Effect of Block Copolymer Composition and Homopolymer Molecular Weight on Ordered Bicontinuous Double-Diamond Structures in Binary Blends of Polystyrene-Polyisoprene Block Copolymer and Polyisoprene Homopolymer

We investigated the effect of block copolymer composition and homopolymer molecular weight on the stability of ordered bicontinuous double-diamond (OBDD) phases in polystyrene-rich polystyrene-polyisoprene block copolymer (PS-PI) and polyisoprene (PI) blend using small-angle X-ray scattering. PS-PI with PI volume fractions (f(PI)) of 31-40 vol % and a degree of polymerization of ca. 320 were prepared. Stable OBDD regions could be observed at f(PI) = 31-35 vol %, but no OBDD could be observed at f(PI) = 40 vol %. Moreover, OBDD morphology was formed only when the chain length of the homopolymer was greater than that of the same constituent chain in the copolymer (dry-brush conditions). Furthermore, OBDD formation required a homopolymer content of at least 11 wt % in the blend. These results indicate that the added homopolymer localized in the middle of the microdomains and relieved packing frustration inside the four-fold nodes of OBDD, resulting in a stable OBDD structure.