Nematic Elastomers: The Dependence of Phase Transformation and Orientation Processes on Crosslinking Topology

Abstract

The orientation process in response to a mechanical field and the phase transformation of nematic elastomers are investigated for different types of crosslinkers and their concentration. For the corresponding linear polymer melt a mechanical field causes a director orientation perpendicular to the external field axis. For networks with rod-like crosslinkers, however, an orientation with the director parallel to the external field is observed. Only a trifunctional, disk-like crosslinker at low concentrations causes the perpendicular orientation as obtained for the linear polymer. At higher concentrations the director again becomes ordered parallel to the field. The orientation behaviour is analysed by stress-optical measurements and the thermal 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” crosslink, but rather as a part of the network chains. Due to interactions with the mesophase it orients the mesogenic groups in an antagonistic manner compared to the orientation of the pure polymer melt.