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
Hideaki Takagi, Katsuhiro Yamamoto
Abstract
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 (fPI) of 31–40 vol % and a degree of polymerization of ca. 320 were prepared. Stable OBDD regions could be observed at fPI = 31–35 vol %, but no OBDD could be observed at fPI = 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.