Processing, network formation, and mechanical behavior of liquid crystal photopolymers for structural applications

Liquid crystalline network polymers are a new class of advanced materials for structural applications. Their rigid molecular structure and liquid crystalline behavior allow these polymers to exhibit useful high temperature properties as well as the possibility of anisotropic physical and mechanical properties. This paper considers the processing parameters that affect molecular ordering and alignment in these polymers, and their effect on the mechanical properties.;Two types of model liquid crystalline diacrylate monomer systems were studied: smectic and nematic. In the nematic phase, the molecules were successfully aligned by a magnetic field of 0.32 Tesla. After mixing with a photo-initiator, a UV laser was used to lock-in the existing orientation in the polymer. This could be done readily by photopolymerization since the photo-reaction rates at low temperatures were large. Network structures were formed from monomers in the following states: non-aligned smectic, non-aligned nematic, aligned nematic, and isotropic.;The degree of molecular order in the aligned nematic monomer was studied with dielectric analysis. Dielectric permittivity was used to estimate the monomer order parameter as a function of temperature. The permittivity was also measured as a function of time after reorientation of an external magnetic field. The change in monomer orientation angle followed an exponential time dependence with the bulk of the monomer obeying an Arrhenius temperature dependence. At temperatures approaching the nematic to isotropic transition, the reorientation occurred rapidly in a matter of seconds.;The molecular order and alignment were locked in by the photopolymerization reaction and infrared dichroism was used to measure the degree of order in the aligned polymer networks. While the order parameter of the nematic monomer could vary from 0.65 to 0.45 with increasing temperature, the order parameter of the polymer samples varied only from 0.60 to 0.50 with increasing polymerization temperature.;Linear viscoelastic behavior of the polymerized networks was investigated with dynamic mechanical analysis. In the aligned networks, the mechanical anisotropy was found to depend not only on the molecular order, but also on free volume and network structure. The difference in the elastic modulus parallel and perpendicular to the alignment direction was as high as a factor of two in the glassy state and a factor of three above T...