Effect of temperature on the kinetics and equilibria of the adsorption of polystyrene on macroporous silica gels

V14

YE. K. BOGACHEVi and:Yu. A. EL'TEKOV

7. A. B. ZELDOVICH and V. V. VOYEVODSKII, Teplovoi vzryv i rasprostranenie plameni v gazakh (Thermal Explosions and Flame Propagation in Gases). Izd. Mesk. mekh. inst., 1947 8. K. AIVIN, Polymer Chem. and Technol., No. 3, 52, 1963 9. Kh. S. BAGDASARYAN, Teoriya radikalnoi polimerizatsii (Theory of Radical Polymerization). Izd. "Nauka", 1966 10. V. I. P E R ~ , Kratkii spravoehnik khimika (Shorter Handbook for Chemists). Gosizdat, 1955 l l . Ye. T. DENISOV, Konstanty skorosti gomolitieheskikh zhidkophaznykh reaktsii (Rate Constants of Homolytical Liquid-Phase Reactions). p. 18, Izd. "Nauka", 1971 t2. K. BA1KFORD, W. BARB, A. JENKINS and P. ONION, Kinetics of the Radical Polymerization of Vinyl Compounds, p. 106, 1961

EFFECT OF TEMPERATURE ON THE KINETICS AND EQUILIBRIA OF THE ADSORPTION OF POLYSTYRENE ON MACROPOROUS SILICA GELS* YE. K. BOGACHEVA a n d Y u . A. EL'TEKOV Physical Chemistry Institute, U.S.S.R. Academy of Sciences

{Received 7 July 1972) The kinetics of adsorption of polydisperso polystyrene (M~300,000) from CC14 ~elutiens on macroporous silica gels were investigated at 20, 50 and 80% I t is shown that the rate of diffusion of the maeromolecules of polystyrene with an~average molecular weight of 300,000 is greatly reduced if the diameter of the macromolecular coils becomes close to that of the silica gel pores. As the temperature rises, the rate of diffusion of the polystyrene macromolecules into silica gel pores is markedly increased owing to the increased segmental mobility of the maeromolecules. It was found that as the temperature rises and the pore dimensions of the adsorbent grew larger, the fraction of the surface ~hat is accessible for the adsorption of the polystyrene (M ~ 300,000) maeromolecules becomes larger as a result of the increased mobility of the macromolecular chains, and on account of the diminished energy of solvent adsorption. ~J~EMPERATURE v a r i a t i o n s occurring in polymer-solvent--filler a d s o r p t i o n systems m a y h a v e a m a r k e d effect on a d s o r p t i o n equilibria [1-8]. This effect a c c o m p a n y i n g a rise in t e m p e r a t u r e is a p p a r e n t l y due m a i n l y to a w e a k e n i n g of t h e polymer-filler a n d solvent-filler a d s o r p t i o n bonds, as well as configurational changes o c c u r r i n g in p o l y m e r macromolecu!es in solutions. However, the effect in question h a s y e t to be explained. Some single f a c t o r could be of p r e d o m i n a t i n g i m p o r t a n c e * Vysokomol. soyed. A16: No. 3, 617-621, 1974.

