EFFECT OF

OF

ORIENTED

LIQUID

MEDIA

I~OLYMETHYL

ON DURABILITY

METHACRYLATE

A . N. T y n n y i , N. A . O l ' k h o v i c h - N o v o s a d y u k , A. I. Soshko, N. G. K a l i n i n , a n d B . V. P e r c y

UDC 620.170 : 678.674:677.521

E x p e r i e n c e in the use of p o l y m e r i c m a t e r i a l s in teclmology has shown that the strength c h a r a c t e r i s t i c s and, in p a r t i c u l a r , the durability of p o l y m e r s , d e t e r i o r a t e under the influence of liquid media. Methods of t r e a t m e n t (e.g., m o l e c u l a r orientation) which lead to an i m p r o v e m e n t in the strength c h a r a c t e r i s t i c s of p o l y m e r s a r e known [1-3]; the i n c r e a s e in strength has been explained mainly by an i n c r e a s e in thenttmbet of chemical bonds involved in the disintegration p r o c e s s [4, 5] and also by a d e c r e a s e in the concentration of s t r e s s e s during the orientation p r o c e s s [6]. T h e r e have been p r a c t i c a l l y no investigations into the durability of oriented p o l y m e r s as a function of the degree of orientation (stretching) under the influence of liquid media except [7, 8] in which a t tempts were made to evaluate the effect of i n t e r m o l e c u l a r interaction forces on the strength of o r i ented p o l y m e r s , where the load was applied at various angles to the direction of orientation; it was shown that if the external load is applied in the direction of orientation liquid media have an insi~aificant effect on the static strength of the p o l y m e r s . It is also known that the nature of the interaction between a liquid m e d i u m and a p o l y m e r i c m a t e r i a l and its effect on the strength c h a r a c t e r i s t i c s of the p o l y m e r s a r e d e t e r m i n e d by the magnitude of the applied load, the length of contact between the s t r e s s e d p o l y m e r and the medium, and the c o n c e r t e d action of t h e s e f a c t o r s [9]. The p u r p o s e of the p r e s e n t work was to investigate the effect of liquid media on the durability of oriented polymethyl m e t h a c r y l a t e (PMMA). The orientation of PMMA specimens of type SOL-95 was r e Mized by uniaxial extension to elongation r a t i o s of ee = 50~o and ee = 80% at 100-105 ~ C, followed by rapid cooling in liquid nitrogen. Water and heptane w e r e used as liquid media, and the e x p e r i m e n t s were also c a r r i e d out in a i r for c o m p a r i s o n . As shown by the r e s u l t s , the f o r m of the log T (e) relationship for the oriented PMMA in air is different f r o m that for the unoriented m a t e r i a l under the same conditions (Fig. 1). In this case t h e r e a r e two c h a r a c t e r i s t i c m o m e n t s . F i r s t , at a s t r e s s g r e a t e r than 6 k g / m m ~ the durability of the oriented PMMA is c o n s i d e r a b l y g r e a t e r than that of the unoriented m a t e r i a l (this difference d e c r e a s e s with d e c r e a s e in the TABLE I

Coefficients o~,~- 10 TM, m m ~

(r tO%ram" U0.kcal mole kcal U0'mole

air

~e:50~

air

water hep[ane

air

waterI;;n2 t,56 ,08 r 0,28

174_

03928

34.6

28,4 34,4 33,2

0,61

25,5

~ater hep.. tane 1,44

0,58

0,96 0,26

28,0 33,9 33,0

27,7

26,8 25,2

21,6

22,0

P h y s i c o m e c h a n i c a l Institute, A c a d e m y of Sciences of the Ukrainian SSR, L'vov. All-Union Institute of Aviation Materials, Moscow. T r a n s l a t e d f r o m F i z i k o - K h i m i c h e s k a y a Mekhanika Materialov, Vol. 7, No. 2, pp. 53-56, M a r c h - A p r i l , 1971. Original article submitted April 28, 1969.

9 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

179

I u

applied s t r e s s ) ; second, the durability of the oriented PMMA i n c r e a s e s with i n c r e a s e in the elongation r a t i o (Fig. 1, c u r v e s 1' and 1").

*"%X\' K.,', A .K w_" \R \ \ ,,

X 8

J

Fig. i. Durability of PMMA

~, kg/ram2

in air, water, and hep-

tane r e s p e c t i v e l y : 1), 2), and 3) unoriented; 1'), 2'), and 3') o r i e n t e d at ee=50%; 1"), 2 " ) , and 3") ~e =

so%.

