Interaction of Polymers with Near-Critical Carbon Dioxide - ACS

Chapter 14

Interaction of Polymers with Near-Critical Carbon Dioxide 1

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Downloaded by UNIV OF ROCHESTER on April 17, 2013 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch014

A. R. Berens and G. S. Huvard

BFGoodrich Company, Research and Development Center, Brecksville, OH 44141

The kinetics and equilibria of carbon dioxide transport in a wide variety of polymers have been studied at pressures up to the saturated vapor pressure at 25°C. Gravimetric data were obtained by rapid weighing of polymer films of varied thickness during desorption at atmospheric pressure, following exposures to compressed CO for various times and pressures. Transport kinetics are Fickian, and diffusivity increases with CO concentration. The solubility of CO generally increases with increasing content of polar groups in the polymer. For several glassy polymers, the isotherms plotted vs. CO activity are sigmoid in shape, combining dual-mode character at low activity with Flory-Huggins form at high activity. The assembled evidence shows that near-critical CO behaves as a polar, highly volatile organic solvent, rather than as a simple gas, in its interactions with polymers. 2

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The transport of caibon dioxide in polymers has historically been analyzed in the same manner as other simple gases (1). Recent studies have shown, however, that the effects of C 0 on polymers include some features commonly associated with organic solvents, including swelling (2-5). and depression of glass transition temperatures, i.e., plasticization (6-8). Moreover, C 0 can be handled as a liquid at room temperature under rather moderate pressures; its critical temperature is 31°C and its saturated vapor pressure at 25°C is 64.6 atm (950 psi). For these reasons it seems appropriate to consider near-critical C 0 as a highly volatile solvent, rather than as a gas, in its interactions with polymers. 2

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Current address: R.D. No. 2, Box 3510, Middlebury, VT 05753 Current address: Ε. I. du Pont de Nemours and Company, P.O. Box 27001, Richmond, VA 23261

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0097-6156/89/Ό406-0207$06.00Λ)

ο 1989 American Chemical Society

In Supercritical Fluid Science and Technology; Johnston, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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This paper describes a simple new technique for obtaining both kinetic and equilibrium data on the transport of C 0 in polymers, and compares results for several polymers with the behavior of organic vapor/polymer systems. In subsequent publications, we will discuss transport data for additional polymer/C0 binary systems and for ternary systems containing an added low molecular weight component It now appears that the high diffusivity, solublity and plastkizing action of compressed C 0 in polymers makes this gas uniquely useful in promoting the impregnation of many polymers with a wide variety of additives (9). 2

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Experimental The general experimental method used here involves sorption of C 0 into polymer film or sheet samples in a simple pressure vessel, followed by rapid venting and transfer of the samples to a balance for recording weight changes during desorption. This gravimetric method does not require a balance capable of operation under high pressure, but can provide kinetic data in both sorption and desorption, as well as equilibrium solubilities, through suitable experimental procedures and data analyses. 2

Procedures. Experiments in this study involved sorption of both gaseous and liquid C 0 and hence covered pressures up to the saturated vapor pressure of C0 . All measurements were carried out at 25°C. Polymer samples included a wide variety of glassy, rubbery, and semicrystalline materials. To avoid use of solvents, the polymers were formed intofilms0.1 to 1 mm. thick by compression-molding from the melt; a sample about 1x2 cm. was used in each experiment. The procedure for high pressure sorption experiments is illustrated schematically in Figure 1. A polymer sample, of 20 - 200 mg. dry weight, w , was placed in a 100 ml. pressure vesselfittedwith a pressure gauge, valve and a screw closure which could be opened quickly. The vessel was evacuated, thenfilledto the desired sorption pressure, P , from a cylinder of liquid C 0 and left at this pressure for an appropriate sorption period, t . Because of the relative sizes of sample and vessel, sorption caused no appreciable pressure drop. For each polymer studied, experiments were run at various pressures, sorption times, and sample thicknesses, / . At the end of the sorption period, the C 0 pressure was rapidly vented to atmospheric, the vessel opened, and the sample quickly placed on the pan of a fast-response electronic digital balance readable to 0.00001 g (Mettler AE163). With a computerized data acquisition system, sample weights during desorption at atmospheric pressure were recorded as a function of desorption time, 1^, at intervals as short as 5 seconds beginning within 10 to 20 seconds after venting the vessel. Supplemental gravimetric sorption/desorption data for C 0 pressures below atmospheric were obtained by conventional procedures using a Cahn recording vacuum microbalance, for greater precision in the low solubility range. 2

