Rheological properties of polyisobutylene

824

G. M. BARTENEV and L. A. VISHNITSKAYA

10. L. BELLAMY, I n f r a k r a s n y y e s p e k t r y molekul. (Infrared Spectra of [Complex] Molecules.) Foreign Literature Publishing House, p. 76, 1957 11. Ibid, p. 16 12. N. A. CHUMAYEVSKH, Optika i spektroskopiya 13: 68, 1962 13. R. BENKESER and LANDESMAN, J. Amer. Chem. Soc. 74: 648, 1952 14. L. I. NELSON, N. C. ANGELATTI a n d D. R. WEYENBERG, J. Amer. Chem. Soc. 85: 2662, 1963

RHEOLOGICAL PROPERTIES OF POLYISOBUTYLENE* G. M. : B A R T E N E V a n d L . A . V I S H N I T S K A Y A V. I. Lenin Moscow State Pedagogic Institute; Scientific Research I n s t i t u t e of the R u b b e r I n d u s t r y

(Received 13 June 1963) T H E v i s c o u s flow o f p o l y i s o b u t y l e n e ( P I B ) , a s a l i n e a r p o l y m e r , h a s b e e n s t u d i e d in various stressed states by numerous investigators [1-6]. In spite of the fact that this linear polymer has attracted such great attention, many of its theological properties are uncertain and contradictory. In this work, our main attention was devoted to the least studied side of the r h e o l o g i c a l p r o p e r t i e s o f P I B - - t h e i n f l u e n c e o f a s h e a r s t r e s s o n i t s v i s c o u s flow. P I B w i t h a h i g h m o l e c u l a r w e i g h t (900,000) w a s i n v e s t i g a t e d . L o w - m o l e c u l a r w e i g h t P I B (60,000) h a s b e e n i n v e s t i g a t e d p r e v i o u s l y [6]. EXPERIMENTAL I n an analysis of previous investigations, attention was drawn to the fact t h a t low stresses were generally used in the experiments. Consequently, a typical deformation curve was always obtained (at a given stress) (Fig. 1). Section I corresponds to an u n s t e a d y stato and I I to a steady state of viscous flow, t h e highly elastic component of the deformation increasing in the first stage simultaneously with viscous deformation until an equilibrium figure is reached. Of the two existing methods of investigating the viscous flow of polymers -- at a constant rate of shear deformation dy/dt= y~-const, and at a constant shear stress P ~ c o n s t , the latter was selected for reasons which will become clear in the discussion of the results. The realization of relatively large stresses is possible in rotation viscometers, one of which [7] was used in our experiments. The range of shear stresses within which the measurements were carried out was between 0"01 and 1 kg/cm ~, and the corresponding rates of deformation changed within a range of five powers of 10. The investigation was carried out in the range of temperatures from 20 to 140 °. The main measurements were carried out at a temperature of a p p r o x i m a t e l y 80 °. * Vysokomol. soyed. 6: No. 4, 751-757, 1964.

825

Rheologieal properties of polyisobutylene

At low stresses, the deformation curves had the usual form (Fig. 1) while at large stresses curves with an inflection were found (Fig. 2). All the results were treated by the graphical method with the object of determining the rate of deformation ~ during the flow- process, ~ -- the total rate of deformation -- consisting of the rate of the viscous components and the rate of the highly elestic component, the rate of the elastic component being neglected since its magnitude is far below the limits of error of the measurements and develops

J

c/

I

J I

]~me , t

FIG. 1. Typical curve of shear deformation 7(t) of a linear polymer at a low constant shear stress P-----const.

A

Time,t

FIG. 2. Curve of the shear deformation 7(t) of a linear polymer at a high constant shear stress, P = const.

during fractions of a second, which are not recorded by the instrument. Accordingly, the initial period of the application of stress from 0 to P = c o n s t , in the course of which the rate of shear deformation rises rapidly to the maximum value, was not measured. Consequently, all the following results relate to readings i-2 see after the application of the stress, when it may be considered that P = c o n s t .

