UDC 820. 172 : 678.674: 877. 521,001.5 E F F E C T OF W O R K I N G M E D I A A N D S T A T I C O R I E N T A T I O N OF P O L Y M E R F I L M S

LOADS

ON S P O N T A N E O U S

S. A. Kazakevich, P. V. Kozlov, and A. P. Pisarenko Fiziko-Khimieheskaya Mekhanika Materiatov, Vol. 4, No. 5, pp. 5~5-590, 1968 It was established that the action of certain liquid media on polymers leads to orientation-relaxation pro= cesses whose character varies with time, To determine the degree of this variation and to elucidate its causes, tests were carried out on polymer films simuitaneously subjected to the action of liquid media and static loads, Optical and X-ray examination of the structure of these specimens showed that additional orientation of polymer structural elements takes place in the first stage of the process, the effects of disorientation phenomena in the second stage being weakly manifested. Spontaneous orientation of structural elements of polymer films may take place under the influence of certain liquid media [1]. Experiment shows that the distribution of the orientation angles of structural elements of polymers under the influence of selected liquids varies in time and remains unchanged after the action of a given polymer has ceased. These phenomena are accompanied by a variation in the polymer film strength. Liquid media can penetrate into microvoids which are always present in polymers (including polymer films) [ 2 - 5], this being accompanied by changes in the surface and bulk energy of structural elements which facilitates relaxation processes [6]. Experiment shows, however, that the orientation-relaxation phenomena do not take place uniformly: orientation of weakly oriented elements predominates in the initial stages of the process, while disorientation of strongly oriented elements takes place in the later stages. If additional stresses are produced in a specimen due to the application of e x ternal loads, the rate of these processes may change. Thus, by accelerating processes of one kind and retarding the other it is possible to separate factors to which they owe their existence.

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a

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,

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Fig. 1. Time dependence of the ultimate tensile stress of polymer films: a) c e l l o phane; b) polystyrene; c) polyethyleneterephthalate; 1) under a static load in air; 2) under a static load in water; 3) unloaded specimens in water. The aim of this investigation was to verify the above views by studying the spontaneous orientation of polymer films under the influence of certain working media and applied loads. The experimental materials included crystalline and amorphous polymers such as polyethyleneterephthalate(PETP), polyethylene, (PE), polyvinylidenechloride(PVDC), polycarbonate (PC), polystyrene(PS), cellophane, etc. t h e e x perimental media and techniques were the same as in our previous work [1] except that the specimens exposed to the action of a given medium were simultaneously subjected to uniaxial tension in the elastic strain range ( i . e . , at a stress equal to 25% of the breaking stress of a given material). The device for loading the specimens in this way is comprised of a set of 1I -shape rigid stainless-steel frames, with two film-strip specimens folded in the form of a loop and mounted on each frame. The upper end of each loop is held by a wedge-shaped clamp (also made of stainless steel) and pulled (relative to the stationary frame) by a weight connected to the clamp through a pulley. Each set consisting of eight frames is placed in a vessel filled with an appropriate m e d i u m so that the specimens are fully immersed in the liquid. Three sets of frames are mounted on the front 429

part of the machine frame, the rear frame carrying the loading pulleys. In this way, 4~ specimens can be simultaneously tested on one m a c h i n e . The composition of working media and their concentration were held constan~ during each experiment [7]. The strains produced by applied loads were measured with a KM-6 cathetometer. As shown in Fig. l a , the variation in the tensile strength of cellophane is represented by curves with sharp m a x i m a . Increasing the time during which this polymer is under a combined influence of a static load and a working medium i n creases its strength from the i n i t i a l level of 12.5 k g / m m 2 to 26 k g / m m 2 after 1000 hr in water and to 19.8 k g / m m 2 after 1000 hr in air. However, during the subsequent 1000 hr under load, the strength of cellophane in water decreases, remaining practically constant in air (curves 2 and 1, respectively). Curve a in Fig. l a shows the variation in the tensile strength of cellophane due to the action of water alone ( i . e . , in the absence of external loads). It will be seen that the shape of curve 8 is similar to that of curve 2 except that the latter is placed above the former. It is evident that processes leading to an increase in strength are promoted by the i n fluence of both working media and external loads. Processes leading to a reduction in strength are promoted only by the working m e d i a , the effect of external loads,being negligible (the strength of cellophane held under load in air for 2000 hr remains p r a c t i c a l l y constant).