Kinetics of adsorption of polystyrene

715

for various systems, a n d accordingly with some systems it is f o u n d t h a t a reduction in adsorption, as is t y p i c a l for a d s o r p t i o n processes, occurs as the t e m p e r a t u r e rises. I t was n o t e d in p a p e r s [1, 4-6] t h a t t h e degree o f a d s o r p t i o n rises with rising t e m p e r a t u r e , in c o n t r a s t to w h a t is characteristic of the physical a d s o r p t i o n of gases, and a d s o r p t i o n f r o m solutions o f o r d i n a r y molecules, assuming unlimited m u t u a l solubility of the components, w h e n one would n o r m a l l y find t h a t the a m o u n t of a d s o r p t i o n decreases as the t e m p e r a t u r e rises. I n particular, it has been d e m o n s t r a t e d t h a t p o l y m e t h y l a c r y l a t e a d s o r p t i o n f r o m toluene solutions on a l u m i n i u m powder, sand a n d glass-fibre increases as t h e t e m p e r a t u r e rises. H o w e v e r , the a d s o r p t i o n of p o l y v i n y l chloride f r o m t h e same solvent b y t h e same a d s o r b e n t s decreases as t h e t e m p e r a t u r e rises, while p o l y s t y r e n e a d s o r p t i o n u n d e r similar conditions is practically i n d e p e n d e n t o f t e m p e r a t u r e [1]. I t was f o u n d t h a t p o l y s t y r e n e a d s o r p t i o n f r o m a p o o r solvent increases as t h e t e m p e r a t u r e rises. This was a t t r i b u t e d to increase in t h e dimensions of seconda r y formations, n a m e l y m a e r o m o l e c u l a r bundles, a n d to a weakening of the intera c t i o n b e t w e e n these bundles [4-6]. I n t h e a u t h o r s ' view a rise in t e m p e r a t u r e leads.in this case to b r e a k d o w n of the s t r u c t u r e of t h e solution a n d t o intensified t r a n s f e r of macromolecules to the surface, i.e. n o t individual macromolecules, b u t bundles o f the latter. T h e increased a d s o r p t i o n o f p o l y m e t h y l a c r y l a t e f r o m dilute aqueous solutions on a l u m i n i u m oxide, which occurs as the t e m p e r a t u r e rises, was a t t r i b u t e d to a lowering of the second virial coefficient, i.e. to r e d u c e d i n t e r a c t i o n of the macromolecules w i t h solvent [7]. I n this case the size of the m a c r o m o l e c u l a r coils is reduced. T h e passage of these coils to the surface leads to increased adsorption. I n this p a p e r we give the results of our investigation of the effect of t e m p e r a t u r e on t h e kinetics a n d equilibrium of p o l y s t y r e n e (PS) a d s o r p t i o n f r o m CCl~ solutions b y u n i f o r m l y porous silica gels with different pore dimensions.

EXPERIMENTAL

The experiments were performed with samples of silica gels having the structural characteristics given in the Table. The specific surface s was calculated by the BET method from the adsorption of erypton vapours at --196 °. The pore volumes V8 were calculated from the limiting value of the adsorption of benzene vapours, and the preferential pore diameters d were obtained from the desorption isotherms of adsorption of benzene vapours, using the Kelvin equation. Prior to use in the experiments the samples were warmed up in a drying chamber at 200 °. The unfraetionated polydisperse PS with M~300,000 used for the adsorption investigations was prepared by bulk polymerization. The solvent was CC14. The method described in paper [8] was used for measuring the kinetics and equilibria of the adsorption of PS. The mixing was done in a temperature-controlled apparatus. The Gibbs adsorption F was

716

YE. K. BOOAC~r~.VA and Yu. A. E L ' ~ x o v

calculated in accordance with the formula

(Co-C)~, F= - - ,

(1)

in which ¢0 a n d c are the PS concentrations in CCI4 before and after adsorption, rag/g, an d mp and ma are the solution (PS) and adsorbent quantities in grams.

DISCUSSION OF RESULTS

As m a y be seen from Fig. 1, negative PS adsorption was observed for C1 in the first minutes of the l~rocess, whether at 20 or 80 °. I n the case of C2 a small a m o u n t of negative adsorption appeared at 20 ° . The negative adsorption in the

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Kinetics of adsorption of polystyrono

