Kinetic control of the phase-separated structure and electro-optical switching properties of (polymer/liquid crystal) composite films prepared by a solvent cast method

Methods of controlling the size and uniformity of low-molecular-weight liquid crystal (LC) channels or domains and the electro-optical switching properties were investigated for (poly(diisopropyl fumarate): (Pdi-iPF)/LC(E8)) composite films prepared by a solvent cast method from a homogeneous solution. The phase-separated structure was controlled on the basis of the degree of intermolecular interaction between the matrix polymer and the solvent. In the case of the composite film prepared from a mixed solution including a small amount of non-solvent to the matrix Pdi-iPF, it was found from morphological observations that the channel or domain sizes of E8 remarkably increased, and the degree of distribution of E8 domain sizes decreased. From the time-resolved light transmittance and light scattering properties during solvent evaporation, it was found that the resulting size and uniformity of the E8 channels or domains were strongly dependent on the degree of solubility between the solvent and the polymer. In other words, the size and uniformity of the E8 domains for the (Pdi-iPF/E8) composite film were strongly dependent on the Pdi-iPF fraction co-existing in the E8-rich phase at an early stage of phase separation which was induced by spinodal decomposition. Therefore, the electro-optical properties of the (Pdi-iPF/E8) composite films were directly dependent on the size and uniformity of the E8 domains in the polymer matrix. The driving voltage decreased with increasing size and uniformity of the E8 domains in the (Pdi-iPF/E8) composite film.