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Study of adsorption of polymer mixtures from solutions in a common solvent on a solid surface
Polymer Science U.S.S.R. Vol. 23, 1~'o. 11, pp. 2841-2649, 1 9 8 1 Printed in Poland
0032-$950/81/112841-09807.50]0 © 1982 Pergamon Press Ltd.
STUDY OF ADSORPTION OF POLYMER MIXTURES FROM SOLUTIONS IN A COMMON SOLVENT ON A SOIJD SURFACE* Yu. S. Ln~ATOV, L. M. SERGEYEVA, G. M. SEMENOVICH, T. T. TODOS~OHUX, L. V. D U B R O W ~ A and V. N. CHORXAYA I,istitute of Chemistry of High Molecular Weight Compounds, IYkr.S.S.R. Academy of Sciences (Received 1 April 1980) The authors have studied the adsorption and determined the fraction of bound segments P in the systems polybutyl methacrylate-butadiene-nitrile rubber-Aerosil and polybutyl methacrylate--polystyrene-Aerosil. The isotherms were obtained with extremes the position of which on the adsorption isotherms depends on the nature of the polymer and concentration of the solution. The value p changes within wide limits with the concentration of the solution from which the adsorption layer is formed. The mutualinfluence of the polymers studied on adsorption from the mixtures was found. I t was established that the nitrile rubbers from mixtures with polybutyl methacrylate are not adsorbed although they have an important influence on the -~dsorption of polybutyl methacrylate and the value p. I t is shown that for a different adsorbent-solution ratio the values of p and the form of the isotherms significantly change both for the individual solutions of the polymers and their mixtures. The results are explained in terms of the molecular-aggregative mechanism of adsorption. ADSOm~rm~ f r o m m u l t i - c o m p o n e n t p o l y m e r s y s t e m s has b e e n s t u d i e d o n l y i,1 a few c o m m u n i c a t i o n s [1-5]. Y e t in t h e p r a c t i c e of o b t a i n i n g filled p o l y m e r s a n d covers as a rule m i x t u r e s of p o l y m e r s or oligomers are used so t h a t s t u d y o f selective a d s o r p t i o n a n d also t h e s t r u c t u r e of t h e a d s o r p t i o n a n d b o u n d a r y l a y e r s f o r m e d f r o m such s y s t e m s a s s u m e s g r e a t i m p o r t a n c e [6-8]. Earlier, in t h e case o f t h e s y s t e m r u b b e r - e p o x i d e r e s i n - a m m o n i u m chloride [9, 10] we e x p e r i m e n t a l l y s h o w e d t h a t t h e selective a d s o r p t i o n of one o f t h e c o m p o n e n t s of t h e c o m p o s i t e on a solid surface leads to a n u n e v e n d i s t r i b u t i o n o f t h e m a c r o m o l e c u l e s o f t h e p o l y m e r s of a different n a t u r e in t h e b o u n d a r y l a y e r o v e r its thickness, w h i c h m u s t seriously affect t h e final p r o p e r t i e s of t h e c o m p o s i t e polymer materials. I n t h e p r e s e n t w o r k we m a k e a d e t a i l e d s t u d y o f t h e a d s o r p t i o n a n d s t r u c t u r e o f t h e a d s o r p t i o n l a y e r s in t w o - c o m p o n e n t p o l y m e r s y s t e m s . For the investigation we chose the following systems: polybutyl methaerylate (PBMA) (2P/~= 24,000) q-nitrile rubber (SKN)-kAeresil and PMBA ( / ~ = 100,000)-bPS-}-Aerosil. Commerical PBMA and PS (~v----50,000) were used and also SKN-26M (/Q'v~ 170,000) and SKN-40 (/~/v----200,000). The polymers were purified by precipitation, Aeorsil immediately * Vysokomol. soyed. A28: No. 11, 2436-2443, 1981.
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V]o'. So LIPATOV e$ ~ .
before the experiments was heated at 800°C in a muffle furnace. For the first system the solvent was chloroform and for the second carbon tetraehloride. The solvents were purified and dried by standard methods. The investigations were by the method of IR spectroscopy with the UR-20 spectrophotometer. The extent of adsorption was judged from change in the concentration of solution before and after adsorption. The analytical bands were those of the valent vibrations of the C = O groups in PBMA, the C--N groups in the rubbers and the deformation vibrations of the substituted aromatic ring in PS. The values of adsorption and the fractions of bmmd segments p were determined both on individual solutions and from a mixture by the method described in reference [11]. For the nitrile rubbers and PS we determined the pK value (K~-8[Mis a constant; 8 is the molar extinction coefficient of the band of valence vibrations of the C ~ N groups in rubber or the bands of the deformation vibrations of the substituted aromatic ring in PS; M is the equivalent weight of the corresponding group). Adsorption was carried out in static conditions. Investigation of the course of adsorption shows that adsorption equilibrium was set up in 30-48 hr depending on the nature of the polymer.
