Ind. Eng. Chem. Res. 2003, 42, 6499-6504
6499
Solubility of Poly(perfluoromonoitaconates) and Poly(perfluorodiitaconates) in Supercritical CO2 Dan Li,† Zhihao Shen,† Mark A. McHugh,*,† John Tsibouklis,‡ and Eugen Barbu‡ Department of Chemical Engineering, Virginia Commonwealth University, Richmond, Virginia 23284, and School of Pharmacy and Biomedical Sciences, University of Portsmouth, Portsmouth PO1 2DT, U.K.
Cloud-point data are reported at 25-150 °C and 100-560 bar for poly(perfluoromonoitaconates) (PFMITn) and poly(perfluorodiitaconates) (PFDITn, where n, the number CF2 groups in the alkyl side chain, is equal to 3, 5, 7, and 9 for both types of itaconates) in supercritical CO2. Cloudpoint curves for the PFMITn-CO2 system shift to lower pressures as the fluorinated alkyl tail length increases; however, this trend reverses going from PFMIT7 to PFMIT9. Hence, there is an optimum fluorocarbon side-chain length that balances energetic polymer-polymer, CO2CO2, and polymer-CO2 interactions with a favorable entropic effect associated with the increased free volume of the longer side chains. The PFDITn polymers readily dissolve in CO2 at pressures as low as 140 bar at 25 °C, although, in this case, the cloud-point curves all superpose. The cloud-point data suggest that supercritical CO2 is a suitable solvent for processing these fluoropolymers. Introduction Fluoropolymers are a unique class of coatings materials used to create nonwettable, low-energy surfaces that also exhibit nonadhesive characteristics for biological foulants, graffiti, soil, ice, and other unwanted contaminants.1 It has been suggested that polymers used for coatings should contain a flexible linear backbone with side chains that exhibit weak intermolecular interactions.2 Tsibouklis and co-workers have reported on the synthesis and surface energy characteristics of several classes of polymeric coatings materials that comply with these molecular design requirements, namely, poly(methylpropenoxyalkylsiloxane)s,2-5 poly (perfluoroalkylacrylate)s,4,5 poly(perfluoroalkyl methacrylate)s,6 and poly(mono- and di-perfluoroalkylitaconate)s.7,8 Their studies demonstrate that a low surface energy is obtained for polymers with long, fluorinated pendant chains, although the average surface roughness increases with increasing side-chain length. Rather than use chlorofluorocarbons, DeSimone and co-workers advocate the use of supercritical CO2 as the solvent of choice for coating surfaces with fluoropolymers.9 The success of a supercritical fluid (SCF) based coatings process depends on a good understanding of the phase behavior of the polymer-CO2 mixture.10 Although CO2 is a poor solvent for most polymers of high molecular weight, whether polar or nonpolar,11-13 results from the literature confirm that CO2 is an effective solvent for silicones and slightly polar, fluorinated polymers.14-22 However, it is still difficult to predict a priori the solubility of polymers in CO2, as many factors affect the phase behavior, including the polymer backbone architecture, the polymer free volume, the polymer molecular weight, and the molecular weight distribution. The temperatures and pressures needed to dissolve a given polymer in CO2 depend on the intermolecular * To whom correspondence should be addressed. Tel.: 804827-7031. Fax: 804-828-3846. E-mail:
[email protected]. † Virginia Commonwealth University. ‡ University of Portsmouth.