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Interaction ofPolymers with Near-Critical C0


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Figure 1. Experimental procedure for following desoiption from polymer samples after exposure to high-pressure CO2. (Schematic).


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Data analysis. Using only sample weights, w , recorded during desorption, it is possible to develop a rather complete picture of sorption equilibria as well as the kinetics of both absorption and desorption. First, Mj ^ = (w - w )/w is plotted vs. the square root of ; for Ficldan diffusion from a plane sheet, this plot should be initially linear. Figure 2, typical of our results on glassy polymers, demonstrates this behavior. Extrapolation to ί^= 0 gives M^ , the weight of C 0 in the sample at the end of the sorption period t . The equilibrium C 0 solubility is established by running several samples at a given P , for successively longer i ^ until a constant value of M ^ establishes the equilibrium uptake, M . The equilibrium sorption isotherm is determined from solubility measurements at various pressures. From the initial slope of Af^j /M vs. Vi^ /I for samples sorbed to equilibrium, one calculates D j , the mean diffusivity for desorption over the concentration interval of that experiment (1). The precision of Mf (intercept) and (slope) determinations depends upon the rate of desorption, and hence on the polymer type and sample thickness. In most of the experiments reported here, at least six points were obtained in the linear region of M ^ j v$Vt; the precision of M^ is generally within ±1 g /100 g polymer, and of log Dj, within ±0.2. While each experimental run gives a complete set of weight vs. time data for desorption, it provides only one point on the weight vs. time curve for absorption. To define the kinetics of absorption, Mf is determined for several samples exposed to a given C 0 pressure for varied t shorter than the equilibration time, as schematically suggested in Figure 3. The mean diffusivity in sorption, D , is obtained from the initial slope of M^ /M vs Vt /I This analysis of the desorption data implicitly assumes that the polymer sample maintains its plane sheet geometry throughout the experiment. For most of the glassy polymers studied, this assumption seems valid, as no visible change in shape or appearance was observed upon release of the C 0 pressure or during desorption. A number of rubbery or highly plasticized polymers, however, expanded and foamed quite noticeably when the pressure was released. Nonetheless, the amount of C 0 absorbed could be estimated by quickly weighing the sample upon removal from the pressure vessel. The determination of equilibrium sorption, and particularly of diffusivity, from the intercept and slope of the weight vs. square-root time plots, however, is clearly imprecise for samples which show such geometric changes. t

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Results Transport kinetics. An example of our kinetic results is shown in Figure 4, as A#| vs Vt / /plots for poly(methyl methacrylate) (PMMA) samples of two different thicknesses exposed to liquid C 0 at 25°C. The superposition of data for different 2


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Figure 2. Representative CO2 desorption data: Weight loss of 0.3 mm cellulose acetate film after 17 hours exposure to CO2 at 500 psi.

0 Time

Figure 3. Determination of adsorption kineticsfromdesorption runs after varied adsorption periods. (Schematic).




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Figure 4.

Adsorption and desorption kinetics for C02 in PMMA films of two

thicknesses at 25°; sorption at 950 psi., desorption to atmospheric pressure.