System PBMA+butadiene nitrile rubber+Aerosil. Analysis o f t h e I R spectra of the gels of the individual solutions o f P B M A established t h a t the earbonyl groups of adsorbed P B M A form a h y d r o g e n b o n d w i t h the h y d r o x y l groups o f Aerosil. I n the s p e c t r u m of the gel in the region of t h e valence vibrations a wide absorption b a n d o f the b o u n d h y d r o x y l groups appears w i t h a m a x i m u m a t 34.20 × 104 m -x, the b a n d of the free h y d r o x y l groups because of the p e r t u r b i n g influence of the solvent shifts to 36.87 x 104 m - l ; t h e a b s o r p t i o n b a n d o f t h e valence vibrations of the carbonyl group splits into two with m a x i m a at 17.30>( × 104 m -1 (absorption of the free C - - O groups) a n d a t 17.08× 104 m -1 ( b o u n d C = O groups). F r o m this b a n d using the F o n t a n a m e t h o d [11] we d e t e r m i n e d t h e fraction of b o u n d segments 10 of PBMA. I n the spectrum o f the gels of rubbers in the b a n d of t h e valence vibrations o f the C - - N groups a high f r e q u e n c y wing appears due to a d s o r p t i o n of the nitrile groups i n t e r a c t i n g w i t h the surface o f Aerosil. T h e shift of the absorption b a n d of the h y d r o x y l groups in the s p e c t r u m o f Aerosil to 3 3 . 7 4 x 10' m -1 indicates t h e i r strong p e r t u r b a t i o n similar to t h e case of f o r m a t i o n of the h y d r o g e n bond [12]. T h e a d s o r p t i o n isotherm of P B M A constitutes a curve with distinct e x t r e m e s (Fig. la). T h e e x p e r i m e n t a l l y d e t e r m i n e d value 10 of the p o l y m e r is 0.4 in the region of equilibrium concentrations of 4-6 g/l., b u t with rise in the c o n c e n t r a t i o n diminishes, falling in the region of the m a x i m u m on t h e adsorption isotherm t o 0.15; f u r t h e r rise in the concentration leads to rise in p (Fig. 13). M a x i m a in t h e adsorption isotherms is observed for a n u m b e r of p o l y m e r systems [13-16] a n d is a characteristic f e a t u r e of the a d s o r p t i o n of pol3mers from soiutions providing adsorption is studied o~ er a wide concentrati,::~ range, xNnmerous e x p e r i m e u t s of ~ ari )us researchers iDdieate t h a t the max;ran ~.r,, n o t t h e result of errors or c,mditions o f nonequiLibrium in the a d s o r p t i o n system, nevertheless the reasons for their appearance have n o t been fully clarified. We consider t h a t t h e n o n - m o n o t o n i e i t y of change in the value o f a d s o r p t i o n w i t h c o n c e n t r a t i o n is due to t h e moleeular-aggregative m e c h a n i s m of adsorption
Adsorption of polymer mixtures from solutions
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assuming passage from the solutions to the surface of the adsorbent both of t h e individual macromolecules and their associates [1, 14, 15]. For the system in question the formation of associates (or aggregates) begins at a concentration of 10 g/1. After this concentration sharp increase in adsorption of the individual PBMA is observed with a considerable fall in the fraction of bound segments which we attribute to the predominant adsorption of the aggregates in this concentration region [16, 16].
A,g,~
o
o.j! 0.! 0
°
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I
I
I
I
0.6
I.B
2.8
3"g
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FzG. 1. Adsorption isotherms (a) and fraction of bound segments (b) of PBMA adsorbed from pure chloroform (1) and mixture (2); 3--absolute values of adsorption of SKN-40. With rise in the concentration of the solution of aggregates enlarge. Evidently, the fraction of segments of such aggregates bound to the surface falls as compared with the smaller aggregates, as witness the fall in p. In the concentration range 18-26 g/l. corresponding to the region of the maximum on the isotherm, the value p does not change, so that it m a y be concluded that the size of the adsorbed aggregates does not change. ]~arther increase in the concentrations of the solutions leads to fall in adsorption. ~Ve would recall that one of our studies [14] showed that the aggregation constant Ka characterizing the ratio of the number of "aggregated" molecules to the total number of molecules falls with increase in concentration, in other words, the degree of aggregatability of the maeromolecules in solution decreases and the ratio of the aggregated and isolated macromoleculcs passing to the surface also changes. Possibly after the point of the maximum on the adsorption isotherm the fraction of isolated molecules passing to the surface rises and, therefore, t h e absolute value of adsorption drops and the value p rises, as Fig. lb shows. Another possible cause of the drop in adsorption after the concentL'ation 26 g/1. m a y be the formation of a fluctuation structural network. In the region of low concentrations of the solutions (1-8 g/1.) judging from the magnitudes of p, which have much higher values, we postulate the adsorption of the isolated PBMA chains. The small maximum and the following minimum in
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Yu. S. l.J2~Tov et al.
this concentration region m a y be explained b y conformationa, l changes in the macromolecules preceding the formation of associates. The adsorption of the polymers from the mixture in solution was studied with a constant content of one of the components in the adsorption system and rise in the concentration of the other. It was shown that the nitrile rubbers introduced into PBMA solution (the concentration of rubber in all cases was 4 g/l.) are practically not adsorbed over the range of compositions of the PBMA solution (0.5-29 g/1.) but have a significant influence both on the absolute values of adsorption of PBMA and the fraction of bound segments characterizing the structure of its adsorption layer. The rubber SKN-40 particularly effectively influenced the adsorption of PBMA. Figure 1 (curve 2) presents the adsorption isotherm for PBMA in presence o f SKN-40 indicating that the rubber not itself adsorbed changes the character of the adsorption of PBMA; in the concentration range 2-14 gfl. the adsorption of PBMA rises b u t with further rise in the concentration diminishes as compared with the value of adsorption of PBMA from pure solvent, i.e. inversion occurs as a function of adsorption against the concentration of solution. In analysing the causes of the observed inversion one must bear in mind that addition to the PBMA solution of a rubber incompatible with it thermod3ummically is analogous to worsening of the quality of the solvent. It is known [1] that from poor solvents in the region of dilute solutions the adsorption of polymers, as a rule, is greater that from good ones because of the fact that in poor solvents the macromolecules interact less with the solvent and are more compact. However, the fraction of segments of the molecules binding to the surface is less as compared with the maeromolecules adsorbed from good :solvents. Probably, therefore, in the initial concentration region the adsorption of PBMA from the mixture rises and the value p considerably falls. In the region of high concentrations where the degree of intermoleeular and interaggregate interaction of PBMA is greater in the poor ("mixed") solvent than in pure chloroform (good solvent) the adsorption of PBMA diminishes. The fraction of bound segments also falls but not so much as in the region of low concentrations. A , g/g
P
a
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1.6
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~1
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:FI(~. 2. Adsorption isotherms (a) and pK ftmotion (b) of rubber adsorbed from chloroform (1) and mixture (2); absolute values of adsorption and fraction of bound segraents of PBM.A (3) adsorbed from mixture with SKN-40.
