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The effect of heat on some of the physico-mechanical properties of polyphenylene sulphide
rl494
V . A . SERGEYEV et al.
16. G. ADAMS and J. H. GIBBS, J. Chem. Phys. 48: I39, 1965 17. V. N. KULEZNEV, V kn.: Mnogokomponentnye polimernye sistemy, pod red. R. F. Gol'da (In the book: Multi-component Polymeric Systems, ed. by R. F. Gold). "Khlmlya" 10, 1974 18. I. I. PEREPECHENKO, Akusticheskie metody issledovaniya polimerov (Acoustic Study Methods of Polymers). "Khimiya", 187, 1973 19. K. HOASHI and R. D. ANDREWS, J. Polymer Sci. C38: 387, 1972 20. Yu. S. LIPATOV, Yu. Yu. KERCHA and L. M. SERGEYEVA, Struktura i svolstva polluretanov (The Structure and Properties of Polyurethanes). "l~aukova Dumka", 170, 1970 21. G. A. GORDON, J. Polymer Sci. 9, A-2: 1693, 1971 22. L. PIMENTEL and A. McCLELLAN, Vodorodnaya svyaz' (The Hydrogen Bond). "Mir", 65, 1964 23. R. C. WIHOIT and lVL DOLE, J. Phys. Chem. 57: 14, 1953 ~24. N. SAITO, K. OKANO, S. IWAYANAGI and T. HIDESHIMA, Solid State Physics, ed. by F. Seitz and D. Turnbull, N.Y., vol. 14, 387, 1963 25. M. P. LETUNOVSKII, Ye. V. MINKIN and Yu. V. ZELENEV, Vysokomol. soyed. A15: 1936, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 9, 2180, 1973)
THE EFFECT OF HEAT ON SOME OF THE PHYSICO-MECHANICAL PROPERTIES OF POLYPHENYLENE SULPHIDE* V. A. SERGEYEV, V. K . SHITXKOV,V. I. NEDEL'KII~, A. A. ASKADSKII, K . A. BYCKKO, G. L. SLOlqIMSKII
and V. V. KoRs-A~ OrganometaUie Compounds Institute, U.S.S.R. Academy of Sciences
(Received 23 September 1976) The heat treatment of linear polyphenylene sulphide (PPS) in air at about 320°C yielded polymers of branched and partly crosslinked structure; this was confirmed by I R spectral data, X-ray structural analysis, and also the results of determining the heat of polymer crystal melting by scanning calorimetry. The physico-mechanical properties of the PPS were investigated as a function of the heat treament conditions and means were found to extend the range utilization of the PPS. W E were able t o show in earlier w o r k t h a t linear a n d b r a n c h e d p o l y p h e n y l e n e s u l p h i d e s (PPS) [1-] show h i g h t h e r m a l s t a b i l i t y a n d h e a t resistance [2]. T h e litera t u r e (chiefly a d v e r t i s e m e n t s a n d p a t e n t s ) claim t h a t t h e m e c h a n i c a l s t r e n g t h o f t h e P P S c a n b e i m p r o v e d b y h e a t t r e a t m e n t [3-5]. T h e c o n t r a d i c t i n g n a t u r e o f t h i s i n f o r m a t i o n does n o t m a k e it possible t o g e t a clear p i c t u r e o f t h e effect o f e l e v a t e d t e m p e r a t u r e s on t h e s t r u c t u r e a n d p r o p e r t i e s of t h e P P S . T h e a i m o f t h e * Vysokomol. soyed. A19: No. 6, 1298--1301, 1977.
Physico-mechanical properties of polyphenylene sulphide
1495
~vork r e p o r t e d here w a s t h e r e f o r e to e s t a b l i s h t h e o p t i m a l conditions o f such h e a t t r e a t m e n t a n d t o s t u d y t h e effects o f t h e l a t t e r on t h e p h y s i e o - m e c h a n i c a l p r o p ~rties. EXPERIMENTAL
The PPS melts were heat treated in air at 320°C for 1 to 300 hr. The resulting products were ground into powders and tabletted under pressure (to a 4 × 10 x 15 m m size); these were subjected to impact and flexing strength tests m a "Dinstat" instrument. The heat of melting of the crystalline phase was measured by means of a DSM-2 scanning calorimeter using 12.5°C/rain heating gradient. The range of utilization of the samples was determined m a device of the Perel'-Dubov type [6] by the usual method [7]. PPS oxidation to polyphenylene sulphone was carried out with a mixture of 30% aqueous hydrogen peroxide and 50% acetic acid in which the polymer was suspended; the method was the same as that used earlier on [8]. RESULTS
T h e results of t h e t h e r m o - m e c h a n i c a l t e s t i n g of t h e h e a t t r e a t e d P P S s a m p l e s a r e s h o w n in Fig. 1. One c a n see t h a t t h e softening t e m p e r a t u r e ~ises as f u n c t i o n o f t h e h e a t t r e a t m e n t t i m e . T r e a t m e n t for 30 h r caused t h e s a m p l e n o t t o s o f t e n u n t i l 410°C w a s r e a c h e d a n d t h a t t h e d e f o r m a t i o n o f t h e s a m p l e a t 500°C w a ~ o n l y a b o u t 16°/o .
