J. Phys. Chem. 1996, 100, 15581-15587
15581
Solubility of Polymers and Copolymers in Supercritical CO2 Frank Rindfleisch, Todd P. DiNoia, and Mark A. McHugh* Department of Chemical Engineering, Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed: May 30, 1996; In Final Form: July 17, 1996X
Cloud-point data to temperatures of 270 °C and 3000 bar are presented for CO2 with the family of poly(acrylates) including methyl, ethyl, propyl, butyl, ethylhexyl, and octadecyl, with poly(butyl methacrylate), with poly(vinyl acetate), with statistically random copolymers of poly(ethylene-co-methyl acrylate) with 41, 31, and 18 mol % acrylate, with poly(tetrafluoroethylene-co-hexafluoropropylene) and poly(vinylidene-cohexafluoropropylene) copolymers, each with ∼20 mol % hexafluoropropylene, and with Teflon AF. Over the same range of conditions, CO2 cannot dissolve polyethylene, poly(acrylic acid), poly(methyl methacrylate), poly(ethyl methacrylate), polystyrene, poly(vinyl fluoride), or poly(vinylidene fluoride). CO2 is a weak solvent that exhibits the temperature-sensitive characteristics observed with polar solvents. The solubility of a nonpolar hydrocarbon polymer or a copolymer in CO2 can be increased by at least partially fluorinating the polymer or by incorporating some polar groups into the backbone architecture of the polymer. Because it is such a weak solvent, CO2 can distinguish differences in polymer architecture even for polymers from the same chemical family, which means that polymer free volume plays a role in determining solubility.
Introduction Carbon dioxide has been touted as the solvent of choice for many industrial applications because of its attractive attributes, e.g., it is environmentally benign, nonhazardous, and very inexpensive. CO2, whose properties are listed in Table 1, has a critical temperature near room temperature, a modest critical pressure, and a higher density than most supercritical fluids, which means that at temperatures slightly above room temperature it is possible to obtain liquidlike densities and, by implication, liquidlike solvent characteristics. CO2 has proven to be a very good supercritical fluid solvent for a select variety of polymers and copolymers. Krukonis has shown that CO2 at or near room temperature and at pressures typically below 600 bar can be used to solubilize polymeric oils, such as many poly(dimethylsilicone) and poly(phenylmethylsilicone), perfluoroalkyl polyethers, and chloro- and bromotrifluoroethylene polymers.1-3 Beckman and co-workers have described the solubility of poly(perfluoropropylene oxide) and poly(dimethylsiloxane) in CO2.4-5 Barton6,7 and Kiran8 have also reported on the high solubility of poly(dimethylsiloxane) in CO2 at approximately 450 bar. It is possible to dissolve very low molecular weight, slightly polar polymers, such as polystyrene or telechelic polyisobutylene, with molecular weights below 10001-3,9,10 in supercritical CO2. It has also been shown recently that it is possible to dissolve poly(tetrafluoroethylene-cohexafluoropropylene) with 19 mol % hexafluoropropylene in CO2 at temperatures in excess of 175 °C and pressures near 1000 bar.11,12 DeSimone and co-workers have generated a large body of work demonstrating that CO2 can dissolve hydrocarbon polymers that contain fluorinated octyl acrylates,13-20 and they have reported on the high solubility of poly(1,1-dihydroperfluorooctyl acrylate) (poly(FOA)) in supercritical CO2.21 In addition, they have developed interesting hydrocarbon-fluorocarbon block copolymers that also exhibit a high degree of solubility in CO2.22 Beckman and co-workers23-26 have also synthesized modest molecular weight block copolymers and graft copolymers that are CO2 soluble. Beckman and co-workers argue that the solubility of the copolymer depends in a somewhat complex X
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01582-1 CCC: $12.00
manner on the number of fluorinated side groups and on the molecular weight of the side groups relative to the molecular weight of the hydrocarbon main chain.26 And very recently, Howdle and co-workers have synthesized polymers with fluorinated heptyl acrylates that are soluble in CO2.27 Although these studies demonstrate how creative, polymer synthetic chemistry can be used to improve polymer solubility in CO2, they only provide limited insight into why most polymers do not dissolve in CO2 regardless of temperature and pressure. For example, although fluorinating side groups or copolymer blocks enhance solubility severalfold in CO2, Mertdogan and coworkers show that fluorination alone does not ensure that the polymer will be soluble in CO2 at temperatures below 100 °C.11 The work in this paper addresses the issue of why CO2 is such a feeble solvent for many polymers and, conversely, why CO2 is such a good solvent for the polymers listed in the previous paragraphs. It is virtually impossible to answer this question unequivocally and quantitatively since the solution theories that describe intermolecular interactions between solvents and solutes in a dense fluid state are still in a nascent state of development. Therefore, detailed modeling studies of the phase behavior measured in this work are left for a later publication. It has been argued that the solvent properties of CO2 should be compared to those of toluene,28 acetone,29 and hexane.28 These seemingly widely varying opinions on the solvent characteristics of CO2 reinforce the complexity of definitively categorizing solvent power.25 Nevertheless, considerable insight into the solvent characteristics of CO2 is reported here based on well-characterized and systematic solubility studies with a wide range of polymers and copolymers. Before proceeding to a presentation of experimental results, a thermodynamic framework is briefly described for interpreting the data presented in this study. The pressures and temperatures needed to dissolve a given polymer in CO2 depend on the intermolecular forces in operation between solvent-solvent, solvent-polymer segment, and polymer segment-segment pairs in solution and on the free volume difference between the polymer and CO2. Rather than present rigorous and complicated expressions for energetic and entropic interactions, approximate expressions are used here that reveal the important physical properties of both the polymer and CO2 that govern whether a © 1996 American Chemical Society
15582 J. Phys. Chem., Vol. 100, No. 38, 1996
Rindfleisch et al.
