Confinement Size Effect on Crystal Orientation Changes of Poly(ethylene oxide) Blocks in Poly(ethylene oxide)-b-polystyrene Diblock Copolymers

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Abstract

A series of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers were designed and synthesized to study the change in crystal orientation of PEO blocks under different confinement sizes. The volume fraction of PEO blocks (fPEO) in these copolymers was kept almost identical (the fPEO values were between 0.45 and 0.48) but with different number-average molecular weights for the PS and PEO blocks (M̄nPEO and M̄nPS). Therefore, the phase morphology of these copolymers was a lamellar structure with different PEO and PS layer thicknesses (dPEO and dPS) detected by synchrotron small-angle X-ray scattering (SAXS) experiments. Since the melting temperature of these PEO crystals is lower than the glass transition temperature of the PS layers, the PEO block crystals could be melted and recrystallized under one-dimensional (1D) confinement at different crystallization temperatures (Tc). The PEO block crystal orientation changes were monitored using synchrotron wide-angle X-ray diffraction (WAXD) experiments. It was found that the crystalline PEO chain orientation (the c-axis in the crystals) underwent a change from being perpendicular (homogeneous) to the layer normal direction (n̂) at low Tcs to parallel (homeotropic) at high Tcs in these 1D confined samples with the dPEO ranging from 8.8 to 23.3 nm. However, with the gradual release of this 1D confinement, i.e., increasing the dPEO, a broad Tc region in which the inclined c-axis orientation was originally observed in the PEO-b-PS with the low M̄nPEO (8.7K g/mol) and M̄nPS (9.2K g/mol) became increasingly narrowed by pushing the starting Tc where the tilting initiates toward higher Tc and reducing the ending Tc where the parallel orientation of the c-axis with n̂ starts. In the PEO-b-PS sample with the highest M̄nPEO (57K g/mol) and M̄nPS (61.3K g/mol) in this study, this Tc region was narrowed to less than 5 °C, suggesting the confinement size effect on the crystal orientation of the PEO crystals. The homogeneous to homeotropic orientation change of the c-axis in the PEO crystals with increasing Tc was explained to be largely governed by the primary nucleation and crystal growth processes of the PEO blocks for developing the maximum crystallinity. A semiquantitative calculation was attempted to illustrate why the homogeneous orientation of the PEO crystals takes place in the 1D confinement based on the SAXS, WAXD, and differential scanning calorimetric results. It was expected that when the dPEO becomes large enough, the homogeneous orientation of the c-axis in the PEO crystals would disappear.

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