ABSTRACT
Dynamic light scattering was used to measure the azimuthal anchoring energy coefficient W(straight phi) of nematic liquid crystal (5CB) on photoaligning poly-(vinyl-cinnamate) layer. Measurements were repeated several times within two months. The results show that W(straight phi) increases in the first few days after filling the cell with liquid crystal. Then it remains approximately constant at W(straight phi)=5 x 10(-6) J/m(2) for at least two months. Also, presence of very large effective surface viscosity is observed. This phenomenon is of transient nature and attributed to swelling and dissolving of photosensitive polymer into the liquid crystal, which gives rise to an inhomogeneity of viscoelastic properties. Numerical modeling of the fluctuation spectrum shows that an inhomogeneous surface layer can account for the observed effective surface viscosity.
ABSTRACT
Surface optical second-harmonic generation measurements were used to determine the change in the surface density and the surface orientational order parameter of the trans-cinnamoyl side groups of unidirectionally photopolymerized poly(vinyl cinnamate) (PVCN) films. The observed surface features were compared to the bulk properties of the films, which were probed by their UV absorption spectra and birefringence measurements. We found that the surface and the bulk photoreaction kinetics of the trans cinnamoyl side groups of the PVCN are very similar. The two photoreaction processes (trans-cis photoisomerization and photo-induced cross linking) appear to affect the linear and nonlinear optical properties of the polymer differently. This may explain the discrepancy between the birefringence and the azimuthal anchoring force of the unidirectionally photopolymerized PVCN.