o;,kgj ...,~ Z kg'c"/cmZ _ 5~~'-....... -
Deformution , %
I
I 0 0 " " 2 0 0 " ~ 3 ~ #00 500 600 T,°C Fro. 1
l
I
l
Time ,he FIe. 2
Fig. 1. The thermomechanieal compression curves for: 1--original PPS; 2--PPS heat treat~ l at 320°C for (hr): 2--12; 3--48; 4--300; 5--for polyphenylene sulphone produced by oxidizing PPS. FIG. 2. / - - T h e specific impact strength a, 2--the flexing strength y of PPS as functions of the duration of heat treatment at 320°C.
Figure 2. shows t h a t a n e x t e n s i o n of t h e h e a t t r e a t m e n t t i m e a t 320°C f r o m 6 ~o 48 hr i m p r o v e d t h e i m p a c t a n d b e n d i n g s t r e n g t h s f r o m 1 a n d 150 t o 4 a n d 650 l r ~ / c m 2 respectively, b u t a n y longer t i m e r e d u c e d t h e m . T h e e x p o n e n t i a l dep e n d e n c e of t h e m e c h a n i c a l p r o p e r t i e s o f P P S o n h e a t t r e a t m e n t t i m e c a n b e p r e l i m i n a r y e x p l a i n e d b y s t r u c t u r a l c h a n g e s w h i c h result in a t r a n s i t i o n f r o m a r i g i d chain, low m o l e c u l a r w e i g h t P P S o f low m e c h a n i c a l s t r e n g t h t o one o f larger m o l .
,1496
V.A.
SERGEYEV et al.
Wt. or partly crosslinkcd and flexible polymer of much improved mechanical strength. The formation of sparse crosslinkages between the macromolecules was also indicated b y earlier findings [2]. An extension of the heat treatment time seems initially to increase the transverse bonds density, after which the rate of secondary processes, such as ageing and decomposition accelerates, and these reduce the strength of the material. T~E D E P E N D E N C E S O F T H E :HEAT A N D T E M P E R A T U R E O F M E L T I N G ON THE DU~aATION OF THE :HEAT TI~EAT~rENT AT 320°C Time of heat treatment, hr
Heat of polymer melting, cal/g
The m.p. of the crystal phase, °C
0
5.12
6 24 96
3.70 0-52 0.00
276 267 240 No melting
The Table contains the results of the calorimetric determinations of the melt~ ing heat of the crystalline phase (These determinations were carried out b y I. I. Dubovik). The Table shows that prolongation of the heat treatment reduces the heat o f melting of the P P S crystalline phase. There was a simultaneous reduction of the melting temperature of the polymer. This can be explained b y larger defects in the crystal structures due to structuration which led to amorphization of the PPS. The I R spectral data (UR-20, K B r tablets) show that the prolongation of th~ heat treatment intensified the 860 cm -1 band absorption lines; these are deformation oscillations of the 1,2,4-trisubstituted benzene rings. There was a simultaneous reduction in intensity in the 820 cm -1 band which typifies the oscillations of the 1,4-disubstituted aromatic rings. The elevation of the softening temperature (Fig. 1), the reduction of the degree of orientation according to X-ray structural analysis [2] (which determined for the first time a transition from the originally crystalline to the completely amorphous polymer as a result of a heat treatment for 96 hr), the results of calorimetry and of I R spectroscopy, confirmed that structuration occurred when linear P P S is heat treated at 320°C for 48 hr. All these results thus point to branching occurring during the initial stages of heat treatment, and also to an increase in mol.wt, of the PPS, which all cause an improvement in the mechanical strength. One could have expected a degree of polymerization greater than that of the mechanical segment and fairly rare branches to give rise to highly elastic properties. The slight deformation changes of the P P S samples heated up to 96 Jar, which start around 100°C (Fig. 1, curves 2 and 3) are probably the first signs o f highly elastic properties. This assumption was confirmed in control experiments i.e. during the plotting of the thermo-mechanical curves of a sample heated t ~
Physieo-mechanical properties of polyphenylene sulphide
1497
170°C, and then exposed to additional stress, so that its deformation increased b y 15~oi the removal to the stress showed however that the deformation returned to its original value, which indicated a highly elastic type. The results of the thermomechanieal experiments (Fig. 1, curve 4) gave no indiaetion of a highly elastic state on a product subjected to a heat for 300 hr, and this was obviously due to an increase in crosslinking.
