Phase Behavior and Morphology of Binary Blend of Block Copolymers having Hydrogen Bonding

Abstract

Block copolymer blends have been extensively investigated to control nanostructures. We employed two types of block components of poly(vinyl pyridine) (PVP) and poly(4-hydroxy styrene) (PHS). Due to the nitrogen atom in PVP and the hydroxy group in PHS group, the hydrogen bonding exists and this bonding could significantly change the phase behavior of binary blends of block copolymers containing hydrogen bonding. In this study, we investigated the phase behavior of binary blends consisting of high molecular weight polystyrene-block-poly(2-vinyl pyridine) copolymer (PS-b-P2VP) and low molecular weight PS-b-PHS by using small angle X-ray scattering and transmission electron microscopy. First experiment, we employed both PS-b-P2VP and PS-b-PHS exhibiting lamellar microdomain over the entire experimental temperatures up to 300oC. 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 spherical (BCC) 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. Secondly, we investigated the phase behavior of binary blends consisting of high molecular weight PS-b-P2VP and low molecular weight PS-b-PHS whose the microdomains are the BCC spherical microdomains of P2VP (or PHS). When the weight fraction of PS-b-PHS in the blend was less than 0.1, the BCC spherical microdomains were maintained. However, with increasing amount of PS-b-PHS, the microdomains in the blends were transformed to HEX cylindrical, lamellar, and finally returned to HEX cylindrical microdomains. On the other hand, when a relatively high molecular weight of PS-b-PHS was used, the lamellar microdomains were maintained even at a large weight fraction of PS-b-PHS in the blend. Very interestingly, we found highly asymmetric lamellar microdomains, despite of blends of highly asymmetric block copolymers with same volume fraction (ΦPS~0.83), not the complementary fraction. The phase behavior which observed experimentally in this study was rationalized by the results of the strong stretching theory.