Adsorption of polymer mixtures from solutions
2645
In the region of high concentrations (above Cequ30/g~l.) from the mixture of polymers rubber begins t.o be adsorbed with a heavy drop in the adsorption of PBMA, i.e. inversion of adsorption due to structural factors is observed. Let us look at the adsorption of the rubber SKN-40 from pure chloroform (Fig. 2, curve 1) and from the mixture with PBMA (Fig. 2, curve 2) the amount of which was constant at all concentrations of rubber (CPB~A4 g/l.). The value of adsorption of SKN-40 falls above the equilibrium concentration of 2 g/]. It may be supposed that in the rubber solutions structuring is more marked than in PBMA solutions in the region of the same concentration as a result of considerable differences in the molecular masses of the polymers studied and therefore aggregates also form at much lower concentrations; the heavy structuring of the rubber starting at C,~u-~3 g/1. lessens adsorption. At the limiting concentration possible for carIsring out the experiment (12 gfl.) the value of adsorption of the rubber is very small but the value p sharply rises, which serves as a basis for concluding that adsorption of isolated molecules and not aggregates is predominant in this concentration region. The introduction into the rubber solution of PBMA sharply reduced adsorption in the interval of equilibrium concentrations 1-4 gfl. and the isotherm shifts to higher concentrations. Probably the introduction of PBMA influences the structuring in solution and shifts the start of the formation of the structural mesh to the region of high concentrations. It is interesting to note that PBMA is adsorbed from the mixture uniformly over the whole concentration range whatever the concentration of rubber. There is only change in the value p which is high for a low concentration of rubber, then sharply falls at ¢equ---~4g/l. i.e. for the same value of adsorption, adsorption layers of different structure form. Thus, the investigations indicate the mutual influence of the polymers on adsorption from the two-component systems. Even when one of the components is not adsorbed, it changes the character of the adsorption of the other, which is connected with complex changes in the structure of the solution (size of associates, degree of their interaction with adsorbent and each other, etc.) with rise in the concentration. System PBMA-~-PS~Aerosil. It is known that the value of adsorption on solid surfaces and the character of the adsorption isotherms of low-molecular weight substauces from solutions with the condition of reaching adsorption equilibrium do not depend oi1 the adsorbent-solution ratio. Differences in the adsorption of polymers from solution from that of low molecular weight substances have now been clearly established [1, 16, 17]. They are determined by the fact that to each concentration of polymer solution correspond different sizes of the polymer coils as a result of their dependence on concentration. This fact and also the possibility of aggregative phenomena bringing into play the molecular-aggregative mechanism of adsorption mean that, on the one hand, at each concentration a new equilibrium is set up between the associated and non-associated macromolecules and, on the other, to each point of the isotherm corresponds a different
Yu. S. LIPATOVet al.
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structure of the elements adsorbed on the surface; from isolated coils of different size to their aggregates or associates and also simultaneous adsorption of both. From this viewpoint one may expect deviations in the behaviour of adsorption of polymers depending on the adsorbent-to-solution ratio. Some published findings support such an assumption [18-20] showing t h a t not only the value of adsorption but also the form of the isotherms depend on this ratio. We examined this problem in relation to the system PBMA+PS+Aerosil. Figures 3-5 present the results of study of adsorption of PS and PBMA for different adsorbent-to-solution ratios. It is worth noting the change in the value of adsorption and the form of the isotherm with the nature of the polymer and amount of adsorbent. We explain such changes on the basis of the molecularaggregative mechanism of adsorption proposed by us earlier. Structure formation in solutions of PS and PBMA before and after adsorption has been studied by the method of the turbidity spectrum [21, 22]. It was established that the initial solutions of polymers contain aggregates the size of which rises with the concentration of the solution. The nature of the polymer in the case studied insiginificantly influenced the size of the associates except for the fact that in the region of high concentrations of PBMA sharper increase in size w a s observed. In the solutions immediately after adsorption aggregates were not detected, which serves as proof of their passage to the surface of the filler.
A,~I/g
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0.3I~ 0.1~ o ~ . . 1.2
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b
c , to-, g/t.
FIG. 3. Adsorptionisotherms (a) and pK value (b) for PS adsorbed from sglution in GC1,for a content of Aerosil of 10 (1); 20 (2) and 40 (3) g/1. Together with the aggregates non-aggregated molecules are also adsorbed. Probably the ratio of aggregated to non-aggregated molecules passing to the surface depending on the concentration of the solutiol~ and the adsorbent will change. This assumption is legitimate if only because the value of the aggregation constant /Ca characterizing the ratio of the aggregated molecules to the total number of molecules in solution changes with concentration. In connexion with this the value of adsorption may change with concentration non-monotonically as Figs. 3 and 4 show. The dependence of the fraction of bound segments on the concentration of the solution changes antibatically to the adsorption isotherms. Let us consider the dependence of adsorption of polymers on the amount of adsorbent in the system. With rise in the concentration of the adsorbent in the
Adsorption of p o l y m e r m i x t u r e s from solutions
2$47
system from 10 to 20 g/l. the absolute value of adsorption of PS changes insignificantly. The forms of the isotherms also change little but at 40 g/1. the value of adsorption is considerably higher and the form of the isotherm substantially changes. For PBMA a somewhat different picture was observed: with increase in the content of Aerosil from 10 to 20 g/l. the value of adsorption declined but with further increase in the amount of adsorbent again rose.
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:Fzo. 4. Adsorption isotherms (a) and fraction of bound segraents (b) for PBMA adsorbed for solutions in CC14 for a content of Aerosil of 10 (•); 20 (2) and 40 (3) g/1.
The fraction of bound segments is least in the adsorption layer formed from the smaller of the content.s of adsorbent studied. On adsorption from a mixture of polymers for a different content of Aerosil (Fig. 5) inversion of such a kind was observed: while for a smaller amount of adsorbent individual PBMA was adsorbed to a lesser degree, from the mixture its adsorption was greater for the same content of Aerosil. While for a high content of Aerosil only PBMA was adsorbed, as in the system considered earlier, for a lower content of Aerosil together with adsorption of PBMA PS was also adsorbed i.e. in the mixture increase in the content of adsorbent promotes decrease in adsorption of PBMA and reduces to zero the adsorption of PS. (2
pK-
P x
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FzG. 5. Adsorption isotherms (a) and fraction of bound segments (b) for P B M A (1-3) and PS (4) adsorbed from mixture in CC14; content of Aerosih 10 (•); 20 (2, 4) and 40 (3) gfl.