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sample thickness indicates that the transport kinetics are Fickian. The relative behavior of the sorption and desorption curves depends on the form of the concentration dependence of diffusivity. The divergence of the sorption and desorption curves, with the desorption curve falling well below the absorption curve at longer times, indicates that the diffusion coefficient is an increasing function of C 0 pressure or concentration (1). Crank (10) shows examples of curves resembling Fig. 4 when there is a maximum in the diffusivity-concentration relationship, as may be suggested by some of our results. More extensive and quantitative data on the variation of diffusivity with concentration has been obtainedfromdesorption runs following equilibration of polymer samples at different C 0 pressures. In Figure 5, values obtained in this way for four glassy polymers (PMMA, poly(vinyl chloride) (PVC), cellulose acetate (CA), and polystyrene (PS)) are plotted against the C 0 concentrations extrapolated to zero desorption time. The order of C 0 diffusivities, PS » PVC > PMMA, is the same as previously found for organic vapors (11). The points at lowest C 0 concentration for each polymer were obtained by conventional gravimetric sorption/desorption experiments using a Cahn vacuum microbalance, and agree well with results of permeation experiments for PS (1), PVC (12X and PMMA (13). The extrapolations of our high-pressure results are quite consistent with these low pressure data, attesting to the validity of the simple new technique described here. Unfortunately, few diffusivity data obtained by other methods are available for direct comparison with our high-pressure kinetic results. With increasing concentration, the diffusivity of C 0 shows parallel increases for the four polymers, and approaches the range of 10" to 10" cm /sec. which is typical for C 0 diffusivity in rubbers (1). This evidence from transport kinetic data is consistent with other indications that C 0 has a substantial plasticizing effect upon glassy polymers (6-8). The downward curvature of the log Dvs C plots is similar to that reportedforseveral organic vapor/polymer systems at temperatures not far above Tg (1). The apparent leveling off or maxima of the curves may also be a result of the opposing effects of increasing C 0 concentration and increasing hydrostatic pressure (14).


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Sorption equilibria. Equilibrium solubilities of C 0 in a variety of different polymers at 25°C have been determined by our gravimetric desorption method. At higher pressures, the solubilities are quite substantial and vary markedly with the polymer type. Equilibrium uptakes of liquid C 0 rangefromabout 3 g/100 g in polyethylene to over 50 g/100g in polyvinyl acetate); other high values have been found for PMMA (27 g/100), ethyl cellulose (30 g/100), and cellulose acetate (27 g/100). To help elucidate the solvent action of C 0 the effects of some systematic variations of polymer structure have been investigated. Informative trends have been observedforseveral series of copolymers, as illustrated in Figure 6. The equilibrium sorption of liquid C 0 in butadiene/aerylonhrile (BAN) copolymer rubbers increases 2

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Wt S AN

Figure 6.

Solubility of liquid CO2 at 25° vs. acrylonitrile content of styrene

(S/AN) and butadiene (B/AN) copolymers.


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regularly with increasing AN content; the same trend was observed for C 0 at atmospheric pressure in the early studies of van Amerongen (15). Figure 6 also shows a similar trend for a series of glassy styrene/acrylonitrile (SAN) copolymers, hence this effect seems independent of the polymer state. Ethylene/vinyl acetate (EVA) copolymers also show a steady increase in C 0 sorption with increasing content of the more polar monomer, vinyl acetate. These trends suggest that relatively high sorption values may result from specific polar interactions of C 0 with carbonyl or nitrile groups in the polymers. The solvent interaction of liquid or dense gaseous C 0 with various organic solutes is a topic of continuing research interest (16-18). The low value of its solubility parameter, 6.0 (cal/cc) (12)» suggests that liquid C 0 should behave like a non-polar hydrocarbon. Our results for polymers, however, show behavior resembling a somewhat polar organic solvent; perhaps the quadrupole moment, or the H-bonding basicity (17) of C 0 may account for its unexpectedly high solubility in polar polymers. Additional data and interpretation of our results will be topics of subsequent publications. 2

