(A) study on phase behavior and its relationship with electrical property of amorphous polymer/LC blends

Abstract

Phase behavior of polymer/low molecular weight liquid crystal blends were studied by DSC, optical microscopy, light scattering technique, SEM, and AC impedance spectroscopy. Electro-optical properties of a polymerdispersed liquid crystal (PDLC) film made by polymerizing PMMA/MMA/ZLI-2061 mixture were also measured. Phase diagrams of the PMMA/7CB and PS/7CB blends were constructed from the DSC and optical microscopy measurements. The low molecular weight liquid crystal, 7CB, showed partial miscibility with PMMA and PS. It was considered that 7CB acted as a plasticizer, judging from the $T_g$ shift of the PMMA and PS. DSC thermograms indicated that the phase separation occurred when the 7CB concentration exceeded 40 weight percent in PMMA and 30 percent in PS. It is important to point out that existence of the polymer/isotropic LC two phases which was undetectable by either DSC or the cross-polarized optical microscopy was verified by the phase-contrast optical microscope at temperature above $T_NI$ and was reconfirmed by the light scattering experiments. It is concluded that the polymer/LC system at certain composition shows several phase states consisted of the homogeneous single-phase, polymer/isotropic LC two phases, polymer/ nematic LC two phases, and polymer/solid crystalline LC two phases depending on temperature. For a mixture of the polymer/7CB in narrow 7CB concentration range, it was observed by the optical microscope that the polymer/nematic 7CB twophase state transformed into the homogeneous single-phase state without going through the nematic-to-isotropic transition of 7CB domains. Electrical properties of the PMMA/7CB blend were measured to investigate the correlation with the phase behavior, by the AC impedance measurement using FRA and the direct measurement of the resistance and capacitance using LCR meter, respectively. It was observed that marked change of the resistance and capacitance occurred near $T_NI$ and $T_{cloud}$ of the polymer/...