Probably the adsorbent materially changes the character of the structures in the system by changing the ratio of aggregates and isolated molecules and also changing the size of the aggregates as a result of their possible destruction, which is especially noticeable with change in the adsorbent-to-solution ratio. One m a y admit the possibility of formation in the adsorption system of aggregates
2648
Ylr. S. L x P A ~ v et a/.
like the maeromolecules of different molecular mass, different "activity" in relation to the adsorbent and change in their share in the total amount for different contents of Aerosi]. Further special aspects arise in analysing adsorption from mixtures of polymers where probably there is complex change in the ebaracter of the thermodynamic interaction of polymers with one another in solution and each polymer with solvent in the systems studied. The thermodynamic incompatibility of the components allows us to consider the solution of the mixtalre of the two polymers as a solution of one polymer in the solvent representing the solution of the second component. Such a solvent will as a result of thermodynamic incompatibility of the components be thermodynamically poor as compared with the pure solvent, which promotes aggregation of the molecules of the first component and their preferential adsorption from the mixture. This is reflected both in the absolute values of adsorption and the character of the isotherms. The diversity and complexity of the processes occurring on adsorption of the polymers studied are indicated by the sizes of the fractions of the bound segments 1o in the adsorption layers formed both from solutions of the polymers in pure solvents and from the mixtm-e of Polymers. As Figs. 3-5 show, there is non-monotonic change in 10 with rise in the concentration of the solution from which the layer formed. At low concentrations of the solutions 0.1-2 g/l., as a rule, the va]ue1~ is considerably higher, which is natural since in this concentration region to the surface probably pass the isolated molecules since aggregates are not detected here. The complex relations of 10 with the concentration of the solution for a different content of adsorbent are due to the fact that a change in the amount of adsorbent possibly changes, the quantitative ratio of the aggregates and the individual molecules passing into the adsorption layer, which is reflected in the value of 10. The values of 10 for the adsorption layer of PBMA formed from the mixture of polymers and the individual solutions also differ significantly. Comparison of the results obtained with the adsorptaion isotherms showed, as stated above, an antibatic relation between the value 10 and the value of adsorption both for the individual solutions and mixtures. Thus, these findings for the first time make it Possible to identify clearly a further fundamental feature of equilibrium adsorption of polymers from solution expressed by the fact that for a different adsorbent-to-solution ratio the values of adsorption, the degree of binding of the segments of the macromolecules by the surface and the form of the isotherms materially change. All these changes occur in that concentration region of solutions where the processes of aggregation of the macromolecules are observed and concentration-dependent equilibrium between the aggregated and non-aggregated macromolecuies is set up. The latter determines the possibility of simultaneous passage to the surface both of the isolated maeromolecuies and their aggregates. The different changes in the free energy on adsorption of both finally lead to the influence of the adsorbent-to-solution ratio on the adsorption of polymers in equilibrium conditions.
Adsorption of polymer mixtures from solutions
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:REFERENCES 1. Yu. S. LIPATOV and L. M. SERGEYEVA, Adsorbtsiya polimerov (Adsorption of Polymers). "Naukova dumka", Kiev, 1972 2, C. THIES, J. Phys. Chem. 70: 12, 3783, 1966 3. C. THIES, Macromolecules'l: 4, 335, 1968 4. M. G. SHICK and E. N. HARVEY, J. Polymer Sci, B7: 7, 495, 1969 5. A. V. UVAROV, V. Yu. ERMAN and N. A. ALEKSANDROVA, Kolloid. zh. 32: 791, 1970 6. Yu, S. LIPATO¥', J. Polymer Sci., Polymer Symp. Ed. 61: 2, 369, 1977 7. Yu. S. LIPATOV, Fizieheskaya khimiya napolnennykh polimerov (Physical Chemistry of Filled Polymers). "Khimiya", Moscow, 1977 8. Yu. S. LIPATOV, J. Adhesion 1O: 1, 85, 1979 9. Yu. S. LIPATOV, In: Adsorption and Adhesion. Part B (Ed. L.H. Lee). p. 601, Plenum Press, New York-London, 1979 10. G. M. SEMENOVICH, Yu. S. LIPATOV, L. M. SERGEYEVA, T. T. TODOSHCHUK, N. A. KORNIYAKA, S. A. VYSOTA and A. V. MARCHENKO, Vysokomol. soyed. A2O: 2375, 1978 (Translated in Polymer Sci. U.S.S.R. A20: 10, 2673, 1978) 11. B. J. FONTANA aud J. R. THOMAS, J. Phys. Chem. 65: 3, 480, 1961 12. V. Ya. DAVYDOV, A. V. KISELEV and V. I. LYGIN, Dokl. Akad. l~auk SSSR 147: 131, 1962 13. N. SHA_MP and J. HUYLEBROECK, J. Polymer Sci., Polymer Syrup. 42: 1, 553, 1973 14. Yu. S. LIPATOV and L. M. SERGEYEVA, Adv. in Colloid and Interface Sci. 6: 1, 1, 1976 15. Yu. S. LIPATOV, Progr. in Colloid and Polymer Sci. 61: 1, 12, 1976 16. Yu. S. LIPATO$', L. M. SEROEYEVA, T. T. TODOSHCHUK and T. S. KHRAMOVA, Dok]. Akad. N a u k SSSR 218: 1144, 1974 17. Yu. S. LIPATOV, T. S. KHRAMOVA, P. T. TODOSIICHUK and L. M. SERGEYEVA, Kolloid zh. 39: 174, 1977 18. J. GREENLAND, J. Colloid Sci. 18: 7, 647, 1963 19. I. L MALEYEV, M. M. SOLTYS, T. M. POLONSKII and V. M. PRISYAZHNYI Struktura i svoistva poverkhnostykh sloyev polimerov (Structure and Properties of Surface Layers of Polymers) p. 74, "Naukova dumka", Kiev, 1972 20. L. Ye. KUZNETSOVA and N. N. SERB-SERBINA, Kolloid. zh. 30: 853, 1968 21. V. I. KLENIN and N. 1~. KOLNIBOLOTCHUK, Mekhanizm protsessov plenkoobrazovaniya iz polimernykh rastvorov i dispersii (Mechanism of Processes of Film Formation from Polymer Solutions and Dispersions). p. 32, " ~ a u k a " , Moscow, 1966 22. V. I. KLENIN, N. V. UZUN and S. Ya. FRENKEL', Vysokomol. soyed. B15: 601, 1973 (l~ot translated in Polymer Sci. U.S.S.R.)
0032-$950/81/112841-09807.50]0 © 1982 Pergamon Press Ltd.