forces in operation between solvent-solvent, solventpolymer segment, and polymer segment-segment pairs in solution, as well as the free volume difference between the polymer and solvent. From an energetic viewpoint, the interchange energy of mixing is the relevant criterion determining whether a polymer will dissolve in an SCF solvent. Solvent power also depends on density, as SCF solvents are highly compressible. In addition to energetic and density effects, it is important to consider the impact of the entropy of mixing, which is related to the free volume difference between the polymer and the solvent and to the stiffness of the polymer chain. The solvent must “condense” around a polymer chain to dissolve it,23 resulting in a large entropy penalty capable of dominating favorable enthalpic interactions and preventing the formation of a single phase. As the rotational flexibility of the chain segments decreases or as the persistence length of the polymer increases, the entropy of mixing a polymer with an SCF solvent becomes more negative. Predicting polymer solubility still remains a formidable challenge, as both energetic and entropic effects depend on temperature in a complex way and, more importantly, it is not possible to decouple rigorously the impact of these two effects. However, it is possible to interpret polymersolvent phase behavior as being dominated by either enthalpic or entropic effects when working with polymers that are members of the same chemical family. In the work presented here, the impact of fluorination of a side chain on polymer solubility in CO2 is demonstrated by comparing the conditions needed to dissolve poly(di-1H,1H,2H,2H-perfluoroalkyl monoitaconate) (PFMITn, where n, the number of CF2 groups in the single fluorinated alkyl side chain, is equal to 3, 5, 7, and 9) and poly(di-1H,1H,2H,2H-perfluoroalkyl diitaconate) (PFDITn, where n, the number of CF2 groups in both fluorinated alkyl side chains, is equal to 3, 5, 7, and 9). The itaconate structures shown in Table 1 indicate that n + 1 is the total number of fluorinated groups in each alkyl side chain and n + 3 is the length of the alkyl chain attached to the ester group. The phase behavior studies are performed at concentrations of
10.1021/ie021063o CCC: $25.00 © 2003 American Chemical Society Published on Web 09/19/2003
6500 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 Table 1. Structures and Melting Temperaturesa of Poly(di-1H,1H,2H,2H-perfluoroalkyl monoitaconate) (PFMITn; n ) 3, 5, 7, and 9) and Poly(di-1H,1H,2H,2H-perfluoroalkyl diitaconate) (PFDITn; n ) 3, 5, 7, and 9) Used in This Study
carbon atom of CO2 and the electron-donating carbonyl oxygen in the polymer. Because polymer-CO2 solutions can exhibit liquidlike densities, this specific interaction is magnified.26 Recent studies also show that the “acidic” carbon atom of CO2 interacts with a lone pair of electrons on fluorine in a C-F bond.27,28 Ab initio molecular orbital energy calculations also predict the formation of a favorable CO2-fluorine quadrupoledipole interaction with a magnitude less than that typically observed for hydrogen bonding but greater than that found with dispersion-type interactions.29 Hence, fluorinating a polymer enhances its solubility in supercritical CO2, although the polymer also must have a measurable polar moment to ensure that the polymer dissolves at low pressures and temperatures.13,21,30,31 Experimental Section
a T values 61 and 112 °C for PFMIT and PFDIT , respectively, m 7 9 and 60 and 109 °C for PFDIT7 and PFDIT9, respectively.