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Fig. 2. Time dependence of the birefringence An of polymer films: a) cellophane; b) polystyrene. 1) under the influence of static loads in air; 2) under the influence of static loads in water; 3) unloaded specimens in water. This is attributable to relatively strong hydrophilic properties of cellophane. Water has in this case a plasticizing effect, increasing the flexibility and mobility of molecules and structural elements and thereby facilitating spontaneous orientation and orientation under the influence of applied loads. Figure Ib shows that the tensile strength of PS acted on by water and an external load (curve 2) increases during the first 800 hr from 7 to 12 kg/mm2; it decreases to 10.5 k g / m m z during the next 1000 hr, after which it remains constant, lhe variation in strength of PS under the influence of air and an applied load is the same (curve I). Fhe strength of PS exposed to the action of water only (curve 8) increases with time to reach II k g / m m z and then decreases again to a constant level of i0 k g / m m 2 [I]. The relatively smaller changes in the strength of PS are evidently associated with a weaker plasticizing action of water on this polymer. Figure Ic shows the corresponding data for PETP whose strength under the combined influence of air and a static load increases during the first 800--400 hr from 20 to 29 k g / m m 2 (curve I), decreases to 21.8 k g / m m z during the next 800-800 hr and then remains constant. The strength of PETP held under load in water increases to 28.5 k g / m m 2 and then falls to a constant level of 21.8 k g / m m ~ (curve 2). The effect of time of exposure to the action of water only is represented by curve 8, which is similar in shape to curves i and 2 but placed above them. It appears that in this case the influence of the working m e d i u m plays the predominant part in promoting processes that lead to an increase in strength. Processes which lead to a reduction in strength after the first 800 hr of the combined action of the working m e d i u m and a static load are promoted to a large extent by the applied load though the m e d i u m also plays a certain part.

430

As shown in [1], such changes in polymer strength may be associated with orientation phenomena. Moreover, it is known that the action of a field of mechanical forces leads to the orientation of polymer structural elements [8], To elucidate the role of the above-described phenomena in changing the strength of polymer films, an examination of their structure was carried out. The results are described below. The orientation of polymer film specimens (including PS and cellophane) and the changes in orientation with time were studied by the birefringence method, k was found (Fig. 2a) that the birefringence An of cellophane exposed to a simultaneous action of water and a static load increases in the first 500-1000 hr from 12.5 x 10 "s to 16.7 X 10 "s, after which the increase in An is insignificant (curve 2). The simultaneous action of air and a static load increases An of cellophane in the first 500-1000 hr to 14.5 • 10 "3, after which An remains practically constant (curve 1). The variation in An of PS is similar (Fig. 2b, curves 1 and 2). If curves 1 and 2 in Fig. 2a, b are compared with curve 3 showing the change in An due re the action of water alone, it will be seen that An of both cellophane and PS increases in the first 500 hr and then decreases. Evidently, applied loads promote orientation, especiaily in the i n i t i a l stages of the process, and inhibit disorientation in the tater stages. Correlation of data on the t i m e dependence of strength and birefringence leads to a conclusion that the increase in strength in the initial stages of the action of working media and static loads is associated both with spontaneous orienration phenomena taking place under the influence of the m e d i a and with orientation due to the action of m e c h a n i c a l loads.

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Fig. 3. Time dependence of the relative number AN/N of variously oriented structural elements of PETP: a) under the influence of static loads in water; b) under the influence of static loads in air. Various curves relate to the f o I lowing angles A~o : 1) 0-15~ 2) 15-30~ 3) 30-45~ 4) 45-60~ 5) 6 0 - 7 5 ~ ; 6) 75-9O". On the other hand, the reduction in strength observed after 1000 hr cannot be attributed to disorientation phenomena since the degree of orientation of PS and cellophane after 1000 hr exposure remains p r a c t i c a l l y constant. Thus, the reduction in strength is associated with the influence of working media and static loads but is not necessarily due to disorientation phenomena. At the same t i m e , in the case of PETP we previously observed [1] a certain degree of disorientation of structural e l e m e n t s strongly oriented in the i n i t i a l state. A simiiar effect was observed on PETP specimens subjected to a s i m u l taneous action of working m e d i a and static loads. In this case X-ray diffraction data were used to compute the d e p e n dence of the distribution of orientation angles on the t i m e of exposure to the action of working m e d i a and static loads AN/N = j~t, ~); the results are reproduced in Fig. 3. It wilt be seen that in the first stage of the process the relative number A N / N of elements oriented in the range AO = 0 . 1 5 ~ is increased from 19 to 25% (Fig. an). and the proportion of elements oriented at A~0 = 15--30 ~ increases in a similar way; this increase is due to a reduction in the proportion of e l e m e n t s oriented at A~ = 4 5 - 9 0 ~ After 2000 hr this trend is reversed in that the proportion of elements oriented at 0 - 4 5 ~ decreases and that of elements oriented at 4 5 - 9 0 ~ increases; however, the relative number of e l e m e n t s oriented at 0 - 4 5 ~ remains larger than their 4al