Features of the unsteady 8rage of flow. The n a t u r e of the change of the rate of def o r m a t i o n w i t h the time (Fig. 3) alters m a r k e d l y w h e n the stress is increased. A t low stresses, t h e rate fails m o n o t o n i c a l l y , which corresponds to the curve in Fig. 1. A t high stresses, curves are f o u n d w i t h a m i n i m u m , w h i c h corresponds to the curve in Fig. 2. The m i n i m u m is t h e m o r e p r o n o u n c e d a n d t h e closer to the axis of ordin a t e s the g r e a t e r the shear stress a n d the higher the t e m p e r a t u r e . A t high stresses it p r a c t i c a l l y fuses w i t h t h e o r d i n a t e axis. * A n a l o g o u s b e h a v i o u r has been f o u n d p r e v i o u s l y for SKS-30 r u b b e r [from b u t a d i e n e ~ - 3 0 % of styrene] a n d is a p p a r e n t l y a general p h e n o m e n o n for all linear p o l y m e r s [7]. The s h a r p fall in the r a t e of d e f o r m a t i o n a t the v e r y beginning of a n e x p e r i m e n t is explained b y the slow d e v e l o p m e n t of the highly elastic c o m p o n e n t , the velocity of w h i c h becomes zero c o m p a r a t i v e l y r a p i d l y (the more r a p i d l y the lower t h e molecular weight a n d t h e higher the t e m p e r a t u r e ) . Simultaneously, the viscous c o m p o n e n t of the r a t e d e f o r m a t i o n c o n t i n u o u s l y rises a n d the viscosity ~ con* In the curves of P against t obtained by the y=const method, maxima must be obtained, instead of minima, as has been observed [8] for low-molecular-weight PIB ( M = 40,000).

826

G. M. BART]~N]~Vand L, A. VISHNITSKAYA

tinuously falls until the steady:state is reached. The superposition of the falling and rising characteristics of the two components of the rate of shear deformation gives the observed rate-of-deformation curve with a minimum. The fall in the viscosity during the flow of P I B is a new effect which at first sight is inconsistent with the opposed effect of the increase in viscosity found b y Kargin and Sogolova for polyisobutylene [2]. The latter, as the authors showed, is connected with the uncoiling and straightening out of the macromolecules during the deformation process. This effect can be explained only b y the monotonically falling nature of the rate of shear deformation observed at low stresses (Fig. 3). ~ " mln

~8~"

n

3

o

0"# 0"4

~ 2

Io.oo ,

0

o

o

o

I

5

Time, hours

10

FIG. 3. Curves of the change in the rate of shear deformation ~ in the deformation of high-molecular-weight polyisobutylene at 83° under various constant shear stresses: 1--0"014; 2--0"31; 3--0.45; 4--1"01 kg/em~. The decrease in the viscosity during flow observed at relatively high stresses can be explained either b y a change in the initial supermolecular structure of the polymer, the role of which has been noted previously [9], or b y a diminution in the molecular weight during flow. The change in the molecular weight was followed b y means of the viscosity. I t was found that under the conditions of our experiments the molecular weight

Rheological properties of polyisobutylene

827

did not change at temperature up to 100 °, and above this the change was slight up to 140 ° . Consequently, the results given, mainly obtained below 1009, cannot be explained by a decrease in molecular weight during flow. Simultaneously, experiments were carried out in order to determine whether "chemical" flow takes place with the "physical" flow. Analogous experiments have previously been carried out for the polymer SKS-30 [7]. Since P I B is difficult to subject to cross-linking, we used a butyl rubber with a similar structure which, under the same experimental conditions, after cross-linking with- chemical - - S - bonds showed the absence of viscous flow (viscosity greater tha~ ~t0la poise). This shows directly t h a t within the range of shear stresses up to several kg/mm 2 rubber-like polymers undergo mainly "physical" flow and the diminution in viscosity during flow is due to a change in the structure of the rubber-like polymer. A molecular model of a rubber-like polymer at high temperature. In the investigations of Kargin et al. [10, 11], molecular orderedness was found not only in solid polymers but also in a series of rubbers. The lower the temperature, the better are supermolecular structures shown in rubbers. I n view of this, the structure of rubber-like polymers cannot be considered as a chaotic interweaving of macromolecules. On the other hand, in contrast to solid amorphous polymers, the bundles in rubber-like polymers are not stable formations. At sufficiently high temperatures (in the viscous-flow state), the bundles, as fibrillar formations, become unstable because of the weak intermolecular forces between the chains of the rubber-like polymers and the intense thermal motion, in consequence of which they disintegrate. However, it is quite likeIy t h a t other processes of ordering still remain at high temperatures. Thus, instead of bundles of fibrils, the length of which is far greater t h a n t h a t of the macromolecules, it is possible t h a t ordered microregions * or "microbundles" are formed. The latter are, as it were, the seeds from which the actual bundles are formed at low temperatures. The microbundles are structures which are constantly broken up at some points and reformed at others, since their lives at high temperatures are short in comparison with the time of observation.¢ After a sufficiently long time of observation, the structure of a polymer at high temperatures is perceived on an average as a structure of chaotically interwoven chains. In view of this, the molecular model of P I B can be considered as a network the temporary nodes of which are the microbundles. Sections of the chain not forming ])art of the microbundles at a given moment change their conformation with the velocity of the thermal motion and during the life of a microbundle * They have the same general nature as the regions of close-range order in liquids but differ by their greater stability and greater orderedness, since the sections of the chains making up a microbundle are arranged roughly parallel to one another. + The lives of the microbundles are apparently considerably longer than the time of passage of the free segments (those not involved in the microbundles) from one equilibrium position to another.

