Photopolymerization of nematic liquid crystal monomers for structural applications: molecular order and orientation dynamics

Abstract

This paper is concerned with the molecular orientation in liquid crystal (LC) monomers and the retention of orientation in cross-linked network polymers subsequently formed by photopolymerization. This is of importance because anisotropic mechanical properties can be beneficial in certain structural applications. We have been using LC monomers for forming structural polymers via stereolithography, a novel rapid prototyping process where three-dimensional objects are built layer-by-layer using a laser to scan the surface of a liquid monomer. To this end the magnetic alignment of liquid crystal photo-monomers was investigated with dielectric measurements. Using the permittivity data, an extrapolation method based on the isotropic transition temperature provided an estimate of the order parameter as a function of temperature. The dielectric permittivity was also measured as a function of time after reorientation of an external magnetic field. The calculated change in monomer orientation angle followed an exponential time dependence with two distinct time constants. The faster of the two time regimes was attributed to bulk reorientation and obeyed an Arrhenius type of temperature dependence. The slower time regime was attributed to inhibition caused by impurities and surface interaction with the electrodes. Finally, dynamic mechanical analysis data showed that the molecular anisotropy induced by the magnetic field was retained after polymerization. © 1997 Elsevier Science Ltd.