Phase and relaxation states of PVC
5. T. MASUDA, N. SASAKI and T. HIGASHIMURA, Macromolecules 8: 717, 1975 6. A. A. BYERLIN and M. I. CHERKASHIN, Vysokomol. soyed. A13: 2208, 1971 (T1anslated in Polymer Sci. U.S.S.R. 13: 10, 2478, 1971) 7. L. A. GRIBOV and V. A. DEMENT'EV, Metody i algo~itmy vychislenii v teorii kolebatel'nykh spektrov molekul (Methods and Algorithms of Calculations in the Theory of Vibratory Spectia of Molecules). Moscow, 1981 8. D. RAKOVIC, S. A. STEPANYAN, L. A. GRIBOV and Yu. N. PANCHENKO, J. Molec. Struct. 90: 363, 1982 9. L. A. GRIBOV and V. A. DEMENT'EV, Tablitsy parametrov dlya rascheta kolebatel'nykh spektrov molekut (Tables of Parameters for Calculating the Vibratory Spectra of Molecules). Issue 1, Moscow, 1979 10. L. A. GRIBOV, Teoriya infiakrasnykh spektrov polimerov (Theory of Infrared Spectra of of Polymers). Moscow, 1977
Polymer Science U.S.S.R. Vol, 31, No. 4, pp. 833-841, 1989 Printed in Poland
0032-3950/89 $10.00 + .00 1990 Pergamon Press pie
PHASE AND RELAXATIONAL STATES OF POLYVINYL CHLORIDE * A.. YE. CHALYKH, I. N. SAPOZHNIKOVAand N. P. BYESSONOVA Institute of Physical Chemistry, U.S.S.R. Academy of Sciences
(Received 25 September 1987) Dynamic calorimetry has yielded the thermograms of commercial PVC S-70 as a function of the prehistory of the samples. From the heat characteristics the authors isolate three stable states of PVC malked by constant parameters of melting and vitrification. Comparison of the PVC structure from the results of DSC and IR spectroscopy shows that the structural states of PVC isolated constitute three stable isomeric forms corresponding to the different conformational organization of PVC with complete identity of the configurational structure.
MANY properties of PVC are still unclear despite a large number of publications. This is particularly true of the phase structure of mixtures of PVC with other components. It may be assumed that the contradictory nature of the findings on the compatibility of PVC with solvents, plasticizers or polymers is primarily linked with the multiplicity of the physicochemical properties of PVC itself characterized by a complex hierarchy of structures at levels from molecular to phase [1-3]. The initial structure of PVC readily changes on processing and annealing [2-5] and, therefore, it is important to establish the directivity of these changes at all structural levels. Recently, we have investigated the patterns of the phase and conformational transitions in PVC on processing through * Vysokomol. soyed. A31: No. 4, 756-762, 1989.
A. YE. CHALYKHet al.
TEMPERATURE CHARACTERISTICS OF P V C *
Conditions of preparation
Types of endothermal peaks and melting characteristics "constant" "induced" T,,;,(ATrap Tmp(ATrap>>30) ~<30) o
To.. > T, T,.~ < T,
"d ..= PVC-I
I Pressing, p = 1 0
T= 323 Pressing, p = 7 MPa, T= 443 MPa, T=463 From solution in MEK, T=293 From solution in THF, T=293
* All t e m p e r a t u r e s g i v e n in K. Note: T h e symbols " - " and " + " denote the absence and presence o f a peak, respectively.
