrance; as a consequence, the chain energy of the isotactic straight c hain is higher than that of the i sotactic he lix. Thisstraight chain conformation is represented by:

    (TR)G^(TR)G (TR)G-(TR)G (TR) eta. or the mirro r image:(TL)G^(TL)G,(TL)G (TL)G7(TL) eta.Figure 5.3. shows pho to graph s of the th ree ch ains. The pi ctures

    indicate that the syndiotactic chain and the isotactic straight chain

    A

    B

    Figure 5.3. Photographs of three different chain conformations.A syndiotactic straight chainB isotactic straight chainC isotactic helix

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    cloSely rese mble each ot he r. The chlori ne-to-chlorine distance alongthe ch ain is almost the same in these two c ases (5.1 A for the s yndiotactic, 5.0 A for the isotactic chain),which is very important forthe for matio n of crystalline st ructures . A second advantage of theisotactic straight chain over the isotactic helix is the bet ter spatialfilling of the former: a closer molecular packing of syndiotactic andsuch straight isotactic parts of the chains is possi ble .

    The disadvantage of the proposed isotactic chain conformation isits higher chain energy because a pair with some steric hi ndrance isused in constructing i t. To est ablish wheth er the chain energy issufficie ntly low for the isot actic straight ch ain to be a pr obablestructure, its chain energy has been calculated and compared with theenergy of the is otactic he lix. This is reported in the next sect ion.5.4. Calculation o fchain energy

    The internal energy of a polymer material is c omposed o f two part s:intermolecular and intramolecular energy.

    The intermolecular energy depends on the packing and alignment ofthe ch ains. No atte mpt was made to calculate this intermolecularenergy. However , one can make this qualit ative statement: whe n straightchains are closely packed the intermolecular energy decre ases. In fact ,this is the very r eason why c rystallizatio n o cc urs . Thus it can beconcluded that -from an intermolecular point of view- straight chainconformations are energetically favourable.

    The intramolecular energy of the c hain is determined by its c onformation.Since this confor matio n can be described (see above) it is nowpossible to calculate the intramolecular energy. The calculations were24)perf ormed acco rding to a meth od used for PVC by Fordham whic h wasimproved later o n by Glazkovskii and Papulov . In this meth od thecontributio n to the intramolecular energy is divided into th e electr ostatic energy and the steri c energy. The ster ic energy is the sum ofall Van der Waals attractions and repulsions of atoms not joined byvalence bonds. The electr ost atic energy was calculated from the dip ole-dipole interaction of the CCl-dipo les.

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    61The calculation requires the knowledge of all i nteratomic dist ance s.

    To this end, the spatial arrangement has to be translated into acoordination scheme of all the atoms of the chain segment underconsideration. For the calculation of the chain energies of the threeimportant conformations one must know the energies of three pairs ,viz. the syndiotactic pair confo rmation (TR)T(TL) and the iso tacticpair co nfor mati ons, (TR)G^(TR) and (TR)G (TR). The syndiotactic straightchain is a combination of the pairs (TR)T(TL) and (TL)T(TR) whic h bothhave the same energy. The isotactic helix is composed of the (TR)Gy(TR)or (TL)G (TL) pai rs ; again, these pairs have the s ame energy. The isotactic straight chain consists of the (TR)G-,(TR) an d (TR)G (TR) pai rs(or their mirror images).These pairs have different energies and thechain energy is the average of the two pair energie s.

    The calculation of the pair energy proce eds as follows:1. Coordinate axes are chose n and the coor dinates of the two chlorine

    atoms,four carbon atoms and six hydrogen atoms are established.2. All interatomic distances are calculated.3. The dipole-dipole interaction is determined.4. The steric energy is calculated.5. The total intramolecular energy is obtained by summation of the

    electrostatic and the steric energy.6. In one c ase rot ation around a C-C bond is introduced to e liminate

    high steric hindrance . The calculation is then repeated until theminimum pair energy is fo und.

    5.4.1. Coordinate systemRectangular space coor dinates were used. However, in the PVC ch ain

    all bond angles are equal to the valence bond of tet rahedral c arbon(4> = 10929'). Especially whe n r otations in the carbon s keleton areintroduced 'tetrahedral axes'are useful since the coordinates thenare very simple and the rotations around C-C bonds are easily described.

