Pronounced effects of cross-linker geometries on the orientation coupling between dangling mesogens and network backbones in side-chain type liquid crystal elastomers

We demonstrate that the cross-linker geometries such as molecular length and functionality have a pronounced effect on the orientation coupling between the dangling mesogens and network backbones in the side-chain type liquid crystal elastomers which are made by the copolymerization of mono-functional mesogens and multi-functional non-mesogenic cross-linkers. For the nematic elastomers (NEs) with planar alignment, the cooling causes an increase in nematic order of the dangling mesogens independently of the cross-linker geometries. Correspondingly, a finite macroscopic elongation along the director is induced for the NEs with the bi-functional (F = 2) and sufficiently long cross-linkers with N >= 6 (N; the number of methylene units between the functional groups), but no macroscopic deformation is driven for the NEs with the short cross-linkers with N <= 4 independently of F. When the corresponding NEs with polydomain alignment are stretched by externally imposed strain, the dangling mesogens are reoriented along the stretching direction independently of N in the case of F = 2 whereas no reorientation of the mesogens is driven in the cases of the short tri- and tetra-functional cross-linkers (N = 3 and F= 3, 4). Such striking and complicated effects of the cross-linker geometries on the orientation-coupling between the dangling mesogens and network backbones is expected from the heterogeneous network structure composed of the dense and sparse regions of cross-links which are characteristic of the polymer networks formed by the copolymerization of mono- and multi-functional monomers. (C) 2015 Elsevier Ltd. All rights reserved.