solutions in M E K and THF. It was found that on passing through the dissolution stage simultaneous with change in the confonnational composition of PVC the character of the phase equilibria PVC-solvent changes . The aim of the present work is to study the thermal properties of PVC as a function of its prehistory in order to determine the totality of complex phenomena with change in the structure and properties of the polymer in the course of processing and annealing. The test object was commercial PVC S-70 with M = l . 4 x 105, M w / M , = 2 and a content of syndiotactic groups, S= 55~/o. The designations of the samples and the conditions of their preparation are given in the Table. The thermograms were obtained with the Perkin Elmer DSC-2. The samples were subjected to cyclic thermal treatment "annealing-quenching" directly in the calorimeter followed by scanning at the rate 40 K/min. The annealing time was 20 min. The glass transition temperatures T. were determined from the point of inflexion in the heat capacity curve in the region of its anomalous change and the point of melting TMp or crystallization T~ from the maximum of the corresponding peak. Figures I - 4 give the thermograms of the samples studied. A notable feature is the variety of forms of the curves and the temperature parameters of the phase and relaxation transitions. Let us look at the properties of the non-processed sample PVC-I (Table)
Phase and relaxation states of PVC
in the cycle studied (Fig. 1). PVC-1 is characterized by two regions ofvittification - w i d e 300-350 K and narrow from 363-371 K (curve 1), the temperature of the onset of devitrification coinciding with T, of isotactic PVC . After quenching from 440 K the first
FIG. 1 Fro. 2 FIG. 1. Thermogtams of PVC-1 measured on cyclic thermal treatment: a-initial; 2-quenched from 440 K; 3 - Ta,, = 373 K; 4 - quenched; 5 - T~,n= 343 K; 6 - T,nn= 433 K; 7 - quenched sample. Here and in Figs. 2-4 aIrows denote T,. Explanations in text. FIo. 2. Thermograms of initial PVC-2 samples (1, 3) and samples quenched from 500 K (2, 4). vitrification degenerates and T~ becomes equal to 363 K (A Tg = 30 K) (curve 2). On heating above 440 K the thermogram (curve 3) shows an endopeak with a maximum at 450-470 K designated in the Figure by the symbol I. We would note that typical of PVC is the appearance of a multiplicity of endopeaks ,-~20 K higher than Ta,, if Tann>T, and also an endopeak on vitrification when Ta,, < T~. We call such peaks "induced". The appearance of endopeaks on vitrification is, as a rule, linked with the relaxation of the free volume. The nature of the endopeaks at T~,, > T~ remains contentious [7, 8]. The induced endopeaks are reproduced in the thermograms after annealing at 373 and 343 K (curves 3 and 5, the symbol H standing for induced). Unlike the induced, endopeak I is a constant characteristic of PVC: its positioo, in the thermogram is confirmed by investigations in individual cycles and coincides with the melting of crystallites of the folded type (structure I, ). After melting and quenching the heat characteristics of PVC change fundamentally. The heat capacity curve assumes a form typical of PVC : a wide region of melting in the interval ~380-490 K (symbol II) following the exothermal peak (373 K) always appearing immediately after vitrification (in the thermogram it is denoted by symbol II', curve 4). Prolonged annealing on melting (region II') does not lead to the appearance of peak I while peak II is readily reproduced even after quenching and heating the sample
A. YE. CHALYKHet aL
-~o. 3 FIG. 4 Fio. 3. Thermograms of PVC-3 measured after cyclicthermal treatment: a - quenched from 440 K; 2 - T,**= 373 K; 3 - quenched; 4 - T,**= 343 K; 5 - T,**= 433 K after quenching from 440 K; 6-T,**=433 K after quenchingfrom 510 K; 7-quenched. Fxo. 4. Thermograms of PVC-4 measured after cyclicthermal treatment: a - quenched from 400 K; 2-Tin--343 K; 3-quenched; 4-T,**= 433 K; 5-quenched.
at the rate 40 K/min (curves 4-7). The peak I' matching peak I in position in the thermogram appears only on prolonged high temperature annealing (curve 6). T8 of the sample quenched from the melt is 5-8 K lower than T= of PVC-1 (Table). It is significant that these values are close to those predicted from the formula 2'=(°(2) = 30 + 93S . The heat capacity curves for the pressed PVC samples (Table) have a specific character (Fig. 2). Peak I is not reproduced but a region of wide melting and a host of induced ertdopeaks appear. From the features of the curves 1 and 3 and also the 1.5-2-fold excess of the total area of the peaks of these curves over that for the peaks I or II it may be stated that on exposure to temperature and pressure, excess enthalpy builds up in the samples and this to a greater degree the higher p and Tused. After heating and quenching the thermograms of the pressed PVC-2 samples do not differ from the corresponding ones for PVC-1 (cf. curves 2 and 4 in Fig. 2 and curve 7 in Fig. 1) both in the form of the melting curve and the value of I"= (Table). From analysis of the thermograms in Figs. 1 and 2 it follows that characteristic of amorphocrystalline PVC are at least two melting points and consequently two types of crystalline structure differing in the rate of formation, temperature and the form of the melting peak~ the slowly crystallizing structure I with TMp= 460 K and the rapidly crystallizing structure II with a wide melting region 380-490 K. Evidently after melting and quenching a structural transition I ~ I I is observed with simultaneous change in 7"= of the system. In this connection, of special interest is the processing of PVC through solutions. The thermograms of the samples passing through the dissolution stage assume unex-