    The distances between the atoms are c alculated wit h the aid of the

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    62cosi ne rule. If coo rdinates X, Y and Z of two atoms diffe r by AX, AYand A Z , re spe ct ively, the dist ance between them is found fro m:

    d^ = AX^ + A Y ^ + A Z ^ + 2cos(()(AXAY + AXAZ + AY AZ) (5.1)Wi th re ctangular axes coscfi = 0, for the te trahedral co ordinatesCOS(j) = - - .

    5.4.2. Electrostatic energyThe electrostatic energy is calculated from dipole-dipole inter

    action. The dipole moment of ethylchloride (2.05 D) is assumed to beconcentrated in the C-Cl dipole. Since all interatomic distances areknown (see previous se ct io n) it is co nvenient to per for m the calculation of the electrostatic factor with a Coulomb-type equation. Thedipole moment y or iginate s from two elect ric charges (+ q) and (- q)located in the carbon and ch lori ne nuclei , res pec tively. Wit h theknown values of y and d . we calculate:

    = 1.16 X 10 ' e.s.u. (5.2)

    The electrostatic energy is then determined with the equation. =q2 J _ ^ _ ^ 1_e l r r r rcc ""cici CCl CCl

    whereq = electric charger = dist ance between the two carbon nuclei of the inter

    act ing CCl-dipolesr .. = distance between the two chlorine nuclei andCCl, and r = dist ance between carbon atom and chlorine atom not

    1 t^Li1 ry connecte d to e ach oth er by a valence bond.2When the factor q is expressed inkcal.A/mole unit s, its value is19.5.

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    63

    5.4.3. Steric energyA set of equati ons assembled by Glazkovs kii and Pap ulov was5)

    ap pl i ed f o r t h e calculat i on o f t h e s t e r i c en e rgy. Th ey u s ed Van de rjotentic68,69)Waals p o t en t ials publi s h ed by Hill. , Cro well , and Me e s o n andKr e e v oy""'"'' f o r t h e in t e r a c t i o n o f non-bonded a t o m s. T h e p o t e n t ials

    we r e m ea su r ed w i t h in e r t gas e s having almo s t t h e same a t om i c s iz e a st h e hydr og en, ca rb on and c hl o r in e a t om s, viz. h e lium, argo n andkryp t o n, r e s p e c t i v ely. Th e in t e ra c t i o n p o t e n t ials all have t h e f o rm:

    E = K est K'r (5.4)W h e r eE = st er ic ene rgy (kcal/mole)r = d i s t anc e b e tw e e n t h e two nucle ia,K,K' = co nst ants (see table 5.2.).

    Table 5.2. Constants in th

    Int e ra c t ingat om s

    H...HC...CCI...CI

    C...HC...C1H...C1

    K X 10 -kcal/mole

    3540220.8

    37.49488

    34 Th e c on s t ant s f o r t h e pcalculated on the basis

    2 i n t e r a c t i o n p o t e n t i a l

    K'kcal'A /mole

    183501430

    79707161

    aA-'

    5.73.63.621

    4.653.64.65

    o t en t ials b e tw e en d i f f e iof the combination rule

    equation (5.4)

    R e f e r e n c e

    Hill6^>Crowell^^)Me e s on and68,69)Kreevoy 'KjfX

    ent atoms were.65) _

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    64

    "'3 ^'i 'g C_l2\ / \ /

    / \ / \H, Hj H^ Hg

    Figure 5.4. Numbering of the atoms with in a pair.

    Table 5.3. Survey ofunits.

    Interactingatoms

    H...H

    ,3H,---4H,...H3H,...Hg2"-3H2---H42---5^ \3---4H3...H3"3---"64---65---6

    the atomic interactions

    C...C

    C,...C^

    CI...CI

    ci,...ci2

    c,.

    s-s-s-s-s-s-^4-^4-^4-

    in a

    .H

    4.,H36612..H,2..H3

    pair

    C .

    S-S-^4-

    of monomer 1

    .Cl

    ..ci,"2..Cl,

    H...C1

    H,...C1,H,...Cl2H^...C1,H^...Cl23-"2H^...C1,H^...Cl2H3...CI,H3...CI2H^...C1,

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    65Figure 5.4. shows a chain segment consisting of two monomer units.

