Crosslinkable liquid crystalline copolymers with variable isotropization temperature

Abstract

Main-chain liquid crystalline polymers were synthesized from diene mesogens using acyclic diene metathesis polymerization to control the thermal properties of an unsaturated polymer that may subsequently be crosslinked to form nematic networks. The mesogen homopolymer forms a nematic phase with a glass transition of 74 °C and an isotropization temperature of 189 °C. Copolymerizing the mesogenic monomer with a more flexible, nonmesogenic monomer lowered the temperatures of the glass and isotropization (clearing) transitions and decreased the latent heat of the clearing transition. The unsaturated backbone of the polymers may be crosslinked using a free-radical initiator, and this was demonstrated for a selected composition. The early stages of the crosslinking reaction were found to obey a first-order rate law, with rate constants that depended on temperature in an Arrhenius manner (E_a = 118.9 kJ mol⁻¹). Measurement of the shear storage and loss moduli using a rotational rheometer during crosslinking showed that free-radical crosslinking can be used to prepare a liquid crystalline network with a subambient glass transition temperature.