I t i s known that the deforming ability of oriented PMMA i n c r e a s e s with a slight i n c r e a s e in the breaking point [10]; the n u m b e r of m o l e cules r e c e i v i n g the load in the c r o s s section of the s p e c i m e n also i n c r e a s e s [4, 5], and t h e c o n figuration of the defects in the p o l y m e r changes, leading to a d e c r e a s e in the s t r e s s c o n c e n t r a tion. On this basis the i n c r e a s e d durability of oriented PMMA can be explained by the i n c r e a s e in the n u m b e r of m o l e c u l e s r e c e i v i n g the load and by the d e c r e a s e in the s t r e s s concentration r e s u l t i n g f r o m the change in configuration and the f a v o r a b l e orientation of the defects in the s p e c i m e n . As seen f r o m the data obtained in the t e s t s the effect of t h e s e f a c t o r s b e c o m e s s t r o n g e r with i n c r e a s e in the d e g r e e of o r i e n tation. With u< 6 k g / m m 2 and, naturally, with d e c r e a s e in the deformation r a t e f a v o r a b l e c o n ditions a r e evidently c r e a t e d f o r a m o r e uniform distribution of the s t r e s s e s in the m a c r o m o l e c u l a r chains as a r e s u l t of the additional f o r c e d e l a s t i c d e f o r m a t i o n and the o c c u r r e n c e of r e laxation p r o c e s s e s .

Analysis of the obtained e x p e r i m e n t a l r e s u l t s f r o m the standpoint of the kinetic conception of strength shows that both the s t r u c t u r e - s e n s i t i v e coefficient (y= r and the activation e n e r g y of the e l e m e n t a r y event in the disintegration p r o c e s s (U 0) d e c r e a s e with i n c r e a s e in the d e g r e e of orientation (Table 1). D u r ing the orientation of the p o l y m e r s the s t r e s s concentration coefficient (fi) and the fluctuation volume (w) change [11]. The high values of the product 0~fi for u < 6 k g / m m 2 (e.g., c u r v e s 1' and 2' and 1 " and 2") is evidently explained by an i n c r e a s e in the n u m b e r of r u p t u r e d c h e m i c a l bonds, since the coefficientfi should d e c r e a s e with d e c r e a s e in the acting s t r e s s owing to the a p p e a r a n c e of conditions for the o c c u r r e n c e of f o r c e d elastic d e f o r m a t i o n s . F o r u> 6 k g / m m 2 and c o n s i d e r a b l e deformation r a t e s , when it i s difficult to r e a l i z e f o r c e d e l a s t i c deformations, a high value is r e t a i n e d for the s t r e s s concentration coefficient with a s m a l l e r value for the fluctuational volume c o m p a r e d with the f i r s t section. Thus, the change in the n a t u r e of the log r (u) r e l a t i o n s h i p for oriented PMMA with d e c r e a s e in the acting s t r e s s can be explained by weakening of the effect of s t r e s s concentration, strengthening of the role of s t r e s s relaxation, and the o c c u r r e n c e of forced e l a s t i c d e f o r m a t i o n s . E a r l i e r it was shown [9] that the strength and durability of PMMA d e c r e a s e under the influence of liquid media; alongside this a change was o b s e r v e d in the f o r m of the log r (u) relationship. The log r (~) relationships for oriented PMMA obtained in the p r e s e n t work a r e s i m i l a r to those o b s e r v e d during the action of liquid m e d i a on unoriented PMMA. Analysis of the r e a s o n s for the change in the f o r m of the log r(u) r e l a t i o n s h i p for oriented PM1VIA in air, c a r r i e d out in the p r e s e n t work, and for unoriented PMMA in liquid m e d i a [12] shows that the effects of orientation p r o c e s s e s and liquid m e d i a on the durability of PMNIA a r e s i m i l a r in f o r m . In fact, if the orientation p r o c e s s e s p r o m o t e a d e c r e a s e in the concentration o f s t r e s s e s and facilitate the o c c u r r e n c e of f o r c e d e l a s t i c d e f o r m a t i o n s and the r e l a x a t i o n of the s t r e s s e s , a p p r o x i m a t e l y the s a m e direction i s found in the action of liquid media, the a t o m s and m o l e c u l e s of which (by being a d s o r b e d on the s u r f a c e of the s p e c i m e n and the defects) r e d u c e the f r e e s u r f a c e energy, weaken the i n t e r m o l e c u l a r bonds, and t h e r e b y facilitate and a c c e l e r a t e the relaxation of the s t r e s s e s , p r o m o t e the local orientation of the m o l e c u l e s , and consequently p r o m o t e the o c c u r r e n c e of f o r c e d elastic deformations. In this r e s p e c t it s e e m e d of i n t e r e s t to i n v e s t i g a t e the effect of liquid m e d i a on the durability of oriented PMNIA. As shown by the data obtained in the t e s t s , the durability of oriented PlVFMA d e c r e a s e s in liquid m e d i a (water, heptane) (Fig. 1, c u r v e s 2' and 3' and c u r v e s 2 " and 3 " ) . This is not, however, a c c o m p a n i e d by a change in the f o r m of the log r (u) relationship for the oriented PNIhUk under the influence of liquid media. 180

