Liquid crystal mobility and PDLC memory effects

Dissertation presented at Faculdade de Ciências e Tecnologia of Universidade Nova de Lisboa to obtain the Degree of Master in Chemical and Biochemical Engineering

PDLCs comprising PEGDA or PEGDMA as polymer matrix to support the liquid crystal E7 from Merck, in a ratio 30/70 (w/w) were studied. These were followed by thermal or photochemical polymerization of these mixtures by FTIR, where there was a similar rate of polymerization for both oligomers. Dielectric relaxation spectroscopy for the PDLC with E7 was used, where the polymer matrix was PEGDMA or PEGDA to study the liquid crystal mobility. The glass transition and the nematic-isotropic transition for E7 in both composites were studied, and the temperatures are consistent with those described in the literature for E7 and already observed by differential scanning calorimetry. The mobility of the liquid crystal alone and confined in the polymer matrix was compared, in a range of temperatures near the glass transition temperature (-60ºC to 0ºC). When an electric field is applied dominates the parallel alignment between all liquid crystal microdomains (the PDLC becomes transparent), after the application of an electric field, if it remains transparent, the PDLC have permanent memory effect. When temperature rise above the clear temperature, there is a phase transition, from the nematic phase to the isotropic phase. When the cell is cooled the liquid crystal molecules in the PDLC tend to align in each microdomain, however the microdomains are not aligned with each (PDLC presents opaque). The electro-optical properties of PDLC using PEGDA were studied in order to find permanent memory effects. It was found that composites have permanent memory effect by both thermal and photochemical polymerization, which was more pronounced than in the case of PEGDMA. The permanent memory effect was related with a parallel alignment of molecules of E7 and the mobility given by Dielectric Relaxation Spectroscopy.