polymer will dissolve in CO2. Consider first the impact of intermolecular forces on solubility. The following simplified expression30 shows how the intermolecular potential energy of an i-j pair of segments or molecules, Γij, depends on the physical properties of the polymer and the solvent.
Γij(r,T) ≈ - C1 C4
RiRj
- C2
r6 µi2Qj2 r8kT
µi2µj2
- C3
r6kT µj2Qi2
- C5
r8kT
Qi2Qj2 r10kT
-
+ complex formation (1)
where subscript ij represents CO2 and a segment of the polymer, the first term on the right-hand side, written in terms of the polarizabilities, Ri, represents dispersion interactions, the second term, written in terms of the dipole moments, µi, represents dipolar interactions, the third, fourth, and fifth terms, written in terms of the quadrupole moments, Qi, represent quadrupolar interactions, and the last term in this equation represents complex formation. In this equation r is the distance between the molecules, k is Boltzmann’s constant, C1-5 are fixed constants, and T is absolute temperature. Neglected from eq 1 are expressions for induction interactions. Polar forces and acceptor-donor complexing are more important at low-to-moderate temperatures because of their inverse temperature dependence. Due to its structural symmetry, CO2 does not have a dipole moment, but CO2 does have a substantial quadrupole moment that operates over a much shorter distance than dipolar interactions. Kazarian and co-workers have used spectroscopic techniques to characterize the interactions between CO2 and polymers.31 They demonstrated that polymers possessing electron-donating functional groups (e.g., carbonyl groups) exhibit specific interactions with CO2, which is similar to the findings of O’Shea and co-workers who also used spectroscopy to study polar and hydrogen-bonding interaction in binary mixtures with supercritical CO2.32 They argued that this complex formation is most probably of a Lewis acid-base nature, where the carbon atom of the CO2 molecule acts as an electron acceptor and the carbonyl oxygen in the polymer as an electron donator. Kazarian and co-workers also show that the strength of the CO2-segment complex is generally less than 1 kcal/mol, which makes it only slightly stronger than dispersion interactions. In certain instances discussed in this paper, CO2polymer complex formation will be an important consideration when interpreting data at low-to-moderate temperatures where complexing is expected to be significant. It is important to remember that eq 1 is only a rough approximation to the more rigorous intermolecular potential energy function, especially since the segmental motion of the polymer is constrained by the connectivity of the segments. Nevertheless, the approximate form of the intermolecular potential energy function provides a useful tool for interpreting experimental solubility data. From an energetic viewpoint, the relevant criteria of whether a polymer will dissolve in CO2 is fixed by the interchange energy of mixing i-j pairs, ω, given by
ω ) z[Γij(r,T) - 1/2(Γii(r,T) + Γjj(r,T))]
(2)
where z is the number of dissimilar solvent-segment pairs. Equations 1 and 2 provide insight as to why temperature plays such a dominant role in determining solubility. CO2 has a large quadrupole moment whose effect is magnified since the nonpolar nature of CO2 is reflected in the value of its polarizability which is approximately equal to the polarizability of methane, a very weak nonpolar supercritical fluid solvent.