    The atoms of thi s pair are numbere d C until C,, H until H,, Cl andCl. The total steri c energy of this pair is found by summation of thepot entials of 37 inte ract ions, listed in table 5.3. Only the inte ractions that depend on the conformation are taken into c onsideration.For example, the dist ance between the atoms C and C does not ch angeupon rotation around a carbon-to-carbon bond and, consequently, theC C, potential is constant. However, the distance between the atomsH and H changes upon rotation around the C. C_ bond. The re fo re ,this H H interaction depends on conformation.5.5. Energies5.5.1. Pair energies

    As we have seen previously three pairs play a role in the occurrenceof straight chain conformatio ns. These pairs are :(TR)T(TL) or (TL)T(TR) , syndiot actic(TR)G^(TR) or (TL)G (TL), is otacti c and(TR)G (TR) or (TL)G^(TL),isotactic.The energies of above pairs were calculated with the aid of an IBM 360/65computer. The res ults are listed in table 5.4.

    Table 5.4. shows a high energy for pair (TR)G (TR). Ac co rding tothis res ult, the conformation of the str aight isotactic chain wouldseem to be unfavourable since this chain conformation contains the

    Table 5.4. Energies of pairs of monomer unit s.(All energies in kcal/mole)

    Pair(TR)T(TL)(TR)G7(TR)(TR)G (TR)

    ^el0.120.780.12

    ^^st- 0.68- 1.0132.25

    \ o t- 0.56- 0.2332.37

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    66

    high e nergy pai r. The detailed informatio n from the co mputer showedthat the hi gh energy for pair (TR)G (TR) is caused by the st eri chindrance between one of the hydrogen atoms and one of the chlorineatoms, viz. H Cl. This was confirmed by the Courteauld At omicModels.Howe ver, it was also found t hat a cloc kwis e rot ation aroundthe C-C bond of the second monomer unit causes the steric energy tofall off rapidly. This rotation was introduced in the pair (TR)G (TR)rand its energy was recalculated in the following manner.

    When deviations from the staggered po siti on occur the potentialenergy due to internal rot ation must be taken into account. This,- ^ - 1 ,- .- J 1. .- 70,71)potential energy is estimated by a functionEE^^j.= (1 - cos 3 Y ) (5.5)

    where E = pot ential barri er , kcal/mole andoY = deviation from the staggered po si ti on.

    6 -

    ai i.z

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    2.Steric interactio ns over long distances are neglect ed. The mostimportant of these is the interacti on C C . However, in thestraight chain conformations considered here the error is quitesmall (correction 2 in table 5.6.).

    Table 5.6. Chain energies(All energy c ontri buti ons in kcal/mole )

    Chain conformatio n

    syndiotactic,straight chainisotactic,helixisotactic,straight chain

    Chain conformation

    syndiotactic,straight chainisotactic,helixisotactic,straight chain

    L

    Pair conformations

    (TR)T(TL)(TL)T(TR)(TR)G (TR)(TR)Gj(TR)(TR)Gj(TR)(TR)G (TR)

    Aver age value

    - 0.56

    - 0.23

    0.8

    Pair energies

    - 0.56- 0.56- 0.23- 0.23- 0.23

    1.8

    corrections1

    0.28

    0.28

    0.25

    2- 0,1

    - 0,1

    - 0,1

    30.40

    0.40

    0.35

    Average value

    - 0,56

    - 0,23

    0,8

    0

    0,3

    1,3

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    H3 C l , He CI2 H g CI3

    C l C3 C sH^ Hg H^ Hg Hy H 8

    Figure 5.6 . Chain segment co ns ist in g of two overla pping p a ir s .

    3 . D i p o le i n t e r a c t i o n s o v e r l o n g d i s t a n c e s a r e n e g l e c t e d t o o , e . g . t h ei n t e r a c t i o n b e tw e e n d i p o l e s C C l a nd C C l . The e l e c t r o s t a t i c2 1 6 3energies are calculated in order to compensate for this effect. Itis found that the corrections have almost the same value for thethree chain conformatio ns (corre ction 3 in table 5.6.).

