Dynamic birefringence of vinyl polymers

The complex strain-optical ratio, O*(omega), and the complex Young's modulus, E*(omega), of four vinyl polymers, atactic polypropylene, 1,2-polybutadiene, poly(ethylethylene), and poly(4-vinylbiphenyl), were measured from the rubbery plateau to the glassy zone to investigate the origin of the birefringence and the chain orientation in the glassy zone. The data, including those for polystyrene and poly(vinyl naphthalene), were analyzed through a modified stress-optical rule: The rule states that O*(omega) and E*(omega) are described as a sum of two component functions (denoted by R and G) and that a proportional rule similar to the ordinary stress-optical rule holds well for each of the R and G components with different stress-optical coefficients. The R and G components have been interpreted to be respectively related to the orientation of the main chain axis toward the stretch direction and tilting of the structural units by a rotational orientation about the main chain axis. According to this interpretation, the stress-optical coefficients for the R and G components are predicted to decrease and increase, respectively, with increasing size of the side chain. The present study shows the validity of this prediction. In addition, it is suggested that the degree of the rotational orientation responsible for the G component becomes larger with increasing size of the side chain.