Phase and relaxation states of PVC
pected as well as typical features. It was found that Tg of PVC-3 and PVC-4 is 25-30 K lower than for PVC-1. It should be particularly emphasized that this phenomenon is not related to the content of residual solvent as was found in special investigations [5, 11]. As Fig. 3 shows Tg of PVC-3 first heated to 440 K is 329 K (curve 1). In conditions of repeated thermal treatment Ts ranges between 329 and 332 K (curves 2-5). Prolonged annealing at T---430 K raises Tg to 345-347 K (curves 6 and 7). However, even after quenching from 500 K it does not exceed 349 K which is 10-15 K lower than for the starting and quenched PVC. Another unexpected property of PVC-3 is the presence of an endothermal peak with a maximum at 400--405 K (in the thermogram it is denoted by the symbol I[I) and the absence of endopeak I (curves 1 and 5). The endopeak II[ is readily reproduced on repeat heating, unlike the induced disappearing a/'ter melting (curves 2-5). The appearance of the endopeak with Tmp'-~460 K (1') is noticeable only after high temperature annealing and preliminary heating of the sample not below 500 K (curves 5 and 6). Evidently on formation of PVC from MEK the structure I degenerates and the structure Iit (I~III) forms. The structure III like I is characterized by a narrow melting interval although it is readily restored after quenching as for structure II. The heat properties of PVC-4 (Fig. 4) are largely similar to those for PVC-3: low T~ values, absence of endopeak I and presence of peak III at 405 K (curves 1-3) and the possibility of passage from peak III to peak I through the melting stage (III--+II~I') curves 3 and 4. Nevertheless, when PVC is obtained from the good solvent THF the characteristic structure is less stable: already in the fourth heating cycle wide melting II appears in the thermogram and the intensity of III sharply diminishes (curve 3). After heating the sample to 470 K and quenching Tg becomes equal to 355 K (curves 4 and 5) like PVC-1 and PVC-2 (Table). From analysis of the experimental findings the enthalpic characteristics of PVC are summarized in the Table by the following classification: from the endopeaks detected we isolate the nominally constant and induced. Among the constant we find peaks I and III with relatively narrow melting range (ATmp~<30 K) and II with a wide one (ATmp>t30 K). Among the induced we distinguish those appearing at T,~ > T~ and also on vitrification if Tann< Ts (Table). Researchers relying in the main on the work  now do not draw any distinctions between endopeaks of types I and II and those induced at T,n, > T~ regarding them as the result of the melting of the crystallites characterized by a wide size distribution. Endopeak III had so far not been detected although in reference  according to X-ray structural analysis and IR dichroism the authors refere to the possible existence in PVC of a structure of two types, folded and micellar. The results taken as a whole allow one to speak not only of the specific nature of the induced endopeaks as in reference  but also of the differences between structures I and II or III more fundamental as compared with the dimensions of the crystallites. While endopeak I as mentioned corresponds to the melting of the crystallite structure of the folded type well known for PVC [9, 10] the structures II or III are different. IR dichroism used in reference  like the technique in reference  showed that the PVC
A. YE. CHALYKHet aL
samples studied have a crystalline structure of the micellar type. In fact, the high rate of formation of structures II and III unlike structure I must be due to fixation of the corresponding crystallites in the amorphocrystaUine system of PVC which is characteristic of the fringed-miceUar type of structure . From the thermograms presented and the Table it will be seen that the induced endopeaks appearing in the last capacity curves for T -~ Tann+20 K do not depend on the mode Of prepaling the samples and the type of crystalline structure. This points to the presence in PVC of a further structural level the nature of which is still unclear [7, 8].
AI •"2 o3
• - . ~
111 I 320
FIG. 6 FIO. 5 FIG. 5. Diagram of the changes in Ts after heating to Tn and quenching of the samples of PVC-1 (1-3); PVC-2 (4); PVC-3 (5, 6); PVC-4 (7, 8). 1, 3-5, 7-Heating rate 40 K/min; 2, 6, 8-anaealing for 20 rain; 1, 4-initial samples, 2, 3, 5, 8-samples first heated to 440 K. Explanations in text. Fro. 6. IR spectra of PVC-I (1), PVC-2 quenched from the melt (2), PVC-4 (3) and PVC-3 (4). Absorption bands, era-t: a-693; b-685; e-640; d-620; e-605; f-637; g-616. Let us look at the patterns of vitrification of PV.C. It is commonly considered that the influence of the conditions of processing on the thermal features of vitrification is governed by kinetic factors on reaching equilibria in the system. Figure 5 gives a generalized diagram characterizing the changes in Ts occurring on prolonged (20 rain annealing)
Phase and relaxation states of PVC
