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Macromolecules 1985, 18, 583-585 583 specific volume of the polymer, and p e is the mole electron density of the solvent. The value of S,(O) was determined by extrapolation of the plot of S,-l(h) vs h2. The value ijq of polystyrene ( M = 5.0 X lo4) in toluene a t infinite dilution was determined to be 0.919 cm3/g a t 25.0 "C. Figure 1 shows the plot of -(1/V,)c3Ap1/ac vs. polymer concentration c. Since a slight molar mass dependence was observed at low concentrations (c 5 0.05 g/cm3), the results for the highest molar mass alone are shown within this range. The smooth and unambiguous plot allows graphical integration with respect to c to yield the values of Apl at given concentrations. Figure 2 shows the interaction parameter x plotted as a function of the volume fraction &. The values determined from osmotic p r e s s~r e ,~.~ light scattering,2 vapor pressure,8 and sedimentation equilibriumg are also plotted by circles, open triangles, open squares, and filled triangles, respectively, for purposes of comparison. The SAXS result is in good agreement with those obtained by other methods over a wide concentration range. Therefore we conclude that SAXS can provide reliable estimates of the interaction parameter x.

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