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 using Raman spectroscopy and differential scanning calorimetry. It is found that the phase transition temperatures inside small pores are depressed 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 replaced by -Si(CH)(3) groups are not sufficient to modify the phase behavior. The restricted space limits the size of a crystal and dominates the phase transitions. Confinement results in broadening of the phase transitions, indicating that in the pores they become weak first-order transitions.