PHASE-TRANSITIONS OF LIQUID-CRYSTAL PAA IN CONFINED GEOMETRIES

Phase transitions of liquid crystal PAA, p-azoxyanisole, in porous sol -gels of average pore diameters 8.9, 16, and 20 nm, are investigated u sing Raman spectroscopy and differential scanning calorimetry. It is f ound that the phase transition temperatures inside small pores are dep ressed and the depressions are pore size dependent. This phenomenon is observed for the solid to nematic and the nematic to isotropic phase transitions of PAA. Surface modifications in which -OH groups are repl aced by -Si(CH)(3) groups are not sufficient to modify the phase behav ior. The restricted space limits the size of a crystal and dominates t he phase transitions. Confinement results in broadening of the phase t ransitions, indicating that in the pores they become weak first-order transitions.