tail in PFOA contains about 25%—CF3branches. (1,1-dihydroperfluorooctyl acrylate) [poly(FOA)] using
CO2 as a continuous phase that yielded homogeneous solutions throughout the course of the reaction [18– 21]. Krukonis and coworkers have demonstrated the solubility of poly(perfluoropropylene oxide) in CO2 [22]. Barton and Kiran have reported on the high solubility of polydimethyl siloxane (PDMS) in CO2at approximately 450 bar [23–25].
Despite the inability of CO2to dissolve most polar or polymeric compounds, its solvency can be enhanced by the addition of surfactants that can drastically strengthen its effective solvating power for such sub- stances. This chapter focuses on the efforts made to- ward the design and synthesis of surfactants for super- critical CO2. These surfactants have been categorized into sections according to the ‘‘CO2-philic’’ segment contained in the polymer (e.g., perfluoropolyethers, PDMS). A range of architectures have been explored, including homopolymers, block and graft copolymers, dendrimers, and small-molecule surfactants. Most often these materials were synthesized for the purpose of sta- bilizing hydrophilic or lipophilic compounds in CO2for such applications as cleaning, separations, or poly- merizations.
II. FLUOROPOLYMER SURFACTANTS
A. Synthesis and Characterization
in scCO2
In 1992 our laboratory reported the homogeneous syn- thesis of a high molar mass amorphous fluoropolymer, poly(1,1⬘-dihydroperfluorooctyl acrylate) (PFOA) in CO2[18]. The reaction scheme is shown in Scheme 1. Polymerizations were extended to include the synthesis of statistical copolymers whereby FOA was copoly- merized with monomers such as methyl methacrylate (MMA), styrene, ethylene, and butyl acrylate (BA). These copolymerizations proceeded homogeneously even with the addition of high concentrations of the comonomer (Table 1). Other fluorinated acrylate poly- mers have been synthesized in a similar manner, including poly[2-(N-methylperfluorooctane-sulfonami- do)]ethyl acrylate, poly[2-(N-ethylperfluorooctane-sul- fonamido)ethyl acrylate, and poly[2-(N-methylperfluo- rooctanesulfonamido)ethyl methacrylate [26]. Along with this investigation, decomposition rates and effi- ciency factors were measured for azobisisobutyronitrile (AIBN) in CO2and comparisons were drawn with con- ventional liquid solvents. It was found that AIBN de- composes at a rate 2.5 times slower in CO2 than in benzene but with greater efficiency. This phenomenon
can be explained in terms of a decreased solvent cage effect that exists in a low-viscosity supercritical me- dium. Hence, CO2can be considered an ideal solvent for free radical polymerizations with no chain transfer to solvent side reactions.
Solution properties of PFOA in CO2 were studied using small-angle neutron scattering (SANS) [27]. McClain et al. demonstrated that SANS, after providing sufficient polymer-solvent contrast, was a viable tech- nique to study homopolymer solutions in CO2. SANS data were generated for a concentration series of both a high- and a low-molecular-weight PFOA in CO2, Mw = 1.4⫻ 106
g/mol and 1.1 ⫻ 105
g/mol, respectively. Samples were measured in dilute solution [0.005 < C (g/mL) < 0.008] over a range of temperatures and pres- sures. The results of the SANS measurements are sum- marized in Table 2. The molecular weights measured by SANS were consistent with values expected from the synthetic conditions. The values for the radius of gyration (Rg) as a function of Mwwere found to follow the function Rg = (0.10 ⫾ 0.02)M
1/2
, where M is the molecular weight. This was the first study confirming that CO2 is a thermodynamically good solvent for PFOA, as indicated by the measurement of positive values for the second virial coefficient A2. In compar- ison, Martino et al. have shown that other CO2-soluble polymers, poly(hexafluoropropylene oxide) (Krytox) and PDMS, have A2values that are zero and negative, respectively, at similar conditions [28]. Studies have also indicated that A2of PFOA not only is positive but
TABLE 1 Statistical Copolymers Containing FOA and Vinyl Monomersa
Copolymer Feed ratio Incorporated
Intrinsic viscosity (dL/g) Poly(FOA-co-MMA) Poly(FOA-co-styrene) Poly(FOA-co-BA) Poly(FOA-co-ethylene) 0.47 0.48 0.53 0.35 0.57 0.58 0.57 — 0.10 0.15 0.45 0.14
aPolymerizations were performed at 59.4⫾ 0.1⬚C and 345 ⫾ 0.5 bar for 48 h in CO 2.
