Deformability of polyvinyl chloride in creep conditions

Deformability of polyvinyl chloride in creep conditions

Polymer Science U.S.S.R. Yol. 24, lq'o.6, pp. 1261-1268, 1 9 8 2 Printed in Poland DEFORMABILITY S. V. PICIIUGII~A, 0032-3950]82]061261-08507.50/0 ...

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Polymer Science U.S.S.R. Yol. 24, lq'o.6, pp. 1261-1268, 1 9 8 2 Printed in Poland

DEFORMABILITY

S. V. PICIIUGII~A,

0032-3950]82]061261-08507.50/0 ~ 1983PergamonPress Ltd.

OF POLYVINYL CHLORIDE IN CREEP CONDITIONS* V. P. LEBEDEV,

N . I. SAMAB~KAYA,

L. I. BATUYEVA,

I. N . RAZleCSKAYA a n d A. I. SLVTSKER

(Received 12 October 1980) Creep and necking processes of PVC have been studied using the direct structural methods: birefringence, X - r a y structural analysis and electron microscopy. I t is shown t h a t the creep process is associated with the disruption of the supramolecular organization of the polymer. A structural p a t t e r n of the processes developing a t different stages o f creep of PVC is proposed. I t is shown t h a t the structural conversions on creep of PVC are based on the process of the local orientation rearrangement with formation of distinctive micronecks on reaching a certain critical concentration of which the macroscopic neck begins to form. The supramolecular organization of the polymer to the point of crystallites in the region of the neck is disrupted. CREEP i.e. s t r a i n o n e x p o s u r e t o a c o n s t a n t a p p l i e d s t r e s s in p o l y m e r s o f t e n c u l m i n a t e s i n t h e f o r m a t i o n o f a n e c k [1, 2]. I t is k n o w n t h a t t h e f o r m a t i o n o f a n e c k is a s s o c i a t e d w i t h s t r o n g s t r u c t u r a l - o r i e n t a t i o n e f f e c t s w h i c h m a y b e p r o dnced by thermofluctuation processes of molecular and supramolecular regrouping [2, 3]. D e s p i t e t h e l a r g e n u m b e r o f s t u d i e s t h e d e t a i l e d m e c h a n i s m o f n e c k f o r m a t i o n e s p e c i a l l y f o r a m o r p h o u s p o l y m e r s is s t i l l n o t a l t o g e t h e r c l e a r . The present work seeks to detail the process of creep and neck formation in t h e c a s e o f P V C . I t is k n o w n t h a t c o m m e r c i a l g r a d e s o f P V C a r e e s s e n t i M l y a m o r p t l o u s a l t h o u g h a s m a l l p e r c e n t a g e o f t h e c r y s t a l l i n e p h a s e ( u p t o 10~/o) w i t h a high melting point (473-493°K) makes it possible to use diffraction methods for observing its structural changes. The test object was PVC of C-70 grade with a molecular mass 140,000t. The material for the tests in plate form was obtained b y the roll-press method at 443°K with addition of 2% mixture of barium and cadmium stearates as heat stabilizer. The density of the material was 1.41 × 10 a kg/m a, the glass transitiou temperature Tg determined b y a t}mrmomechanical method at a t e m p e r a t u r e rise rate of 1.66 × 10 -2 deg/sec was 353°K, the degree o f crystallinity determined radiographically [5] was 8 per cent. Tile samples for plotting the creep curves were in the form of bilateral blades 6 × 10 -2 m tong of section (2 × 2) × 10 -a m prepared on a miling machine. Before the test the samples for standardization were annealed at Tg for 12 hr. The effect of standardization of each * Vysokomol. soyed. A24: No. 6, 1123-1129, 1982. * The molecular mass was determined viseometrically in solution in cyclohexanon~ from the formula [,/]=4.622× lO-SM °'~ 1261

