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Fractionation of styrene-methyl methacrylate copolymer
European Polymer Journal. Vol. 12. pp. 189 to 193. Pergamon Press 1976. Printed in Great Britain•
FRACTIONATION OF STYRENE-METHYL METHACRYLATE COPOLYMER LJ. LOVRI~ INA, Research and Development Institute, Zagreb, Yugoslavia Z. GRUBIgI~-GALLOT Centre de Recherches sur les Macromol6cules, Strasbourg, France and B. KUNST* Institute of Physical Chemistry, University of Zagreb, P.O. Box 179, 41001 Zagreb, Yugoslavia
(Received 16 May 1975) Abstract--Styrene-methyl methacrylate copolymer was fractionated by the column elution and coacervate extraction techniques. Different solvent-nonsolvent combinations were used; both batchwise and continuous column elution fractionations were carried out. Successful resolution of fractions according to molecular weight was achieved. The effect of the solvent-precipitant pair on the fractionation efficiency, as tested by gel permeation chromatography, is discussed. The results of continuous column elution depend significantly on the elution rate.
INTRODUCTION Fractionation of a polymer sample is usually carried out with the goal [1] of obtaining its molecular weight distribution or preparing samples of reduced polydispersity. The process itself is not simple, and may become very complicated for copolymer samples, which may exhibit heterogeneity in chemical composition, in molecular weight and in monomer sequence length. The process of separation of a sample into materials having the same structure is most often based on difference in solubility. Since the solubility behaviour of the individual molecules depends on the three mentioned variables, there may be fractionation according to all three or to two of them. Proper selection of the eXperimental conditions is needed for fractionation to be done only with respect to the one of the variables. Two of the fractionation conditions especially affect the solubility of a copolymer, viz. the nature of solvent-precipitant combination and the fractionation temperature. The selection of these two parameters is therefore very important. The purpose of this paper is to report a study of various methods and conditions for fractionation of a styrene-methyl methacrylate copolymer. Papers concerning the fractionation of styrene-methyl methacrylate copolymer have already been published [ 2 4 ] but using different techniques. In the present study, column elution and coacervate extraction methods have been investigated using different solvent-precipitant systems and fractionation temperatures. EXPERIMENTAL
Materials A styrene-methyl methacrylate copolymer was obtained from the R/Shm & Haas Co., Darmstadt, W. Germany, * To whom correspondence should be addressed. • 189
and used without preliminary purification. The homopolymer samples u~ed for the solubility determinations were fractions of polystyrene (OKI, Zagreb) and poly(methyl methacrylate) (R6hm & Haas, Darmstadt) with viscosity molecular weights of approx 230,000. The solvents were the best commercially available and were used without further purification.
Fractionations The fractionating column was approx 120 cm long and 2.6 cm i.d., and fitted with a glass jacket for the circulation of thermostated water. The column temperature was maintained within 0.1 °. Glass wool or glass beads (Jencons, Hertfordshire, England) of 0-1 mm diameter, cleaned by the usual procedure, were used as the polymer support in the column. For the batchwise fractionations, the polymer sample was deposited on glass wool by solvent evaporation. Twelve grams of polymer in the form of 3~o solution in acetone, dichloromethane or carbon tetrachloride were introduced into the column and solvent was evaporated in vacuum at 30°. The nonsolvent (methanol) was then put in the column. The solvent power of solvent-precipitant system was changed stepwise, with the volume of each fraction amounting to 500 ml; the elution rate was 150 ml/hr. The continuous fractionations were performed with 3 g samples. The sample was deposited on the glass beads outside the column by the evaporation of solvent. After drying the beads were placed in the column by the wet method. The elution was carried out using the logarithmic solvent gradient produced by a solvent mixing system similar to those reported in the literature [5, 61 The solventprecipitant system used in the continuous fractionations was carbon tetrachloride-methanol at 30°, with the initial and final volume fractions of precipitant being 0'57 and 0.47, respectively. The volume of solvent mixture in the mixing chamber was 650 ml and the elution rate was varied from 60 to 150 ml/hr. The volume of each fraction was 150 ml, and 15-20 fractions were taken in each experiment.