828

G.M. BAItTENEVand L. A. VISHNITSKAYA

they have time to change their conformation several times. Since the life of a microbundle is considerably greater than the time of passage of segments not forming parts of microbundles from one equilibrium position to another, the wdocity of viscous flow depends mainly on the rate of disintegration and restor~tion of the microbundles, and the viscosity on the average number of them in unit of the polymer. At tow s~resses, the flow of the polymer takes place with a practically undisturbed structure, since during slow flow the mean number of microbundles does not change. In accordance with this, at low stresses Kargin and Sogolova's effect prevails, as a result of which monotonic relationships are found (Fig. 3). However, at high stresses, the distintegrated microbundles do not have time for complete recovery, and flow takes place under conditions Of a partially disintegrated polymer structure. The disintegration process goes the faster and the further the higher the stress. With an increase in the stress, the process of disintegrktion of the supermoleeular structure and the associated effect of a diminution of viscosity is the main one, in consequence of which the rate of viscous flow rises in spite of the increase in the highly elastic component of the deformation. In the steady process of flow, when the rate of flow becomes constant (the curve enters a rectilinear section parallel to the stress axis), the mean number of distintegrating microbundles is equal to the mean number regenerated per unit time. The nature of the steady flow process. In the steady flow process, the viscous component of the rate of deformation is accurately equal to the observed rate of shear deformation ~, since the highly elastic component of the rate of deformation is zero. Besides the well-known power law of Ostwald and de Vil expressing the connection between the stress and the rate of shear deformation, polymers are often treated b y means of Eisenschitz's equation [12]:

dT/dt =P/rlo (1 + t i p 2 ) ,

(1)

and also b y Ferry's equation [3]

dy/dt = P~ ~lo( 1 + ~P) ,

(2)

where fl and ~ are constants. Both equations have been tested at low shear stresses and predominantly on polymers w i t h low molecular weights. Leaderman [4] reports that Ferry's formula is more suitable for polymers. In the region of extremely low shear stresses, according to these equations, flow takes place with a viscosity ~/o which does not depend on the stress, and this has been found mainly for polymers with a low molecular weight [1, 5, 6, 13]. For other po!ymers~ no region of Newtonian flow is found, since as the molecular weight increases or when an active filler is introduced, this region practically disappears [6]. Our results also confirm this.

829

Rheological properties of p o l y i s o b u t y l e n e

Neither of the equations proposed describes the viscous flow of P I B in t h e range of stresses which we have investigated. The best equation for the flow of P I B is the following one, proposed by one of us [7]: dT/dt = (P/~lo) e~P, (3} where P is the shear stress ( P ~ 0), rj0 is the viscosity at an infinitely low stress, and ~ is a constant which does not depend on the temperature and is 5.5 cm2/kg for PIB. At low stresses (P<
poise log~],_)oise

8l

9 6

.o

log ~l/~l~o •

2

~0

8

4

zxS

7 2

-2

. . , d x

--

6 fl

0"5 P, kg/cm z FIC. 4

1"0

2.6

I

lob~r, °K

3-0

1

3.4

FIG. 5

FIG. 4. Viscosity (1) and l o g a r i t h m of t h e viscosity (2) of p o l y i s o b u t y l e n e u n d e r s t e a d y flow conditions at 80 ° as f u n c t i o n s of the shear stress. FIG. 5. L o g a r i t h m of the r a t i o b e t w e e n t h e viscosity of p o l y i s o b u t y l e n e at a g i v e n t e m p e r a t u r e , ~/, a n d t h e viscosity at 20 °, ~/20, as a f u n c t i o n of t h e reciprocal absolute t e m p e r a t u r e . R e s u l t s o b t a i n e d at various shear stresses: 1--0"17; 2--0"31; 3--0.51; 4--0"73 k g / c m ~.