original number even after 5000-6000 hr exposure to the action of working media and static loads. Similar disorientation effects were observed on PETP specimens exposed to the action of static loads atone(Fig. 3b); in this case, however, the disorientation phenomena are much slower due to the absence of the plasticizing action of the liquid medium. It will be seen that the rate of change in the orientation of structural elements of PETP in the initial exposure stages coincides with the rate of increase in strength. However, the disorientation processes are much slower than the decrease in strength after 800-500 hr exposure to the action of working media and static loads. This means that the decrease in strength of PETP is not due to disorientation phenomena. Experiments whose aim was to study the effect of applied loads on the strength, birefringence, and distribution of the orientation of polymer structural elements made it possible to separate the influence of orientation processes on polymer strength from the influence of other phenomena, It was found that, in spite of the increasing degree of orientation, the strength of polymer films after the initial stage (500-1000 hr) of working media (wi~ and without applied loads) decreases; during the initial stage the polymer strength increases to an extent which in many cases is increased by the application of external loads. It may be postulated that two competing processes are simultaneously taking part. One of them leads to an increase in strength due to the orientation phenomena (both spontaneous and induced); the other process, whose nature is still obscure, is not associated with disorientation of structural elements of polymer materials although it also leads to a certain reduction in strength at a rate which increases under the influence of working media and applied loads. This is clearly illustrated by the fact that the birefringence of PS and cellophane decreases with time and by the variation in the relative orientation of structural elements of PETP (as revealed by X-ray diffraction measurements). However, it is known that orientation phenomena (especially in hard polymers) are accompanied by a reduction in the packing density of structural elements of polymers [9, i 0 ] . It may be postulated that with the passage of time (especially under the influence of working media and applied load~) the number of microvoids wils increase even when the degree of orientation remains constant. This is the only plausible explanation of the reduction of polymer strength with time, although it needs to be verified by experiment. CONCLUSIONS 1. Fhe strength of polymer films under the influence of working media is substantially increased during the first 500-1000 hr and reduced in the later stages. 2. The increase in strength is associated with orientation phenomena. The subsequent reduction in strength is intensified under the influence of liquid media and applied loads. This effect is attributable to the formation and growth of cracks and microvoids and to a general increase in the heterogeneity of the system. REFERENCES S . A . Kazakevich, P. V. Kozlov, and A. P. Pisarenko, FKhMM [Soviet Materials Science], no. 3, 1968. D. Heikens, P. H. Hermans, and A. Weidinger, J. Polymer. Sci. 86, 145, 1959. P. V. Kozlov and B. N. Korostylev, Trudy VKhO ira. Mendeleeva, no. 3, 57. 1955. D. G. Grand, Amer. Chem. Soc. Potymer. Propert. 6, no. 1, 116-120, 1956. V. A. Kargin and T. V. Gatovskaya, ZhPKh, 29, 889, 1955. A. I. Soshko and A. N. Tynnyi, FKhMM [Soviet Materials Science], no. 5, 512, 1967. B. Dolezhel, Corrosion of Plastics and Resins [in Russian], Izd. Moscow, 175, 1964. 8. P. V. Kozlov and G. I. Braginskii, Chemistry and Technology of Polymer Films [in Russian], Izd. Iskusstvo, MOSC0W, 140-170, 1968. 9. James Balzer, Chemistry and Technology of Polymers [Russian translation], tzd. Mir, no. 7, 87-100, 1966. 10. Kh. U. Usmanov and V. A. Kargin, Proceedings of the 7-th Conference on High-Molecular Compounds [in lJ 2. 3. 4. 5. 6.

Russian], Izd. A N SSSR, 169, 1952. 18 December 1967

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Moscow State University