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:FIG. 3. The range of utilization of PPS heat treated at 320°C for (hr) a--6; b--24; c--300. d--the range of utilization of polyphenylene sulphone produced by oxidizing PPS. The range of utilization of P P S heat treated for 6 and 24 hr is reproduced in Fig. 3a, b, which shows it to have a temperature of about 100°C as its upper limit; this is associated with the appearance of high elasticity which greatly reduced the stresses in the polymer. The fact that the curves limiting the range of utilization of the sample treated for 6 hr approach 240°C (at a lower than 100 kgf/cm ~ stress application) can be explained b y some residual crystallinity of the sample. The determination of the range of utilization of crystalline samples gave secondary (dashed) curves which limit a very narrow and curving part in the range of high temperature and low stress. This progress of the relaxation curves is typical of rigid chain, crystalline polymers [9]; this t y p e of curve remains in the low temperature range in such eases, and the reason is the softening of the amorphous par~ of the polymer. A broadening of the range of utilization towards higher tempera-
1498
V . A . SE~aO~.YEVe$ al.
tures required a large increase in the crosslinking density and "suppresion" o f high elasticity b y this means, or alteration of the chemical composition of the system b y polymer-analogue reactions ~vhich would cause strong reactions between the chains due to the presence of polar groups in the main chain. Completely amorphous P P S not softening until well above 400°C were produced for this purpose b y long heat treatment (up to 300 hr) at 320°C, as indicated b y the thermomeehanical curves. The range of utilization of this sample is sho~n in Fig. 3c, where t h e curve limiting the range is strongly displaced into a higher temperature range when compared with the control sample (Fig. 35). As there is no oxidation of the P P S when heat treated in air at 320°C for a long time (based on elemental analysis), the elevation of the softening temperature over all the studied range of stresses can be explained only b y the formation of branched and partly crosslinked PPS. We had also shown in earlier work [8] that the heat resistance of P P S can b e improved b y oxidizing it with hydrogen peroxide to the respective polyphenylene sulphone. This raised its softening point up to its decomposition temperature (above 500°C). The results of the thermomechanical tests (Fig. 1, curve 5) show t h a t the range of the highly elastic state of the oxidized polymer disappeared. The transition from P P S to polyphenylene sulphone resulted, as expected, in a wider range of utilization (Fig. 3d) which is given b y approximately the same temperat u r e limits as applicable to the P P S heat treated at 320°C in air for 300 hr. H e a t resistant polymer systems based on P P S can thus be produced b y heat treatment at elevated temperature, which results in the formation of a frequent .transverse network, as well as b y polymer-analogue reactions, which will yield a polymer with strong intermolecular reactions. Our results could form the basis for the development of a P P S processing technology into various articles. Translated by K. A. _~T.T.I~,I.hT
REFERENCES
1. V. A. SERGEYEV, V. K. SHITIKOV, V. I. NEDEL'KIN and V. V. KORSHAK, Vysokomol. soyed. A17: 2420, 1975 (Translated in Polymer Scl. U.S.S.R. 17: 11, 2783, 1975) 2. V. A. SERGEYEV, V. K. SHITIKOV, V. I. NEDEL'KI~, N. V. BATENINA and V. V~ KORSHAK, Vysokomol. soyed. BI7: 710, 1975 (Not translated in Polymer Sci. U.S.S.I%.~ 3. U.S. Pat. 3793256, 1974; Chem. Ab. 81: 14277, 1974 4. Europlas~. Mon. 46: 77, 1973 5. Plastvarlden, No. 2, 44, 1973 6. A. A. ASKADSKII, l~iziko-khimiya poliarilatov (The Physical Chemistry of Polyarylates). "Khimiya", 1968 7. G. L. SLONIMSKII and A. A. ASKADSKII, Mekhanika Po]imerov, No. 1, 36, 1965 8. V. A. SERGEYEV, V. K. SHITIKOV, V. I. NEDEL'KIN and V. V. KORSHAK, Yysokotool. soyed. A18: 533, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 3, 609, 1976) 9. S. V. VINOGRADOVA, S. N. SALAZKIN, L. A. BERIDZE, A. N. MZHEL'SKII et al, Izw Akad. Nauk SSSR, Seriya khim., 931, 1969
V . A . SERGEYEV et al.