190
LJ. LOVRIC, Z. GRUBI~I~-GALLOTand B. KUNST
Fractionation by coacervate extraction was carried out in a thermostated cylindrical container of 2 1. The system benzene-triethylene glycol at 50° was used. A 5% solution of the polymer (10 g) was placed in the stirred vessel and warmed up to 50°. The nonsolvent was added dropwise with vigorous stirring. Two layers were allowed to separate and the lower layer containing the first fraction was withdrawn from the bottom of the vessel. The procedure was repeated with the solvent-precipitant mixtures, each of a slightly greater solvent power until the fractionation was completed. All the fractions were recovered by precipitation with methanol, filtration and drying to constant weight. Fraction analysis Dilute solution viscosities were measured at four or more concentrations in butanone at 25'0 ___0"05°. A semimicro dilution viscometer (Cannon Ubbelohde type CUSMDC-50) with a flow rate for pure solvent of more than 135 sec was used. A Waters Associates M-200 gel permeation chromatograph equipped with 5 columns (107, 106, 10s, 104 and 3 x 103 A) at room temperature was employed. Tetrahydrofuran was used as eluting solvent at a flow rate of 1 ml/min. To characterize the GPC fractions, an automatic viscometer and u.v. spectrophotometer were coupled with the gel permeation chromatograph in the manner described earlier I-7]. A universal calibration curve I-8] was used. A Mechrolab osmometer model 201 was used for number-average molecular weights. Measurements were made at 30° using toluene as solvent. A Fica 42000 light scattering photometer was used for weight-average molecular weights. Measurements were made in tetrahydrofuran at 25° using 5460 A light. The chemical compositions of fractions and of the original sample were determined by NMR measurements using a Varian A-60 spectrometer. RESULTS AND DISCUSSION
Selection of solvents The procedures devised to select the solventprecipitant system for the fractionation of copolymers have been reported [9,10]. They generally consist of solubility determinations for the parent homopolymers and for a series of copolymers using cloud-point titration. In this work three solvent-precipitant pairs have been selected for the column fractionation. The first, suitable to enhance fractionation with respect to molecular weight, was chosen according to the procedure recommended by Jurani~ova et al. [10]. The most convenient combination appeared to be dichloromethane-methanol with Vcrit values* for polystyrene and poly(methyl methacrylate) of 0"282 and 0'338, respectively. The other two solvent-nonsolvent combinations for the column fractionation were selected by reasoning similar to that of Teramachi et al. [11] who showed that the phase separation conditions of a copolymer are sensitive to change in chemical composition if the difference between the interaction parameters of solvent with both monomer units, ZA and ZB, is large. The convenient solvent-precipitant pairs which * 7crlt is the value of the volume fraction of nonsolvent at the cloud point extrapolated to zero concentration, the value that could be put equal to the composition of Omixture.
should give such conditions seem to be carbon tetrachloride-methanol and acetone-methanol. In the first combination, carbon tetrachloride is solvent for polystyrene and nonsolvent for poly(methyl methacrylate), so the difference between the interaction parameters of the solvent pair with polystyrene and poly(methyl methacrylate) may be quite large. The analogous conditions may be achieved with the second combination chosen, where acetone is nonsolvent for polystyrene and solvent for poly(methyl methacrylate). Using these two combinations, one can expect therefore that fractionation of styrene-methyl methacrylate copolymer according to chemical composition would be favoured. The choice of the solvent combination and the appropriate temperature for the fractionation by coacervate extraction has always been tedious. Although guidelines, both theoretical [12] and practical r13,14], exist, the proper choice has to be made by the slow trial-and-error procedure. It seems to be particularly true with copolymers, so in this work many nonsolvents in combination with two suitable solvents-benzene and dichloromethane--as well as the convenient temperatures have been tested. The best coacervate appeared to form with the combination benzene-triethylene glycol at 50 °, which was eventually used for fractionation by this method. Fractionation The results of the column fractionations by the batchwise variation of the solvent-precipitant mixture and those obtained by the coacervate extraction in the form of cumulative weight distribution data, constructed in the usual way [15], are shown in Figs. 1 and 2, respectively. Experimental points in Fig. 1 representing three column fractionations using different solvent-nonsolvent pairs fall practically along a single distribution curve. There are some minor deviations from the common curve but they can hardly be considered as systematic• Such agreement of experimental results for different solvent-nonsolvent pairs indicates that fractionation took place with respect to molecular weight only. The good fractionation is also confirmed by the
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Fractionation of styrene methyl methacrylate copolymer
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Fig. 2. Comparison of coacervate extraction and column fractionation results. absence of fraction reversal, i.e. there is always a regular progression of intrinsic viscosities through the fractions. The distribution curve obtained by the coacervate extraction is basically identical (Fig. 2) to that shown in Fig. 1. There is however a slight tendency for low intrinsic viscosity values to fall below the distribution curve obtained by the column fractionations, and the high intrinsic viscosity values to move above it. The general agreement of the distribution curves obtained by two methods suggests that the fractionation conditions chosen for the coacervate extraction were good and the slight deviations originate most probably from the inherently broader fractions produced by the coacervate extraction method. Composition determinations for all the fractions are summarized in Table 1. These results show the same composition for all fractions. They are chemically identical with the original sample, substantiating the chemical homogeneity of the latter. This is why all the four carefully performed fractionations, regardless of the solvent-nonsolvent pair selection or the method, gave the same distribution curve. Such a finding is expected to hold [3] for a copolymer sample polymerized by a radical process carried to only low conversion. If the conclusion on chemical homogeneity of the copolymer sample is correct, the results obtained by the molecular weight fractionations using different solvent-precipitant combinations may be examined in more detail for fractionation efficiency. This can be done by comparing the distribution shown in Fig. 1 with the result obtained by the GPC determination
~LECUL.Z~ v~r~r, M,,.~o-5 Fig. 3. Comparison of cumulative weight distributions of the sample as the result of column fractionation and gel permeation chromatography.
