Figure 5. (Eq.8) vs. T-0 for polymer Figure 6. (Eq.8) vs. T-0 for polymer solutions in 0 solvents specified in the inset. solutions in poor solvents specified in the inset.

conveniently varied with pressure over a much wider range (factors of 2 to 10) than is possible for organic liquid solvents where the range is at most a few percent. This property of SCF solvents should offer an unparalleled means of controlling polymer solubility, but unfortunately, only two classes of polymeric materials, i.e. fluoropolymers and silicones have been shown to exhibit appreciable solubility in SC CO2 [27]. The first SANS measurements of the second vinal coefficient A2 were performed in mixtures of hexafluoropropylene oxide (HFPPO) and poly( 1,1 -dihydroperfluoro-octylacrylane) (PFOA) with SC CO2 at T-65 OC and P=34.5 MPa (see [28] and references therein). They have shown that A2=~0 for the former and A2 > 0 for the latter systems and thus SC CO2 is a © solvent and a good solvent for HFPPO and PFOA, respectively at these particular values of (T,P). In a recent work [4] SANS was applied to investigate the structure and thermodynamic properties of PDMS - SC CO2 solutions as a function of pressure and temperature. Fig.7 illustrates the (T,P) dependence of the radius of gyration of PDMS in SC CO2. Rg, of the polymer remains invariant during both pressure and temperature quenches extending to the immediate vicinity of the (T,P) polymer-SCF demixing locus and agrees well with the dimension of unperturbed chains

(Rg= 0.267Mw/ =~40 A). This observation indicates the universality of the constancy of Rg in semidilute polymer solutions below the © condition previously demonstrated for liquid solvents (see Figs.3, 4) and now extended to supercritical fluids. The dimensions of polymer chains increase when T and/or P exceed the appropriate values of the © parameters (© = 65 ± 5 OC and [email protected] = 52 ± 4 MPa at p co2=0.95 g/cm3 [4])due to excluded volume effects, indicating that SC CO2 becomes a good solvent for PDMS. The experimental data agree with the results of Monte Carlo simulations which suggest that polymer chains may adopt unperturbed and expanded conformations at high densities [29]

A typical variation of \ vs. x in PDMS-SC CO2 solutions is shown in Fig.8. As is seen, the critical index v exhibits a sharp crossover from the mean-field value (v=0.5) in the © region to the Ising model value (v =0.630±0.001 [30]) in the region around Tc. Accordingly, the critical index g=1.23±0.02 for the susceptibility (i.e. the osmotic compressibility) agrees within experimental errors with the Ising model value (1.239±0.002 [30]). The crossover takes place at 2, - Rgand thus reproduces the main features of the crossover observed in solutions of PS in CH-d [3]. The dimension of the poor solvent domain may be made infinitely large (as in PS-AC) or small (as in PS-CH) depending on the density of SC CO2. These observations certainly establish basic similarities of behavior in solutions of polymers in SCFs and in sub-critical solvents, and show that SCF/polymer solutions belong to the Ising model universality class.

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