0.95-2.06 wt % PFMITn and PFDITn (n ) 3, 5, and 9), which is likely on the dew-point side of the phase diagram. Phase behavior studies of the PFMIT7 and PFDIT7 systems are performed at concentrations ranging from 0.96 to 11.00 wt % to ascertain the impact of concentration on the location of the cloud point. The concentration effect exhibited by the PFMIT7 and PFDIT7 systems is expected to be similar to that of the other itaconate systems considered in this study. The phase behavior studies presented for the itaconates in CO2 offer an opportunity to determine the effect of chain branching, both the length of the branch as well as the number of branches, on polymer solubility. As the number of chain branches or the length of a branch is increased, polymer free volume is expected to increase, which reduces the entropic mismatch between supercritical CO2 and the polymer. Hence, polymers with longer side chains should require lower pressures to dissolve in CO2. However, polymer solubility can decrease with increasing side-chain length for two reasons. If the intermolecular interactions between CO2 and the repeat groups on a side chain are not energetically favorable, higher pressures and temperatures will be required to dissolve the polymer.12 Also, the polarity per repeat group decreases as the side-chain length increases, as the dipole moment of the ester group in the itaconates is distributed over larger molar volumes.24 Hence, polar side chain-CO2 interactions diminish with increasing chain length, and solubility is expected to decrease as is found with the poly(alkyl methacrylate)CO2 systems.21 The Eckert group has demonstrated that temperature-sensitive specific interactions, such as complex formation, also contribute to the attractive, pairpotential energy between CO2 and a polymer segment.25 Polymers with electron-donating groups, such as carbonyls, exhibit specific CO2-segment interactions on the order of 1 kcal/mol between the electron-accepting
The synthesis of the polymers used in this study has been discussed in detail elsewhere.8 It was not possible to determine the molecular weight of the fluorinated itaconates because these fluoropolymers are not soluble in common organic solvents. However, gel permeation chromatography was used to determine the weightaverage molecular weight (Mw) (150 000) and the number-average molecular weight (25 000) of the nonfluorinated diitaconate with n ) 9 relative to polystyrene standards.7 Judging from the synthetic technique used to make these fluoropolymers, Mw is estimated to be near 150 000. Hence, the effect of molecular weight on polymer solubility is expected, on the basis of previous polymer-SCF studies reported in the literature,13 to be secondary relative to the effect of chain branching. The melting temperatures (Tm) for the mono- and diitaconates were determined by the second and third DSC heating scans at 10 °C/min. Only PFMIT7, PFMIT9, PFDIT7, and PFDIT9 exhibited melting points at 61 °C for the two polymers with seven CF2 groups and at 112°C for the two polymers with nine CF2 groups. It is apparent that the length of the side chain fixes Tm regardless of whether there is one or two chains per repeat group. The other fluoropolymers did not exhibit melting points. The carbon dioxide, obtained from Roberts Oxygen (99.8% minimum purity), was used as received. Described elsewhere are the apparatus and technique used to obtain polymer-CO2 cloud-point curves that represent the pressures and temperatures needed to obtain a single phase for a polymer-CO2 solution of fixed concentration.30,32 The main component of the experimental apparatus is a high-pressure, variablevolume cell (Nitronic 50, 7.0 cm o.d. × 1.6 cm i.d., ∼30 cm3 working volume). The cell is first loaded with polymer measured to within (0.002 g. The cell is then flushed very slowly with CO2, at pressures less than 3 bar, to remove entrapped air. CO2 is then transferred into the cell gravimetrically to within (0.02 g using a high-pressure bomb. The mixture in the cell is viewed with a borescope (Olympus Corporation, model F100024-000-55) placed against a sapphire window secured at one end of the cell. A stir bar activated by an external magnet is used to mix the contents of the cell. The mixture in the cell is compressed to a single phase, and the pressure is then slowly decreased until a second phase appears. The cloud-point pressure is the point at which the solution becomes so opaque that the stir bar is no longer visible. The cloud-point temperature is held
Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6501 Table 2. Experimental Temperatures, Pressures, and Densities at the Phase Boundary for Poly(perfluoromonoitaconate) (PFMIT3, PFMIT5, PFMIT7, and PFMIT9) in CO2 T (°C)
P (bar)
density (g/cm3)
0.98 wt % PFMIT3 127.8 450 0.850 109.3 487 0.927 93.0 498 0.982 79.3 518 1.030 62.6 539 1.114
1.00 wt % PFMIT7 114.2 335 0.733 96.4 318 0.809 77.1 298 0.855 64.3 275 0.883 49.8 244 0.941 28.0 183 0.963 20.1 142 1.001 5.50 wt % PFMIT7 101.1 326 0.744 76.1 284 0.835 70.2 274 0.844 52.3 243 0.882 41.8 217 0.935 23.0 171 0.972 1.29 wt % PFMIT9 128.9 401 0.804 96.5 361 0.884 86.1 357 0.923 75.5 345 0.961 69.2 335 0.983 52.9 320 1.047 46.4 306 1.069 27.0 287 1.147
T (°C)
P (bar)
density (g/cm3)
1.03 wt % PFMIT5 115.4 355 0.766 97.4 356 0.854 86.5 359 0.907 74.4 364 0.971 62.9 345 1.009 48.6 318 1.057 32.9 252 1.120 20.3 265 1.154 3.05 wt % PFMIT7 125.5 351 0.702 92.0 306 0.817 73.3 283 0.870 64.4 267 0.887 50.3 237 0.930 33.4 210 0.950 23.0 170 0.978 9.92 wt % PFMIT7 106.6 344 0.752 83.6 297 0.822 67.7 258 0.897 58.4 246 0.909 49.4 227 0.919 36.8 186 0.928
constant to within (0.3 °C, and the cloud-point pressure is reproduced two to three times with a scatter of (4.0 bar, as measured with a Heise pressure gauge accurate to within (0.7 bar. Solution density data are also determined at the phase transition using a technique reported by Diguet and co-workers in 198533 and subsequently by Kiran and co-workers in 1990.34 Solution densities are calculated knowing the amount of material loaded into the cell and the volume of the cell at a given pressure and temperature by detecting the location of the internal piston with an LVDT coil (Lucas Schaevitz Co., model 2000-HR) that fits around a 1.43-cm high-pressure tube and tracks the magnetic tip of a steel rod connected to the piston. The solution densities have an accumulated uncertainty of (1.5%.35 Results and Discussion Tables 2 and 3 list the cloud-point temperatures, pressures, and solution densities for the PFMITn-CO2 and PFDITn-CO2 mixtures investigated in this study. All of the transitions reported in Tables 2 and 3 represent fluid w liquid + liquid cloud points. Figure 1 shows a comparison of the cloud-point curves for the PFMIT and PFDIT systems. Note that the difference in cloud-point pressures is greatest for a CF2 side-chain length of 3 and that the difference decreases as the CF2 side-chain length increases, although the curves again diverge from one another for a CF2 side-chain length of 9. The acid group in the PFMIT polymers is expected to have the largest impact on the phase behavior, especially for PFMIT3 as is demonstrated in Figure 1a.
Table 3. Experimental Temperatures, Pressures, and Densities at the Phase Boundary for Poly(perfluorodiitaconate) (PFDIT3, PFDIT5, PFDIT7, and PFDIT9) in CO2 T (°C)
P (bar)
density (g/cm3)
1.00 wt % PFDIT3 136.2 388 0.675 108.8 343 0.709 85.7 294 0.744 69.4 252 0.773 60.3 228 0.783 51.9 215 0.806 45.6 182 0.818 32.9 137 0.820 21.5 95 0.838 0.96 wt % PFDIT7 151.3 398 0.635 122.9 359 0.676 99.0 321 0.710 84.9 929 0.736 69.6 260 0.770 55.3 238 0.816 42.9 218 0.859 21.9 118 0.870 6.70 wt % PFDIT7 146.3 396 0.711 116.5 355 0.752 82.4 287 0.802 61.5 237 0.838 41.7 180 0.872 23.8 126 0.907 0.95 wt % PFDIT9 113.2 293.1 76.8 225.5 51.7 161.7 24.1 81.4 -
T (°C)
P (bar)
density (g/cm3)
1.00 wt % PFDIT5 148.1 402 0.644 131.0 379 0.669 100.2 327 0.713 85.3 313 0.757 69.0 256 0.761 42.5 177 0.810 33.1 146 0.827 21.7 106 0.847 1.50 wt % PFDIT7 122.2 364 102.4 336 83.0 326 72.6 309 61.7 291 55.3 276 46.1 258 33.0 221 11.00 wt % PFDIT7 112.5 349 0.749 79.7 281 0.798 64.0 239 0.826 52.1 209 0.847 37.5 163 0.877 24.9 124 0.901 2.06 wt % PFDIT9 104.3 282.4 72.6 221.7 39.3 130.2 24.7 88.3 -
