Liquid crystal and polymer dispersions (LCPD) have potential application as flat panel displays, switchable lenses, optical switches, Bragg gratings, photonic crystals, diffractive optics, hyper-spectral filters, and etc. Precise morphological control of the phase-separated morphology of LCPDs is required to meet the rigorous requirements for the numerous applications. Liquid crystal and polymer dispersions (LCPDs) are micro or nano-structured materials fabricated using one of several phase separation techniques. The micro- or nano-structured morphology of LCPDs ranges from a polymeric network suspended in a liquid crystal solvent (polymer stabilized liquid crystals [PSLC] to random or spatially periodic micron or sub-micron sized liquid crystal droplet dispersions in a solid polymer matrix of polymer dispersed liquid crystals (PDLC) and the holographically formed PDLC (H-PDLC). The goal of this investigation is to identify the material properties and processing conditions required for more precise control of the phase-separated morphology of PDLCs. The investigation entailed construction of thermal phase diagrams for liquid crystal and monomer/pre-polymer mixtures to identify the compositionally dependent phase separation temperatures. The investigation also entailed inducing phase separation of the mixtures via ultra-violent light initiated polymerization at carefully chosen cure temperatures based on the thermal phase diagrams. The phase-separated morphology was correlated with the cure temperature, liquid crystal component, and mixture composition.
This paper investigates the effects of cure temperature and composition on the morphology of polymer dispersed liquid crystals (PDLCs) composed of the liquid crystal, K21 (4-heptyl-4'-cyanobiphenyl), in a thiol-ene based pre-polymer. PDLCs are composites composed of micron-size liquid crystal droplets dispersed in a solid polymer matrix. The PDLCs were fabricated by photo-initiated, polymerization induced phase separation using a differential photo-calorimeter (DPC). The cure temperatures were chosen based on the temperature-composition phase diagrams of the liquid crystal/pre-polymer mixtures. The characterization of the PDLCs included differential scanning calorimeter (DSC) to determine the nematic-to-isotropic transition temperature (TNI) of the liquid crystal and the glass transition temperature (Tg) of the polymer matrix. Scanning electron microscopy (ESEM) was used to examine phase separated morphology.
We have investigated the single components and binary component mixtures of the liquid crystal E7 in NOA65 and UV1 thiol-ene pre-polymers. E7 is composed of K15 (4-pentyl-4'- cyanobiphenyl), K21 (4-heptyl-4'-cyanobiphenyl), M24 (4- octyloxy-4'-cyanobiphenyl), and T15 (4-pentyl-4'- cyanoterphenyl). The single liquid crystal components and binary liquid crystal mixtures that were investigated include K15, K21, K15-K21, K15-M24, and K21-M24. The liquid crystal/pre-polymer phase diagrams were developed using thermally induced phase separation to determine the temperature of phase separation. Then, PDLC samples were prepared using photo-polymerization induced phase separation, and were polymerized at varying increments above the liquid crystal/pre-polymer phase separation temperature to determine how the morphology changes with the polymerization temperature, the liquid crystal component, and liquid crystal percentage. Polarized optical microscopy was used to determine the phase separation temperatures for the liquid crystal/pre- polymer samples and the nematic-to-isotropic transition temperatures for the PDLC samples. Laser light transmission measurements were performed to determine the electro-optic properties of the PDLC samples.