STUDY OF ADSORPTION OF POLYMER MIXTURES FROM SOLUTIONS IN A COMMON SOLVENT ON A SOIJD SURFACE* Yu. S. Ln~ATOV, L. M. SERGEYEVA, G. M. SEMENOVICH, T. T. TODOS~OHUX, L. V. D U B R O W ~ A and V. N. CHORXAYA I,istitute of Chemistry of High Molecular Weight Compounds, IYkr.S.S.R. Academy of Sciences (Received 1 April 1980) The authors have studied the adsorption and determined the fraction of bound segments P in the systems polybutyl methacrylate-butadiene-nitrile rubber-Aerosil and polybutyl methacrylate--polystyrene-Aerosil. The isotherms were obtained with extremes the position of which on the adsorption isotherms depends on the nature of the polymer and concentration of the solution. The value p changes within wide limits with the concentration of the solution from which the adsorption layer is formed. The mutualinfluence of the polymers studied on adsorption from the mixtures was found. I t was established that the nitrile rubbers from mixtures with polybutyl methacrylate are not adsorbed although they have an important influence on the -~dsorption of polybutyl methacrylate and the value p. I t is shown that for a different adsorbent-solution ratio the values of p and the form of the isotherms significantly change both for the individual solutions of the polymers and their mixtures. The results are explained in terms of the molecular-aggregative mechanism of adsorption. ADSOm~rm~ f r o m m u l t i - c o m p o n e n t p o l y m e r s y s t e m s has b e e n s t u d i e d o n l y i,1 a few c o m m u n i c a t i o n s [1-5]. Y e t in t h e p r a c t i c e of o b t a i n i n g filled p o l y m e r s a n d covers as a rule m i x t u r e s of p o l y m e r s or oligomers are used so t h a t s t u d y o f selective a d s o r p t i o n a n d also t h e s t r u c t u r e of t h e a d s o r p t i o n a n d b o u n d a r y l a y e r s f o r m e d f r o m such s y s t e m s a s s u m e s g r e a t i m p o r t a n c e [6-8]. Earlier, in t h e case o f t h e s y s t e m r u b b e r - e p o x i d e r e s i n - a m m o n i u m chloride [9, 10] we e x p e r i m e n t a l l y s h o w e d t h a t t h e selective a d s o r p t i o n of one o f t h e c o m p o n e n t s of t h e c o m p o s i t e on a solid surface leads to a n u n e v e n d i s t r i b u t i o n o f t h e m a c r o m o l e c u l e s o f t h e p o l y m e r s of a different n a t u r e in t h e b o u n d a r y l a y e r o v e r its thickness, w h i c h m u s t seriously affect t h e final p r o p e r t i e s of t h e c o m p o s i t e polymer materials. I n t h e p r e s e n t w o r k we m a k e a d e t a i l e d s t u d y o f t h e a d s o r p t i o n a n d s t r u c t u r e o f t h e a d s o r p t i o n l a y e r s in t w o - c o m p o n e n t p o l y m e r s y s t e m s . For the investigation we chose the following systems: polybutyl methaerylate (PBMA) (2P/~= 24,000) q-nitrile rubber (SKN)-kAeresil and PMBA ( / ~ = 100,000)-bPS-}-Aerosil. Commerical PBMA and PS (~v----50,000) were used and also SKN-26M (/Q'v~ 170,000) and SKN-40 (/~/v----200,000). The polymers were purified by precipitation, Aeorsil immediately * Vysokomol. soyed. A28: No. 11, 2436-2443, 1981.
2~42
V]o'. So LIPATOV e$ ~ .
before the experiments was heated at 800°C in a muffle furnace. For the first system the solvent was chloroform and for the second carbon tetraehloride. The solvents were purified and dried by standard methods. The investigations were by the method of IR spectroscopy with the UR-20 spectrophotometer. The extent of adsorption was judged from change in the concentration of solution before and after adsorption. The analytical bands were those of the valent vibrations of the C = O groups in PBMA, the C--N groups in the rubbers and the deformation vibrations of the substituted aromatic ring in PS. The values of adsorption and the fractions of bmmd segments p were determined both on individual solutions and from a mixture by the method described in reference [11]. For the nitrile rubbers and PS we determined the pK value (K~-8[Mis a constant; 8 is the molar extinction coefficient of the band of valence vibrations of the C ~ N groups in rubber or the bands of the deformation vibrations of the substituted aromatic ring in PS; M is the equivalent weight of the corresponding group). Adsorption was carried out in static conditions. Investigation of the course of adsorption shows that adsorption equilibrium was set up in 30-48 hr depending on the nature of the polymer.
System PBMA+butadiene nitrile rubber+Aerosil. Analysis o f t h e I R spectra of the gels of the individual solutions o f P B M A established t h a t the earbonyl groups of adsorbed P B M A form a h y d r o g e n b o n d w i t h the h y d r o x y l groups o f Aerosil. I n the s p e c t r u m of the gel in the region of t h e valence vibrations a wide absorption b a n d o f the b o u n d h y d r o x y l groups appears w i t h a m a x i m u m a t 34.20 × 104 m -x, the b a n d of the free h y d r o x y l groups because of the p e r t u r b i n g influence of the solvent shifts to 36.87 x 104 m - l ; t h e a b s o r p t i o n b a n d o f t h e valence vibrations of the carbonyl group splits into two with m a x i m a at 17.30>( × 104 m -1 (absorption of the free C - - O groups) a n d a t 17.08× 104 m -1 ( b o u n d C = O groups). F r o m this b a n d using the F o n t a n a m e t h o d [11] we d e t e r m i n e d t h e fraction of b o u n d segments 10 of PBMA. I n the spectrum o f the gels of rubbers in the b a n d of t h e valence vibrations o f the C - - N groups a high f r e q u e n c y wing appears due to a d s o r p t i o n of the nitrile groups i n t e r a c t i n g w i t h the surface o f Aerosil. T h e shift of the absorption b a n d of the h y d r o x y l groups in the s p e c t r u m o f Aerosil to 3 3 . 7 4 x 10' m -1 indicates t h e i r strong p e r t u r b a t i o n similar to t h e case of f o r m a t i o n of the h y d r o g e n bond [12]. T h e a d s o r p t i o n isotherm of P B M A constitutes a curve with distinct e x t r e m e s (Fig. la). T h e e x p e r i m e n t a l l y d e t e r m i n e d value 10 of the p o l y m e r is 0.4 in the region of equilibrium concentrations of 4-6 g/l., b u t with rise in the c o n c e n t r a t i o n diminishes, falling in the region of the m a x i m u m on t h e adsorption isotherm t o 0.15; f u r t h e r rise in the concentration leads to rise in p (Fig. 13). M a x i m a in t h e adsorption isotherms is observed for a n u m b e r of p o l y m e r systems [13-16] a n d is a characteristic f e a t u r e of the a d s o r p t i o n of pol3mers from soiutions providing adsorption is studied o~ er a wide concentrati,::~ range, xNnmerous e x p e r i m e u t s of ~ ari )us researchers iDdieate t h a t the max;ran ~.r,, n o t t h e result of errors or c,mditions o f nonequiLibrium in the a d s o r p t i o n system, nevertheless the reasons for their appearance have n o t been fully clarified. We consider t h a t t h e n o n - m o n o t o n i e i t y of change in the value o f a d s o r p t i o n w i t h c o n c e n t r a t i o n is due to t h e moleeular-aggregative m e c h a n i s m of adsorption
Adsorption of polymer mixtures from solutions
2643
assuming passage from the solutions to the surface of the adsorbent both of t h e individual macromolecules and their associates [1, 14, 15]. For the system in question the formation of associates (or aggregates) begins at a concentration of 10 g/1. After this concentration sharp increase in adsorption of the individual PBMA is observed with a considerable fall in the fraction of bound segments which we attribute to the predominant adsorption of the aggregates in this concentration region [16, 16].
