Optical properties of a low birefringence polyester containing fluorene side chain I

Abstract

We examined optical properties of a new polyester resin synthesized from 9,9-bis[4-(2-hydroxy-ethoxy)-phenyl]fluorene (BPEF), ethylene glycol, and dimethyl terephthalate (DMT) by a melt-condensation polymerization. When the mol ratio of BPEF to the total diol (k) was increased above 0.2, the refractive index exceeded 1.6. From rheooptical measurements carried out below the glass transition temperature, the intrinsic birefringence was found to decrease with increasing k. For the sample with k = 0.7, the intrinsic birefringence was as small as 0.07, about one third the value commonly observed for poly(ethylene terephthalate). Molecular modeling of the BPEF unit indicates that the fiuorene side chain is perpendicularly aligned to the main chain and this stereo structure appears responsible for the small birefringence of the polyester studied.