Spacer Length Dependence of Tilt Angle of Mesogenic Chromophores of Side-Chain Liquid-Crystalline Polysiloxanes

The orientational behavior for a series of side-chain liquid-crystalline polysiloxanes incorporating azo chromophores, i.e., poly(3-{4-[(4-nitrophenyl)azo]phenyl]oxycarbonyl} propylmethylsiloxane)-co-(3-carboxypropylmethylsiloxane) (P1), poly{3-[(6-{[4-[(4-nitrophenyl)azo] phenyl]oxy}hexyl)oxycarbonyl] propylmethylsiloxane}-co-( 3-carboxypropylmethylsiloxane) (P2), and poly{3-[(11-{[4-[(4-nitrophenyl)azo]phenyl]oxy}undecyl)oxycarbonyl]propylmethylsiloxane}-co-(3-carboxypropylmethylsiloxane) (P3) were investigated by UV-Vis and polarized FTIR-ATR spectroscopy as well as X-ray diffraction measurement using their thin films. P1 exhibited highly-ordered smectic and nematic phases as the mesophases. On the other hand, P2 and P3 showed only smectic C phase. The mesogenic chromophores of these polymers in smectic phases and the once-annealed solid were spontaneously aligned nearly normal to a substrate surface. In the case of P1, the tilt angle of the mesogenic chromophores in the nematic phase was found to be larger than that in the smectic phase. The tilt angles of the mesogenic chromophores of these polymers in the smectic phase and in the once-annealed solid increased with the increase in the side-chain methylene spacer units. The dependence of the tilt angle on the spacer length was concluded to be attributable to the changes in intermolecular interactions with relation to the degree of the motion decoupling between the side-chain and main-chain.