UDC 620. 172:678. 674:67% 521 001.5 E F F E C T OF C E R T A I N OF P O L Y M E R F I L M S

WORKING

MEDIA

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

ORIENTATION

S. A. Kazakevich, P. V. Kozlov, and A. P. Pisarenko Eiziko-Khimicheskaya Mekhanika Materialov, Vol. 4, No. 3, pp. 247-252, 1968 A study was carried out of the tensile strength of plane-oriented amorphous and crystalline polymer films previously acted on by water and aqueous solutions of salts and organic acids, The dependence of strength on the t i m e of exposure to the action of liquids was found to be extremal in character. X - r a y diffraction anaIysis and optical measurements established that the increase in strength is accompanied by an increase in the optical anisotropy of polymers and is associated with supplementary spontaneous orientation of a part of their structural d e m e n t s , The action of certain m e d i a m a y lead to spontaneons orientation of polymers [1, 2]. It was demonstrated on p o l y styrene and polyvinylchloride [3, 4] that the orientation of polymers produces a loosening of their structure, i, e , , a reduction in the packing density of their structural elements. Studies of the sorption of vapors by plane-oriented polymer films [6] proved that the orientation of polymers m a y lead to the formation of macrovoids or increase the initial void size. It.is evident that the influence exerted on polymers by m e d i a in which t h e s e p o l y m e r s are not soluble cannot be attributed only to surface phenomena. Liquid m e d i a can penetrate into microvoids filling them c o m p l e t e l y or partially [6]. The liquid adsorbed on the pore surface can change the m a g nitude of the surface energy [7] and so create conditions favorable for the parts of polymer chains adjacent to m i c r o pores to approach closer the state of equilibrium due to changes in the balance between the surface energy and forces C: of m o l e c u l a r interaction of these chain segments. z5

b

tO

5-0

o

I

|

2000

"

!

4~

"

i,,

t, hr

Fig. 1. Polymer strength plotted against the t i m e of previous exposure to the action of water: a) PETP; b) PS; c) cellophane.

However, these changes in the state of equilibrium may be manifested not only by disorientation but also by straightening of chains in the vicinity of pore surfaces and by the alignment of these straightened segments in the direction of drawing (unrelaxed stresses), i . e . , by additional orientation. The aim of this investigation was to Verify these postulates and to carry out a more detailed study of the laws governing the phenomena under consideration.

The experimental materials including the following p o l y mers in their amorphous and crystalline forms: polyethylenetetraphthalate (PETP), polyethylene (PE), polyvinylidenechloride, polycarbonate (PC), polystyrene (PS), polyvinylchloride (PVC), cellophane, and cellulose t r i a c e t a t e , The main c h a r acteristics of these materials are given in a table. All the polymers studied were exposed to the action of the followMain Characteristics of Industrial Polymer Films ing media: d/stilled water, tap water, 22% aqueous solution Studied of NaC1 with 2 % a c e t i c acid, 3% l a c t i c acid solution, and Tensile 40% solution of ethyl alcohol with 1% tartaric acid. Structural Polymers ~ strength, state ~9 kg/mm2 The specimens (30 • 100 ram) were immersed in the m e d i a tested and held in them for 9 0 - 1 0 000 hr, after which they were washed with water, dried with filter papers and Polyethylenetetra80 20 Crystalheld for 50 hr in a desiccator at a relative humidity of 65~. phthalate line The strength of the films studied was determined on a d y n a Polyethylene 100 2.0 m o m e t e r constructed at the Institute of High-Molecular C o m Polyvinylpounds AS USSR [9]. The character of changes in the orienideneehloride 35 7.0 tation were studied by X - r a y diffraction using filtered C u - r a Polyearbonate 3O 6.0 Amorphous diation. The intensity of diffractions was determined with an Polystyrene 60 6.0 MF recording microphotometer, EZ-3 potentiometer and an Polyvinylchloride 60 2.2 attachment for a u t o m a t i c recording of the microphotograms 40 12.0 Cellophane along the circle. 60 10.0 Triacetate