of the mass distribution curve. For this purpose, the intrinsic viscosities determined experimentally were in both cases converted to molecular weights using the equation of Stockmayer[2], valid for the 1:1 S-MMA copolymer in butanone at 25 °. Comparison of the integral mass distribution curves (Fig. 3) shows evident differences, due to the different natures of the fractionation processes. The integral mass distribution curve obtained by gel permeation chromatography shows more lower molecular weight material, and also better resolution of higher fractions. The highest intrinsic viscosity was 2"94 dl/g compared to only 1.44 dl/g for the highest fraction obtained by column fractionation. The observed differences can be partly attributed to the general tendency 1-16] of gel permeation chromatography to give wider distributions. The more important reason for the observed deviation, i.e. for the worse resolution achieved by the column fractionation, seems to be incomplete elution of higher fractions, so called 'entrainment'. This effect may be caused by adsorption of polymer on the glass carrier [17] which, together with a finite number of evaluated fractions, affects the mass distribution curve. There are also some subtle differences within the column ffactionation results arising from the application of various solvent-nonsolvent combinations. They can be identified in Fig. 1 by the highest values of the measured intrinsic viscosities. The maximum intrinsic viscosity was 1.44 dl/g obtained by the acetone-methanol system, which may be therefore considered the most efficient solvent pair. On the other hand, the last experimental point obtained with the dichloromethane~methanol system showing an upward shift of the curve indicates the relatively worse
Table 1. Chemical compositions of sample and fractions Styrene in the unfract, sample (mol %)
49.7
Solvent-precipitant system
No. of fractions and styrene determinations
Styrene in fractions (mol %)
Acetone-methanol
22 14 13 28
49.8 + 0.77 49.6 4- 0"95 49.8 +_ 0-91 49.9 + 0.87
CH2C12 methanol CCI4 methanol
Benzene-TEG
192
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Fig. 4. Gel permeation chromatograms for various fractions. resolution. There is support for these conclusions. It manifests in the more uniform distribution of the amount of polymer among the fractions from the fractionation using the acetone-methanol system. The total polymer quantity used in the other two column fractionations was accumulated in several fractions only. Such findings are consistent with previous views [5, 18, 19] on the selection of the optimal solvent-nonsolvent system for the fractionation and its effect on the results. Such views may be summarized that the optimum solvent-nonsolvent combination is formed from a poor solvent and a mild precipitant• The closest approximation to this condition seems to be achieved by the acetone-methanol combination• It is important to note that the results obtained here show the applicability of this idea to copolymers, if the fractionation was performed with respect to molecular weight only. The best comparison of the fractionation efficiencies can be obtained by analysis of the heterogeneity of fractions• This was determined for several representative fractions using GPC, light scattering and osmotic pressure measurements. The results in Fig. 4 and Table 2 show the appreciably narrower distributions of all the fractions in comparison with that of the original sample. The same conclusion comes from the heterogeneity values of fractions. This means that good fractionation was accomplished by both the column and coacervate extraction methods. The fraction heterogeneity values obtained by GPC are as a rule higher than those calculated from the
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Fig. 5. Cumulative weight distributions of the sample obtained by continuous column fractionations. light scattering and osmotic measurements (last column in Table 2), most probably because of the broadening effect typical for the former characterization. There are also differences in the resolution accomplished by the various fractionation methods and different solvent pairs. The resolution is practically identical for lower and middle fractions (see for example fractions 1-6 and 111-9). The differences arise for higher fractions, indicating the superior efficiency of the acetone-methanol pair over dichloromethane-methanol and carbon tetrachloridemethanol• This follows from the width of the distribution curves for fractions 1-22, 11-14 and 111-13, the narrowest is that obtained by the column fractionation method using the acetone-methanol combination, which also correspond to the lowest heterogeneity (Table 2). The distribution curves for fractions I L l 4 and 111-13 (the same or even lower Mw values) obtained by using dichloromethane-methanol and carbon tetrachloride-methanol pairs, respectively, are wider. The poorest efficiency is obtained by the coacervate extraction method (fraction IV-9 compared by II-12), which could be easily explained by the nature of this procedure [20]. The results of continuous column fractionations compared with those obtained by the stepwise variation of solvent power are presented as integral weight distribution data in Fig. 5. Two features of
Table 2. Results of fraction analysis Fraction no.
