Thermo-optical properties of polymer dispersed liquid crystals

Polymer dispersed liquid crystal (PDLC) films serve as the basis of a variety of highefficiency electro-optical effects. In the most general sense, as PDLC materials consist of micro-sized domains of a liquid crystal, with the domain size determined by a continuous polymer matrix running through the film. Depending on the choice of liquid crystal and polymer, a variety of electro-optical effects have been demonstrated. PDLC materials do not require the use of polarizers and as such can offer advantages of high optical efficiency and wide viewing angles compared with the liquid crystal display. Therefore, the aim of this study is to prepare and characterize mesotropic liquid crystals and polymer dispersed liquid crystals by application of solubility parameters through solvent induced phase separation, followed by thermally induced phase separation to obtain the suitable morphologies of PDLCs.

The thermal properties of four low molar mass mesogens were studied by DSC and the morphologies were investigated by polarized optical microscope (POM). There were significant super cooling/heating effects on the first order phase transitions but not on the mesophase transitions, which are second-order or weak first-order transitions. The structural effects on the transition temperature were investigated. Between the two 4-alkoxybenzoic acids mesogens, the clearing temperature of 4- (octyloxy)benzoic acid was higher than 4-(decyloxy)benzoic acid because of the increasing chain length. Trans-4-methoxycinnamic acid had the highest melting temperature among the four mesogens despite the molar mass because the carboxylic acid termini of trans-4-methoxycinnamic acid gave rise to strong intermolecular attractions.

The different liquid crystal structures were captured by POM. Some typical mesophase structures, like Schilieren nematic texture, “focal-conic” and “threading” smectic textures, were discussed. The smectic phases of 4-(octyloxy)benzoic acid were classified as head-to-head bilayer orientational smectic structures, SmA2 and - XX - SmC2, respectively, by wide angle X-ray diffraction through measuring the d spacing of the liquid crystal.

The total solubility parameter was used to evaluate matching a polymer-LC-solvent combination. However, the polar component of the solubility parameter was most important in predicting the solubility limit in a polymer matrix and the LC fraction in the droplets, while the other components, dispersive and hydrogen bonding were not critical.

PDLC films were prepared by the solvent induced phase separation method and suitable morphologies were achieved by thermal induced phase separation. The phase transition temperatures of PDLCs were shifted to a lower temperature due to the polymer dispersion effects. Different mesophases were observed in PDLC films when LC exhibited different mesophases.

There are different factors that affect the size of the droplet dispersed in the polymer matrix. The thermal cooling rate and the liquid crystal concentration in the PDLC composites were investigated in the PVC dispersed 4,4’-azoxyanisole. The results showed that fast cooling rate and low LC concentration more readily produced a smaller and narrower distribution size of droplet, which would be an advantage for commercial applications. The LC fractions in the droplets were calculated from the nematic to isotropic enthalpies through the Smith equation. The other phase enthalpies, such as crystallization and smectic to nematic, were not achieved because they were not sufficiently isolated from other endotherms to allow enthalpies to be determined accurately due to the complex sequences of solid transitions.

The phase transitions, first-order crystallization to smectic, second-order smectic to nematic and a weak first-order nematic to isotropic phase transition, of 4- (octycloxy)benzoic acid and its pHEMA dispersion were investigated by DSC, TMDSC and quasi-isothermal TMDSC. The TMDSC results were analysed by the two approaches, reversing and non-reversing heat flow and complex heat capacity. The C-S and N-I transition temperatures of the PDLC were shifted to lower temperature and the enthalpies decreased compared with those of the pure LC due to the effects of the polymer. However, the S-N transition temperature of the PDLC was almost the same as its pure LC and the enthalpies of the mesophase change decreased.

The 4,4’-azoxyanisole and PVC dispersed 4,4’-azoxyanisole were investigated by TMDSC. The non-reversible C-N transition and reversible N-I transition were investigated by DSC and TMDSC. The significant peaks in the loss heat capacity curve and the phase lag curve represented the C-N transition which was a nonreversing first-order transition. On the contrary, the N-I transition was reversing transition which proved that there were no obvious peaks could be observed in the loss heat capacity curve and in the phase angle curve. POM was employed to investigate the morphologies of the PDLC film and four different self-organized extinction patterns were found in the 4,4’-azoxyanisole dispersed PVC film caused by the different orientations of droplet axes, but not in the different internal structures.

The results of the phase transitions of the two PDLCs illustrated that PDLCs involved both non-reversing, melting, and reversing, clearing and the transition between two mesophases. In the non-reversing transition, the transition temperature would be affected by super cooling/heating and the results obtained in the experiments were dependent on the experimental conditions, such as the heating or cooling rate, sample size and purge gas flow rate. However, in the reversing transition process, there were no super cooling/heating effects observed and it seemed that the experimental conditions were not so critical. Results could be monitored by Lissajous figures obtained from the quasi-isothermal TMDSC. The plots of modulated heat flow versus the derivative of modulated temperature can be used to alert to unfavorable experimental conditions where loss of system linearity could be seen. The quasiisothermal TMDSC were provided accurate results for phase transition temperature and the transition ranges in addition.