Anomalous Difference in the Order−Disorder Transition Temperature Comparing a Symmetric Diblock Copolymer AB with Its Hetero-Four-Arm Star Analog A2B2

The order−disorder transition temperature ( T ODT ) of symmetric styrene−isoprene diblock copolymers (AB) and its hetero-four-arm star analog (A 2 B 2 ) was measured using dynamic mechanical spectroscopy and small-angle X-ray scattering (SAXS). Care was taken to ensure that the arm molecular weights of both AB and A 2 B 2 were matched. Contrary to mean-field theory predictions, a significant difference in T ODT, Δ T ODT ≈ 30 °C, is found between the two systems (the arm molecular weight for both the styrene and isoprene is about 10 kg mol -1 ). We show that this difference is too large to be explained by 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 incorporate the 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. We demonstrate that a simple model calculation that incorporates sufficient chain stretching to match the experimentally determined shifts in the SAXS peak for the two systems also reproduces a difference in the T ODT which is comparable to what is measured experimentally.