Phase Behavior of Binary Blends of Block Copolymers Having Hydrogen Bonding

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 degrees 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.