The quadrupolar nature of CO2 works against it in solubilizing polymers that are predominantly comprised of nonpolar repeat units, since CO2 quadrupolar interactions dominate the interchange energy as the temperature is lowered. Conversely, CO2 is expected to be a feeble solvent for polymers with repeat units that have large dipole moments since the interchange energy is dominated by polymer-polymer interactions rather than polymer-CO2 interactions, especially at low temperatures where polar interactions are magnified. Hence, CO2 has the characteristics of both a weak nonpolar solvent and a weak polar solvent. Supercritical CO2 is highly compressible, so it is important to be aware of how its solvent power depends on its density. McQuarrie33 shows how the internal energy of a mixture, Umixture, depends on the density of the solvent for a homogeneous-isotropic solution.
umixture kT
F(P,T) ≈ A0 + A1
kT
∑ij xixj∫Γij(r,T) gij(r,T,F)r2 dr
(3)
where gij (r,T,F) is the radial distribution function, A0 and A1 are constants, xi is the mole fraction of component i, and F is the solvent density if the solution is moderately dilute in solute. This simplified formula suggests that the internal energy is directly related to density, which explains the heuristic that, to a first approximation, polymer solubility is proportional to CO2 density. However, this formula also shows why this heuristic is only true to a first approximation since the type and strength of interactions are buried in the Γij(r,T) and gij(r,T,F) terms. Equations 1-3 demonstrate that it is important to match the physical properties of CO2 with those of the polymer so that the interchange energy is of sufficient strength to ensure finite polymer solubilities at a given pressure and temperature. However, if the interchange energy is large and unfavorable for polymer solubility, it is not possible to manipulate solubility by changing CO2 density with changes in the system pressure. Finally, eq 1 shows that if the temperature is varied, the interchange energy can be adjusted through the Γij, Γii, and Γjj values to allow for the solubility of the polymer. In addition to energetics, the entropy of mixing is a very important, and often overlooked, consideration when assessing the possibility of polymer solubility in supercritical CO2. The entropy of mixing is related to the free volume difference between the polymer and CO2. Roughly speaking, CO2 must condense around the polymer in order to dissolve it.34 This condensation process represents a large entropy penalty that can dominate favorable enthalpic interactions and prevent the formation of a single phase. As the rotational flexibility of the chain segments decreases, the number of possible configurations available to the polymer is expected to decrease, which makes the entropy of mixing the polymer with CO2 more negative. Qualitatively, the entropy penalty paid by CO2 is related to the free volume of the polymer which is proportional to T - Tg.35 Predicting polymer solubility based solely on entropic or enthalpic arguments is not without pitfalls since both of these considerations depend on temperature in complex ways, and more importantly, decoupling these two considerations is not rigorous. However, in this paper we show that it is possible to interpret CO2-polymer phase behavior as being dominated by either enthalpy or entropy, especially in the extremes of temperatures and when working with polymers that are members of the same chemical family. Tables 1 and 2 list the properties of CO2 and the polymers and copolymers used in this study. Polymer solubilities are reported as cloud points obtained at a fixed polymer composition
Polymers and Copolymers in Supercritical CO2
J. Phys. Chem., Vol. 100, No. 38, 1996 15583
TABLE 1: Physical Properties of CO230,39 a Q × 1026 Tc (°C) Pc (bar) Fc (g/cm)3 R × 1025 (cm3) µ (D) (erg1/2 cm5/2) 31.0
73.8
0.468
27.6
-4.3
0.0
a
The polarizability of CO2 was calculated using the method of Miller and Savchik.40
TABLE 2: Weight-Average Molecular Weight (Mw), Molecular Weight Polydispersity, and Glass Transition or Melting Temperatures for the Polymers and Copolymers Investigated in This Studye poly(ethylene) poly(styrene) poly(vinyl fluoride) poly(vinylidene fluoride) poly(vinyl acetate) poly(acrylic acid) poly(methyl acrylate) poly(ethyl acrylate) poly(propyl acrylate) poly(butyl acrylate) poly(ethylhexyl acrylate) poly(octadecyl acrylate) poly(methyl methacrylate) poly(ethyl methacrylate) poly(butyl methacrylate) poly(butyl methacrylate) EMA18 EMA31 EMA41 TFE-HFP19 VDF-HFP22 Teflon AF
Mw
Mw/Mn
Tg (°C)
108 500 106 000 125 000 530 000 124 800 450 000 30 700 119 300 140 000 61 800 112 800 23 300 93 300 340 000 320 000 102 300 185 200 99 000 96 400 210 000 85 000 ∼400 000
5.40 1.03 3.12
-12335 10535 Tm ≈ 187 °Cb Tm ≈ 168 °Cc 3035
2.37 2.90 4.83 3.78 2.99 2.97 1.79 2.01 2.70 4.35 1.02 4.90 3.00 3.22
9a -23a -37b -49a -55a 10535 63a 2035 2035 -2741 -1841 Tm ≈ 147 °C42 -20d 16043