Original sample 1-6 1-9 1-22 11-12 11-14 III-9 III-11 II1-13 IV-19
Solvent pair
Acetone-methanol Acetone-methanol Acetone-methanol CH2C12-methanol CH2Clz-methanol CC14-methanol CCl4-methanol CC14-methanol Benzene~TEG
Mw x 10-'* (GPC)
M. x 10 -4 (GPC)
(Mw/M.) (GPC)
(M~/M.) (LS, OS)
28-84
12.26
2.35
1.81
11.25 16.00 86.57 32.20 61.95 11.97 20.20 58.56 32.38
9.15 13.00 67.58 25.55 43.66 9.62 15.40 39.34 21.33
1.23 1.22 1.29 1.29 1.42 1.24 1.31 1.49 1.48
1.19 1 "20 1 '29
1"36
Fractionation of styrene-methyl methacrylate copolymer these curves should be discussed. Firstly, the highest intrinsic viscosities in both continuous column fractionations were 1.56 dl/g, i.e. above those observed in the batchwise operations. This result, indicating the improved fractionation of tail fractions of the sample, is most probably due to better accommodation of the solvent gradient applied in the continuous fractionation procedure to the solubility of the sample. The second result in Fig. 5 needing explanation is an obvious deviation of experimental points obtained by the continuous procedure from the "reference" integral weight distribution constructed from the data of batchwise column elution; the term reference used here relates to the distribution supposedly determined under conditions of thermodynamic equilibrium between the sol and gel phase in the fractionating column. Such reasoning is based on the relatively long elution period (more than 3 hr) for each fraction during the batchwise procedure. The observed shift of experimental points obtained by continuous elution toward lower intrinsic viscosities can be attributed to too short contact time between gel and sol phase in the column. The duration of contact plays a very important role in column fractionation [21], and its minimum value, necessary for equilibrium in the column to be achieved, can be estimated [22] from the column dimensions and physico-chemical parameters of a sample. Such an estimation, performed in our case with the. results already transformed to the limiting elution rate, required for the establishing of equilibrium conditions in the column, gave the value of approx. 120 ml/hr. The elution rates used in two experiments were 150 and 60 ml/hr, respectively, i.e. only the latter value may allow good fractionation conditions in the colunm. The results of this second run are better indeed, although the experimental points still do not coincide with the reference integral weight distribution. This is not surprising because the applied elution rate still does not differ significantly from the limiting value, and the uncertainties of the estimation procedure, particularly when dealing with copolymers, may be quite high. The observed results of continuous column fractiona-
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tions justify therefore the use of very low elution rates of approx. 20-25 ml/hr, as applied by Cantow [16] and Ueda [17]. REFERENCES
1. M. J. R. Cantow (editor), Polymer Fractionation. Academic Press, New York (1967). 2. W. H. Stockmayer, L. D. Moore, M. Fixman and B. N. Epstein, J. Polym. Sci. 56, 517 (1955). 3. F. C. Baines and J. C. Bevington, Europ. Polym. J. 3, 593 (1967). 4. L. G. Kudryavtseva, A. D. Litmanovich, A. V. Topchiev and V. Ya Shtern, Neftekhimiya 3, 343 (1963). 5. N. S. Schneider, J. Polym. Sci. C8, 179 (1965). 6. J. Pola~k, Colin Czech. chem. Commun. 28, 1838 (1963). 7. Z. Grubi~i~-Gallot, M. Picot, Ph. Gramain and H. Benoit, J. appl. Polym. Sci. 16, 2931 (1972). 8. Z. Grubi~i6, P. Rempp and H. Benoit, J. Polym. Sci. B5, 753 (1967). 9. A. D. Litmanovich, V. Ya. Shtern and A. V. Topchiev, Neftekhimiya 3, 217 (1963). 10. V. Jurani~ova, S. Florian and D. Berek, Europ. Polym. J. 6, 57 (1970). 11. S. Teramachi, H. Tomioka and M. Sotokawa, J. Macromol. Sci. A6, 97 (1972). 12. H. Bamford and H. Tompa, Trans. Faraday Soc. 46, 310 (1950). 13. E. Turska and L. Utracki, J. appl. Polym. Sci. 2, 46 (1959). 14. O. Redlich, A. L. Jacobson and W. H. McFadden, J. Polym. Sci. A1, 393 (1963). 15. G. V. Schulz and A. Dinglinger, Z. phys. Chem. B 43, 47 (1939). 16. M. J. R. Cantow, R. S. Porter and J. F. Johnson, J. Polym, Sci. C, 16, 13 (1967). 17. M. Ueda, Polym. J. 3, 431 (1972). 18. J. M. Hulme and L. A. McLeod, Polymer 3, 153 (1962). 19. T. Ogawa, S. Tanaka and S. Hoshino, J. appl. Polym. Sci. 16, 2257 (1972). 20. J. Klein and U. Wittenberger, Makromolek. Chem. 131, 217 (1970). 21. J. Klein and G. Weinhold, Angew. Makromol. Chem. 10, 49 (1970). 22. G. V. Schulz, P. Deussen and A. G. R. Scholz, Makromolek. Chem. 69, 47 (1963).
