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

•The crosslinker geometries influence the orientation-coupling in side chain nematic elastomers.•The length of crosslinker significantly affects the thermal deformability of nematic elastomers.•The nematic elastomers with the short crosslinkers exhibit no thermal deformation.•The functionality of crosslinker significantly affects the stretching driven LC realignment.•The marked crosslinker-effect on orientation coupling is due to nonuniform network structure.

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.

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