Aldrich Chemical Co. b Scientific Polymer Products, Inc. c Polysciences, Inc. d 3M Corp., product information. e EMAx is a poly(ethyleneco-methyl acrylate) copolymer with x mol % methyl acrylate, TFEHFP19 is a poly(tetrafluoroethylene-co-hexafluoropropylene) copolymer with 19 mol % hexafluoropropylene, VDF-HFP22 is a poly(vinylidene fluoride-co-hexafluoropropylene) copolymer with 22 mol % hexafluoropropylene, and Teflon AF is a fluorinated (ethylenic-cyclooxyaliphatic substituted ethylenic) copolymer. a
of ∼5 wt %, the expected maximum in the P-x isotherms. The temperature range of these experiments is extended to 270 °C to modulate the impact of polar interactions. In addition, the pressure range of these experiments is extended to 3000 bar to obtain CO2 densities that are high enough to dissolve the polymer at very high temperatures. Experimental Section Described in detail elsewhere are the techniques used to obtain cloud-point curves using a high-pressure, variable-volume view cell.36 While being maintained at room temperature, the cell is loaded with (0.3-0.7) ( 0.02 g of polymer and is purged first with nitrogen at pressures of 30-50 bar and then with CO2 at 3-6 bar to remove any entrapped air. Approximately (8-16) ( 0.02 g of CO2 is then transferred into the cell. The system pressure is measured to within (2.8 bar, and the system temperature is measured to within (0.2 °C but is maintained to within (0.2 °C below 200 °C and (0.4 °C above 200 °C. The cloud-point pressure is defined as the point at which the solution becomes so opaque that it is no longer possible to see the stir bar in solution. Cloud points obtained in this manner are identical to those defined as the point where there is a 90% drop in transmitted light through the solution. Cloud-point measurements are repeated at least twice at each temperature and are typically reproducible to within (5 bar. The lowest temperature of the cloud-point curves occurs either at the highest
Figure 1. Critical-mixture curves for octane (C8), hexadecane (C16), and squalane (C30) obtained by Liphard and Schneider.44 The closed square represents the critical point of CO2, and the closed circle is the critical point of octane. Below each of the three curves in this figure two phases exist, and at pressure above the curves a single phase exists. Smoothed data curves are presented rather than individual data points.
operating pressure of the experimental apparatus or at room temperature, whichever comes first. Normally, experimental data are taken first at a high temperature and then at lower temperatures. After obtaining a cloud point at the lowest desired operating temperature, the temperature is increased, and one or two high-temperature data points are measured. If there is a discrepancy between data taken at the beginning and the end of an experiment, another independent experiment with fresh solution is performed to determine which data are correct. Materials The poly(alkyl acrylates), poly(alkyl methacrylates), and poly(vinyl acetate) were obtained from Aldrich Chemical Co. The low molecular weight poly(butyl methacrylate) was purchased from Polymer Source, Inc. Poly(propyl acrylate), poly(vinyl fluoride), and poly(vinylidene fluoride) were obtained from Scientific Polymer Products, Inc. Poly(styrene) was purchased from Pressure Chemical Co. and poly(acrylic acid) from Polysciences, Inc. The poly(ethylene), poly(ethylene-co-methyl acrylate), fluorinated (ethylenic-cyclooxyaliphatic substituted ethylenic) copolymer (Teflon AF), and poly(tetrafluoroethyleneco-hexafluoropropylene) copolymers were supplied by DuPont Co. The poly(vinylidene fluoride-co-hexafluoropropylene) was supplied by 3M Corp. CO2 was obtained from Potomac Airgas, Inc. (bone dry grade, 99.8% minimum purity). Since the acrylate polymers were supplied in a toluene solution, the polymer solution was placed under vacuum for at least 8 h for solvent removal. The other polymers and the CO2 were used as received. Results and Discussion The experimental results are presented starting with polyethylene (PE), the most nonpolar hydrocarbon polymer, proceeding to the polar polymers and copolymers, and ending with the fluorinated copolymers. PE is not soluble in CO2 even to temperatures of 270 °C and 2750 bar. The poor solvent power of CO2 for PE becomes readily apparent by considering the solubility of low molecular weight nonpolar hydrocarbons in CO2 as shown in Figure 1. Octane and CO2 have a continuous critical-mixture curve that extends between the two pure component critical points. However, if the molecular weight of octane is doubled to hexadecane, the critical-mixture curve is shifted to higher pressures, and the low-temperature branch of the curve exhibits a steep rise in slope near 30 °C. At these cold temperatures the resultant phase behavior is usually attributed to enthalpic interactions. Evidently, the sharp increase
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Figure 2. Impact of polar methyl acrylate groups on the location of the cloud-point curves of CO2-EMAx mixtures, where x represents the mole fraction of methyl acrylate groups in the copolymer.
