Crystal Orientation Change and Its Origin in One-Dimensional Nanoconfinement Constructed by Polystyrene-block-poly(ethylene oxide) Single Crystal Mats

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Abstract

Utilizing crystalline−amorphous block copolymers, such as in the case of polystyrene-block-poly(ethylene oxide) (PS-b-PEO), under a large amplitude shear process provides an opportunity for investigating crystal growth and orientation within nanoconfinements at different supercoolings. However, the internal stress generated during the shearing process and the structural defects embedded in the phase-separated morphology inevitably play roles in affecting the confinement effect on the crystallization of the crystalline blocks. In this study, we designed a one-dimensional (1D), defect-free confinement constructed by PS-b-PEO single crystal mats collected in dilute solution. Each single crystal possessed a square-shaped, “sandwiched” lamellar structure, and it consisted of a PEO single crystal layer between two PS nanolayers formed by the tethered PS blocks on the PEO single crystal top and bottom fold surfaces. Furthermore, in these single crystal mats the glass transition temperature of the PS blocks is higher than the melting temperature of the PEO single crystals. We melted the PEO crystals between the two vitrified PS nanolayers, and the PEO blocks were recrystallized isothermally by quenching the mats to preset recrystallization temperatures (Trx). The orientation change of the PEO crystals with respect to the “sandwiched” lamellar normal at different Trx values was investigated via 2D small-angle and wide-angle X-ray scattering experiments. It was observed that the PEO c-axis orientation underwent a sharp change from homogeneous (perpendicular to the lamellar normal) in the low Trx region to approximately homeotropic (parallel to the lamellar normal) in the high Trx region. The results showed that this change of the PEO crystal orientation takes place within a few degrees Celsius. Microscopically, the crystal orientation might be determined from the status of critical nuclei formation due to the size and shape of this 1D confinement. This likely included a competition between the high tethering density (the junctions) of the PEO blocks at the PS interfaces leading to the homeotropic orientation with an anisotropic conformational orientation of the PEO blocks in the melt and the anisotropic density fluctuations within the 1D confined layer which could lead to an anisotropic ability for the PEO blocks to overcome the nucleation barrier to form the homogeneous orientation. The resolution of these two factors might explain the origin of the crystal orientation change in 1D confined crystallization. Analysis of the apparent crystallite size in these mats along both the orthogonally oriented [120] directions of the PEO crystals after recrystallization indicated that a change in the PEO crystal growth dimension from 1D to 2D occurs within this narrow Trx region corresponding to the location of the crystal orientation change.

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