FRACTIONATION OF STYRENE-METHYL METHACRYLATE COPOLYMER LJ. LOVRI~ INA, Research and Development Institute, Zagreb, Yugoslavia Z. GRUBIgI~-GALLOT Centre de Recherches sur les Macromol6cules, Strasbourg, France and B. KUNST* Institute of Physical Chemistry, University of Zagreb, P.O. Box 179, 41001 Zagreb, Yugoslavia
(Received 16 May 1975) Abstract--Styrene-methyl methacrylate copolymer was fractionated by the column elution and coacervate extraction techniques. Different solvent-nonsolvent combinations were used; both batchwise and continuous column elution fractionations were carried out. Successful resolution of fractions according to molecular weight was achieved. The effect of the solvent-precipitant pair on the fractionation efficiency, as tested by gel permeation chromatography, is discussed. The results of continuous column elution depend significantly on the elution rate.
INTRODUCTION Fractionation of a polymer sample is usually carried out with the goal [1] of obtaining its molecular weight distribution or preparing samples of reduced polydispersity. The process itself is not simple, and may become very complicated for copolymer samples, which may exhibit heterogeneity in chemical composition, in molecular weight and in monomer sequence length. The process of separation of a sample into materials having the same structure is most often based on difference in solubility. Since the solubility behaviour of the individual molecules depends on the three mentioned variables, there may be fractionation according to all three or to two of them. Proper selection of the eXperimental conditions is needed for fractionation to be done only with respect to the one of the variables. Two of the fractionation conditions especially affect the solubility of a copolymer, viz. the nature of solvent-precipitant combination and the fractionation temperature. The selection of these two parameters is therefore very important. The purpose of this paper is to report a study of various methods and conditions for fractionation of a styrene-methyl methacrylate copolymer. Papers concerning the fractionation of styrene-methyl methacrylate copolymer have already been published [ 2 4 ] but using different techniques. In the present study, column elution and coacervate extraction methods have been investigated using different solvent-precipitant systems and fractionation temperatures. EXPERIMENTAL
Materials A styrene-methyl methacrylate copolymer was obtained from the R/Shm & Haas Co., Darmstadt, W. Germany, * To whom correspondence should be addressed. • 189
and used without preliminary purification. The homopolymer samples u~ed for the solubility determinations were fractions of polystyrene (OKI, Zagreb) and poly(methyl methacrylate) (R6hm & Haas, Darmstadt) with viscosity molecular weights of approx 230,000. The solvents were the best commercially available and were used without further purification.
Fractionations The fractionating column was approx 120 cm long and 2.6 cm i.d., and fitted with a glass jacket for the circulation of thermostated water. The column temperature was maintained within 0.1 °. Glass wool or glass beads (Jencons, Hertfordshire, England) of 0-1 mm diameter, cleaned by the usual procedure, were used as the polymer support in the column. For the batchwise fractionations, the polymer sample was deposited on glass wool by solvent evaporation. Twelve grams of polymer in the form of 3~o solution in acetone, dichloromethane or carbon tetrachloride were introduced into the column and solvent was evaporated in vacuum at 30°. The nonsolvent (methanol) was then put in the column. The solvent power of solvent-precipitant system was changed stepwise, with the volume of each fraction amounting to 500 ml; the elution rate was 150 ml/hr. The continuous fractionations were performed with 3 g samples. The sample was deposited on the glass beads outside the column by the evaporation of solvent. After drying the beads were placed in the column by the wet method. The elution was carried out using the logarithmic solvent gradient produced by a solvent mixing system similar to those reported in the literature [5, 61 The solventprecipitant system used in the continuous fractionations was carbon tetrachloride-methanol at 30°, with the initial and final volume fractions of precipitant being 0'57 and 0.47, respectively. The volume of solvent mixture in the mixing chamber was 650 ml and the elution rate was varied from 60 to 150 ml/hr. The volume of each fraction was 150 ml, and 15-20 fractions were taken in each experiment.