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sample was checked against the uniformity of colour in polarized light in the KPS-500 polariscope. The creep plots were recorded with the modified unit of S. 1~. Zhurkov [1] on uniaxial stretching of the sample in the temperature interval 178-293°K. To study the character of deformabibility of the sample in different portions along its length special labels were applied to the working part of the blade every (2-5) × 10 -3 m and during the creep process (at room temperature) the sample was photographed at definite time intervals. The extent of the deformation was measured on the photographic negatives of the sample using the IZA-2 comparator with an accuracy to 0.01 × 10-* m. Birefringence and the diffraction patterns in creep conditions were measured at room temperature using the PKS-125 polariscope and the URS-1.0 X-ray unit. In an attempt to trace separately the orientation of the maeromolecules in the amorphous and c~wstalline parts of the sample we used the internal label method. For this purpose crystalline tribasie lead sulphate (TPbS) was added to the polymer. The anisodimetric TPbS crystals must be oriented in the sample in line with the orientation of the polymer matrix surrotmding them and since PVC is 90 per cent amorphous then the orientation of the TPbS crystals must characterize the orientation of the amorphous phase of PVC. The morphological structure of the samples in different portions of the creep curve was judged from the electron-microscopic films of replicas of the surfaces of a brittle chip. The samples were cleaved at the temperature of liquid nitrogen in the direction of stretcldng. The replicas (platinum-carbon) were studied with the ZMV-100L microscope. T h e creep curves in t h e t e m p e r a t u r e a n d stress i n t e r v a l s studied are q u i t e u n i f o r m . F i g u r e 1 p r e s e n t s some creep curves one of which (curve 3) i n d i c a t e s t h e four stages of creep isolated b y us a n d the t i m e of n e c k f o r m a t i o n v, : I, s t a g e of elastic d e f o r m a t i o n ; I I , elastic after-effect I I I , s t e a d y creep; I V , n e c k formation. A f t e r r e m o v i n g t h e load, p a r t of t h e d e f o r m a t i o n relaxes a n d the o t h e r p e r sists in t h e f o r m of d e l a y e d d e f o r m a t i o n (Fig. 1, curve 1') t h e c o m p l e t e r e l a x a t i o n of which requires h e a t i n g a b o v e Tg. P h o t o g r a p h y e n a b l e d us to t r a c e the course o f creep of i n d i v i d u a l p o r t i o n s o f t h e s a m p l e a n d establish t h e n o n - u n i f o r m i t y o f d e f o r m a b i l i t y . I n t h e course o f creep on the w o r k i n g p a r t of the s a m p l e the a p p e a r a n c e o f one or m o r e clouded spots m e a s u r i n g ( 1 - 2 ) × 10-~m, t h e site o f f o r m a t i o n t h e neck, was noted. T h e c h a r a c t e r of creep in t h e region of t h e t u r b i d s p o t (zone I) significantly differed f r o m t h e d e f o r m a b i l i t y of the r e m a i n d e r of the s a m p l e (zone 2). Figure 2 p r e s e n t s t h e creep curves s e p a r a t e l y for zone 1 (curve 1) zone 2 (2) a n d for s a m p l e as a whole (curve 3). T h e c h a r a c t e r of creep for zone 1 is t h e s a m e as for t h e whole s a m p l e b u t is distinguished b y t h e high r a t e of s t e a d y creep a n d large d e f o r m a t i o n values. Characteristic o f zone 2 is v i r t u a l absence of g r o w t h o f d e f o r m a t i o n in p o r t i o n I I I of t h e creep curve. R e m o v a l of t h e l o a d for times u p t o ~, led t o c o m p l e t e r e l a x a t i o n of d e f o r m a t i o n in zone 2 while in zone 1 t h e r e r e m a i n e d its d e l a y e d p a r t w h i c h was set u p in t h e p o r t i o n of s t e a d y creep (broken lines in Fig. 2). T h e initial s a m p l e s before loading u s u a l l y d i s p l a y e d m i n o r p o s i t i v e birefringence (0.5-1.0) × 10 -4 n o t r e m o v e d b y t h e r m a l t r e a t m e n t a t 353°K; o n l y rise i~l

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t e m p e r a t u r e to 443°K a n d higher led to its d i s a p p e a r a n c e . This initial birefringence is due to t h e m i n o r residual o r i e n t a t i o n set u p on processing t h e m a t e r i a l a n d in o u r e x p e r i m e n t s did n o t influence t h e q u a l i t a t i v e c h a r a c t e r of the relations studied.

~,% I0 -

q

3 _

22(

t

I

I

f I

I

i

5o

1oo

~-¢

75o

2oo r/me

7 sec

FIG. 1. Curves of creep of PVC at 293 (1), 263 (2), 233 (3) and 193 (4)°K a~d relaxation of

deformatio~ (1'). a = 5 4 (1), 74 (2), 97 (3) and 138 (4) MPa.

9

3

yL /S

.