eosity ~ (P) and 7 = P / t l ( P ) are unambiguous functions of this parameter. Sine~ there is a completely definite connection between 7 and P under the conditions of steady flow, it is immateriM b y what m e t h o d steady flow is achieved (by

~30

G. M. BARTENEV a n d L . A. VISHNITSKAYA

¢he ~=const. method or the P = e o n s t . method). However, the investigation of unsteady flow is preferably carried out by the P-----eonst. method, since this gives a physically more definite picture of viscous flow. The viscosity r]0 does not depend on the shear stress and is a function only o f the temperature 77o-----Aev/k~ (5) where U is the activation energy of viscous flow, T is the absolute temperature, k is Boltzman's constant, and A is a constant which does not depend on the stress or the temperature. Since ~ and A do not depend on the temperature, the ratio t]/~20, where t/ is the viscosity at any temperature and ~/20 is the viscosity at 20 ° should not d e p e n d on the shear stress. The data of Fig. 5 confirm this, since at all stresses £he points lie on a single straight line corresponding to the equation U 1 l°g-~-q 72o -- 2Z-'~( ~

1 293)"

According to these results, the activation energy is 13.4 kcal/mole. The rule of logarithmic additivity. The viscosity of the steady flow of poly:isobutylene, as follows from the results of the preceding section and other investigations [1, 6, 7, 14] obeys the rule of the logarithmic additivity of viscosities or log t / / c = ~ log t/i(Xi) (6)

tT----c~l(T)~ (P) ~/3(M) ~4(~)

(7)

where c is a constant characterizing the structure of the polymer, T is the absolute temperature, P is the shear stress, M is the weight-average molecular weight, a n d ~ is the content by volume of active filler, denoted in formula (6) by the parameters Xi; ~l----exp (U)/kT); ~2=exp (--aP); ~ 3 ~ M n, where n-~3.5; and ~/4 expresses a complex relationship [14]. The rule of logarithmic additivity means that the mechanism of the flow o f linear polymers is such that all the parameters act on the viscosity independe n t l y of one another. It is applicable, of course, within certain, although wide, limits of change of the parameters--for example, when the molecular weight is greater than some critical value characterizing the transition from a lowmolecular-weight compound to a high-molecular-weight compound, when the content of active filler lies within the limits of compatibility of the filler and the polymer, and when the shear stress and the temperature are not so great that t h e y cause chemical flow and decomposition of the polymer, and so on. A very important practical conclusion follows from the rule of logarithmic additivity: the addition of an active filler and a change in the molecular weight o r the stress do not change the temperature coefficient of viscosity of the material,

/=theological properties of polyisobutylene

83I

even though they change the viscosity as much as can be desired in practice. It is known that the addition of an active filler causes the viscosity of P I B to rise sharply, b u t the activation energy remains constant [14]. A similar phenomenon is observed with an increase in the shear stress, with the only difference that the viscosity does not rise b u t falls. The mechanism of the viscous flow of polyisobutylene. In addition to the diffusion-segmental mechanism of flow proposed b y Eyring, another view has been developed of the mechanism of the viscous flow of a linear polymer in connection with the conception of a linear polymer as a structure cross-linked b y subsidiary or temporary connections [7, 15], which m a y be microbundles. According to this view, the flow of a polymer is regarded as the result of the rearrangement of temporary connections of a three-dimensional lattice under the action of thermal motion and external forces. The disintegration and restoration of the temporary connections of the lattice take place even in the absence of a stress. The higher the temperature the lower the number of unbroken nodes and, consequently the lower the initial viscosity ~/0. A stress displaces the equilibrium between the number of disintegrated and undisintegrated nodes in t h e direction of increasing the former and thereby affects the viscosity in accordaace with Formula (4). This has been explained [7] b y the fact that the shear stress decreases t h e probability of the restoration of the disintegrated lattice nodes, since after the disintegration of a node a stress favours the removal of the active sections of the macromolecules from one another at a velocity which is the greater the greater the stress. Thus, the probability of the restoration of the nodes diminishes and the average number of disintegrated nodes rises. The activation energy does not change under these conditions, since it is determined not b y the number of nodes b u t b y their nature. From this, it follows that the stress affects not t h e energy b u t the entropy of activation of the flow process. On the other hand, there is no doubt that the kinetic unit of flow in polymers is the chain segment, since the magnitude of the activation energy is small. The question arises of how the diffusion-segmental mechanism of viscous flow agrees with the role of temporary nodes, which, in all probability, are microbundles. For this purpose it is only necessary to assume that the mean length of a microbundle in polyisobutylene corresponds to the length of a segment (30-40 carbon atoms in the main chain), and its disintegration and formation takes place b y the detachment or coherence of the segments as individual kinetic units. CONCLUSIONS

