DIELECTRIC-RELAXATION OF LIQUID-CRYSTALLINE POLYMERS USED IN NONLINEAR-OPTICAL MATERIALS

Dielectric permittivity and loss have been measured over the frequency range 0.1 - 1000 Hz between 80 K and 300 K for side-chain and network liquid crystalline polyacrylates obtained by photopolymerization of s mectic monomers. The polyacrylates were based on mixtures of chiral mo nofunctional monomers with a lateral nitro group attached to the outer phenyl group of the mesogen and an ester group in the mesogen and non chiral bifunctional monomers, either with an ester group in the mesoge n or with an ester group in the mesogen and a lateral nitro group, cre ating networks with different crosslink densities. Poled copolymers ba sed on these monomers exhibit second-order nonlinear optical propertie s. Polymers based only on monomers with the chiral and nitro groups ex hibited a pronounced gamma process, associated with segmental motions in the methylene spacer groups and only vague signs of the high-temper ature subglass process (beta) associated with reorientation of the mes ogen. The polymers with the bifunctional monomer without the nitro gro up exhibited clearly both gamma and beta processes. The dielectric dat a indicate that the ester groups in the bifunctional units undergo con ventional reorientation according to the beta mechanism and that the b eta process is suppressed in the pendant chains with the lateral nitro groups. The beta process involves a coordinated torsion about two bon ds in the mesogen and the swept-out volume involved in the motion beco mes extensive when a lateral nitro group is present. (C) 1997 Elsevier Science Ltd.