Competitive Effects of Hydrogen Bonds and Molecular Weights on the Phase and Crystallization Behaviors of Binary Block Copolymers

We investigated the competitive effects of the hydrogen-bonding interaction and molecular weight on the phase and crystallization behaviors of polystyrene-block-poly(ethylene oxide) (PS-b-PEO)/polystyrene-block-poly(acrylic acid) (PS-b-PAA) blends. The hydrogen-bonding interaction between PEO and PAA chains improved their miscibility, enabling the distinct block copolymers to co-organize into common PS and PEO/PAA microdomains. When the molecular weight ratio of PS-b-PEO to PS-b-PAA exceeded 4.5, the entropy penalty (caused by long PEO chains organizing into the PEO/PAA microdomains) suppressed the hydrogen-bonding interaction, inducing macrophase separation. Localization of large PS-b-PEO chains and small PS-b-PAA chains at the common interface caused the PEO/PAA microdomains to be composed of coexisting PEO/PAA and PEO regions (Regions I and II, respectively). Hydrogen bonds hampered PEO crystallization in Region I, whereas the PEO in Region II was crystallizable. When the molecular weight ratio of PEO to PAA was decreased, PEO crystallization in Region II became strongly confined, reducing the crystallization temperature and kinetics. Increasing the PS-b-PAA content expanded the separation distance between PEO chains in Region I. Consequently, PEO chains in Region II became more stretched to maintain the microdomain incompressibility, which inhibited PEO crystallization in Region II.