I

5-

\

l

I

50

100

FIG. 2

z

/lnxlO -a 10-

\.

T/me,sec

0

T

-700

-~/ ~5

200

300 r/me. sec

FIG. 3

FIo. 2. Curves of creep of the individual portions of the PVC sample at 293°K and ~ = 54 MPa. 1 -- Region of turbid spot (zone 1); 2-- remaining region of sample (zone 2); 3-- for sample as a whole. Broken lines denote relaxation of deformation. Fro. 3. Change in birefringence on creep of PVC sample at 293 K and e = 5 0 MPa (1--zone 1, 2--zone 2); 3,4--value of birefringence after unloading sample. On a p p l y i n g t h e l o a d to t h e s a m p l e n e g a t i v e birefringence i m m e d i a t e l y a p p e a r ed; it r e m a i n e d a t a c o n s t a n t level t h r o u g h o u t t h e e x p e r i m e n t in t h a t p a r t o f t h e s a m p l e n o t affected b y n e c k f o r m a t i o n (zone 2) (Fig. 3, s t r a i g h t line 2). I n zone 1 b i r e -

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fringence changed sign to the opposite and greatly increase in absolute terms (Fig. 3, curve 1). I n the neck itself birefringence could not be measured owing to its complete non-transparency. After removal of the load in zone 2 the initial value of birefringenee was instantly set up but in zone 1 and apparently in the neck itself the positive value of birefringenee reached was maintained (broken lines in

Fig. 3). Negative birefringence appearing in the sample simultaneously with the application of stress is apparently caused by the elastic shift of the atoms through distortion of the lengths and angles of the chemical bonds and is known as "distorted" birefringence [6, 7].

[(010) a

c

~(110)

b

d

FIG. 4. Schemes of difDaction patterns of PVC recorded in different stages of creep of sample (a, b, d) and oriented at T>Tg (c). a - - I n i t i a l sample and deformed to ~0; b - - n e c k region; c - - o r i e n t e d at 373°K by 250~o; d - - n e c k annealed at 383°K in fixed state.

Since the polarizability of the PVC macromolecule is higher along the chain axis [7] change in the sign of birefringence to positive in zone 1 of the sample may signify the start of the orientation mechanism of birefringence. Additional information on orientation phenomena on creep may be given by the X-ray diffraction method. Figure 4 presents schemes for the diffraction patterns* recorded at different stages of the process of creep of the PVC. The * The diffraction patterns are not given since the difference between t h e m is lost on phot~reproduction because of the low erystallinity of PVC.

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initial sample was isotropic (Fig. 4a)--against the background of the amorphous halo two rings of the reflexions (200) and (110) of the crystalline PVC lattice were traced. The diffraction patterns of the loaded sample in stages I - I I I of the creep curve do not reveal explicit qualitative changes. The same result is given by analysis of the diffraction patterns of the samples containing a crystalline "label": TPbS crystals in the initial and deformed samples before neck formation are not oriented (Fig. 5a). On passing to the neck region the diffraction patterns of PVC proper (Fig. 4b) and its composite with TPbS (Fig. 5b) clearly display orientation effects--the internal "intermolecular" diffusion ring (0.54 nm) of PVC is drawn into an arc close to the equator of the diffraction patterns while TPbS crystals form an ~xial texture in the direction of the stress applied. This means that in the neck

ii

lvm. 5. Diffraetiol~ p a t t e r n s o f D~ C s a m p l e s c o n t a i n i n g c r y s t a l l i n e label (TPbS) a - - I n i t i a l a n d d e f o r m e d t o r,; b - - n e c k r e g i o n ; c - - a n n e a l i n g o f n e c k a t T~Tg in free s t a t e .

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region there is preferential orientation of segments of the chains in the direction of the applied stress. Annealing of the neck above Tg in the free state leads to disruption of the texture of the TPbS crystals (Fig. 5c) and hence to disorientation of the PVC-matrix. It is worth noting the amorphous character of the PVC diffraction pattern in the neck (Fig. 4b). Comparison of it with the diffraction pattern of the samples stretched above T~ (at 373°K) (Fig. 4c), indicates that in the course of neck formation there is amorphization of the originally partially crystalline PVC sample. Earlier a similar influence of an external mechanical agent--grinding in a mortar and pressing--on the erystallinity was detected by X-ray diffraction, I R spectroscopy and DTA for PVC in the powder state [8]). Diffractometric study of the samples unloaded at different stages of creep confirmed the qualitative observations of change in the erystallinity of PVC on creep: the degree of crystallinity of the initial sample 74-1.5%, in zones 1 and 2 of the sample before neck formation 5 ± 1 % and 54-1.5% respectively, and in the neck 0%. Thus, mechanical melting of the erystallites occures only when in the material under the influence of the mechanical field of stress the structure is transformed from an isotropie to an oriented anisotropic structure. Because of the absence of the thermal segmental mobility of the maeromolecules in the experiments considered (Ttest
Deformability of polyvinyl chloride in creep conditions