A,g,~
o
o.j! 0.! 0
°
|
I
I
I
I
0.6
I.B
2.8
3"g
o.lo-',,qp.
FzG. 1. Adsorption isotherms (a) and fraction of bound segments (b) of PBMA adsorbed from pure chloroform (1) and mixture (2); 3--absolute values of adsorption of SKN-40. With rise in the concentration of the solution of aggregates enlarge. Evidently, the fraction of segments of such aggregates bound to the surface falls as compared with the smaller aggregates, as witness the fall in p. In the concentration range 18-26 g/l. corresponding to the region of the maximum on the isotherm, the value p does not change, so that it m a y be concluded that the size of the adsorbed aggregates does not change. ]~arther increase in the concentrations of the solutions leads to fall in adsorption. ~Ve would recall that one of our studies [14] showed that the aggregation constant Ka characterizing the ratio of the number of "aggregated" molecules to the total number of molecules falls with increase in concentration, in other words, the degree of aggregatability of the maeromolecules in solution decreases and the ratio of the aggregated and isolated macromoleculcs passing to the surface also changes. Possibly after the point of the maximum on the adsorption isotherm the fraction of isolated molecules passing to the surface rises and, therefore, t h e absolute value of adsorption drops and the value p rises, as Fig. lb shows. Another possible cause of the drop in adsorption after the concentL'ation 26 g/1. m a y be the formation of a fluctuation structural network. In the region of low concentrations of the solutions (1-8 g/1.) judging from the magnitudes of p, which have much higher values, we postulate the adsorption of the isolated PBMA chains. The small maximum and the following minimum in
2644
Yu. S. l.J2~Tov et al.
this concentration region m a y be explained b y conformationa, l changes in the macromolecules preceding the formation of associates. The adsorption of the polymers from the mixture in solution was studied with a constant content of one of the components in the adsorption system and rise in the concentration of the other. It was shown that the nitrile rubbers introduced into PBMA solution (the concentration of rubber in all cases was 4 g/l.) are practically not adsorbed over the range of compositions of the PBMA solution (0.5-29 g/1.) but have a significant influence both on the absolute values of adsorption of PBMA and the fraction of bound segments characterizing the structure of its adsorption layer. The rubber SKN-40 particularly effectively influenced the adsorption of PBMA. Figure 1 (curve 2) presents the adsorption isotherm for PBMA in presence o f SKN-40 indicating that the rubber not itself adsorbed changes the character of the adsorption of PBMA; in the concentration range 2-14 gfl. the adsorption of PBMA rises b u t with further rise in the concentration diminishes as compared with the value of adsorption of PBMA from pure solvent, i.e. inversion occurs as a function of adsorption against the concentration of solution. In analysing the causes of the observed inversion one must bear in mind that addition to the PBMA solution of a rubber incompatible with it thermod3ummically is analogous to worsening of the quality of the solvent. It is known [1] that from poor solvents in the region of dilute solutions the adsorption of polymers, as a rule, is greater that from good ones because of the fact that in poor solvents the macromolecules interact less with the solvent and are more compact. However, the fraction of segments of the molecules binding to the surface is less as compared with the maeromolecules adsorbed from good :solvents. Probably, therefore, in the initial concentration region the adsorption of PBMA from the mixture rises and the value p considerably falls. In the region of high concentrations where the degree of intermoleeular and interaggregate interaction of PBMA is greater in the poor ("mixed") solvent than in pure chloroform (good solvent) the adsorption of PBMA diminishes. The fraction of bound segments also falls but not so much as in the region of low concentrations. A , g/g
P
a
0.2~- o
3
°"I 0.4
z 1.0
1.6
O.J o
0"I
I
~1
O'tl
1.0
°
~=1 3 1"6
c. lo-', g/l.
:FI(~. 2. Adsorption isotherms (a) and pK ftmotion (b) of rubber adsorbed from chloroform (1) and mixture (2); absolute values of adsorption and fraction of bound segraents of PBM.A (3) adsorbed from mixture with SKN-40.
