Effect of Crosslinker Geometry on Equilibrium Thermal and Mechanical Properties of Nematic Elastomers

We study three monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic groups but with different types of crosslinking: (i) short flexible siloxane linkage affine to the network backbone, (ii) short flexible aliphatic crosslinks miscible with mesogenic side chain groups, and (iii) long segments of main-chain nematic polymer. Equilibrium physical properties of these three systems are very different, especially the spontaneous thermal expansion and anisotropic stress-strain response along and perpendicular to the uniform nematic director. In the latter case, we examine the soft elastic plateau during the director reorientation. We compare the nematic order-parameter Q(T), provided primarily by the side mesogenic groups and relatively constant between the samples, and the average backbone chain anisotropy r(T)=l( parallel)/l( perpendicular), which is strongly affected by the crosslinking geometry. The experimental data is compared quantitatively with theoretical models of nematic elastomers.