Intrinsic viscosity determinations were conducted in 1,1,2-trifluorotrichloroethane (Freon-113) at 30⬚C.
TABLE 2 SANS Results for Concentration Series of Poly(FOA) in CO2at Various Conditions
Poly(FOA) sample P (bar) T (⬚C) a (g cm⫺3) Rg (A˚ ) A2 (⫻105cm3mol g⫺2) Mw (⫻10⫺6g mol⫺1) Low Mw High Mw High Mw High Mw 395 340 395 340 60 65 60 40 0.888 0.842 0.888 0.934 35⫾ 0.15 120⫾ 13 100⫾ 6 114⫾ 9 9.5⫾ 0.5 1.9⫾ 0.4 4.1⫾ 0.8 2.5⫾ 0.3 0.113 ⫾ 0.006 1.5⫾ 0.4 1.2⫾ 0.3 1.6⫾ 0.3 a
Density of pure CO2at these conditions.
Source: Adapted from Ref. 27.
also is increased as the density of the CO2increases (S. Wells, M. Adam, M. Rubenstein, and J. M. DeSimone, in preparation).
DeSimone and coworkers also studied the synthesis of a CO2-philic/hydrophilic amphiphilic graft copoly- mer [29]. Using the macromonomer technique, poly(FOA-g-PEO) copolymers were made and found to be completely soluble in CO2 at 238 bar and 60⬚C (10 wt%). The microenvironment of these solutions was studied using solvatochromic characterization. Methyl orange (MO), a water-soluble dye that is in- soluble in CO2, was added to the CO2 system in a so- lution with water. It was found that the PEO grafts enabled the solubility of the dye in CO2, yielding bright orange solutions. This phenomenon was further con- firmed by ultraviolet (UV) spectroscopy data that yielded a max of 418 nm for the colored solution (MO(aq)has amaxof 464 nm). This blue shift is a result of decreasing solvent polarity, which has been observed by Zhu and Scheely [30].
The discovery of uptake of both methyl orange and water into the scCO2 continuous phase by the PFOA- g-PEO graft copolymer led to studies using small-angle
x-ray scattering (SAXS) [31]. Many copolymers mo- lecularly designed for CO2applications are difficult to characterize because of their limited solubility in con-
ventional solvents. Both SAXS and SANS have proved to be powerful techniques for obtaining structural in- formation about aggregated systems in solution. The SAXS study by Fulton et al. was the first of many subsequent scattering experiments performed on aggre- gated amphiphilic polymeric systems in CO2.
The PFOA-g-PEO polymer synthesized with 5 kg/ mol PEO grafts was studied by SAXS. The experi- ments were performed at 60⬚C and three different pres- sures (470, 300, and 255 bar) in CO2 in the presence of water. The water-to-surfactant ratio was 0.32. The data are shown in Fig. 2. As the pressure of the system was decreased, there was an increase in the scattered intensity. The increase is a result of not only the greater particle-solvent contrast at lower CO2densities but also the small increase in the size of the particles as pressure was decreased, as evidenced by the shift in the scat- tering peaks at lower scattering vectors (q). The oscil- latory nature of the scattering curves is indicative of spherical core-shell structures. A depiction of the spher- ical micelle of PFOA-g-PEO with the collapsed PEO chains and water molecules within the core is shown in Fig. 3. A core-shell model was used to fit the SAXS data at low q. From the fits, the outer radii of the ag- gregates were found to be approximately 125 A˚ with relatively low polydispersities. The radii of the particles
FIG. 2 Small-angle x-ray scattering spectra for 1.9% (w/w) PFOA-g-PEO in supercritical CO2 at 60⬚C and three different
pressures, 255, 300, and 470 bar. (Adapted from Ref. 30.)
