EVALUATION OF SEGMENTAL INTERACTION BY SMALL-ANGLE X-RAY-SCATTERING BASED ON THE RANDOM-PHASE APPROXIMATION FOR ASYMMETRIC, POLYDISPERSE TRIBLOCK COPOLYMERS

The binary segmental interaction parameter, chi-S-B, for poly(styrene-block-butadiene-block-styrene) (SBS) triblock copolymer was evaluated by analyzing the small-angle X-ray scattering (SAXS) in the disordered state based on the random-phase approximation (RPA) for asymmetric and polydisperse triblock copolymers. The parameter fitting allowed us to determine also the radius of gyration, R(g), of an entire copolymer from the peak position of the scattered intensity. Samples used in this study are Shell's TR-1102 and Kraton D-1102 having the weight fractions of polystyrene 0.31 and 0.28, respectively. The effects of asymmetry in the size of the repeating units and polydisperity in molecular weight on the scattering function and on the evaluation of chi and R(g) for these particular SBS's were examined. The analyses gave the temperature (T) dependence of chi-S-B as chi-S-B = 6.59 X 10(-3) + 13.6/T and the statistical segment length of polybutadiene, b(PB), calculated from the value of R(g) as b(PB) = 0.65 +/- 0.02 nm, for both TR-1102 and D-1102. The theory with no polydispersity or no asymmetry correction turned out to overestimate chi-S-B and the R(g) values for these block copolymers.