190
LJ. LOVRIC, Z. GRUBI~I~-GALLOTand B. KUNST
Fractionation by coacervate extraction was carried out in a thermostated cylindrical container of 2 1. The system benzene-triethylene glycol at 50° was used. A 5% solution of the polymer (10 g) was placed in the stirred vessel and warmed up to 50°. The nonsolvent was added dropwise with vigorous stirring. Two layers were allowed to separate and the lower layer containing the first fraction was withdrawn from the bottom of the vessel. The procedure was repeated with the solvent-precipitant mixtures, each of a slightly greater solvent power until the fractionation was completed. All the fractions were recovered by precipitation with methanol, filtration and drying to constant weight. Fraction analysis Dilute solution viscosities were measured at four or more concentrations in butanone at 25'0 ___0"05°. A semimicro dilution viscometer (Cannon Ubbelohde type CUSMDC-50) with a flow rate for pure solvent of more than 135 sec was used. A Waters Associates M-200 gel permeation chromatograph equipped with 5 columns (107, 106, 10s, 104 and 3 x 103 A) at room temperature was employed. Tetrahydrofuran was used as eluting solvent at a flow rate of 1 ml/min. To characterize the GPC fractions, an automatic viscometer and u.v. spectrophotometer were coupled with the gel permeation chromatograph in the manner described earlier I-7]. A universal calibration curve I-8] was used. A Mechrolab osmometer model 201 was used for number-average molecular weights. Measurements were made at 30° using toluene as solvent. A Fica 42000 light scattering photometer was used for weight-average molecular weights. Measurements were made in tetrahydrofuran at 25° using 5460 A light. The chemical compositions of fractions and of the original sample were determined by NMR measurements using a Varian A-60 spectrometer. RESULTS AND DISCUSSION
Selection of solvents The procedures devised to select the solventprecipitant system for the fractionation of copolymers have been reported [9,10]. They generally consist of solubility determinations for the parent homopolymers and for a series of copolymers using cloud-point titration. In this work three solvent-precipitant pairs have been selected for the column fractionation. The first, suitable to enhance fractionation with respect to molecular weight, was chosen according to the procedure recommended by Jurani~ova et al. [10]. The most convenient combination appeared to be dichloromethane-methanol with Vcrit values* for polystyrene and poly(methyl methacrylate) of 0"282 and 0'338, respectively. The other two solvent-nonsolvent combinations for the column fractionation were selected by reasoning similar to that of Teramachi et al. [11] who showed that the phase separation conditions of a copolymer are sensitive to change in chemical composition if the difference between the interaction parameters of solvent with both monomer units, ZA and ZB, is large. The convenient solvent-precipitant pairs which * 7crlt is the value of the volume fraction of nonsolvent at the cloud point extrapolated to zero concentration, the value that could be put equal to the composition of Omixture.
should give such conditions seem to be carbon tetrachloride-methanol and acetone-methanol. In the first combination, carbon tetrachloride is solvent for polystyrene and nonsolvent for poly(methyl methacrylate), so the difference between the interaction parameters of the solvent pair with polystyrene and poly(methyl methacrylate) may be quite large. The analogous conditions may be achieved with the second combination chosen, where acetone is nonsolvent for polystyrene and solvent for poly(methyl methacrylate). Using these two combinations, one can expect therefore that fractionation of styrene-methyl methacrylate copolymer according to chemical composition would be favoured. The choice of the solvent combination and the appropriate temperature for the fractionation by coacervate extraction has always been tedious. Although guidelines, both theoretical [12] and practical r13,14], exist, the proper choice has to be made by the slow trial-and-error procedure. It seems to be particularly true with copolymers, so in this work many nonsolvents in combination with two suitable solvents-benzene and dichloromethane--as well as the convenient temperatures have been tested. The best coacervate appeared to form with the combination benzene-triethylene glycol at 50 °, which was eventually used for fractionation by this method. Fractionation The results of the column fractionations by the batchwise variation of the solvent-precipitant mixture and those obtained by the coacervate extraction in the form of cumulative weight distribution data, constructed in the usual way [15], are shown in Figs. 1 and 2, respectively. Experimental points in Fig. 1 representing three column fractionations using different solvent-nonsolvent pairs fall practically along a single distribution curve. There are some minor deviations from the common curve but they can hardly be considered as systematic• Such agreement of experimental results for different solvent-nonsolvent pairs indicates that fractionation took place with respect to molecular weight only. The good fractionation is also confirmed by the
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Fractionation of styrene methyl methacrylate copolymer
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Fig. 2. Comparison of coacervate extraction and column fractionation results. absence of fraction reversal, i.e. there is always a regular progression of intrinsic viscosities through the fractions. The distribution curve obtained by the coacervate extraction is basically identical (Fig. 2) to that shown in Fig. 1. There is however a slight tendency for low intrinsic viscosity values to fall below the distribution curve obtained by the column fractionations, and the high intrinsic viscosity values to move above it. The general agreement of the distribution curves obtained by two methods suggests that the fractionation conditions chosen for the coacervate extraction were good and the slight deviations originate most probably from the inherently broader fractions produced by the coacervate extraction method. Composition determinations for all the fractions are summarized in Table 1. These results show the same composition for all fractions. They are chemically identical with the original sample, substantiating the chemical homogeneity of the latter. This is why all the four carefully performed fractionations, regardless of the solvent-nonsolvent pair selection or the method, gave the same distribution curve. Such a finding is expected to hold [3] for a copolymer sample polymerized by a radical process carried to only low conversion. If the conclusion on chemical homogeneity of the copolymer sample is correct, the results obtained by the molecular weight fractionations using different solvent-precipitant combinations may be examined in more detail for fractionation efficiency. This can be done by comparing the distribution shown in Fig. 1 with the result obtained by the GPC determination
~LECUL.Z~ v~r~r, M,,.~o-5 Fig. 3. Comparison of cumulative weight distributions of the sample as the result of column fractionation and gel permeation chromatography.
