Basic Research and Development of Model Polymers for Optical Use with Low Birefringence

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In order to clarify the origin and the relaxation mechanism of strain-induced birefringence, we measured the dynamic viscoelasticity and the dynamic birefringence in oscillatory deformation for more than 30 polymeric materials. Systematic studies for a series of vinyl polymers revealed that the birefringence as well as the modulus are composed of two relaxational components ; one dominant at high temperatures and long times and the other at low temperatures and short times. An equation was derived to estimate the birefringence associated with two relaxation modes from the anisotropy of polarizability and the orientation of monomer units. By combining the theoretical expression and the observed results, a method was obtained to estimate the orientation function from the geometrical anisotropy of monomer unit and then to estimate the stress-optical coefficient from the molecular structure.According to the predicted relation between the degree of birefringence and molecular structure, a few polymers were synthesized. Some of the polyolefins including cyclic structures in the main chain were revealed to be low birefringent. Also hydrogenated polystirene was found to be low birefringent. It was suggested that mixtures of hydrogenated polystirene of different degrees of hydrogenation would be excellent materials in view of the optical as well as the mechanical quality.