The physical and chemical nature of bare surfaces and of thin films of polymers deposited in solid substrates plays a key role in the alignment of liquid crystal (LC). The ability to define the polar and azimuthal direction of the LC director at substrate-LC interface is at the heart of electro-optical applications of liquid crystals. However, the precise mechanism(s) of LC alignment have remained shrouded in mystery. The research conducted under this project was aimed at gaining insight into the relationship between LC alignment and the surface structure/morphology and to understand the dynamics of polymer chains at the polymer-air interface and under confinement to small dimensions. The results presented here show that properties of thin polymer films, such as glass transition temperature, Tg, and chain mobility are significantly different from bulk polymers. However, the understanding of the underlying reasons for these effects is far from complete. The studies of various polymers (weight molecular weight, radius of gyration) in different film geometries (supported on substrate or free standing) using different techniques reveal either an increase or a decrease in Tg and/or chain mobility with decreasing film thickness. Although the shifts of the Tg are a clear indication of the differences between the polymer chain mobility in the bulk and thin films, the measurements of Tg’s are only an indirect method to probe their relaxation dynamics.
The measurement of optical retardation, a simple optical technique requiring no physical contact with the polymer film, has been employed to directly probe the relaxation dynamics of variously treated thin polymer films routinely used for liquid crystal alignment. Using the relaxation model based on Kohlrausch-Williams-Watts, the Tg’s of polystyrene films were determined as a function of the thickness and rubbing strengths (depths). Tg’s are significantly reduced by 15-20 K for films of thickness less than 250 Å. This correlates very well with previous studies on similar systems but using different methods. In addition, Tg decreases with rubbing depth indicating that polymer molecules close to the film-air interface relax faster than the molecules farther from the interface. The dependence of Tg on the rubbing strength provides clear evidence that polymer chains at the surface relax more readily.
The morphology of polymer films was determined using high-resolution x-ray reflectivity, which is a sensitive and direct probe of surface structure at submicron levels. Making use of the inherent anisotropy in the coherence area of the x-ray beam, the morphological anisotropy of the films was quantitatively determined. The films’ surfaces were modified by a specific treatment (i.e., rubbing or exposing to linearly polarized UV). Knowledge of the vertical root mean square (rms) roughness parallel and perpendicular to the treatment direction is of great importance in understanding the exact role in liquid crystal alignment of the nature of the surface. Reflectivity scans on the treated surfaces reveal roughness anisotropy, i.e., roughness is significantly different in the two orthogonal directions on the substrate plane. This direction of a more roughness is perpendicular to the azimuthal direction of the liquid crystal alignment. Hence, liquid crystal molecules align in the smoother direction.