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

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 (f PEO) in these copolymers was kept almost identical (the f PEO values were between 0.45 and 0.48) but with different number-average molecular weights for the PS and PEO blocks (M̄ n PEO and M̄ n PS). Therefore, the phase morphology of these copolymers was a lamellar structure with different PEO and PS layer thicknesses (d PEO and d PS) 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 (T c). 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 T cs to parallel (homeotropic) at high T cs in these 1D confined samples with the d PEO ranging from 8.8 to 23.3 nm. However, with the gradual release of this 1D confinement, i.e., increasing the d PEO, a broad T c region in which the inclined c-axis orientation was originally observed in the PEO-b-PS with the low M̄ n PEO (8.7K g/mol) and M̄ n PS (9.2K g/mol) became increasingly narrowed by pushing the starting T c where the tilting initiates toward higher T c and reducing the ending T c where the parallel orientation of the c-axis with n̂ starts. In the PEO-b-PS sample with the highest M̄ n PEO (57K g/mol) and M̄ n PS (61.3K g/mol) in this study, this T c 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 T c 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 d PEO becomes large enough, the homogeneous orientation of the c-axis in the PEO crystals would disappear.

Impact of Nanoscale Confinement on Crystal Orientation of Poly(ethylene oxide)

“…An on-edge orientation of the PEO crystals was also observed in PS-PEO block copolymers at much lower crystallization temperatures (À10 8C), [27,28] however the orientation was somewhat different. In the block copolymers, the c-axis was parallel to the layer plane, but the b-axis was 458 with respect to the layer plane.…”

Crystallization of Polystyrene‐block‐[Syndiotactic Poly(propylene)] Block Copolymers from Confinement to Breakout

“…It is clear that the final morphology is a consequence of microphase separation, crystallization, and vitrification. For systems with T ODT > T g a > T c c (i.e., a hard confinement for crystallization), [6][7][8][9][10][11][12][13][14][15][16] the ordered nanostructure is preserved because of the vitrified microdomains of amorphous blocks. In contrast to the hard confinement, morphological evolution upon crystallization is more complicated if T ODT > T c c !…”

Composition Dependence of Crystallization Behavior Observed in Crystalline-Crystalline Diblock Copolymers

“…Therefore the crystallized morphology formed by high-T m blocks is a kind of spatial confinement for the subsequent crystallization of low-T m blocks, which will be qualitatively different from the confinement imposed by amorphous or glassy microdomains observed in crystallineamorphous diblock copolymers. [9][10][11] There are some studies so far on the morphology formation of double crystalline block copolymers with different T m values, [12][13][14][15][16][17][18][19][20][21][22][23][24] where they are mainly focused on the characteristic morphology finally formed in the system.…”