Vapor deposition of a smectic liquid crystal: highly anisotropic, homogeneous glasses with tunable molecular orientation

Abstract

Physical vapor deposition (PVD) has been used to prepare glasses of itraconazole, a smectic A liquid crystal. Glasses were deposited onto subtrates at a range of temperatures (T_substrate) near the glass transition temperature (T_g), with T_substrate/T_g ranging from 0.70 to 1.02. Infrared spectroscopy and spectroscopic ellipsometry were used to characterize the molecular orientation using the orientational order parameter, S_z, and the birefringence. We find that the molecules in glasses deposited at T_substrate = T_g are nearly perpendicular to the substrate (S_z = +0.66) while at lower T_substrate molecules are nearly parallel to the substrate (S_z = −0.45). The molecular orientation depends on the temperature of the substrate during preparation, allowing layered samples with differing orientations to be readily prepared. In addition, these vapor-deposited glasses are macroscopically homogeneous and molecularly flat. We interpret the combination of properties obtained for vapor-deposited glasses of itraconazole to result from a process where molecular orientation is determined by the structure and dynamics at the free surface of the glass during deposition. Vapor deposition of liquid crystals is likely a general approach for the preparation of highly anisotropic glasses with tunable molecular orientation for use in organic electronics and optoelectronics.