We study the monodomain (single-crystal) nematic elastomer materials, all
side-chain siloxane polymers with the same mesogenic groups and crosslinking
density, but differing in the type of crosslinking. Increasing the proportion
of long di-functional segments of main-chain nematic polymer, acting as network
crosslinking, results in dramatic changes in the uniaxial equilibrium thermal
expansion on cooling from isotropic phase. At higher concentration of main
chains their behaviour dominates the elastomer properties. At low concentration
of main-chain material, we detect two distinct transitions at different
temperatures, one attributed to the main-chain, the other to the side-chain
component. The effective uniaxial anisotropy of nematic rubber, r(T)
proportional to the effective nematic order parameter Q(T), is given by the
average of the two components and thus reflects the two-transition nature of
thermal expansion. The experimental data is compared with the theoretical model
of ideal nematic elastomers; applications in high-amplitude thermal actuators
are discussed in the end