Crosslinked PLCs

In addition, more recently, an interesting approach to fast response of photorespon-sive LCs has been reported. Crosslinked PLC networks containing azobenzene molecules were prepared by polymerization of ternary mixtures of monofunctional and difunctional LC monomers together with a LMW azobenzene LC, as shown in Figure... 

Several classes of crosslinked PLCs have been prepared ... 

PLC elastomers are a special class of lightly crosslinked one-comb PLC which were first made by Finkelmann et al [69]. A permanently oriented PLC may be achieved by mechanical means by stretching a lightly crosslinked polymer, and the orientation is made permanent by further crosslinking in the distorted state. The crosslinks are attached to the backbone chains. The end product is crosslinked polydomain PLC which can be further aligned by drawing. 

Dense PLC networks (thermosets) are often prepared from mixtures of mono- and bifunctional monomers resulting in a one-comb polymer-like structure with crosslinks. 

Oriented PLC thermosets based on epoxy chemistry and with the mesogens in the backbone chain have been prepared by aligning an oligoether prepolymer either mechanically or in a magnetic field (13.5 T) prior to and during curing [135]. The temperature associated with a drop in order parameter increased with increasing crosslink density. Samples with few crosslinks showed a drop in order parameter at 130 C... 

Figure 10.20 Normalized order parameter (s is the original order parameter) of aligned PLC networks with different molar contents of crosslinks as shown in the graph as a function of temperature during the first heating and cooling scans. The order parameter was assessed by IR spectroscopy. (After data of Sahlen et al. [129].)...

Viscoelasticity is perhaps the most ubiquitous characteristic of high molecular weight polymers at temperatures above Tg. Here we consider the implications of coupling rubber elasticity and mesomorphism via synthesizing covalent networks from conventional elastomers (siloxanes, isoprenes, etc.) and typical MLC mesogenic cores such networks are thermotropic PLCs. At low levels of crosslink densities in the PLC network, there is no appreciable change in the transition temperatures (Tg, Td, etc.) from those of the uncross-linked PLC (and its ancestral MLC) [68]. Below Td, rather modest mechanical deformations (extension ratio A < 1.5) may convert an initial random and disclination-ridden texture into a... 

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