of the mass distribution curve. For this purpose, the intrinsic viscosities determined experimentally were in both cases converted to molecular weights using the equation of Stockmayer[2], valid for the 1:1 S-MMA copolymer in butanone at 25 °. Comparison of the integral mass distribution curves (Fig. 3) shows evident differences, due to the different natures of the fractionation processes. The integral mass distribution curve obtained by gel permeation chromatography shows more lower molecular weight material, and also better resolution of higher fractions. The highest intrinsic viscosity was 2"94 dl/g compared to only 1.44 dl/g for the highest fraction obtained by column fractionation. The observed differences can be partly attributed to the general tendency 1-16] of gel permeation chromatography to give wider distributions. The more important reason for the observed deviation, i.e. for the worse resolution achieved by the column fractionation, seems to be incomplete elution of higher fractions, so called 'entrainment'. This effect may be caused by adsorption of polymer on the glass carrier [17] which, together with a finite number of evaluated fractions, affects the mass distribution curve. There are also some subtle differences within the column ffactionation results arising from the application of various solvent-nonsolvent combinations. They can be identified in Fig. 1 by the highest values of the measured intrinsic viscosities. The maximum intrinsic viscosity was 1.44 dl/g obtained by the acetone-methanol system, which may be therefore considered the most efficient solvent pair. On the other hand, the last experimental point obtained with the dichloromethane~methanol system showing an upward shift of the curve indicates the relatively worse
Table 1. Chemical compositions of sample and fractions Styrene in the unfract, sample (mol %)
49.7
Solvent-precipitant system
No. of fractions and styrene determinations
Styrene in fractions (mol %)
Acetone-methanol
22 14 13 28
49.8 + 0.77 49.6 4- 0"95 49.8 +_ 0-91 49.9 + 0.87
CH2C12 methanol CCI4 methanol
Benzene-TEG
192
LJ. LOVRI~, Z. GRUBIgI~-GALLOT and B. KUNST
I
100
,_9O ~•
I-6
111:9
%0
IV-19 II-12 111-131-22 11-11.
u.I
0-70
,6o ~so ~o 20 36
3l,
32
30 28 26 ELUTION VOLUME, COUNTS
10
2':
Fig. 4. Gel permeation chromatograms for various fractions. resolution. There is support for these conclusions. It manifests in the more uniform distribution of the amount of polymer among the fractions from the fractionation using the acetone-methanol system. The total polymer quantity used in the other two column fractionations was accumulated in several fractions only. Such findings are consistent with previous views [5, 18, 19] on the selection of the optimal solvent-nonsolvent system for the fractionation and its effect on the results. Such views may be summarized that the optimum solvent-nonsolvent combination is formed from a poor solvent and a mild precipitant• The closest approximation to this condition seems to be achieved by the acetone-methanol combination• It is important to note that the results obtained here show the applicability of this idea to copolymers, if the fractionation was performed with respect to molecular weight only. The best comparison of the fractionation efficiencies can be obtained by analysis of the heterogeneity of fractions• This was determined for several representative fractions using GPC, light scattering and osmotic pressure measurements. The results in Fig. 4 and Table 2 show the appreciably narrower distributions of all the fractions in comparison with that of the original sample. The same conclusion comes from the heterogeneity values of fractions. This means that good fractionation was accomplished by both the column and coacervate extraction methods. The fraction heterogeneity values obtained by GPC are as a rule higher than those calculated from the
~/
_J, 02
*
60 mr/hour
, Q,'-
06 0.8 tO ( 'r/ }, d u g
1.2
1/,
1.6
Fig. 5. Cumulative weight distributions of the sample obtained by continuous column fractionations. light scattering and osmotic measurements (last column in Table 2), most probably because of the broadening effect typical for the former characterization. There are also differences in the resolution accomplished by the various fractionation methods and different solvent pairs. The resolution is practically identical for lower and middle fractions (see for example fractions 1-6 and 111-9). The differences arise for higher fractions, indicating the superior efficiency of the acetone-methanol pair over dichloromethane-methanol and carbon tetrachloridemethanol• This follows from the width of the distribution curves for fractions 1-22, 11-14 and 111-13, the narrowest is that obtained by the column fractionation method using the acetone-methanol combination, which also correspond to the lowest heterogeneity (Table 2). The distribution curves for fractions I L l 4 and 111-13 (the same or even lower Mw values) obtained by using dichloromethane-methanol and carbon tetrachloride-methanol pairs, respectively, are wider. The poorest efficiency is obtained by the coacervate extraction method (fraction IV-9 compared by II-12), which could be easily explained by the nature of this procedure [20]. The results of continuous column fractionations compared with those obtained by the stepwise variation of solvent power are presented as integral weight distribution data in Fig. 5. Two features of
Table 2. Results of fraction analysis Fraction no.
