Anomalous domain spacing difference between AB diblock and homologous A2B2 starblock copolymers

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Abstract
Based on weak- and strong-segregation theories, symmetric AB diblock copolymers and the homologous A 2 B 2 starblock copolymers formed by joining pairs of diblocks together by their junctions should have identical order–disorder transitions (ODT’s) and lamellar periods. However, recent experiments report that the starblock melts have elevated ODT’s and domain spacings. The ODT shift has been attributed to the increase in chain stretching, but there is still no convincing explanation for the larger domain size. Here, we show that this latter effect is correctly predicted by self-consistent field theory (SCFT) and that it results due to a reduction in translation entropy of the junctions. Furthermore, we derive a strong-segregation based approximation for this anomalous domain spacing difference.