717

first stage of the process is due to solvent molecules diffusing rapidly into silicagel pores and then, moments later, being expelled by the small PS macromoleeules. I t is characteristic t h a t a rise in temperature accelerates the process of expulsion of solvent molecules b y polymer molecules. However, the amount of PS adsorption by C1 (in 4 hr) still remains quite small (~0.01 mg/m2). In the case of C8 adsorption equilibria are rapidly established at 20, 50 and 80 °. For C8 the adsorption equilibrium appears in only 1 hr at 80°; the increased adsorption of PS by C8 when the temperature rises is apparently due, for the most part, to a weakening of solvent adsorption. The "limiting" degrees of PS adsorption (F~) from solutions in CC14 for the studied samples are equal to the mixing of a suspension of a 0.5% PS solution with 0.5 g of an adsorbent (see Table) over a 2-week period. In the case of the wide-pore samples the F~ values m a y be regarded as being the equilibrial values. As the temperature rises, the values of F~ are doubled or trebled for C4 and C5, while for C7 and C8 the values in question remain practically constant when the temperature is raised. Given a contact time of ~-----1 hr, the relative adsorption ratios (F/F~) and similarly the To.5 values, which correspond to the time required for a degree of adsorption equal to haft the limiting (equilibrial) degree of adsorption to take place, show t h a t pore size and temperature have a major effect on the kinetics of adsorption (see Table). The calculated effective coefficients of internal diffusion [9] of PS macromolecules into silica gel pores D=0.049R2~.~

(2)

(R -- a silicagel grain radius equal to 4.5 ram) v a r y according to pore size, more than 200-fold variations appearing at 20 °. The value of D rises with rising temperature; for C6 D is increased by one order of magnitude if the temperature rises from 20 to 80 ° . The markedly increased dependence of ¢0.5 on the size of the silicagel pores at 20 °, and when d = 2 2 0 / ~ , points to very slow diffusion of the PS macromolecules into the pores, in view of the fact that the dimensions of the macromoleeules are similar to those of the pore apertures (Fig. 2). In dilute CC14 solutions the size of the coiled PS macromoleeules is close to 300 /~, as these globules enter fairly easily into sflieagel pores with apertures larger than 400 A, and only with difficulty into silicagel pores with apertures smaller than 250 A [7]. The D value for PS macromolecules in respect to pores of 200 /~ is lower by three orders than the D value for diffusion into wide pores (500-800/~). As m a y be seen from Fig. 3, the degree of'equilibrium adsorption rises with rising temperature for all three samples. A particularly marked increase in PS adsorption at 50 ° is observed for C5. As the temperature rises, the physical adsorption of solvent molecules weakens to a greater extent than the adsorption of polymer macromoleeules. This leads to increased PS adsorption. This is explained by the fact that the macromoleeules become more mobile, diffuse more

718

YE. K. BOGACHEVA and Y u . A. EL'TEKOV

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Kinetics of adsorption of polystyrene

719

rapidly into the pores of the adsorbent, and expel solvent molecules from the pores, so t h a t t h e degrees o f a d s o r p t i o n come dose to t h e equilibrium values. T h e r e f o r e as t h e t e m p e r a t u r e rises, the e x t e n t of t h e accessible surface of t h e

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FIG. 2. Dopondonco of v0.5 on d at 20 (•); 50 (2) and 80° (3). FIo. 3. Isotherms of PS adsorption from solutions in C,C14 by silieagols C7 (1, 2); C5 (3, 4) a~d C2 (6, 6) at 50 (1, 3, a) and 20 ° (2, 4, 6). porous silieagels is increased. As t h e average d i a m e t e r o f the silicagel pores increases, t h e e x t e n t of the accessible surface s' of the silicagels, d e t e r m i n e d f r o m the P S a d s o r p t i o n at 20 °, is increased (see Table). Naturally, for C2 t h e a m o u n t o f surface accessible for P S macromolecules (s'/s) a m o u n t s to several percentages. F o r 07 almost the entire inner surface is accessible for a d s o r p t i o n of t h e p o l y m e r macromolecules a t 50 °. The ratio of t h e accessible surface, determ i n e d f r o m t h e a d s o r p t i o n o f P S macromolecules, to the surface d e t e r m i n e d f r o m the low t e m p e r a t u r e a d s o r p t i o n of nitrogen (the B E T method) rises m a r k edly, p a r t i c u l a r l y for C2, as the a d s o r p t i o n t e m p e r a t u r e is raised.