in the critical-mixture pressure is a consequence of a large energy mismatch between CO2 and hexadecane; that is, the interchange energy, ω, is dominated by CO2-CO2 quadrupolar interactions rather than CO2-hexadecane dispersion or induction interactions. In other words, CO2 is too polar for hexadecane once the temperature is reduced below 30 °C as indicated in eq 1 where quadrupolar interactions scale with inverse temperature. If the molecular size of the nonpolar hydrocarbon is approximately doubled again to a C30 molecule, the critical-mixture curve is shifted to higher pressures, and the sharp increase in the critical pressure occurs at 60 °C. Extrapolating the results shown in Figure 1 to PE, a moderately branched hydrocarbon with approximately 1400 CH2 groups, it is apparent that CO2 is too polar for PE even at temperatures in excess of 270 °C. Hence, we conjecture that a polymer or copolymer must have some polarity before it will exhibit any solubility in CO2. The question of how much polarity is needed for CO2 solubility is addressed with the cloud-point curves for the poly(ethylene-co-methyl acrylate) (EMAx, where x represents the mole fraction of methyl acrylate groups in the backbone)-CO2 mixtures shown in Figure 2. As the methyl acrylate content increases, the cloud-point curve shifts to lower temperatures, suggesting that the energy mismatch is relaxed by dipolequadrupole interactions between methyl acrylate repeat units and CO2. In this instance, the acrylate groups in the backbone of the copolymer act as an internal cosolvent. It is also interesting that the cloud-point curves appear to level off in pressure at approximately 1400 bar, which indicates that CO2 must be highly compressed before it can dissolve these copolymers. This high-temperature, high-pressure behavior with CO2 shows up with other polymers and copolymers, and it is more than likely a consequence of the very low polarizability of CO2 which, in fact, is similar to that of methane. Determining the cloud-point behavior of the poly(acrylate) family in CO2 provides another means of testing the hypothesis that some polarity is needed for a polymer to dissolve in CO2. Figure 3 shows cloud-point curves for poly(ethyl acrylate) (PEA), poly(butyl acrylate) (PBA), poly(ethylhexyl acrylate) (PEHA), and poly(octadecyl acrylate) (PODA) in CO2. Also shown in this figure are the weight-average molecular weight, Mw, and glass transition temperature, Tg, of the polymers. As the alkyl tail on the acrylate increases, the effective polarity decreases since the reduced dipole moment scales inversely with the square root of the molar volume.30 It is readily apparent from the ordering of the curves in pressure-temperature space that the cloud-point behavior is not fixed by Mw nor by Tg. If it were, the PODA-CO2 curve would be at the lowest temperature. The PODA curve turns up sharply in pressure at 215 °C, which is surprising since it suggests that even at this very high temperature, CO2 is too polar to dissolve PODA.
Rindfleisch et al.
Figure 3. Impact of the nonpolar alkyl tail of the acrylate group on the cloud-point curves of poly(ethyl acrylate) (PEA), poly(butyl acrylate) (PBA), poly(ethylhexyl acrylate) (PEHA), and poly(octadecyl acrylate) (PODA) in CO2. Shown in the legend is the glass transition temperature, Tg, of the polymers, and listed on each curve is the respective weight-average molecular weight, Mw.
Figure 4. Impact of polymer free volume on the cloud-point curves of poly(methyl acrylate) (PMA), poly(ethyl acrylate) (PEA), poly(propyl acrylate) (PPA), and poly(butyl acrylate) (PBA) in CO2 at high temperatures. Shown in the legend is the glass transition temperature, Tg, and the weight-average molecular weight, Mw, of the polymers.
Conversely, as the alkyl tail on the acrylate is decreased, the polymer remains in solution to lower temperatures, suggesting that dipole-quadrupole interactions between the acrylate group and CO2 are favorable and, thus, promote solubility. At high temperatures, where polar interactions are decreased, the free volume of the polymer should have a large influence on the location of the cloud-point curve. Figure 4 compares the cloud-point curves of the poly(methyl acrylate) (PMA), PEA, poly(propyl acrylate) (PPA), and PBA systems. Comparing the cloud-point pressure of PMA to PEA at 165 °C shows the large impact that polymer free volume, defined as T - Tg, has on the phase behavior. Note that the Mw of PEA is approximately 4 times greater than that of PMA, which should cause the PEA cloud-point curve to be at higher pressures than the PMA curve if molecular weight were controlling phase behavior. Even the cloud-point curve of PPA, the polymer with the highest molecular weight shown is this diagram, is located at lower pressures than the PEA curve. Comparing the cloud-point curves of these poly(alkyl acrylates) shows that the cloud-point pressure decreases almost linearly with decreasing Tg. Figure 5 shows the cloud-point curve of the PMA-CO2 system extended to lower temperatures relative to the other poly(acrylate)-CO2 systems. The PMA curve is shifted to higher pressures, and it does not exhibit a sharp rise in pressure although the PMA curve should increase rapidly in pressure if it is extended to temperatures below 30 °C. Relative to the other poly(acrylates) considered in this study, PMA has the highest Tg, suggesting that it has the lowest rotational flexibility of chain segments which makes it difficult to dissolve in CO2. But, PMA is the most polar of the poly(acrylates) since its
Polymers and Copolymers in Supercritical CO2
Figure 5. Comparison of the CO2-poly(methyl acrylate) (PMA) cloudpoint curve to the other CO2-poly(acrylate) cloud-point curves. To avoid clutter, the other poly(acrylate) curves are shown with dashed lines.
