Viscoelasticity and birefringence of low birefringent polyesters

Dynamic viscoelasticity and dynamic birefringence were investigated for a s eries of polyesters with various combinations of dimethyl decalin 2,6-carbo xylate, DMHN and dimethyl. terephthalate, DMT as acid component and 9,9-bis [4-(2-hydroxy-ethoxy)phenyl] fluorene, BPEF and ethylene glycol, EG as dio l component. The lowering of the stress-optical coefficient was achieved by the displacement of phenylene (DMT) with a hydrogenated naphthylene ring ( DMHN) in the main chain and by the introduction of BPFE in the place of EG. The former choice lowered the intrinsic birefringence but did not affect t he chain flexibility. The latter enhanced the flexibility but did not affec t the intrinsic birefringence. The systematic result clearly revealed the i mportance of two different factors, the anisotropy of structural units and the chain flexibility, in molecular designs for polymers with low stress-op tical coefficient.