FIG. 3 Proposed structure of a PFOA-g-PEO graft copoly- mer micelle in supercritical CO2. (Adapted from Ref. 30.)
increased when either the concentration was reduced or the pressure was decreased. The radius of the core was estimated to be 105 A˚ at the higher pressures and 125 A˚ at lower pressures. The number of PEO segments in the core was estimated to be about 600 based on the measured volume of the micelle core and PEO bulk density.
In attempts to mimic and further study the PFOA-
g-PEO system, Chillura-Martino et al. performed
SANS experiments on the same sample [28]. By taking advantage of the contrast differences between H2O and D2O, more conclusive data were obtained. The data showed not only that the micelles were swollen in the core but also that the shell was slightly swollen, sug- gesting that PEO segments also penetrate the shell.
Measurements were also performed on the polymer in the absence of water and showed that the micelle dimensions were smaller. Figure 4 shows the differ- ences in the SANS data when H2O and D2O were pres- ent versus when there was no added water. The radius of gyration taken from a core-shell fit increased from ⬃56 A˚ (no added water) to 86 A˚ (H2O swollen) and 136 A˚ (D2O swollen).
FIG. 4 The differential cross section per unit sample vol- ume (d⌺/d⍀(Q)) versus Q for PFOA-g-PEO graft copolymer in CO2 before and after swelling with H2O and D2O.
(Adapted from Ref. 28.)
FIG. 5 Scanning electron micrograph of PMMA particles produced by a dispersion polymerization in scCO2 using
PFOA as stabilizer.
B. Application in Heterogeneous
Polymerizations
Materials useful as stabilizers in colloidal dispersions of lipophilic or hydrophilic polymers usually employ amphiphilic molecules. They contain anchoring seg- ments that have an affinity for the polymer particles, most likely by physical adsorption, and a segment that is highly soluble in the continuous phase. DeSimone et al. demonstrated the amphiphilicity of PFOA and car- ried out the first successful dispersion polymerization in scCO2[32,33]. The polymerization of methyl meth- acrylate was carried out in a 10-mL high-pressure re- action view cell at 65⬚C and 204 bar using AIBN or a fluorinated derivative of AIBN as the initiator. Poly- merizations conducted in the absence of stabilizer pro- duced polymers in low yield. In remarkable contrast, the reactions with added stabilizer produced free-flow- ing powders in high yields upon removal of CO2. Scan- ning electron micrographs displayed micrometer-sized particles with spherical morphologies and a relatively narrow size distribution (Fig. 5). Indeed, the amphi- philicity of PFOA contributed to the success of particle formation.
DeSimone et al. have also reported the successful dispersion polymerization of styrene in scCO2 using amphiphilic diblock copolymers containing poly- (styrene) (PS) and PFOA segments [34]. These mate- rials were prepared via a controlled free radical method known as the iniferter technique. The detailed synthesis has been described by Guan and DeSimone and is il- lustrated in Scheme 2 [35]. With such block copoly- mers, polydisperse submicrometer-sized PS particles were produced by dispersion polymerization with spherical morphologies. It was also found that as the
length of the stabilizing moiety increased, the particle size distribution decreased.
The PS-b-PFOA surfactants were further character- ized by small-angle neutron scattering studies and found to self-assemble in solution to form multimo- lecular micelles [36]. Both SAXS and SANS measure- ments were performed on the samples at 65⬚C and 338 bar. The scattering curves confirm that spherical mi- celles are formed in solution. As with the PFOA-g-PEO samples, a core-shell model was used to fit the data (Fig. 6). Table 3 shows the polymer dimensions as well as the results from the fits. As the PFOA block length increases, as expected the thickness of the shell and size of the overall micelle increase. A core-shell model could not be applied to the 4K-b-245k copolymer be- cause of the large asymmetry, which was expected to give rise to a different morphology. The form factor for an f-arm star polymer was applied to the data and gave results (Rg⬃ 200 A˚ and f ⬃ 7.7 arms) consistent with those for other diblock copolymer samples.
After confirmation of micelle formation, the PS-b- PFOA surfactants were used to emulsify CO2-insoluble PS oligomer [27]. SANS characterization of micelles of PS-b-PFOA surfactants in CO2(65⬚C, 340 bar) with
SCHEME 2 Synthesis of PS-b-PFOA via the iniferter tech-