Phase Behavior of Binary Blends of Block Copolymers Having Hydrogen Bonding

Abstract

Phase behavior of binary blends consisting of high molecular weight polystyrene- block -poly(2-vinylpyridine) copolymer (PS- b -P2VP) and low molecular weight polystyrene- block -poly(4-hydroxystyrene) copolymer (PS- b -PHS) was investigated by using small-angle X-ray scattering and transmission electron microscopy. Both PS- b -P2VP and PS- b -PHS exhibited lamellar microdomain over the entire experimental temperatures up to 300 °C. When the weight fraction of PS- b -PHS in the blend was less than 0.1, the lamellar microdomains were maintained. However, with increasing amount of PS- b -PHS, the microdomains in the blends were transformed to hexagonally packed (HEX) cylindrical microdomains and body-centered cubic (BCC) spherical microdomains. On the other hand, when a relatively high molecular weight of PS- b -PHS was used, the BCC spherical microdomains were not observed even at a large weight fraction of PS- b -PHS in the blend, but HEX cylindrical microdomains were formed. The phase behaviors observed experimentally were rationalized by the results of the self-consistent mean-field theory. Based on the theoretical results, some of the PS- b -PHS dissolves into P2VP microdomains and thereby increases the effective volume of P2VP/PHS phase because of hydrogen bonding between P2VP and PHS blocks. In the case of smaller molecular weight of PS- b -PHS, the dissolved amount is significant because the favorable interactions of P2VP/PHS chains becomes dominant over the relatively small enthalpic penalty between PS/P2VP in the P2VP microdomains as well as the gain of the translational entropy of the PS- b -PHS chains. The increase in the effective volume of the PHS/P2VP phase leads to a transformation of the microdomains in the blends from lamellar to HEX cylindrical microdomains and BCC spherical microdomains. On the other hand, when a relatively higher molecular weight of PS- b -PHS was used, the effective volume of the P2VP/PHS phase was not increased greatly because of a large enthalpic loss arising from the contact between PS/P2VP as well as a small gain in the translational entropy. Consequently, only lamellar and HEX cylindrical microdomains are observed.