reduced dipole moment is scaled with the smallest molar volume, which enhances its solubility in CO2 as the temperature is lowered. If the cloud-point temperature at 1850 bar is plotted as a function of methyl acrylate content using the PMA data from Figure 5 and the EMA data from Figure 2, it is possible to show that the extrapolated cloud-point temperature for PE is 420 °C. We conclude that CO2 will not dissolve nonpolar, hydrocarbon polymers at temperatures below 300 °C, regardless of pressure. As noted in the Introduction, CO2 can dissolve polystyrene (PS) if the molecular weight of the PS is less than 1000. However, the experiments performed to determine PS solubility were only operated to 524 bar and 100 °C.1 The poor solubility of PS in CO2 was checked in the present study using PS with Mw of 106 000. This PS is also insoluble in CO2 even to temperatures of 225 °C and 2100 bar. At first glance this result is somewhat surprising since the aromatic ring in PS has a quadrupole that interacts favorably with the quadrupole of CO2. However, PS has a Tg of approximately 103 °C, which indicates a very high hindrance potential for chain segment rotations. Hence, PS has a stiffer chain backbone relative to the previously described poly(acrylates) and EMA copolymers, which means that there is a higher entropy penalty for CO2 to dissolve PS compared to these other polymers. Poly(methyl methacrylate) (PMMA) with an Mw of 93 300 is also insoluble in CO2 to temperatures of 255 °C and 2550 bar even though PMA dissolves in CO2, and the strengths of PMA-CO2 intermolecular interactions are not expected to be very different than those of PMMA-CO2. But, PMMA, with a Tg of approximately 105 °C, has a stiffer chain than PMA with a Tg of ∼9 °C. Poly(ethyl methacrylate) (PEMA) with an Mw of 340 000 also does not dissolve in CO2 to temperatures of 285 °C and 2550 bar since PEMA, with a Tg of 63 °C, is a stiffer chain polymer compared to PEA, which has a Tg of -23 °C. CO2 can, however, dissolve poly(butyl methacrylate) (PBMA) as shown in Figure 6. The cloud-point curve for PBMA with Mw of 100 000 and Tg of 20 °C is shifted by 45 °C to higher temperatures than the curve for PBA, which has a Tg of -49 °C. Also shown in this figure is the cloud-point curve for PBMA with an Mw of 320 000. Comparing the two PBMA curves shows that Mw has a modest effect on the location of the cloud-point curve. The effect of Mw can be quantified by plotting the inverse of the cloud-point temperatures of the two PBMA curves at a fixed pressure with respect to 1/Mw0.5.37 At a fixed pressure of 2500 bar, an estimated cloud-point temperature of 116 °C is obtained for PBMA with an Mw of 62 000. The cloud-point temperature of PBA is 33 °C lower than that of PBMA of the same molecular weight, which demonstrates the impact of free volume or, conversely, chain stiffness on
J. Phys. Chem., Vol. 100, No. 38, 1996 15585
Figure 6. Comparison of the CO2-poly(butyl acrylate) (PBA) cloudpoint curve to two CO2-poly(butyl methacrylate) (PBMA) curves. The weight-average molecular weights, Mw, of these polymers are given in the figure.
Figure 7. Comparison of the CO2-poly(methyl acrylate) (PMA) and CO2-poly(vinyl acetate) (PVAc) cloud-point curves. The weightaverage molecular weights, Mw, of these polymers are given in the figure.
solubility in CO2. It is interesting to note that the impact of Tg is still apparent, but its effect is much less in the hightemperature region in Figure 6. On the basis of the phase behavior results presented in the previous figures, it is apparent that CO2 is a very weak supercritical solvent that is sensitive to polymer architecture and to the chemical type and intermolecular potential energy of the repeat units. Figure 7 shows an interesting comparison of the cloud-point curves of PMA and poly(vinyl acetate) (PVAc) in CO2. The difference between the location of these two curves is quite dramatic. At 30 °C the PMA cloud-point curve is more than 1500 bar higher than the PVAc curve even though the molecular weight of PVAc is 4 times greater than that of PMA. Both PMA and PVAc are polar, but the Tg for PVAc is approximately 21 °C higher than the Tg of PMA. The slightly higher Tg of PVAc is a reflection of the stronger polar interactions between vinyl acetate groups as compared to methyl acrylate groups when these groups are tethered to a polymer chain. CO2 can more easily access the carbonyl group in PVAc than in PMA which, we conjecture, makes PVAc effectively more polar than PMA and more soluble in CO2 with decreasing temperature. In addition, as shown by Kazarian et al.,31 easier access to the carbonyl group in PVAc makes it easier for CO2 to form a weak complex with PVAc especially at near room temperatures. The preceding examples of polymer solubility in CO2 demonstrate that hydrocarbon polymers and copolymers must have some polarity to dissolve in CO2. However, very polar polymers or polymers that are water-soluble are not expected to dissolve in CO2 even at high temperatures. For example, poly(acrylic acid) did not dissolve in CO2 to temperatures of 272 °C and pressures of 2220 bar.
