Thermoelastic and photoelastic properties of crosslinked liquid‐crystalline side chain polymers

Abstract

Measurements of stress and birefringence of crosslinked liquid-crystalline side chain polymers are reported. The results are compared with those obtained for conventional elastomers. It is found that the thermoelastic behaviour in the nematic state completely differs from that observed for common polymer networks due to the anisotropic ordering of the mesogenic side chains. The retractive force in the nematic state is found to be independent of path, indicating that the nematic phase is a homogeneous phase. Photoelastic investigations prove the proportionality of stress and birefringence above the clearing temperature T, which is similar to common elastomers. The stress-optical coefficient, however, strongly depends on temperature in the pretransformation region. X-ray measurements show a uniform orientation of the mesogenic side groups in the nematic state. The diffraction patterns observed contain many more reflexes than observed for common nematics.

Cholesteric Liquid Single‐Crystal Elastomers (LSCE) Obtained by the Anisotropic Deswelling Method

“…This allows mechanical fields to directly affect the liquid crystalline (LC) phase structure and state of order. [2,3] Piezoelectricity and the variation of the helicoidal pitch emerge as mechanical field effects. [4 -7] To analyze these anisotropic mechanical effects, cholesteric liquid single-crystal elastomers (cholesteric LSCE) are necessary that are macroscopically uniformly ordered with respect to the cholesteric phase structure.…”

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

“…For these crosslinked systems, a parallel orientation nearly always occurs, while only some experiments prove a perpendicular orientation similar to the corresponding linear polymer. [7,8] Unfortunately, using divinyl-oligosiloxane (ViSi) as a crosslinker that favours a perpendicular orientation of the mesogenic groups induces a microphase separation. [9] On the other hand, a crosslinker that does not cause phase separation, a hydroquinone derivative substituted with two alkenoxy groups (V1), causes a parallel orientation.…”

Mechanism of strain retention and shape memory in main chain liquid crystalline networks

“…By appropriate molecular engineering, LCEs can be designed to place the liquid crystalline (mesogenic) unit either pendant to the polymer main chain or directly in the main chain itself. Additionally, the LCE can be prepared either in a polydomain state in which there is local ordering but no preferred macroscopic ordering or in a monodomain state in which there is both local and macroscopic ordering in the specimen [2][3][4]. In the absence of an orienting field during its synthesis, the LCE will have a polydomain structure.…”