Polymers for Fibers and Elastomers - American Chemical Society

22 Effects of Annealing Below T on the Properties of Poly(vinyl chloride) g

Downloaded by EAST CAROLINA UNIV on January 4, 2018 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch022

DEREK A. OTT and ECKHARD W. HELLMUTH Polymer Section, Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO

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The r e l a t i o n s h i p between observed enthalpy-volume relaxations and thermal treatment of s l i g h t l y oriented i n d u s t r i a l PVC films was investigated. D i f f e r e n t i a l scanning calorimetry at 20°C per minute and s p e c i f i c volume analysis (density gradient column) were used to study the e f f e c t s of annealing near and below Tg. Nonlinear e f f e c t s i n the volume relaxation at r e l a t i v e l y long times and temperatures close to the glass t r a n s i t i o n produce deviations i n the s p e c i f i c heat curves at temperatures far above Tg i n addition to the normal overshoot e f f e c t s .

For many years, nonlinear relaxation e f f e c t s have been observed i n thermally treated poly (vinyl chloride) (PVC), a common commercial p l a s t i c . The influence of annealing i n and below the glass t r a n s i t i o n temperature (Tg) on the physical and s t r u c t u r a l properties of PVC are well established and documented (1-8). Numerous i n v e s t i g a tions using d i f f e r e n t i a l scanning calorimetry (DSC) of PVC annealed beneath Tg have revealed a v a r i e t y of enthalpy relaxations occurring near or below Tg (1,3-6). Workers have also discovered density alterations which were induced by suitable heat treatment below Tg (1-2,6). Since the polymer i s generally considered amorphous, theories which o f f e r explanation of this phenomenon usually involve free volume changes and molecular c l u s t e r s . However, l i t t l e agreement exists concerning the exact mechanisms behind these e f f e c t s and the rate of their influence. This project was carried out as an i n v e s t i g a t i o n of the r e l a tionship between thermal prehistory or treatment of the sample and properties of poly v i n y l chloride. The rate of annealing on e f f e c t s such as enthalpy relaxations and density reduction was investigated. 0097-6156/84/0260-0345$06.00/0 © 1984 A m e r i c a n C h e m i c a l Society

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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POLYMERS FOR FIBERS AND ELASTOMERS

Experiments and Results Materials. Clear, unplasticized PVC was obtained from B.F. Goodrich Vynaloy V 453 as .35mm sheets. Density (determined by a density gradient column p r i o r to annealing) was 1.3342 g/ml (± .0006 g/ml). C NMR was used to determine the t a c t i c i t y of the material. The PVC used i s 46.4% heterotactic, 38.0% syndiotactic, and 15.6% i s o t a c t i c . Tg was approximately 85°C. 1 3

Annealing. Samples of PVC were annealed at 65, 70, 75, 80, and 85°C (±.3°C). Annealing times were based on a logarithmic progression (10°, 1 0 - , 10 , 1 0 - , 10 , 1 0 * , and 10 h r . ) . Thus, the given storage times were 1.00, 3.16, 10.0, 31.6, 100, 316, and 1000 hours.


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D i f f e r e n t i a l Scanning Calorimetry. Thermograms were recorded with a Perkin Elmer D i f f e r e n t i a l Scanning Calorimeter 1-b. A l l scanning rates were 20°C/min. The DSC results are i l l u s t r a t e d i n Figures 1-8. The reference l i n e which appears i n each figure i s used as the baseline f o r comparison, because i t s prehistory i s known (maintained at 150°C for 7 minutes and then cooled at the rate of 40°C/min through the glass t r a n s i t i o n temperature). Annealing at 65°C produced the c h a r a c t e r i s t i c endothermic peaks ("overshoot"). Annealing times ranging from 10° to 10 hours are i l l u s t r a t e d i n Figures la-b. This figure shows the increase i n peak height at a constant temperature. At longer annealing rates, the enthalpy peaks s h i f t e d to higher temperatures and the magnitude continued to increase as at shorter times. This behavior of the DSC curves at 70°C (Figure 2) i s s i m i l a r to those at 65°C. I n i t i a l increase i n the maxima i s followed by a s h i f t of the peak to a higher temperature which remained stationary between 31.6 and 1000 hours. At 75°C (Figure 3) the trend continued as at the lower temperatures, but a s i g n i f i c a n t change occurred at longer times. The minima following the peaks disappeared and the peaks s p l i t into two portions with a broad component at a higher temperature. Figures 4a-b shows s i m i l a r behavior for 80°C. At this temperature the s p l i t t i n g of the peak occurred at a shorter annealing time. The minimum disappeared as the second relaxation process began. The e f f e c t s of annealing at 85°C (Figure 5) seems much more complicated. The peak at the glass t r a n s i t i o n temperature decreased rapidly and the broad peak dominated behavior above Tg. This trend i s more e a s i l y seen i n Figures 6-8. S p e c i f i c heat curves of PVC annealed at 1, 31.6, and 1000 hours at the various annealing temperatures are i l l u s t r a t e d . In the 1 hour f i g u r e , the s p e c i f i c heat peak increased from 65-80°C. However, at 85°C the magnitude of the peak suddenly decreased and became much broader. For those samples annealed for 31.6 hours, the process appeared at a lower temperature (75°C). Once again, the peak increased then suddenly decreased and broadened. The 1000 hour graph i s dominated by the o v e r a l l change. A s l i g h t decrease i n temperature f o r the f i r s t peak and the Tg step i s followed by a pronounced maxima at higher temperatures. 3


22. OTT AND HELLMUTH

Annealing Below T on PVC

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Temperature (°C) F i g . 3 DSC curves of PVC annealed at 75°C f o r d i f f e r e n t times