It i s c h a r a c t e r i s t i c that the durability of o r i e n t e d PMMA in liquid m e d i a is m u c h g r e a t e r than that of unoriented PMMA under the s a m e conditions. Thus, w a t e r g r e a t l y r e d u c e s the durability of unoriented PMMA and at the s a m e t i m e has a weak effect on the oriented PMMA. Heptane r e d u c e s the durability of o r i e n t e d PM1VfA m o r e s t r o n g l y (Fig. 1, c u r v e s 3' and 3 ' 9 , but this d e c r e a s e is c o n s i d e r a b l y l e s s than in the c a s e of the unoriented PMMA. T h e s e data show that p o l a r and nonpolar m e d i a have different effects on the durability of PMMA. The r e s u l t s f r o m the investigations into the durability of oriented PiVfMA under the influence of liquid m e d i a a r e of i n t e r e s t in that they c o n f i r m the view, e x p r e s s e d in [12], that the m e d i a have an initiating effect on the o c c u r r e n c e of p l a s t i c i z i n g p r o c e s s e s in the s t r e s s e d p o l y m e r ; the intensity of this p r o c e s s is conditioned by the magnitude of the s t r e s s and the length of contact between the m e d i u m and the p o l y m e r . On the basis of an a n a l y s i s of the e x p e r i m e n t a l data it can be concluded that the durability of PM1ViA i n c r e a s e s with i n c r e a s e in the d e g r e e of orientation, while the effect of the liquid m e d i a d e c r e a s e s . Thus, orientation of p o l y m e r s can p r o v i d e a method for i m p r o v i n g t h e i r strength and r e s i s t a n c e to the action of liquid m e d i a .

LITERATURE i. 2. 3. 4. 5. 6. 7. 8. 9. i0. ii.

12.

CITED

N.V. Mikhailov, V. A. Kargin, and V. N. Bukhman, Zh. Fiz. }(him., 14, 205 (1940). B.V. Percy and ]VI. M. Gudimov, Oriented Plastics [in Russian], Oborongiz (1961). T. Alfrey, Mechanical Behavior of High Polymers, Wiley (1948). V.A. Kargin and N. V. Mikhailov, Zh. Fiz. Khim., 14, 195 (1940). S.N. Zhurkov and S. A. Abasov, Vysokomolek. Seed., 3, 450 (1961); 4, 1703 (1962); Fiz. Tverd. Tela, 4, 2184 (1962). P . P . Kobeko, Amorphous Substances [in Russian], Izd. AN SSSR (1952). A.I. Soshko and A. N. Ty~nyi, Fiz.-Khim. Mekhan. Mat., No. 5 (1968). A.I. Soshko, Ya. M. Spas, and A. N. Tynnyi, Fiz.-Khim. Mekhan. Mat., No. 5 (1968). A.I. Soshko and A. N. Tyrmyi, Fiz.-I~fim. Mekhan. Mat., No. 3 (1965); No. 5 (1965). A.V. Stepanov, Zh. }~sper. Tekh. Fiz., 19, 973 (1949). G.M. Bartenev and Yu. S. Zuev, Strength and Failure of Highly Elastic Materials [in Russian], Izd. Khimiya (1964). A.N. Tyrmyi, N. A. Ol'khovich-Novosadyuk, A. I. Soshko, et al., Fiz.-Khim. Mekhan. Mat., No. 4 (1969).

181