In this instance, the PFMIT3 cloud-point curve exhibits a negative slope similar to that found for polar polymernonpolar SCF solvent mixtures.13 As the temperature is decreased, interpolymer acid-acid hydrogen bonding occurs, making it more difficult for CO2 to dissolve PFMIT3 as compared to PFDIT3, which does not have an acid group. Because the acid group is directly tethered to the main chain in PFMIT, it is less likely that intrapolymer hydrogen bonding will occur because of the steric hindrance afforded by the side chains. Figure 1a also shows that the PFMIT3 and PFDIT3 curves converge at temperatures in excess of 130°C where hydrogen-bonding interactions are diminished. Figure 1b shows that the addition of two more CF2 groups on the side chain, from PFMIT3 to PFMIT5, lowers the cloud-point pressure from ∼540 to ∼325 bar at 60 °C. Notice that the cloud-point curves for PFDIT3 (Figure 1a) and PFDIT5 (Figure 1b) essentially superpose. Also, Figure 1b shows that the PFMIT5 and PFDIT5 curves merge at temperatures in excess of ∼100 °C, which is ∼30 °C lower than the temperature needed to merge the PFMIT3 and PFDIT3 curves. As the length of the side chain increases in both the mono- and diitaconates, the strength of polymer-polymer polar interactions decreases because the dipole moment of the ester group is distributed over a larger molar volume, the free volume of the polymer increases, and impact of acid-acid hydrogen bonding in the monoitaconate is reduced. The behavior exhibited in Figure 1a and 1b suggests that the reduced polarity and increased polymer free volume both enhance the solubility of PFMIT5 and PFDIT5 in CO2. If two more CF2 groups are added to the side chain, giving PFMIT7 and PFDIT7, the cloud-point curves
6502 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
Figure 1. Comparison of the phase behaviors of poly(di-1H,1H,2H,2H-perfluoroalkyl monoitaconate) (PFMITn; n ) 3, 5, 7, and 9; filled symbols) and poly(di-1H,1H,2H,2H-perfluoroalkyl diitaconate) (PFDITn; n ) 3, 5, 7, and 9; open symbols) in CO2 obtained in this study. For the systems shown in a and b, the PFMIT and PFDIT polymer concentrations are all ∼1.00 wt %. For the systems shown in c, the concentrations are 1.00 (filled triangles), 3.05 (filled squares), 5.50 (filled circles), and 9.92 (filled diamonds) wt % for the PFMIT7 mixtures and 0.96 (open circles), 6.7 (open triangles), and 11.00 (open squares) wt % for the PFDIT7 mixtures. For the systems shown in d, the concentrations are 0.95 wt % (filled circles) for the PFMIT9 mixture and 0.95 (open circles) and 2.06 (open squares) wt % for the PFDIT9 mixtures.
essentially superpose for concentrations ranging from ∼1.0 to 11.0 wt %. Even though the 1.0 wt % curves represent dew-point transitions, these transition pressures are very close to the maximum in the pressurecomposition (P-x) isotherms that can be generated from the data in Figure 1c. These very flat P-x isotherms are similar in shape to the P-x isotherms observed for fluorinated monomers of 2,5-dichlorobenzoates in CO2.36 These benzoates have alkyl and fluoroalkyl side chains ranging from 3 to 10 carbons, similar to the lengths of the side chains of the itaconates considered in the present study. Interestingly, the P-x isotherms for the benzoates with nonfluorinated side chains have very pronounced maxima. Hence, even with low-molecularweight monomers, a fluorinated side chain can have a very large impact on solubility behavior in CO2. Figure 1d shows that higher pressures are required to dissolve PFMIT9 than PFDIT9 in CO2. Therefore, there is an optimum side-chain length for PFMIT to balance the reduced acid group hydrogen bonding, the reduced polymer polarity, and the increased polymer free volume. No such optimum exists for the diitaconates for the range of side-chain lengths investigated in this study. The volumetric data provide further insight into the underlying mechanisms controlling polymer solubility. Figure 2 shows a comparison of the PFDIT-CO2 solution volume at the cloud point with the volume of pure CO2 calculated at the cloud-point pressure and temper-
Figure 2. Comparison of densities of mixtures of poly(perfluorodiitacanoate) (PFDITn; n ) 3, 5, 7, and 9) and CO2 with calculated CO2 densities determined at the corresponding mixture cloud points. The open circles are densities of 0.96-2.06 wt % PFDIT3,5,7,or9-CO2 mixtures, and the filled circles are densities of 6.70 and 11.00 wt % PFDIT7-CO2 mixtures.