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Figure 8. Comparison of the poly(tetrafluoroethylene-co-hexafluoropropylene) (TFE-HFP19)-CO2 cloud-point curve, obtained by Mertdogan et al.,11 and the poly(vinylidene fluoride-co-hexafluoropropylene) (VDF-HFP22)-CO2 cloud-point curve. The subscripts on the chemical formulas represent the mole fraction of each comonomer in the backbone of the copolymer.
Figure 9. Comparison of the poly(tetrafluoroethylene-co-hexafluoropropylene) (TFE-HFP19)-CO2 cloud-point curve, obtained by Mertdogan et al.,11 and the fluorinated (ethylenic-cyclooxyaliphatic substituted ethylenic) (Teflon AF)-CO2 cloud-point curve. The subscript on the Teflon AF chemical formula represents the mole fraction of each comonomer in the backbone of the copolymer.
As mentioned in the Introduction, a large body of work has recently been developed on the solubility of fluorinated polymers and copolymers in CO2. However, in the present study, we find that CO2 cannot dissolve poly(vinyl fluoride) (PVF) or poly(vinylidene fluoride) (PVDF) even at temperatures of 300 °C and pressures of 2750 bar. This result is not too surprising since, as shown in Table 2, each of these fluoropolymers has a very high melting temperature, which is a consequence of strong segment-segment polar interactions. Figure 8 shows the results of further experiments performed with statistically random fluorocopolymers of approximately 20 mol % hexafluoropropylene (HFP) with tetrafluoroethylene (TFE-HFP19)11 and with vinylidene fluoride (VDF-HFP22) where the subscript on HFP indicates the mole fraction of HFP in the copolymer. In this instance the melting point of TFE-HFP19 is only ∼147 °C, and VDF-HFP22 is an amorphous copolymer. These two fluorocopolymers are soluble in CO2, but there are dramatic differences in the location of the two cloud-point curves. Both cloud-point curves virtually superpose at temperatures in excess of 210 °C. However, the nonpolar TFE-HFP19 curve has a very sharp increase in pressure at temperatures near 185 °C. Even though TFE-HFP19 can crystallize, Mertdogan and coworkers11 showed experimentally that the sharp rise in the cloudpoint curve is not a crystallization boundary but, rather, a liquid + liquid f liquid boundary. Mertdogan et al. argue that although it is difficult to see solid polymer in solution in the two-phase region, it is possible to deduce whether solid polymer is present. At temperatures less than the crystallization temperature, increasing pressure does not clear the solution while the same pressure increase at a temperature that is slightly higher than the crystallization temperature rapidly clears the solution. Also, if the system is heated isobarically at a fixed rate, the solution clears with a 1-2 °C increase if two liquid phases are present. In contrast, at the same heating rate, it typically takes a 10-20 °C increase to clear the solution if solid polymer is present.11 The sharp pressure increase in the TFE-HFP19CO2 cloud-point curve occurs because “hot” CO2 is too polar to dissolve this nonpolar fluorocopolymer. In other words, CO2-CO2 polar interactions dominate the interchange energy even at very high temperatures. In contrast, the VDF-HFP22 curve exhibits a slightly positive slope down to temperatures near 100 °C. The major difference between these two copolymers is that VDF-HFP22 has a dipole moment that makes it energetically more favorable to remain in solution in CO2 as the temperature is lowered and polar forces increase. Figure 9 compares the phase behavior of fluorinated (ethylenic-cyclooxyaliphatic substituted ethylenic) copolymer (Teflon
AF)38 to TFE-HFP19. Teflon AF is a copolymer of 35 mol % tetrafluoroethylene and 65 mol % dioxole monomers consisting of oxygen, carbon, and fluorine arranged in a five-member ring structure.38 The very high Tg of Teflon AF is a consequence of the bulky size of the dioxole groups which reduces the rotational flexibility of the chain segments. However, as the temperature is lowered, the polar dioxole groups in Teflon AF interact favorably with polar CO2 so that the copolymer remains in solution. A sharp increase in pressure is observed at temperatures less than 70 °C, which suggests that the interchange energy becomes dominated either by CO2 quadrupole-quadrupole interactions or by Teflon AF dipole-dipole interactions. The cloud-point data in Figures 8 and 9 show quite dramatically that fluorinating a polymer or a copolymer does indeed make it CO2-soluble. However, as also demonstrated with the same data, the