Adsorption of polymer mixtures from solutions
2645
In the region of high concentrations (above Cequ30/g~l.) from the mixture of polymers rubber begins t.o be adsorbed with a heavy drop in the adsorption of PBMA, i.e. inversion of adsorption due to structural factors is observed. Let us look at the adsorption of the rubber SKN-40 from pure chloroform (Fig. 2, curve 1) and from the mixture with PBMA (Fig. 2, curve 2) the amount of which was constant at all concentrations of rubber (CPB~A4 g/l.). The value of adsorption of SKN-40 falls above the equilibrium concentration of 2 g/]. It may be supposed that in the rubber solutions structuring is more marked than in PBMA solutions in the region of the same concentration as a result of considerable differences in the molecular masses of the polymers studied and therefore aggregates also form at much lower concentrations; the heavy structuring of the rubber starting at C,~u-~3 g/1. lessens adsorption. At the limiting concentration possible for carIsring out the experiment (12 gfl.) the value of adsorption of the rubber is very small but the value p sharply rises, which serves as a basis for concluding that adsorption of isolated molecules and not aggregates is predominant in this concentration region. The introduction into the rubber solution of PBMA sharply reduced adsorption in the interval of equilibrium concentrations 1-4 gfl. and the isotherm shifts to higher concentrations. Probably the introduction of PBMA influences the structuring in solution and shifts the start of the formation of the structural mesh to the region of high concentrations. It is interesting to note that PBMA is adsorbed from the mixture uniformly over the whole concentration range whatever the concentration of rubber. There is only change in the value p which is high for a low concentration of rubber, then sharply falls at ¢equ---~4g/l. i.e. for the same value of adsorption, adsorption layers of different structure form. Thus, the investigations indicate the mutual influence of the polymers on adsorption from the two-component systems. Even when one of the components is not adsorbed, it changes the character of the adsorption of the other, which is connected with complex changes in the structure of the solution (size of associates, degree of their interaction with adsorbent and each other, etc.) with rise in the concentration. System PBMA-~-PS~Aerosil. It is known that the value of adsorption on solid surfaces and the character of the adsorption isotherms of low-molecular weight substauces from solutions with the condition of reaching adsorption equilibrium do not depend oi1 the adsorbent-solution ratio. Differences in the adsorption of polymers from solution from that of low molecular weight substances have now been clearly established [1, 16, 17]. They are determined by the fact that to each concentration of polymer solution correspond different sizes of the polymer coils as a result of their dependence on concentration. This fact and also the possibility of aggregative phenomena bringing into play the molecular-aggregative mechanism of adsorption mean that, on the one hand, at each concentration a new equilibrium is set up between the associated and non-associated macromolecules and, on the other, to each point of the isotherm corresponds a different
Yu. S. LIPATOVet al.
2646
structure of the elements adsorbed on the surface; from isolated coils of different size to their aggregates or associates and also simultaneous adsorption of both. From this viewpoint one may expect deviations in the behaviour of adsorption of polymers depending on the adsorbent-to-solution ratio. Some published findings support such an assumption [18-20] showing t h a t not only the value of adsorption but also the form of the isotherms depend on this ratio. We examined this problem in relation to the system PBMA+PS+Aerosil. Figures 3-5 present the results of study of adsorption of PS and PBMA for different adsorbent-to-solution ratios. It is worth noting the change in the value of adsorption and the form of the isotherm with the nature of the polymer and amount of adsorbent. We explain such changes on the basis of the molecularaggregative mechanism of adsorption proposed by us earlier. Structure formation in solutions of PS and PBMA before and after adsorption has been studied by the method of the turbidity spectrum [21, 22]. It was established that the initial solutions of polymers contain aggregates the size of which rises with the concentration of the solution. The nature of the polymer in the case studied insiginificantly influenced the size of the associates except for the fact that in the region of high concentrations of PBMA sharper increase in size w a s observed. In the solutions immediately after adsorption aggregates were not detected, which serves as proof of their passage to the surface of the filler.
A,~I/g
a
0.3I~ 0.1~ o ~ . . 1.2
3 2.q
pK.
b
c , to-, g/t.
FIG. 3. Adsorptionisotherms (a) and pK value (b) for PS adsorbed from sglution in GC1,for a content of Aerosil of 10 (1); 20 (2) and 40 (3) g/1. Together with the aggregates non-aggregated molecules are also adsorbed. Probably the ratio of aggregated to non-aggregated molecules passing to the surface depending on the concentration of the solutiol~ and the adsorbent will change. This assumption is legitimate if only because the value of the aggregation constant /Ca characterizing the ratio of the aggregated molecules to the total number of molecules in solution changes with concentration. In connexion with this the value of adsorption may change with concentration non-monotonically as Figs. 3 and 4 show. The dependence of the fraction of bound segments on the concentration of the solution changes antibatically to the adsorption isotherms. Let us consider the dependence of adsorption of polymers on the amount of adsorbent in the system. With rise in the concentration of the adsorbent in the
Adsorption of p o l y m e r m i x t u r e s from solutions
2$47
system from 10 to 20 g/l. the absolute value of adsorption of PS changes insignificantly. The forms of the isotherms also change little but at 40 g/1. the value of adsorption is considerably higher and the form of the isotherm substantially changes. For PBMA a somewhat different picture was observed: with increase in the content of Aerosil from 10 to 20 g/l. the value of adsorption declined but with further increase in the amount of adsorbent again rose.
x1~× ~ x " ~
0.3 L
1"2
3
" !
2"4
1"2
2"q C~IO'~g/I.
:Fzo. 4. Adsorption isotherms (a) and fraction of bound segraents (b) for PBMA adsorbed for solutions in CC14 for a content of Aerosil of 10 (•); 20 (2) and 40 (3) g/1.
The fraction of bound segments is least in the adsorption layer formed from the smaller of the content.s of adsorbent studied. On adsorption from a mixture of polymers for a different content of Aerosil (Fig. 5) inversion of such a kind was observed: while for a smaller amount of adsorbent individual PBMA was adsorbed to a lesser degree, from the mixture its adsorption was greater for the same content of Aerosil. While for a high content of Aerosil only PBMA was adsorbed, as in the system considered earlier, for a lower content of Aerosil together with adsorption of PBMA PS was also adsorbed i.e. in the mixture increase in the content of adsorbent promotes decrease in adsorption of PBMA and reduces to zero the adsorption of PS. (2
pK-
P x
b _
o
3
...4
g
'l
I
1"2
2"4
oq
oz l I
I
I
1.2
I,
i
I
2"4
c , 1o- glt.
FzG. 5. Adsorption isotherms (a) and fraction of bound segments (b) for P B M A (1-3) and PS (4) adsorbed from mixture in CC14; content of Aerosih 10 (•); 20 (2, 4) and 40 (3) gfl.
