Order-disorder transition of polystyrene-block-polyisoprene. I. Thermal concentration fluctuations in single-phase melts and solutions and determination of χ as a function of molecular weight and composition

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Abstract
Small‐angle x‐ray scattering (SAXS) from single‐phase melts and solutions of polystyrene‐block‐polyisoprene (SI) having various degrees of polymerizationN and compositions f were analyzed on the basis of Landau‐type mean‐field theory developed by Leibler. The effective Flory interaction parameter χeff per segment in melts and in semidilute Θ solutions were determined from the mean‐field analysis of the SAXS profiles obtained at temperatures higher than T MF, the crossover temperature above which the non‐mean‐field effects become less significant and the mean‐field theory becomes a good approximation. It was found that χeff thus determined depends on N, f, volume fraction of the SI in the solution φ p and the interaction parameter χ0 in melt: χeff=φ p χ0 and χ0 is a function of f and N; χ0 at a given f is a decreasing function of N and that at a given N is approximately a quadratic function of f with a minimum at f≂1/2.