The magnitude of double refraction An (as the ratio of

181

the path I of the beam to the specimen thickness d was estimated with an MIN-8 microscope using quartz compensating plates of the higher orders and a Berek compensator [10]. The density of the polymer specimens was determined by the method of graduated tubes [11] and by hydrostatic weighing, The resuhs of some investigations [12, 13] showed that the action of water and other liquid media on block polymers and organic glasses produces a gradual reduction in their strength, the dependence of strength on the exposure time approaching an exponential relation. As shown by our experiments (Fig. 1), the dependence of the strength of polymer films on the time of previous exposure to liquid media differs from exponential. For instance, in the initial stages (500 hr) the action of water increases the strength of PETP from 20 to 30 kg/mm2; an additional exposure of 1000 hr reduces the strength of this polymer to 25 k g / m m 2 (which is still higher than the initial level), no significant changes being produced by further exposure (up to ! 0 000 hr and more). Similar results were obtained for other polymers, especially amorphous polystyrene and cellophane (Fig lb and c). It can be postulated that the initial increase in the strength of polymer films is associated with orientation phenomena [1], and that the subsequent reduction in strength is due to a predominant contribution of competing disorientation phenomena and by the weakening of the force of interaction of structural elements as a result of increased microporosiry. ro

%

7

5

.....

i

~

I

1.06

I

I

i __

Fig. 2. Variation in a) double refraction coefficient and b) density of PS in relation to the time of exposure to 1) water and 2) aqueous NaC1 solution with acetic acid. Studies of the polymer films (e. g., PS and cellophane) by the double refraction method showed that in the first 500 hr of exposure to liquid media the value ~m increases from g x 10 "s to 9. 6 x 10 "s (in the case of water) and to 8.6 x x 10 "3 (in the case of an NaC1 solution with acetic acid), the measurements being accurate to a - 5 % (Fig. 2). Further exposure to the action of liquid media produces a reduction in An. And so, the double refraction measurements made it possible to establish that the relation An = Zm(t) is analogous in character to the relation o b = Ob(t), which supports the view that orientation phenomena take place in the initial stages of the action of liquid media, disorientation processes taking place in the later stages. In general the orientation is determined by the distribution function of structural elements (molecules, bundles, molecular chains, crystals) in respect to the orientation angles [14] ~(~o) = ZXN/N, where ZXN/N is the relative number of such elements oriented in the angle interval ~0i + ZXO. X-ray diffraction measurements were used to find this distribution function. Comparison of X-ray diffraction patterns revealed changes in the magnitude of equatorial reflections with Increasing time of exposure to the action of liquid media. To find whether there is any correlation between the variation in the strength and orientation of polymer films: we used the Kargin-Mikhailov method [2] to calculate the orientation with the aid of normalized distribution curves. Curves obtained by normalization (Fig. 3) made it possible to compute the distribution functions ~O) - ZXN/N for a series of selected angles ~0i between 0 and 90 ~ relative to the direction of the initial orientation for various times of exposure to the action of liquid media. In Fig. 4 the percentage of oriented structural elements is plotted against their orientation. In these calculations the number of elements AN was taken as proportional to the surface area

r

As = ~ E~ (~) d,~

J 182

E

with A(g taken to be equal 15". The total number of elements oriented at angles between 0 and 90* corresponded to the surface area of a rectangle with the ordinate Ee/E = 1.

Z.0

180

90

50 o

0

zSO

~0

~

0

Fig. 3. Normalized values of the optical density E of photomicrographs (along the circle) plotted against the orientation angle of structural elements of PETP exposed to the action of a) water and b) NaC1 solution containing acetic acid for the following times: .) 500 hr; (])) 1000 hr; O) 2000 hr; @) 6000 hr, Thedashed line relates to PETP in the initial s t a t e . The distribution functions of structural elements in respect of their orientation angles were converted into a set of curves (Fig. 5) showing how the time of exposure to the action of various media affects the proportions of elements with different orientations, i . e . , plotted in coordinates that make it possible to compare curves o b = Ob(t) and An = An(t). Structural elements whose orientation in the as-received polymer is, say, in the range ~7" to the direction of the initial drawing undergo disorientation with time; however, its magnitude is small (about 1-:3% for such an initial angle). At the same time, chain segments initially oriented at angles 8 0 - 9 0 ~ spontaneously orient themselves in the direction of the initial drawing; the magnitude of this additional orientation (for this angle) is 5 - 6 % . And so, two processes take place simultaneously: the orientation of elements weakly oriented in the direction of the initial drawing and disorientation of elements that were initially strongly oriented. Graphs in Fig. 5 show also that the contribution of orientation pre-

0

ii

il r 9

18

"

i

74

7a

,

,_

,

\l , ~1

Fig. 4. Dependence of the percentage of oriented structural elements of PETP on their orientation angle at various stages of the action of a liquid medium: 1) initial state; 2) 500 hr; 3) 1000 hr; 4) 6000 hr.