New data are presented on the influence of the shear stress and the temperature on the viscosity of high-mo]ecnlar-weight polyisobutylene, and hypotheses have been p u t forward on the mechanism of viscous flow of linear polymers. A law of logarithmic additivities of viscosities has been formulated. Translated by B. J. HXZZARI)

832

G. II-IARDYet al.

REFERENCES 1. 2. 3. 4. 5.

6. 7. 8, 9. 10.

11. 12. 13. t4. 15.

T. FOX and P. FLORY, J. Amer. Chem. Soc. 70: 2384, 1948 V. A. KARGIN and T. I. SOGOLOVA, Zh. fiz. khim. 23: 540, 551, 1949 J. D. FERRY, Physica 6: 356, 1935; J. Amer. Ceram. Soe. 64: 1330, 1942 H. LEADERMAN, J- Polymer Sci. 13: 371, 1954 L. V. IVANOVA-CHUMAKOVA and P. A. REBINDER, Kolloidn. zh. 18: 429, 1956 N. V. ZAKHARENKO, F. S. TOLSTUKHINA and G. M. VARTENEV, Kolloidn. zh. 24: 168, 1960 G. M, BARTENEV, Dokl. Akad. Nauk SSSR 133: 88, 1960 H. A. POHL and C. G. GOGOS, J. Appl. Polymer Sci. 5: 67, 1961 T. I. SOGOLOVA, Thesis, Moscow, 1963 V. A. KARGIN, A. I. KITAIGORODSKII and G. L. SLONIMSKII, Kolloidn. zh. 19: 131, 1957 V. A. KARGIN, B. G. ZHURAVLEVA and Z. Ya. BERESTNEVA, Dokl. Akad. Nauk SSSR 144: 1089, 1962 R. EISENSCHITZ, Kolloidn. zh. 64: 184, 1933 L. H. TUNG, J. Polymer Sci. 46: 409, 1960 G. M. BARTENEV and N. V. ZAKHARENKO, Kolloidn. zh. 24: 121, 1962 Sh. HAYASHI, Suppl. Progr. Theoret. Phys. No. 10, 82, 1959

SYNTHESIS, POLYMERIZATION, AND COPOLYMERIZATION OF VINYL THIOACETATE* G. ~ R D Y ,

J . VARGA, K . N Y T R A I , I. T S A J L I K a n d L. ZUBONYAI

Scientific Research Institute for the Plastics Industry, Budapest; Budapest Polytechnic Institute (Received 21 October 1963)

T H E r e s u l t s of radiobiological i n v e s t i g a t i o n s h a v e s h o w n t h a t m e r c a p t o a m i n o d e r i v a t i v e s s u c h as 4 - m e r c a p t o b u t y l a m i n e , 3 - m e r c a p t o p r o p y l a m i n e [1], 1-merC a p t o p r o p y l a m i n e , a n d 2 - m e r c a p t o p r o p y l a m i n e [2], a n d t h e i r d e r i v a t i v e s , red u c e t h e h a r m f u l effect o f r a d i a t i o n on living o r g a n i s m s a n d are effective p r o t e c tive agents against radiation. T o a l t e r s o m e of t h e u n d e s i r a b l e p r o p e r t i e s of l o w - m o l e c u l a r - w e i g h t m e r c a p t o c o m p o u n d s ( u n p l e a s a n t odours, r a p i d e l i m i n a t i o n f r o m t h , o r g a n i s m ) O v e r b e r g e r et al. [3, 4] h a v e s y n t h e s i z e d p o l y m e r s c o n t a i n i n g a m e r c a p t o g r o u p which a r e also effective p r o t e c t i v e a g e n t s a g a i n t s radiations. I t a p p e a r e d to be o f i n t e r e s t to s y n t h e s i z e m a c r o m o l e c u l e s similar to the m e r c a p t o a m i n o c o m p o u n d s m e n t i o n e d a b o v e . Since t h e c o r r e s p o n d i n g m o n o m e r s ( v i n y l a m i n e , v i n y l m e r c a p t a n ) are c h e m i c a l l y u n s t a b l e c o m p o u n d s , p o l y m e r s * :Vysokom0L soyed. 6: No. 4, 758-765, 1 964.