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Temperature (°C) F i g . 4a and 4b DSC curves annealed at 80°C f o r d i f f e r e n t times Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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Temperature (°C) F i g . 5 DSC curves of PVC annealed at 85°C for d i f f e r e n t times

Temperature (°C) F i g . 7 DSC curves f o r PVC samples annealed f o r 31.6 hours at d i f f e r e n t temperatures Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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S p e c i f i c Volume. Sample densities were obtained at 25.0°C using a density gradient column of an aqueous solution of calcium c h l o r i d e . The measured s p e c i f i c volumes are shown i n Figure 9. Annealing decreased the volume of the samples. For each curve or annealing temperature, a c r u c i a l time period where the volume decreased at a d i f f e r e n t rate process seemed to e x i s t . The volume of untreated samples i s also included i n the f i g u r e . During annealing, molecules moved more often and more f r e e l y than at room temperature. After i n i t i a l expansion, the density increased, which has also been seen i n polystyrene by Ibar (9) and Kovacs (10). At higher temperatures (75-85°C) relaxations occurred i n at least two stages. The material seemed to relax to apparent constant values with a d d i t i o n a l density changes at longer annealing times. A change i n the nonequilibrium substructures of the material with d i f f e r e n t packing densities may influence the complex rate of these volume changes. A s i m p l i f i e d model might involve an a l t e r ation of the hole structure and the c l u s t e r i n g of the molecules with a smaller o v e r a l l free volume d i s t r i b u t e d into larger microscopic voids. The r e s u l t s of wide angle X-ray d i f f r a c t i o n confirmed that c r y s t a l l i n i t y did not change during annealing below Tg. Structural changes i n the microsystem of the glassy structure could not be observed by wide or small angle X-ray d i f f r a c t i o n a n a l y s i s . However, some alterations (not necessarily c r y s t a l l i n e changes) are suggested by preliminary positronium l i f e t i m e measurements. Results i n d i c a t e an increase i n the s i z e of i n d i v i d u a l microscopic voids between molecular c l u s t e r s , but an accompanying o v e r a l l decrease i n void volume. Discussion The normal explanation f o r s p e c i f i c heat deviation above Tg as e f f e c t of c r y s t a l l i z a t i o n or melting can not be used here. A possible explanation of the observed data could involve c l u s t e r formation. The reported s p e c i f i c heat and volume data indicate s t r u c t u r a l changes i n the glassy state with d i f f e r e n t m o b i l i t i e s nonequilibrium v i t r i f i c a t i o n process and altered by the following thermal treatment of the samples. This process, an a l t e r a t i o n of cooperative motions due to c l u s t e r formation, leads to a decrease i n mobility of the chains and prevents equilibrium enthalpies above Tg to be d i r e c t l y attained at the selected scanning rate. A comparison of the volume and enthalpy data substantiates t h i s hypothesis. The second step of volume reduction i s c l e a r l y related to the development of the s p e c i f i c heat peaks above Tg. The overshoots of enthalpy below the equilibrium value are moved from a lower temperature to a higher one i f the i n i t i a l volume i s decreased below a c r i t i c a l value. The clusters i n the rubbery material above Tg behave s i m i l a r l y to the time dependent process found i n or below the glass t r a n s i t i o n . The k i n e t i c e f f e c t s i n the breakdown of cooperative regions are rate and temperature dependent and can apparently exist between the glass and l i q u i d - l i q u i d t r a n s i t i o n as introduced by Boyer (11) years ago.


OTT AND HELLMUTH


22.

Annealing Below T on g

PVC

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Log Annealing Time F i g . 9 S p e c i f i c volume of PVC samples at 25°C f o r sample annealed at d i f f e r e n t temperatures as function of annealing time.


POLYMERS FOR FIBERS A N D E L A S T O M E R S

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The theories of the glass t r a n s i t i o n should account f o r observed e f f e c t s below and above the main glass t r a n s i t i o n . Annealing or other prehistory a f f e c t s sub-Tg, Tg and post-Tg anomalies i n s p e c i f i c heat and the expansion c o e f f i c i e n t s . Nonl i n e a r relaxation e f f e c t s i n the glass t r a n s i t i o n k i n e t i c s depend on d i f f e r e n t parameters i n models which can be used to describe the data. An extension of this theory must include the data reported i n this paper. The proposed clusters which can not be observed by X-rays are important for a l t e r a t i o n s of the ultimate properties of glassy materials.


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Illiers, K. H. Makromol. Chem. 1969, 127, 1. Gray, A.; Gilbert, M. Polym. 1976, 17,-44. Foltz, Carl R.; McKinney, Paul V. J. Appl. Polym. Sci. 1969, 13, 2235. Straff, R.; Uhlmann, D. R. J . Polym. Sci., Polym. Phys. Ed. 1976, 14, 1087. Berens, Alan R.; Hodges, Ian M. Macromolecules 1982, 15, 756. Philips, R.; Cox, R. L . ; Heiberger, C. A. Proc. 26th SPEANTEC Meeting New York. 1968, p. 216. Hyndman, John R. Polym. Eng. Sci. 1966, 6, 169. Matsuoka, S. Polym. Eng. Sci. 1974, 14, 162. Ibar, J . P. J . Macromol. Sci., Phys. Ed. 1982, B21, 481. Kovacs, A. J . Fortschr. Hochpolym. Forsch. 1963, 394, 3. Boyer, R. F.; Gillham, J . K. J . Macromol. Sci., Phys. Ed. 1977, B13, 494.

RECEIVED January 30, 1984


Polymers for Fibers and Elastomers - American Chemical Society

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