16. G. ADAMS and J. H. GIBBS, J. Chem. Phys. 48: I39, 1965 17. V. N. KULEZNEV, V kn.: Mnogokomponentnye polimernye sistemy, pod red. R. F. Gol'da (In the book: Multi-component Polymeric Systems, ed. by R. F. Gold). "Khlmlya" 10, 1974 18. I. I. PEREPECHENKO, Akusticheskie metody issledovaniya polimerov (Acoustic Study Methods of Polymers). "Khimiya", 187, 1973 19. K. HOASHI and R. D. ANDREWS, J. Polymer Sci. C38: 387, 1972 20. Yu. S. LIPATOV, Yu. Yu. KERCHA and L. M. SERGEYEVA, Struktura i svolstva polluretanov (The Structure and Properties of Polyurethanes). "l~aukova Dumka", 170, 1970 21. G. A. GORDON, J. Polymer Sci. 9, A-2: 1693, 1971 22. L. PIMENTEL and A. McCLELLAN, Vodorodnaya svyaz' (The Hydrogen Bond). "Mir", 65, 1964 23. R. C. WIHOIT and lVL DOLE, J. Phys. Chem. 57: 14, 1953 ~24. N. SAITO, K. OKANO, S. IWAYANAGI and T. HIDESHIMA, Solid State Physics, ed. by F. Seitz and D. Turnbull, N.Y., vol. 14, 387, 1963 25. M. P. LETUNOVSKII, Ye. V. MINKIN and Yu. V. ZELENEV, Vysokomol. soyed. A15: 1936, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 9, 2180, 1973)
THE EFFECT OF HEAT ON SOME OF THE PHYSICO-MECHANICAL PROPERTIES OF POLYPHENYLENE SULPHIDE* V. A. SERGEYEV, V. K . SHITXKOV,V. I. NEDEL'KII~, A. A. ASKADSKII, K . A. BYCKKO, G. L. SLOlqIMSKII
and V. V. KoRs-A~ OrganometaUie Compounds Institute, U.S.S.R. Academy of Sciences
(Received 23 September 1976) The heat treatment of linear polyphenylene sulphide (PPS) in air at about 320°C yielded polymers of branched and partly crosslinked structure; this was confirmed by I R spectral data, X-ray structural analysis, and also the results of determining the heat of polymer crystal melting by scanning calorimetry. The physico-mechanical properties of the PPS were investigated as a function of the heat treament conditions and means were found to extend the range utilization of the PPS. W E were able t o show in earlier w o r k t h a t linear a n d b r a n c h e d p o l y p h e n y l e n e s u l p h i d e s (PPS) [1-] show h i g h t h e r m a l s t a b i l i t y a n d h e a t resistance [2]. T h e litera t u r e (chiefly a d v e r t i s e m e n t s a n d p a t e n t s ) claim t h a t t h e m e c h a n i c a l s t r e n g t h o f t h e P P S c a n b e i m p r o v e d b y h e a t t r e a t m e n t [3-5]. T h e c o n t r a d i c t i n g n a t u r e o f t h i s i n f o r m a t i o n does n o t m a k e it possible t o g e t a clear p i c t u r e o f t h e effect o f e l e v a t e d t e m p e r a t u r e s on t h e s t r u c t u r e a n d p r o p e r t i e s of t h e P P S . T h e a i m o f t h e * Vysokomol. soyed. A19: No. 6, 1298--1301, 1977.
