An investigation into the thermal and mechanical behavior of some structurally unique linear liquid crystal polyesters and liquid crystal elastomers
While most known materials expand laterally when stretched, the possibility for materials in which there is a lateral expansion when stretched does exist. These materials display a negative Poisson's ratio and have been termed "auxetic" by Evans. Some liquid crystal elastomers display unique properties when placed under stress. One of these unique properties is that they are able to undergo large amounts of strain with little or no increase in the applied stress. Another is that during this period of extension, the elastomer undergoes a polydomain to monodomain transition. This transition causes the polymer to go from optically opaque to optically transparent. Warner and Terentjev have called polymers with these properties "soft elasticity" and to date, these systems have mostly been studied using smectic main-chain or nematic/smectic side-chain liquid crystal polymers. This dissertation will focus on the thermal and mechanical properties from a series of main-chain liquid crystal polymers designed and synthesized in order to examine their potential as auxetic or soft elastic polymers. These polymers consisted of four types of materials: linear polymers, linear polymer with transverse rods incorporated into their main-chain, crosslinked elastomers, and crosslinked elastomers with transverse rods incorporated into their main-chain. Thermally, it has been noted that in incorporation of crosslinks into the parent polymer did not destroy the mesophase and, in the case of low crosslink density (<3%), actually enhanced the thermal stability of the mesophase. The introduction of transverse rods into the polymer systems reduce the phase transition temperatures, destroyed all signs of crystallinity, but did not destroy the liquid crystal phase. The mechanical results showed that only those polymers with less than or equal to 3% crosslink density, with or without the incorporation of transverse rods, display soft elastic properties. These elastomers display both stress/strain requirements as well as the transition from optically opaque to optically transparent. In addition, the linear polymers with transverse rods incorporated display lower moduli than those of the linear or crosslinked systems and that modulus was independent of the length or amount of incorporation of transverse rod, in all polymers but one.