Probably the adsorbent materially changes the character of the structures in the system by changing the ratio of aggregates and isolated molecules and also changing the size of the aggregates as a result of their possible destruction, which is especially noticeable with change in the adsorbent-to-solution ratio. One m a y admit the possibility of formation in the adsorption system of aggregates
2648
Ylr. S. L x P A ~ v et a/.
like the maeromolecules of different molecular mass, different "activity" in relation to the adsorbent and change in their share in the total amount for different contents of Aerosi]. Further special aspects arise in analysing adsorption from mixtures of polymers where probably there is complex change in the ebaracter of the thermodynamic interaction of polymers with one another in solution and each polymer with solvent in the systems studied. The thermodynamic incompatibility of the components allows us to consider the solution of the mixtalre of the two polymers as a solution of one polymer in the solvent representing the solution of the second component. Such a solvent will as a result of thermodynamic incompatibility of the components be thermodynamically poor as compared with the pure solvent, which promotes aggregation of the molecules of the first component and their preferential adsorption from the mixture. This is reflected both in the absolute values of adsorption and the character of the isotherms. The diversity and complexity of the processes occurring on adsorption of the polymers studied are indicated by the sizes of the fractions of the bound segments 1o in the adsorption layers formed both from solutions of the polymers in pure solvents and from the mixtm-e of Polymers. As Figs. 3-5 show, there is non-monotonic change in 10 with rise in the concentration of the solution from which the layer formed. At low concentrations of the solutions 0.1-2 g/l., as a rule, the va]ue1~ is considerably higher, which is natural since in this concentration region to the surface probably pass the isolated molecules since aggregates are not detected here. The complex relations of 10 with the concentration of the solution for a different content of adsorbent are due to the fact that a change in the amount of adsorbent possibly changes, the quantitative ratio of the aggregates and the individual molecules passing into the adsorption layer, which is reflected in the value of 10. The values of 10 for the adsorption layer of PBMA formed from the mixture of polymers and the individual solutions also differ significantly. Comparison of the results obtained with the adsorptaion isotherms showed, as stated above, an antibatic relation between the value 10 and the value of adsorption both for the individual solutions and mixtures. Thus, these findings for the first time make it Possible to identify clearly a further fundamental feature of equilibrium adsorption of polymers from solution expressed by the fact that for a different adsorbent-to-solution ratio the values of adsorption, the degree of binding of the segments of the macromolecules by the surface and the form of the isotherms materially change. All these changes occur in that concentration region of solutions where the processes of aggregation of the macromolecules are observed and concentration-dependent equilibrium between the aggregated and non-aggregated macromolecuies is set up. The latter determines the possibility of simultaneous passage to the surface both of the isolated maeromolecuies and their aggregates. The different changes in the free energy on adsorption of both finally lead to the influence of the adsorbent-to-solution ratio on the adsorption of polymers in equilibrium conditions.
Adsorption of polymer mixtures from solutions
2645
:REFERENCES 1. Yu. S. LIPATOV and L. M. SERGEYEVA, Adsorbtsiya polimerov (Adsorption of Polymers). "Naukova dumka", Kiev, 1972 2, C. THIES, J. Phys. Chem. 70: 12, 3783, 1966 3. C. THIES, Macromolecules'l: 4, 335, 1968 4. M. G. SHICK and E. N. HARVEY, J. Polymer Sci, B7: 7, 495, 1969 5. A. V. UVAROV, V. Yu. ERMAN and N. A. ALEKSANDROVA, Kolloid. zh. 32: 791, 1970 6. Yu, S. LIPATO¥', J. Polymer Sci., Polymer Symp. Ed. 61: 2, 369, 1977 7. Yu. S. LIPATOV, Fizieheskaya khimiya napolnennykh polimerov (Physical Chemistry of Filled Polymers). "Khimiya", Moscow, 1977 8. Yu. S. LIPATOV, J. Adhesion 1O: 1, 85, 1979 9. Yu. S. LIPATOV, In: Adsorption and Adhesion. Part B (Ed. L.H. Lee). p. 601, Plenum Press, New York-London, 1979 10. G. M. SEMENOVICH, Yu. S. LIPATOV, L. M. SERGEYEVA, T. T. TODOSHCHUK, N. A. KORNIYAKA, S. A. VYSOTA and A. V. MARCHENKO, Vysokomol. soyed. A2O: 2375, 1978 (Translated in Polymer Sci. U.S.S.R. A20: 10, 2673, 1978) 11. B. J. FONTANA aud J. R. THOMAS, J. Phys. Chem. 65: 3, 480, 1961 12. V. Ya. DAVYDOV, A. V. KISELEV and V. I. LYGIN, Dokl. Akad. l~auk SSSR 147: 131, 1962 13. N. SHA_MP and J. HUYLEBROECK, J. Polymer Sci., Polymer Syrup. 42: 1, 553, 1973 14. Yu. S. LIPATOV and L. M. SERGEYEVA, Adv. in Colloid and Interface Sci. 6: 1, 1, 1976 15. Yu. S. LIPATOV, Progr. in Colloid and Polymer Sci. 61: 1, 12, 1976 16. Yu. S. LIPATO$', L. M. SEROEYEVA, T. T. TODOSHCHUK and T. S. KHRAMOVA, Dok]. Akad. N a u k SSSR 218: 1144, 1974 17. Yu. S. LIPATOV, T. S. KHRAMOVA, P. T. TODOSIICHUK and L. M. SERGEYEVA, Kolloid zh. 39: 174, 1977 18. J. GREENLAND, J. Colloid Sci. 18: 7, 647, 1963 19. I. L MALEYEV, M. M. SOLTYS, T. M. POLONSKII and V. M. PRISYAZHNYI Struktura i svoistva poverkhnostykh sloyev polimerov (Structure and Properties of Surface Layers of Polymers) p. 74, "Naukova dumka", Kiev, 1972 20. L. Ye. KUZNETSOVA and N. N. SERB-SERBINA, Kolloid. zh. 30: 853, 1968 21. V. I. KLENIN and N. 1~. KOLNIBOLOTCHUK, Mekhanizm protsessov plenkoobrazovaniya iz polimernykh rastvorov i dispersii (Mechanism of Processes of Film Formation from Polymer Solutions and Dispersions). p. 32, " ~ a u k a " , Moscow, 1966 22. V. I. KLENIN, N. V. UZUN and S. Ya. FRENKEL', Vysokomol. soyed. B15: 601, 1973 (l~ot translated in Polymer Sci. U.S.S.R.)