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Thus the former supramolecular organization of the polymer (to the point of ,:rystallites) is disrupted in the region of the neck and the new one bears within it only one element of order--preferential arrangement of the axes of the segmr-uts along the direction of the acting force. From the results of the investigation one may try to represent the structural pic~,ure of the processes developping in different stages of creep of PVC. On applying a load to the PVC sample fast processes of deformation are first realized in it, not associated with rearrangement of the supramoleeular structure of the material--which takes much time. As well as the purely Hooke's component this deformation apparently also comprises shift of certain elements of the supram(,tecular structure without their disruption. There is no doubt t h a t a definite li~hit of such a shift exists for every test condition, which probably accounts ibr the gradual slowing of the rate of creep in portion I I (Fig. 1). Since real samples are not ideally homogeneous in the distribution of the (~mlponents of the composite, the degree of working over the material on process-

t"~lt~. 6. Electrol~ photomicrographs of chip replicas of samples at different stages of creep: (~--hlitial sample; b, c--samples deformed to w, (zones 2 and 1 respectively); d--neck region.

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S . v . PxcHuGn~A e~ a/.

ing, the presence o f impurities and, finally, in g e o m e t r y , t h e stress d i s t r i b u t i o n o v e r t h e v o l u m e o f t h e sample will also be uneven. A t the sites of t h e superstresses the limiting value o f the shift of the s t r u c t u r e s will be reached earlier a n d in t h e m the process of local o r i e n t a t i o n r e a r r a n g e m e n t will begin w i t h f o r m a t i o n o f distinctive micronecks possibly as f o u n d in the p h o t o g r a p h s o f t h e chip replicas (Fig. 6b). I n such a zone of the sample the c o n c e n t r a t i o n o f t h e micronecks will begin t o rise, as reflected in rise in positive birefringence a n d t h e irreversibility o f p a r t of t h e d e f o r m a t i o n . On reaching a certain critical concentrat i o n of micronecks in this zone a macroscopic neck begins to f o r m c h a r a c t e r i z e d b y complete r e a r r a n g e m e n t o f the s t r u c t u r e of the material. REFERENCES

1. V. R. REGEL', A. I. SLUTSKER and E. Ye. TOMASHEVSKII, Kineticheskaya prirod, prochnosti tverdykh tel (Kinetic Nature of the Strength of Solids). p. 560, l~auka, Moscow 1974 2. A. A. ASKADSKII, Defornaatsiya polimorov (Deformation of Polymers). p. 448, Khimiya, Moscow, 1973 3. G.P. ANDRIANOVA, Entsiklopediya polimerov (Polymer Encyclopedia). Vol. 3, Moscow, Soviet Ecyclopedia, p. 887, 1977 4. B. P. SHTARKMAN and Y. N. LEBEDEV, Polucheniye i svoistva PVKh (Preparation and Properties of PVC) (Ed. Ye. N. Zil'borman) p. 199, Khimiya, Moscow, 1968 5. V. P. LEBEDEV, N. A. OKLAI)NOV and M. N. SHLYKOVA, Vysokomol. soyod. Ag: 495, 1967 (Translated in Polymer Sci, U.S.S.R. 9: 3, 553, 1967) 6. V. N. TSVETKOV, Polymer Ecyclopedia. Vol. 1, Moscow. Soviet Encyclopedia p. 670, 1977 7. A. UTSUO and R. S. STEIN, J. Polymer Sci. 5: 583, 1967 8. Yu. V. OLAZKOVSKII, A. N. ZAV'YALOV, V. P. LEBEDEV and N. A. OKLADNOV. Vysokomol. soyed. A10: 910, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 4, 1059, 1968) 9. V. V. GUZEYEV, D. M. BeRT and S. I. PEREDEREYEVA, Kolloid. zh. 33: 349, 1971

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