and fast (40 K/rain) thermal exposures. It will be seen that on rapid heating of the PVC-1, PVC-3 and PVC-4 samples the Tg values are constant to Tn=470-500 K. The scanning conditions chosen at the heating rate 40 K/min allow one to avoid the structural changes in PVC usually attendant on thermal treatment. Prolonged annealing leads to gradual fall in Tg in PVC-1 and rise in PVC-3 and PVC-4 when Tn>~430 K. The direction of the changes in Tg towards T~ characteristic of PVC-1 and PVC-2 samples quenched from the melt is indicated in the Figure by arrows (broken lines for annealing and continuous on heating at the rate 40 K/min). Thus, in the diagram one may distinguish three groups of Tg stable over a wide temperature region: I, 363-370; II, 350-358 and III 329-340 K and also an intermediate region at 430< Ta~480 K. In the irttermediate region the most marked are the relaxation processes due to possible changes on annealing in the degree of crystallinity, the density of the amorphous phase and the regularity of the physical network and to other factors . The Tg values for groups I-III are discrete and indicate abrupt change in the whole structure of PVC on passing from one type of sample to another. It is significant that each of the Tg groups isolated is peculiar to PVC with a constant melting characteristic (Table). Thus, from the results taken as a whole it follows that PVC S-70 may be present as a minimum in three stable structural states characterized by constant parameters of melting and vitrification: the first state I* is formed during synthesis, it is characterized by the highest T~ and Trap of a structure of the folded type. States II* and III* arise from the melt and the solution and are characterized by micellar morphology and lower Ts and Trap values. These states differ in the form of the melting curve of the structure and the T, values. Questions arise concerning the mechanism of the formation of the stable states of PVC and to which level of heterogeneity they correspond and what are the conditions of the transitions I* II*~III* In the state I* the PVC studied has the most ordered crystalline structure. Thus, ~, the degree of crystallinity determined from the equation ~=l-ACp/ACpam is equal to 0"95 in the state I* and is 2-2.5 times smaller in the states II* and III*. Earlier it had been shown that in the conformationally sensitive region of the IR spectrum of powdered PVC-I the bands 640 and 605 cm -1 are detected corresponding to the long fiat T T T T conformations of the syndiotactic chain sequence* (Fig. 6, curve 1). Evidently in the conditions of polymerization there is selection of the long trans-sequences of the chain ensuring orderliness maximum for the given PVC. It is significant that already in reference  the authors noted that the highest degree of crystallinity is displayed precisely by non-processed latex PVC particles. Melting and dissolution lead to degeneration of the cortformational set specific to PVC-1 (Fig. 6, curves 2-4). It will be seen that quenching of the fused samples freezes * Figure 6 is taken from references [5, 11].
A. YE. CHALYKH et al.
the PVC structure with a wide set of conformers peculiar to the melts which probably also determines the characteristic form of the wide melting peak II. From the solutions in MEK and THF, PVC macromolecules form with perturbed TTTG and TTTG' conformations of the syndiotactic chain sequence (637 cm-1). In reference  it was shown that the mechanism of formation of PVC from solutions is determined by the specific interaction between polymer and solvent as witness the appearance of the band at 637 cm-1. It is worth noting that complexing in solution ensures the regular sequence of perturbed trans-conformations of the chains, the length of which however, is insufficient for folding  which also leads to crystallization of PVC with a micellar morphology of peak Ill (like peak II but with a narrow melting interval). The conformational organization of PVC specific to the three states isolated also determines the character of vitrification. According to Gibbs and di Marcio  TBdepends on the energy of the intermolecular interaction ~n and the flexibility of the chain determined by the difference in the energies of the gauche and trans conformers Ae where 2~h+ As/kT~ =const (1) As a relative characteristic of A8 one may take change in the ratios of the gauche and trans conformers in the syndiotactic chain sequence on passing from PV(2-1 to PVC-2, PVC-3 and PVC-4. From the IR spectra we determine the parameter K=D637/ /D64o where D~37 is the optical density of the I k band at 637 cm -z in the samples PVC-2-PVC-4 and D64o is the optical density of the band at 640 cm- 1 in PVC-1. The function K=f(Aa) shows the degree of deviation of the long trans sequences of the chain from the lowest level. The greater K the smaller A8 and the lower must be Tg. In fact, the most perturbed conformational state (in relation to PVC-1) and the lowest Tg are peculiar to PVC-3: the K values amount to 1.55, 1.75 and 2.0 for PVC-2, PVC-4 and PVC-3 respectively. The magnitude K'/Tg (K'= l/K) is practically constant in agreement with equation (1). From the results it may be concluded that the states characteristic of PVC designated I*, II* and III* constitute three stable isomeric forms corresponding to the different conformational organization of PVC with complete identity of the configurational structure peculiar to PVC S-70. PVC in the states I*, II* and III* was designated respectively as p-, m- and s-isomers in accord with the conditions of their formation found on polymerization from the melt and solutions. Thus, the transitions singled out may be represented in the form of isomeric structures p-PVC m-PVC~S-PVC. Evidently the processes of the back transitions (broken arrows) like the forward established in the work (continuous arrows) will be determined by the conditions of stabilization of certain sequences of the trans rotatory chain conformations.
Translated by A. CROZY
Phase and relaxation states of PVC
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