| Abstract
| - The order−disorder transition temperature (TODT) of symmetric styrene−isoprene diblockcopolymers (AB) and its hetero-four-arm star analog (A2B2) was measured using dynamic mechanicalspectroscopy and small-angle X-ray scattering (SAXS). Care was taken to ensure that the arm molecularweights of both AB and A2B2 were matched. Contrary to mean-field theory predictions, a significantdifference in TODT, ΔTODT ≈ 30 °C, is found between the two systems (the arm molecular weight for boththe styrene and isoprene is about 10 kg mol-1). We show that this difference is too large to be explainedby polydispersity, segmental asymmetry, or the Fredrickson−Helfand theory of compositional fluctuations(which assumes that the polymer chains are Gaussian). Instead, we suggest that one needs to incorporatethe differing degrees of non-Gaussian chain stretching in the two systems at the order−disorder transition(as confirmed by SAXS) into the fluctuation theory for a proper understanding of the effect. Wedemonstrate that a simple model calculation that incorporates sufficient chain stretching to match theexperimentally determined shifts in the SAXS peak for the two systems also reproduces a difference inthe TODT which is comparable to what is measured experimentally.
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