Photoelastic studies of polymers and copolymers in the rubbery state

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Abstract
Photoelastic investigations have been carried out on a number of polymers in the crosslinked rubbery state. These include cis‐polyisoprene, two polybutadienes of different structural configurations, an ethylene‐propylene terpolymer, and a range of styrene—butadiene copolymers. From the measured values of stress‐optical coefficients, together with calculated values of the optical anisotropies of the monomer units in the chain (based on Denbigh's values of bond polarizabilities), estimates are made of the number of monomer units in the equivalent random link (of the Kuhn‐Grün theory) for the polymeric chains considered. Further, from a study of the temperature dependence of the stress‐optical coefficient, the temperature dependence of the lenght of the equivalent random link, and hence of the statistical length of the chain, is derived. The interpretation of the temperature dependence data is discussed in a semiquantitative manner in terms of internal energy differences between different rotational states.