pressures needed to solubilize fluorinated copolymers can be very high, and they can be extremely temperature sensitive depending on the polarity of the polymer. The results shown in Figures 8 and 9 explain the relatively mild pressures and temperatures needed to dissolve the fluorinated acrylates mentioned in the Introduction since these fluorinated acrylate polymers are partially fluorinated, and they have carboxyl groups that give them some polarity. Conclusions Supercritical CO2 is a slightly polar solvent that has a polarizability similar in value to that of methane, which suggests that CO2 is indeed a very weak supercritical fluid solvent. Because of its low polarizability, the quadrupole of CO2 plays a dominant role in determining polymer solubility. The ability to dissolve polymers in CO2 is very temperature dependent since polar interactions roughly scale with inverse temperature. The interchange energy, that is the balance between segmentsegment, segment-CO2, and CO2-CO2 interactions, can be adjusted by varying the system temperature over wide ranges. Although CO2 exhibits the temperature-dependent characteristics of a polar solvent, it is only weakly polar which means that it cannot dissolve very polar or hydrogen-bonded polymers, such as poly(acrylic acid), even to 300 °C and 2750 bar. Also, because it is such a weak solvent, CO2 can readily distinguish differences in polymer architecture which means that polymer free volume plays a role in determining solubility. The stiffer the main chain, the more difficult it is to dissolve the polymer in supercritical CO2. The solubility of a polymer or a copolymer in CO2 can be increased if the polymer is at least partially fluorinated. But,
Polymers and Copolymers in Supercritical CO2 fluorination does not ensure that the polymer will dissolve at room temperature or that the cloud-point pressures will be low. For example, PE does not dissolve in CO2 even at 300 °C, whereas totally fluorinated ethylene-propylene copolymer (TFE-HFP19) does dissolve in CO2 as long as the temperatures remain above 190 °C and the pressures are greater than 1000 bar. In contrast, partially fluorinated ethylene-propylene copolymer (VDF-HFP22) remains dissolved in CO2 at temperatures as low as 100 °C and pressures less than 1000 bar. The difference between these two fluorocopolymers is that one of them, VDF-HFP22, is polar which allows it to interact with the quadrupole of CO2. Likewise, Teflon AF, a fluorinated copolymer with polar dioxole groups which interact with CO2, can be dissolved in CO2 at temperatures as low as 70 °C and pressures less than 600 bar. Challenges remain in quantifying the observations made in this study. Work is in progress to model CO2-polymer cloudpoint behavior using various polymer solution models such as the Sanchez-Lacombe, statistical associating fluid theory, and the Flory-Orwell-Vrij equations of state. It remains to be seen whether these equations have the ability to predict the extreme difference in the cloud-point behaviors of PMA and PVAc in CO2 using only pure component parameters. Acknowledgment. DiNoia and McHugh acknowledge the National Science Foundation for partial support of this project under Grants CTS-9509608 and GER-9454136. DiNoia also acknowledges the Strategic Environmental Research and Development Program through Project PP-660 for partial support. Rindfleisch acknowledges the DFG for financial support during his postdoctoral stay at Johns Hopkins University. The authors acknowledge C. Mertdogan, who measured the CO2-Teflon AF cloud-point curve, and they also acknowledge Dr. W. H. Tuminello of the DuPont Co. for enlightening technical discussions on the data and their interpretation. References and Notes (1) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principle and Practice, 2nd ed.; Butterworths: Stoneham, MA, 1994. (2) Yilgor, I.; McGrath, J. E.; Krukonis, V. J. J. Polym. Bul. 1984, 12, 499. (3) Krukonis, V. J. J. Polym. News 1985, 11, 7. (4) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127. (5) Hoefling, T. A.; Stofesky, D.; Reid, M.; Beckman, E. J.; Enick, R. M. J. Supercrit. Fluids 1992, 5, 237. (6) Dris, G.; Barton, S. W. Polym. Mater. Sci. Eng. 1996, 74, 226. (7) Zhao, X.; Watkins, R.; Barton, S. W. J. Appl. Pol. Sci. 1995, 55, 773. (8) Xiang, Y.; Kiran, E. Polymer 1995, 36, 4817. (9) Gregg, C. J.; Stein, F. P.; Radosz, M. Macromolecules 1994, 27, 4972. (10) Gregg, C. J.; Stein, F. P.; Radosz, M. Macromolecules 1994, 27, 4981.
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