ature. In all cases, the solution volume is less than the volume of pure CO2, indicating that CO2 must “condense” around the polymer to solubilize it. The effect is magnified as the concentration of PFDIT increases to 6 and 11 wt %. These volumetric data suggest that the location of the cloud-point curve is a function more of strong fluoropolymer-CO2 interactions than of CO2
Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6503
Figure 3. Comparison of densities of mixtures of poly(perfluoromonoitacanoate) (PFMITn; n ) 3, 5, 7, and 9) and CO2 with calculated CO2 densities determined at the corresponding mixture cloud points. The open circles are densities of 0.98-1.29 wt % PFMIT3,5,7,or9-CO2 mixtures, and the filled circles are densities of 3.05, 5.50, and 9.92 wt % PFMIT7-CO2 mixtures.
density alone. Figure 3 shows that this “condensation” process is magnified for the PFMIT-CO2 systems. It is interesting that the condensation effect is quite pronounced even for PFMIT solutions with as little as 1.0 wt % polymer. The results shown in Figures 1-3 demonstrate the large impact of fluorination on polymer solubility in CO2. Conclusions Supercritical CO2 is a high-quality solvent for slightly polar, fluorinated polymers. As shown here, only modest pressures and temperatures are needed to dissolve mono- and diitaconate polymers that can be used in a variety of coatings applications. A major advantage with CO2 is that the solvent can be quantitatively removed from the polymer. CO2 is a unique solvent that has characteristics of a weak alkane with some polarity. For certain fluoropolymers, CO2 is a better solvent than a hydrofluorocarbon because CO2 can form a weak complex with carbonyl groups in the fluoropolymer. This extra bit of interaction can have a significant effect on the solubility behavior because the density of CO2 is so high at moderate temperatures and pressures. The potential of CO2-based processing will continue to be enhanced as a better understanding of the solution properties of polymer-SCF mixtures is developed. Literature Cited (1) Scheirs, J., Ed. Modern Fluoropolymers, High Performance Polymers for Diverse Applications; Wiley Series in Polymer Science; John Wiley & Sons: Chichester, U.K., 1997. (2) Thorpe, A. A.; Nevell, T. G.; Young, S. A.; Tsibouklis, J. Surface Energy Characteristics of Poly(methylpropenoxyfluoroalkylsiloxane) Film Structures. J. Appl. Surf. Sci. 1998, 136, 99. (3) Thorpe, A. A.; Nevell, T. G.; Tsibouklis, J. Surface Energy Characteristics of Poly(methylpropenoxyalkylsiloxane) Film Structures. J. Appl. Surf. Sci. 1999, 137, 1. (4) Tsibouklis, J.; Stone, M.; Thorpe, A. A.; Graham, P.; Peters, V.; Heerlien, R.; Smith, J. R.; Green, K. L.; Nevell, T. G. Preventing Bacterial Adhesion onto Surface: The Low-Surface-Energy Approach. Biomaterials 1999, 20, 1229. (5) Graham, P.; Stone, M.; Thorpe, A.; Nevell, T. G.; Tsibouklis, J. Fluoropolymers with Very Low Surface Energy Characteristics. J. Fluorine Chem. 2000, 104, 29.
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Received for review December 30, 2002 Revised manuscript received August 5, 2003 Accepted August 7, 2003 IE021063O
