Tuning oxygen permeability in azobenzene-containing side-chain liquid crystalline polymers

A series of poly[4-(4-cyanoazobenzene-4'-oxy)alkyl methacrylate]s, side-chain liquid crystalline polymers (azoLCPs) were synthesized with methylene groups as spacers varying from 5 to 12. The thermal properties and phase transition temperatures of the polymers were characterized with differential scanning calorimetry and polarized optical microscopy and a relationship between spacer lengths, glass transition (T-g) and clearing temperatures (T-c) of the polymers was established. The T-g decreased with increasing spacer length, while the T-c exibited an odd-even effect with varying spacer length. X-ray diffraction was used to determine the degree crystallinity above and below T-c. The azoLCPs exhibited smectic <-> nematic <-> isotropic phases. Increasing crystallinity correlated linearly with decreasing gas permeability as measured using an oxygen permeation analyzer, which was used to measure the films' permeability to oxygen (O-2) gas. Switching of the azoLCPs from a liquid crystalline to an isotropic state was accomplished by heating the films above their T-c, which resulted in at least a 10-fold increase in the O-2 permeability coefficient (P-O2). Increasing the methylene spacer length of the azoLCP had little or no effect on gas permeability however it did decrease the T-c, allowing fine control of the temperature at which the switch in P-O2 takes place by tuning the mesophase between nematic and isotropic states.