Relationship between mesogenic side chain fraction in liquid crystalline copolymer and bistable electro-optical switching for (Liquid crystalline copolymer/liquid crystals) composite systems

The influence of the mesogenic side chain fraction in a side chain liquid crystalline copolymer (LCcoP) on electro-optical effects for (LCcoP/low-molecular-weight nematic liquid crystals (nematic LCs)) composite systems in a smectic state was studied in order to improve reversible and bistable electro-optical switching characteristics driven by ac electric fields with two different frequencies. In the case of the LCcoP fraction lower than 40 wt %, the magnitudes of glass transition temperature (T_g) and the mesophase temperature range for the binary composite systems were independent of the mesogenic side chain fraction in LCcoP, that is, ∼220 K and 220-340 K, respectively. In the case of the LCcoP fraction of 30 wt % and, also, its mesogenic side chain fraction of 52.5 and 32.5 mol %, the binary composite systems showed a homogeneous smectic phase at room temperature. On the other hand, the LCcoP with the mesogenic side chain fraction 16.5 mol % did not exhibit any mesomorphic characteristics at any temperature without any addition of LCs. Then, this LCcoP was named as the pseudo-LCcoP in this study. However, the (pseudo-LCcoP/nematic LCs) composite system showed a novel type of induced smectic phase over a wide range of both mixing concentration (35-85 wt % pseudo-LCcoP) and temperature (220-320 K). This composite system showed a fast switching speed from a transparency state to a light-scattering state. Therefore, it was reasonably concluded that the application of LCcoP with the optimum substituent fraction of mesogenic side chain is useful to realize a high electro-optical switching speed (∼50 ms) as well as a stable memory effect (∼years) at room temperature.