Original sample 1-6 1-9 1-22 11-12 11-14 III-9 III-11 II1-13 IV-19
Solvent pair
Acetone-methanol Acetone-methanol Acetone-methanol CH2C12-methanol CH2Clz-methanol CC14-methanol CCl4-methanol CC14-methanol Benzene~TEG
Mw x 10-'* (GPC)
M. x 10 -4 (GPC)
(Mw/M.) (GPC)
(M~/M.) (LS, OS)
28-84
12.26
2.35
1.81
11.25 16.00 86.57 32.20 61.95 11.97 20.20 58.56 32.38
9.15 13.00 67.58 25.55 43.66 9.62 15.40 39.34 21.33
1.23 1.22 1.29 1.29 1.42 1.24 1.31 1.49 1.48
1.19 1 "20 1 '29
1"36
Fractionation of styrene-methyl methacrylate copolymer these curves should be discussed. Firstly, the highest intrinsic viscosities in both continuous column fractionations were 1.56 dl/g, i.e. above those observed in the batchwise operations. This result, indicating the improved fractionation of tail fractions of the sample, is most probably due to better accommodation of the solvent gradient applied in the continuous fractionation procedure to the solubility of the sample. The second result in Fig. 5 needing explanation is an obvious deviation of experimental points obtained by the continuous procedure from the "reference" integral weight distribution constructed from the data of batchwise column elution; the term reference used here relates to the distribution supposedly determined under conditions of thermodynamic equilibrium between the sol and gel phase in the fractionating column. Such reasoning is based on the relatively long elution period (more than 3 hr) for each fraction during the batchwise procedure. The observed shift of experimental points obtained by continuous elution toward lower intrinsic viscosities can be attributed to too short contact time between gel and sol phase in the column. The duration of contact plays a very important role in column fractionation [21], and its minimum value, necessary for equilibrium in the column to be achieved, can be estimated [22] from the column dimensions and physico-chemical parameters of a sample. Such an estimation, performed in our case with the. results already transformed to the limiting elution rate, required for the establishing of equilibrium conditions in the column, gave the value of approx. 120 ml/hr. The elution rates used in two experiments were 150 and 60 ml/hr, respectively, i.e. only the latter value may allow good fractionation conditions in the colunm. The results of this second run are better indeed, although the experimental points still do not coincide with the reference integral weight distribution. This is not surprising because the applied elution rate still does not differ significantly from the limiting value, and the uncertainties of the estimation procedure, particularly when dealing with copolymers, may be quite high. The observed results of continuous column fractiona-
~.l,,l
12/~
I
193
tions justify therefore the use of very low elution rates of approx. 20-25 ml/hr, as applied by Cantow [16] and Ueda [17]. REFERENCES
1. M. J. R. Cantow (editor), Polymer Fractionation. Academic Press, New York (1967). 2. W. H. Stockmayer, L. D. Moore, M. Fixman and B. N. Epstein, J. Polym. Sci. 56, 517 (1955). 3. F. C. Baines and J. C. Bevington, Europ. Polym. J. 3, 593 (1967). 4. L. G. Kudryavtseva, A. D. Litmanovich, A. V. Topchiev and V. Ya Shtern, Neftekhimiya 3, 343 (1963). 5. N. S. Schneider, J. Polym. Sci. C8, 179 (1965). 6. J. Pola~k, Colin Czech. chem. Commun. 28, 1838 (1963). 7. Z. Grubi~i~-Gallot, M. Picot, Ph. Gramain and H. Benoit, J. appl. Polym. Sci. 16, 2931 (1972). 8. Z. Grubi~i6, P. Rempp and H. Benoit, J. Polym. Sci. B5, 753 (1967). 9. A. D. Litmanovich, V. Ya. Shtern and A. V. Topchiev, Neftekhimiya 3, 217 (1963). 10. V. Jurani~ova, S. Florian and D. Berek, Europ. Polym. J. 6, 57 (1970). 11. S. Teramachi, H. Tomioka and M. Sotokawa, J. Macromol. Sci. A6, 97 (1972). 12. H. Bamford and H. Tompa, Trans. Faraday Soc. 46, 310 (1950). 13. E. Turska and L. Utracki, J. appl. Polym. Sci. 2, 46 (1959). 14. O. Redlich, A. L. Jacobson and W. H. McFadden, J. Polym. Sci. A1, 393 (1963). 15. G. V. Schulz and A. Dinglinger, Z. phys. Chem. B 43, 47 (1939). 16. M. J. R. Cantow, R. S. Porter and J. F. Johnson, J. Polym, Sci. C, 16, 13 (1967). 17. M. Ueda, Polym. J. 3, 431 (1972). 18. J. M. Hulme and L. A. McLeod, Polymer 3, 153 (1962). 19. T. Ogawa, S. Tanaka and S. Hoshino, J. appl. Polym. Sci. 16, 2257 (1972). 20. J. Klein and U. Wittenberger, Makromolek. Chem. 131, 217 (1970). 21. J. Klein and G. Weinhold, Angew. Makromol. Chem. 10, 49 (1970). 22. G. V. Schulz, P. Deussen and A. G. R. Scholz, Makromolek. Chem. 69, 47 (1963).