Translated by

R. J. A. I~ENDI%Y

REFERENCES

1. E. $ENCKEL and B. RUM~ACH, Z. Elektroohem. 55: 612, 1951 2. Yu. S. LIPATOV, L. M. SERGEYEVA and V. P. MAKSIMOVA, Vysokomol. soyed. 2: 1569, 1960 (Not translated in Polymer Sci. U.S.S.R.) 3. Yu. S. LIPATOV, Fiziko-khimiya napolnennykh polimerov (Physical Chemistry of Filled Polymers). Izd. "Naukova dymka", 1967 4. Yu. S. LIPATOV and L. 1%I.SERGEYEVA, Kolloidn. zh. 27: 217, 1965

720

V . P . BuDTov and A. M. LOBANOV

5. lVL N. SOLTYS, I. I. MALEYEV, T. M. POLONSKII and I. M. MJKITYUK, Poverkhnostnye yavleniya v polimerakh (Surface Phenomena in Polymers). Izd. "Naukova dumka", 1970 6. T. M. POLONSKTI, M. N. SOLTYS and I. I. M2tI.I~.YEV,Vestnik Lvov Univ., chem. series, No. 9, 1966 7. Ye. K. BOGACHEVA, A. V. KISELEV, Yu. S. NIKITIN and Yu. A. EL'TEKOV, Vysokotool soyed. AIO: 574, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 3, 610, 1968) 8. Ye. K. BAGACKEVA, A. V. KISELEV and Yu. A. EL'TEKOV, Kolloidn. zh. 26: 458, 1964 9. J. CRANK, The Mathematics of Diffusion, London, 1956 i

ANALYSIS OF CONCENTRATION DEPENDENCES OF DIPOLESEGMENTAL AND DIFFUSION PROCESSES AND THE VISCOUS FLOW OF POLYMER SOLUTIONS* V. P. BUDTOV and A. M. LOBANOV Plastpolymer Scientific and Industrial Association

(Received 10 July 1972) Correlations were obtained for the concentration dependence of the dipole-segmental relaxation time, the "activation heat" of dil2ole-segmental relaxation and the self-diffusion of solvents in polymer solutions, as well as for the "activation heat" of viscous flow. The obtained correlations are in agreement with experimental results. I t is shown that in cases where the kinetic units of chain molecules and solvent are equal, the coefficients in the correlations agree with the theoretical coefficients. I t is generally necessary to take account of structural dissimilarities in the elementary segments of the polymer chains and solvents. I~VV.STmATIONS Of m o l e c u l a r r e l a x a t i o n processes in p o l y m e r solutions are o f i m p o r t a n c e w h e t h e r f r o m t h e p o i n t of v i e w of d e t e r m i n i n g t h e kinetic p r o p e r t i e s a n d s t r u c t u r e of p o l y m e r chains, or w i t h a v i e w to t h e elucidation o f m a n y p h e n o m e n a r e l a t i n g to t h e field of a p p l i e d p h y s i c a l c h e m i s t r y of p o l y m e r s . T h e processes in question are d e t e r m i n e d b y t h e r a t e of m o l e c u l a r r e a r r a n g e m e n t o f small s e g m e n t s of c h a i n molecules, a n d this in t u r n c h a r a c t e r i z e s t h e "local v i s c o s i t y " in c o n c e n t r a t e d solutions. Changes in local v i s c o s i t y r e l a t i v e t o p o l y m e r c o n c e n t r a t i o n s m u s t also b e t a k e n into a c c o u n t in t h e process of interp r e t i n g t h e v i s c o s i t y of c o n c e n t r a t e d solutions [1]. This p a p e r relates to o u r analysis a n d t h e correlations o b t a i n e d for t h e conc e n t r a t i o n d e p e n d e n c e of t h e m o s t p r o b a b l e t i m e a n d " a p p a r e n t h e a t of a c t i v a t i o n " of d i p o l e - s e g m e n t a l losses, a n d t h e self diffusion of solvents a n d viscous flow of p o l y m e r solutions o v e r a wide r a n g e of c o n c e n t r a t i o n s . * Vysokomol. soyed. AI6: No. 3, 622-626, 1974.