2 /

3--

75

14 12 0.

.

t000 . .

2000

31oo

4000 '

t," hr

Fig, 5. Distribution of the orientation angles of polymer structural elements plotted against the time of exposure to the action of liquid media: 1) 0-15 ~; 2) 15-30~ 3) 30-45~ 4) 45-60~ 5) 60-75~ 6) 75-90*.

dominates in the first 500-1000 hr, disorientation being the predominant phenomenon in the next 1000 hr; the influence of these two factors in the later stages of the action of liquid media is practically the same. The changes in the strength of films observed can therefore be attributed to the spontaneous orientation of structural elements weakly oriented during the initial drawing in the course of their manufacture. There is some evidence [15, 16] that the process of orientation of polymers is accompanied by a loosening of their structure and a reduction in their density while disorientation has an opposite effect. Dflatometric measurements on films acted on by various liquid media (Fig. 2) revealed a decrease in the density during the initial stages, i . e . , during the spontaneous orientation stage, and an increase in the later stages. This data is in good agreement with the results of X-ray diffraction studies of spontaneous orientation. 183

The experimental results can be explained starting from the relaxation character of the behavior of structural elements of polymer films after their manufacture. The action of Iiquid media (e. g., water) facilitates re-grouping of structural elements under the influence of local (in the vicinity of pores) unrelaxed stresses. As a result, stresses of this kind are relieved, the system is shifted toward the state of equilibrium, and the additionally produced orientation becomes stable. Before concluding the authors wish to thank A. V. ~molina and N. F. Bakeev for providing facilities to carry out certain measurements and for their comments on the results of this investigation. Summary 1. The strength of polymer films under the influence of liquid media varies with time. 2.

In the initial stages of the process the polymer strength increases.

3. The initial increase in the polymer strength is due to spontaneous orientation of its structural elements. 4. The spontaneous orientation persists even when a p01ymer is no longer acted on by a liquid medium. This may be attributed to the relaxation character of the relief of localized internal stresses. REFERENCES 1, V, A, Kargin and G. L. Slonimskii, A Short Outline of Physical Chemistry of Polymers [in Russian], Moscow, Izd. Khimiya, 171-173, 1967, 2. V. A. Kargin and N. V. Mikhailov, ZhFKh, 14, 195, 1940. 3. Yu. S. Lipatov, V. A. Kargin, and G. L. Slonimskii, ZhFKh, 30, 1075, 1956. 4. G. S. Park, I. Polymer ScL, lt, 91, 1953. 5. P. V. Kozlov and B. P. Korostylev, Report on the Scientific Research of the Mendeleev All-Union Chemical Society, [in Russian], 3, 57, 1955. 6. V. A. Kargin and T. V. Gatovskaya, ZhFKh, 29, 889, 1955. 7. A. L Soshko and A. N. Tynnyi, FKhMM [Soviet Materials Science], no. 5, 512, 1967. 8. A. V. Goryainova, The All-Union Scientific and Technical Conference on the Applications of Plastics and Improving their Qualiry [in Russian], Erevan, 1966. 9. A. p. Rudakov and N. A. Semenov, Mekhanika polymerov [Polymer Mechanics], no. 3, 155, 1965. 10. F. Rinne and M. Berek, Optical Investigations with the Aid of a Polarizing Microscope [Russian translation], ONTI, 1937. 11, I. M. Mills, J. Polymer Sci., 19, 598, 1956. 12. S. V. Shchutskii and V. S. purkin, Vinyl Plastics [in Russian], Goskhimizdat, 1953. 13. S. M. Perlin, Plastmassy, no. 8, 62, 1966. 14. V. I. Selikhova, G. S. Markova, and V. A. Kargin, VMS, no. 8, 1214, 1959. 15. R. G. Miller and H. A. Willis, I. Polymer Sci., 19, 485, 1956. 16. I. Sandeman and A. Keller, L Polymer Sci., 19, 405, 1956. 27 September 1967

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