Abstract
The variation of photoelastic and mechanical behavior of cross-linked polymers, such as, phenolic, epoxy resins, diallylphthalate (DAP) polymer, styrene-divinylbenzene (S-DVB) copolymers, and methylmethacrylate (MMA)-DAP copolymers, has been studied between 20°and 200°C. A large change of the stress-optical coefficient is observed in the glass transition region. At temperatures above this region, suitably cross-linked polymers show photoelastic behavior corresponding to entropy elasticity, but too much cross-linked ones do not show it perfectly. The effect of copoly-merization ratio on the stress-optical coefficient has been studied in S-DVB and MMA-DAP copoly-mers. The DVB content affects the coefficient considerably in the entropy elasticity region but negligibly in the glassy state. The value of Δn/σ extrapolated to zero degree of cross-linkage agrees well with that of the linear polymer. The reversal of the sign of stress-optical coefficient in the glass transition region is observed for S-DVB copolymers but not for the others. The stress-optical coefficient of MMA-DAP copolymers in both glassy and rubbery states and the stress-optical coefficients of respective monomors are not additively related. Generally the degree of cross-linkage affects remarkably the stress-optical coefficient in rubbery state, differing from the result of statis-tical theory of rubbery photoelasticity. From photoelastic data, the values of the optical anisotropy (αl-α2) are derived. The photoelastic behavior of polymers can be explained by applying a model in which two photoelastic effects corresponding to glassy and rubbery elasticities are considered.