Physico-mechanical properties of polyphenylene sulphide
1495
~vork r e p o r t e d here w a s t h e r e f o r e to e s t a b l i s h t h e o p t i m a l conditions o f such h e a t t r e a t m e n t a n d t o s t u d y t h e effects o f t h e l a t t e r on t h e p h y s i e o - m e c h a n i c a l p r o p ~rties. EXPERIMENTAL
The PPS melts were heat treated in air at 320°C for 1 to 300 hr. The resulting products were ground into powders and tabletted under pressure (to a 4 × 10 x 15 m m size); these were subjected to impact and flexing strength tests m a "Dinstat" instrument. The heat of melting of the crystalline phase was measured by means of a DSM-2 scanning calorimeter using 12.5°C/rain heating gradient. The range of utilization of the samples was determined m a device of the Perel'-Dubov type [6] by the usual method [7]. PPS oxidation to polyphenylene sulphone was carried out with a mixture of 30% aqueous hydrogen peroxide and 50% acetic acid in which the polymer was suspended; the method was the same as that used earlier on [8]. RESULTS
T h e results of t h e t h e r m o - m e c h a n i c a l t e s t i n g of t h e h e a t t r e a t e d P P S s a m p l e s a r e s h o w n in Fig. 1. One c a n see t h a t t h e softening t e m p e r a t u r e ~ises as f u n c t i o n o f t h e h e a t t r e a t m e n t t i m e . T r e a t m e n t for 30 h r caused t h e s a m p l e n o t t o s o f t e n u n t i l 410°C w a s r e a c h e d a n d t h a t t h e d e f o r m a t i o n o f t h e s a m p l e a t 500°C w a ~ o n l y a b o u t 16°/o .
o;,kgj ...,~ Z kg'c"/cmZ _ 5~~'-....... -
Deformution , %
I
I 0 0 " " 2 0 0 " ~ 3 ~ #00 500 600 T,°C Fro. 1
l
I
l
Time ,he FIe. 2
Fig. 1. The thermomechanieal compression curves for: 1--original PPS; 2--PPS heat treat~ l at 320°C for (hr): 2--12; 3--48; 4--300; 5--for polyphenylene sulphone produced by oxidizing PPS. FIG. 2. / - - T h e specific impact strength a, 2--the flexing strength y of PPS as functions of the duration of heat treatment at 320°C.
Figure 2. shows t h a t a n e x t e n s i o n of t h e h e a t t r e a t m e n t t i m e a t 320°C f r o m 6 ~o 48 hr i m p r o v e d t h e i m p a c t a n d b e n d i n g s t r e n g t h s f r o m 1 a n d 150 t o 4 a n d 650 l r ~ / c m 2 respectively, b u t a n y longer t i m e r e d u c e d t h e m . T h e e x p o n e n t i a l dep e n d e n c e of t h e m e c h a n i c a l p r o p e r t i e s o f P P S o n h e a t t r e a t m e n t t i m e c a n b e p r e l i m i n a r y e x p l a i n e d b y s t r u c t u r a l c h a n g e s w h i c h result in a t r a n s i t i o n f r o m a r i g i d chain, low m o l e c u l a r w e i g h t P P S o f low m e c h a n i c a l s t r e n g t h t o one o f larger m o l .
,1496
V.A.
SERGEYEV et al.
Wt. or partly crosslinkcd and flexible polymer of much improved mechanical strength. The formation of sparse crosslinkages between the macromolecules was also indicated b y earlier findings [2]. An extension of the heat treatment time seems initially to increase the transverse bonds density, after which the rate of secondary processes, such as ageing and decomposition accelerates, and these reduce the strength of the material. T~E D E P E N D E N C E S O F T H E :HEAT A N D T E M P E R A T U R E O F M E L T I N G ON THE DU~aATION OF THE :HEAT TI~EAT~rENT AT 320°C Time of heat treatment, hr
Heat of polymer melting, cal/g
The m.p. of the crystal phase, °C
0
5.12
6 24 96
3.70 0-52 0.00
276 267 240 No melting
The Table contains the results of the calorimetric determinations of the melt~ ing heat of the crystalline phase (These determinations were carried out b y I. I. Dubovik). The Table shows that prolongation of the heat treatment reduces the heat o f melting of the P P S crystalline phase. There was a simultaneous reduction of the melting temperature of the polymer. This can be explained b y larger defects in the crystal structures due to structuration which led to amorphization of the PPS. The I R spectral data (UR-20, K B r tablets) show that the prolongation of th~ heat treatment intensified the 860 cm -1 band absorption lines; these are deformation oscillations of the 1,2,4-trisubstituted benzene rings. There was a simultaneous reduction in intensity in the 820 cm -1 band which typifies the oscillations of the 1,4-disubstituted aromatic rings. The elevation of the softening temperature (Fig. 1), the reduction of the degree of orientation according to X-ray structural analysis [2] (which determined for the first time a transition from the originally crystalline to the completely amorphous polymer as a result of a heat treatment for 96 hr), the results of calorimetry and of I R spectroscopy, confirmed that structuration occurred when linear P P S is heat treated at 320°C for 48 hr. All these results thus point to branching occurring during the initial stages of heat treatment, and also to an increase in mol.wt, of the PPS, which all cause an improvement in the mechanical strength. One could have expected a degree of polymerization greater than that of the mechanical segment and fairly rare branches to give rise to highly elastic properties. The slight deformation changes of the P P S samples heated up to 96 Jar, which start around 100°C (Fig. 1, curves 2 and 3) are probably the first signs o f highly elastic properties. This assumption was confirmed in control experiments i.e. during the plotting of the thermo-mechanical curves of a sample heated t ~
Physieo-mechanical properties of polyphenylene sulphide
1497
170°C, and then exposed to additional stress, so that its deformation increased b y 15~oi the removal to the stress showed however that the deformation returned to its original value, which indicated a highly elastic type. The results of the thermomechanieal experiments (Fig. 1, curve 4) gave no indiaetion of a highly elastic state on a product subjected to a heat for 300 hr, and this was obviously due to an increase in crosslinking.
