Boundaries and cross-link densities modulate domain sizes of polydomain nematic elastomers

Abstract

Nematic liquid crystal elastomers (LCEs) crosslinked at their isotropic phase, when they are quenched to the nematic phase, show polydomain patterns, in which nematic microdomains with different orientations self-organize into a three-dimensional mosaic with characteristic correlation patterns. The orientational correlation length of the domain, which is usually in the micrometer range, is believed to emerge as a result of a competition between liquid crystalline ordering and frozen network inhomogeneity. Although polydomain patterns show potentials as the basic platform for optical, memory, and mechanical devices, no study exists regarding how they are modulated by experimentally accessible parameters. Here, using confocal polarized fluorescence microscopy, we study the effects of a solid-wall or open boundary on the domain size in conjunction with effects of cross-link density. The LCE bounded by a solid glass shows the reduced domain size near the boundary. In contrast, increased domain size appears at the free surface. With increasing cross-link density, the domain size decreases, also exhibiting the boundary effects. Guided by theoretical considerations, the results are explained by a picture that the effective strength of the inhomogeneity frozen in the polymer network, i.e., the effective disorder strength, varies depending on the cross-link density and constraint states at boundaries. The results offer the first experimental approach to global and local modulation of the polydomain pattern in nematic LCEs.