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Sorption Isotherms. The dependence of equilibrium sorption uponC0 pressure has been studied for several glassy polymers; representative data are shown in Figure 7, for PVC, polycarbonate (PC), PMMA, and polyvinyl acetate) (PVA). These 25°C sorption isotherms show features well-known for both gases and organic solvents in glassy polymers: At low pressures or low concentrations, the curvature is concave downward, as is typical for gases in glassy polymers and generally described as "dualmode" behavior. At high concentrations, the curvature is upward, in the nature of the Flory-Huggins isotherms typical for vapors of swelling solvents in rubbery polymers. In the case of PMMA, the isotherm seems to show an inflection, changing from dualmode to Flory-Huggins form with increasing pressure. Sigmoidal sorption isotherms combining dual-mode behavior at low concentrations with Flory-Huggins form at higher concentration werefirstreported for the system vinyl chloride monomer (VCM)/PVC (20); examples are reproduced here in Figure 8. It was suggested that this behavior is related to the depression of the glass transition temperature, Tg, by the dissolved VCM. It was later demonstrated (21) that the isotherm inflection, which occurs attowerconcentrations as temperature is increased, coincides with independent measurements of Tg in the VCM/PVC system. Dual-mode sorption thus was shown to be characteristic of the glassy state, and the Flory-Huggins form, of the rubbery state. Observations of similar sigmoidal isotherms for several other polymer-vapor systems have prompted the recent suggestion that this may be the general form describing sorption of penetrants in glassy polymers whenever a sufficiently broad range of concentrations is covered (22). In view of the other similarities between C 0 and organic vapors in their effects on glassy polymers, further investigation of the applicability of the generalized sigmoidal isotherm to CO^polymer systems seems appropriate. The conventional and 2

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Figure 8.

Sorption isotherms for vinyl chloride in PVC (20).


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thermodynamically sound practice for polymer/vapor systems is to plot equilibrium solubility against thermodynamic activity, rather than against pressure as gas/polymer isotherms are generally published. While activity of organic vapors is adequately approximated by the ratio of actual partial pressure to saturated vapor pressure, ρ/ρ the high-pressure non-ideality of C 0 requires use of fugacities to accurately express activity. We have used the following approach in calculating C 0 activities: At temperatures below T the reference state is taken to be liquid 0 0 at p . The reference state at higher temperatures is selected by extrapolation of p into the supercritical region; the linear relation between p and IIΤ is valid for this extrapolation up to about T= 2 T (12). For each temperature, the fugacity at either the actual or extrapolated p is taken as the reference state fugacity, f , and evaluated by interpolation from published f / P data (23). Using these f values and the published f/Pdata, a table of activities, f/f , vs. Ρ is calculated for Τ at 10K intervals. Activities at each experimental Ρ and Τ are then obtained by double parabolic interpolation from this activity table. σ

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In exploring the sorption isotherms, we have utilized both our own data for gas and liquid C 0 solubility at 25°C and published data at other temperatures for a number of glassy polymers. Figures 9-11 show afewexamples, plotted both against pressure and against activity. Isotherms for G0 in polyvinyl benzoate) (PVBz), replottedfromthe data of Kamiya et al. (24), are shown in Figure 9. Conversionfrompressure to activity coordinates has the effects of compressing the higher temperature isotherms along the activity axis, increasing the upward curvature, and superimposing the isotherms at higher activities. Theresultingset of isotherms is remarkably similar to the VCM/PVC curves of Figure 8. In both cases, the isotherms show dual-mode curvature at lower activity, then appear to converge to a common curve of Flory-Hugginsformat higher activity, as indicated by the dashed line in Figure 9b. With increasing temperature, the apparent Langmuir or hole-filling portion of the dual-mode isotherm diminishes, and the apparent inflection shifts to lower concentration or activity. As in the VCM/PVC case, the inflections presumablyrepresentthe glass transitions of the C0 /(PVBz) system; Kamiya (24) has applied the term glass composition, or Cg, to the penetrant concentration which lowers Tg to a specified temperature. In Figure 10, data for C 0 solubility in polycarbonate (PC) from the present work and two published studies (2,22) are plotted against both pressure and activity. Here, the sigmoidal form, not apparent in the pressure plot,is revealed by conversion to the activity coordinate. Data in the low-activity, dual-mode range show both a decreasing solubility with increasing temperature and a sorption/desorption hysteresis (25). The higher pressure data are brought nearly into coincidence when replotted against activity. 2

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Figure 9. et al. (24).