    Table 5.6. summarizes the final results of the calculation of thechain energies after correctio n for the three sources of erro r. Thedifference in the c hain energies of the isotactic he lix and the isotactic straight chain is but small (1.0kcal/mole).Therefore it isvery likely that -below the glass transition temperature- a largefraction of the isotactic material has assumed the straight chainconformation.

    The chain structure can be investigated experimentally by nuclearmagneti c res onance and infrared measurements . A survey of the lite raturereports on this subject is given below. It is of interest to learn fromthese experimental results whether the isotactic straight chain conformation indeed exists and to what extent it oc curs in PVC.5.6. Chain structure analysis5.6.1.NMRanalysi s

    The chain confor matio n can be s tudied o n the basis of the observedcoupling constants between the meth ylene and the meth ine pr ot ons. Wit h

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    70liquid model compounds like the stereoisomers of 2,4-dichloropentane(DCP) ' ' and 2,4,6-tr ic hloro he pt ane (TCH) ' ' it was found that thetrans conformation is the stable form of the syndiotactic compoundsof DCP and TCH whe reas the isotactic compounds have the he lix confor mation.

    These res ults indicate that the s table, low-energy co nformatio nwill be all-trans or helix-like for syndiotactic and isotactic PVC,res pec tively. This applies to PVC in s olutio n. However , as was mentio ned in the previous section, other conformations may also occur in thebulk. Unfortunately, the conformation of the chains in solid PVC samplescannot be studied with NMR because solid samples give broad spectralbands from which no information about the chain st ructure can bederived.

    5.6.2. IRanalysi s5.6.2.1. CCl-stretching vibrations

    Almost all the liter ature repo rts in which information is given onthe chain structure deal with the region in the infrared spectrumwhere the carbon-to-chlorine stre tching vibratio ns abso rb. This partof the s pec trum is shown in figure 5.7. whic h was taken from the wor k73)of Krimm . The s pec trum consist s of thre e bands located at 615, 635and 690 cm , re sp ec ti vely. The bands at 615 and 635 cm st ronglyoverlap. Moreover, the three bands are all complex since each bandconsists of at least two different absorption frequencies.The absorption frequency depends on the spatial arrangement of theatoms around the carbon-to-chlorine bond. Wi th the aid of the infor mation from the s pect ra of low molecular weight model co mpounds it ispos sible to assign the absorpt ion bands to diffe rent 'local' co nformations around the CCl-bond. The se local arrangements occur in diffe re ntchain conformatio ns and, he nce , the sp ectr um can be interpret ed interms o f chain confor mati ons. Ther ef or e, a qualitative impressio n canbe obtained co ncerning the o ccurrence of the vario us chain confor mations in bulk PVC.

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    7 0 0 6 0 0 5 0 0FREQUENCY, CM

    Figure 5.7. Infrared spe ctrum of PVC obtained by polymerization at 50 C, KBr p ellet731(According to Krimm ).

    Xc c^ \ ^ V

    Figure 5.8. Nomenclature of CCl-stretching vibrations.

    5.6.2.2. NomenclatureTh e n om en clatur e o f t h e f r equen c i e s o f t h e CCl-s t r e t c h ing v ib ra t i on s

    is base d o n the geo met ri cal arr angeme nt around the CCl-bond. The symbolS is used bec ause the c hlor ine ato m is att ached to a se co ndary carbonatom, and two subscripts are added for the two atoms in trans positionr elat iv e t o t h e c hl o r in e a t om.

    Figur e 5.8. s h ow s t h e c on f o rmat i on o f t h e c ha in in t h e n e ig hb ou r h o odo f on e c h l o r i n e a t om. Th e two t rans subs t i tu en t s a r e X and Y , r e s p e c tiv ely; h e n c e , t h e IR abs o r p t i o n o f t h i s c o n f o rmat i on i s called S .

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    72The substi tuents X and Y can be carbon or h ydro gen. Wh en the carbonbackbone is planar there are two hydrogen atoms in trans position andthe conformati on is then indicated by S,,. Ther e is anoth er co nfor ma-Hntion in which two hydrogen atoms are in trans position but here thecarbon chain is not p lanar. This c onfor mati on is indicated by S ,.HHThirdly, an arrangement exists with one carbon and one hydrogen atomin the trans po si ti on, S . These three co nformatio ns are sho wn inOnfigure 5.9. On paper it is pos sible to construct t