kg/cruz
~,k @m2
000 -
/¢ "\
300 -
/
200-
//
100
jill
\\ /"
\
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--.
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\\
~ °c d
X
~oo
200
1
100
500
t_--%.\
I
ty, k,q/crne
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200 ; . i . /
//
200 7;,°C
300 ~/
100
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/
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//
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200 - / / / ' ~ , ~
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lot?
200
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:FIG. 3. The range of utilization of PPS heat treated at 320°C for (hr) a--6; b--24; c--300. d--the range of utilization of polyphenylene sulphone produced by oxidizing PPS. The range of utilization of P P S heat treated for 6 and 24 hr is reproduced in Fig. 3a, b, which shows it to have a temperature of about 100°C as its upper limit; this is associated with the appearance of high elasticity which greatly reduced the stresses in the polymer. The fact that the curves limiting the range of utilization of the sample treated for 6 hr approach 240°C (at a lower than 100 kgf/cm ~ stress application) can be explained b y some residual crystallinity of the sample. The determination of the range of utilization of crystalline samples gave secondary (dashed) curves which limit a very narrow and curving part in the range of high temperature and low stress. This progress of the relaxation curves is typical of rigid chain, crystalline polymers [9]; this t y p e of curve remains in the low temperature range in such eases, and the reason is the softening of the amorphous par~ of the polymer. A broadening of the range of utilization towards higher tempera-
1498
V . A . SE~aO~.YEVe$ al.
tures required a large increase in the crosslinking density and "suppresion" o f high elasticity b y this means, or alteration of the chemical composition of the system b y polymer-analogue reactions ~vhich would cause strong reactions between the chains due to the presence of polar groups in the main chain. Completely amorphous P P S not softening until well above 400°C were produced for this purpose b y long heat treatment (up to 300 hr) at 320°C, as indicated b y the thermomeehanical curves. The range of utilization of this sample is sho~n in Fig. 3c, where t h e curve limiting the range is strongly displaced into a higher temperature range when compared with the control sample (Fig. 35). As there is no oxidation of the P P S when heat treated in air at 320°C for a long time (based on elemental analysis), the elevation of the softening temperature over all the studied range of stresses can be explained only b y the formation of branched and partly crosslinked PPS. We had also shown in earlier work [8] that the heat resistance of P P S can b e improved b y oxidizing it with hydrogen peroxide to the respective polyphenylene sulphone. This raised its softening point up to its decomposition temperature (above 500°C). The results of the thermomechanical tests (Fig. 1, curve 5) show t h a t the range of the highly elastic state of the oxidized polymer disappeared. The transition from P P S to polyphenylene sulphone resulted, as expected, in a wider range of utilization (Fig. 3d) which is given b y approximately the same temperat u r e limits as applicable to the P P S heat treated at 320°C in air for 300 hr. H e a t resistant polymer systems based on P P S can thus be produced b y heat treatment at elevated temperature, which results in the formation of a frequent .transverse network, as well as b y polymer-analogue reactions, which will yield a polymer with strong intermolecular reactions. Our results could form the basis for the development of a P P S processing technology into various articles. Translated by K. A. _~T.T.I~,I.hT
REFERENCES
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