Activity Sorption isotherms for CO2 in polyvinyl benzoate); data from Kamiya,



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Figure 10. Sorption isotherms for C02 in polycarbonate; data from Fleming and Koros (2), Kamiya et al. (25). and this study.


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Equilibrium solubility data for the CO^NfMA system are assembled in Figure 11; as in the above cases, results of our gravimetric desorption experiments are again in line with published data obtained by other methods (5* 26). The sorption of G0 in PMMA, at equal activity, is substantially greater than in PVBz or PC. Transformation of the abscissafrompressure to activity results in close superposition of the PMMA data over a 55° temperature range. Over most of the activity range, the isotherms show upward curvature, yet inflections are apparent in the data for the lower temperatures. The identification of the inflections with the glass transition seems quite consistent with experimental and calculated values of Tg in the PMMA/C0 system (7). At corresponding temperatures, Cg may be somewhat higherforPMMA than for PVBz because of the difference in Tg of the pure polymers (105°C for PMMA, 65.5° for PVBz (24)), but occurs at lower activity in PMMA because of the higher solubility of CC>2' Dual-mode behavior in PMMA appears relatively less pronounced than in PVBz or PC, probably reflecting a greater contribution of the Henry's Law mode, relative to the Langmuirian portion, in PMMA compared to the other two polymers. These three examples, and others to be included in subsequent publications, seem clearly to demonstrate that the sorption isotherms for C 0 follow the general sigmoid form observed for organic vapors in glassy polymers. This behavior seems to be a reasonable consequence of the high solubility and plasticizing action of C 0 at high pressures. At sufficiently high pressures, this gas indeed produces many of the same effects as an organic solvent. Use of the activity scale, rather than pressure, accentuates the similarity between C 0 and conventional solvents and demonstrates the continuity of behavior below and above the critical temperature. This continuity allows estimation of polymer behavior in supercritical C 0 at high pressuresfromdata obtained at subcritical temperatures and much lower pressures. The close superposition of C0 /polymer isotherms at various temperatures, when plotted vs. activity, suggests that most of the apparent temperature dependence of the isotherms plotted vs. pressure is related to the activity change of C 0 with temperature. At constant activity, the actual mixing of C 0 with PMMA, PC, or PVBz appears to be nearly athermal; i.e., the energy of interaction of C 0 with these polymers seems to be essentially that associated with the compression of the gas to its molar volume in the sorbed state. This aspect of polymer interactions with C 0 will also be considered further in forthcoming publications. The effects of high-pressure C 0 upon polymers, as demonstrated in this study, seem relevant in several practical applications. Because of its plasticizing action and its rapid absorption and desorption, compressed C 0 dramatically accelerates the transport of other small molecules in glassy polymers. This effect has recently been applied to the impregnation of glassy polymers with a wide variety of additives (2). The same action of C 0 also mavtpiav a large role in its effectivness in supercritical extractions and fractionations of polymers, and in the resistance of barrier polymers to attack by swelling agents during service in high-pressure C 0 environments. 2


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Figure 11. Sorption isotherms for C02 in PMMA; data from Wissinger and Paulaitis (5), Koros, Smith and Starmett (26). and this study.


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Conclusions 1. Useful data on the kinetics and equilibria of C 0 transport in polymers can be obtained by rapidly weighing film or sheet samples during desorption after exposure to liquid or gaseous C 0 at high pressure. 2

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2. The diffusion of C 0 in polymers follows Fickian kinetics over the entire pressure range; diffusivity increases with concentration, in line with other evidence of the plasticizing action of C 0 2


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3. Liquid C 0 is a swelling agent for a wide variety of polymers; equilibrium solubilities at 25°C rangefrom3 to at least 50 g per 100 g polymer. The variation of swelling with polymer type and polarity indicates that C 0 behaves as a somewhat polar, rather than hydrocarbon-like, solvent. 2

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4. Sorption isotherms plotted against C 0 activity in several glassy polymers show the sigmoid form observed for a number of glassy polymer/organic vapor systems. Isotherm curvature is downward (dual-mode form) at low concentrations, as is typical of the glassy state, and upward (Flory-Huggins form) at higher concentrations, characteristic of the rubbery state. The inflection corresponds to a composition having its glass transition at the isotherm temperature. 2

