Nematic elastomers: The dependence of phase transformation and orientationprocesses on crosslinking topology

The orientation process in response to a mechanical field and the phase tra nsformation of nematic elastomers are investigated for different types of c rosslinkers and their concentration. For the corresponding linear polymer m elt a mechanical held causes a director orientation perpendicular to the ex ternal field axis. For networks with rod-like crosslinkers, however, an ori entation with the director parallel to the external field is observed. Only a trifunctional, disk-Eke crosslinker at low concentrations causes the per pendicular orientation as obtained for the linear polymer, At higher concen trations the director again becomes ordered parallel to the field. The orie ntation behaviour is analysed by stress-optical measurements and the therma l expansion behaviour of the network, where the thermal expansion directly reflects the change of the network conformation at the isotropic to nematic phase transformation. The different orientation behaviour of the networks can be simply explained. Because of the contour length of the crosslinking molecules the crosslinker can no longer be considered as "point-like" cross link, but rather as a part of the network chains. Due to interactions with the mesophase it orients the mesogenic groups in an antagonistic manner com pared to the orientation of the pure polymer melt.