5. For several polymers, replotting of C 0 solubilities against activity, rather than pressure, nearly removes the temperature dependence of the isotherms above their inflection. This result indicates a near-zero heat of mixing; variation of C 0 activity with temperature apparently accounts for most of the temperature dependence of the isotherms plotted vs pressure.. 2

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Acknowledgements: The authors appreciate the experimental assistance of F. W. Kunig and the permission of The BFGoodrich Company to publish this work. Literature Cited 1. 2. 3. 4.

Crank, J.; Park, G. S. Diffusion in Polymers, Academic Press, London, 1968. Fleming, G. K.; Koros, W. J. Macromolecules 1986, 19, 2285. Sefcik, M. D. J. Polym. Sci. Polym. Phys.1986,24,935. Hirose,T.; Mizoguchi, K.; Kamiya, Y. J. Polym. Sci. Polym. Phys.1986,24, 2107. 5. Wissinger, R. G.; Paulitis, M.E. J. Polym. Sci., Polym. Phys. 1987, 25, 2497. 6. Wang, W. V.; Kramer, E. J.; Sachse, W. H. J. Polym. Sci. Polym. Phys. 1982, 20, 1371. 7. Chiou,J.S.; Barlow,J.W.; Paul, D. R. J. Appl. Polym Sci. 1985, 30, 2633.



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8. Sefcik, M. D. J. Polym. Sçi. Polym. Phys. 1986, 24, 957. 9. Berens, A. R.; Huvard, G. S.; Korsmeyer, R. W. AIChE National Meeting, Washington, DC, November 28 - December 2, 1988 (to be published). 10. Crank, J. The Mathematics of Diffusion, Clarendon Press, Oxford, 1956; p. 283. 11. Berens, A. R.; Hopfenberg, H. B. J. Membrane Sci., l982, 10, 283. 12. Tikhomirov, B. P.; Hopfenberg, H Β.; Stannett, V.; Williams, J. L. Makromol. Chem. 1968, 118, 177. 13. Patel, V. M., et al. Makromol. Chem. 1972, 158, 65. 14. Rogers, C.E., in Polymer Permeability, Comyn, J., Ed.; Applied Science Publishers: London, 1984. 15. van Amerongen, G. J. J. Polym. Sci. 1950, 5, 307. 16. Francis, A. W. J. Phys. Chem. 1954, 58, 1099. 17. Hyatt, J. A.J.Org.Chem. 1984, 19, 5097. 18. Dandge, D. K.; Heller, J. P.; Wilson, Κ. V. Ind. Eng. Chem. Prod. Res. Dev. 1985,24,162. 19. Prausnitz, J. M.; Shair, F. H. A.I.Ch.E. Journal 1961, 7, 682. 20. Berens, A. R. Angew. Makromol. Chem. 1975, 42, 97. 21. Berens, A. R. Polym. Eng. Sci., 1980, 20, 95. 22. Connelly, R. W.; McCoy, N. R.; Koros, W. J.; Hopfenberg, H. B.; Stewart, M. E. J. Appl. Polym. Sci., 1987, 34, 703. 23. International Thermodynamic Tables of the Fluid State: Carbon Dioxide, Angus, S.; Armstrong, B.; de Reuck, K.M., Eds., Pergamon Press, Oxford, 1976. 24. Kamiya, Y.; Mizoguchi, K.; Naito, Y.; Hirose, T. J. Polym. Sci. Polym. Phys., 1986, 24, 535. 25. Kamiya, Y.; Hirose, T.; Mizoguchi, K.; Naito, Y. J. Polym.Sci.Polym. Phys., 1986, 24, 1525. 26. Koros, W. J.; Smith, G. N.; Stannett, V. T. J. Appl. Polym. Sci. 1981, 26, 159. RECEIVED May 1, 1989


Interaction of Polymers with Near-Critical Carbon Dioxide - ACS

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