Highlights of the optical anisotropy of unstressed transparent polymeric plates

The photoelastic effect is cumulative, therefore the presence of material birefringence in unstressed transparent polymers might lead to erroneous photoelastic analyses. This presence is more influential in the scattered-light photoelasticity. Direct-transmission polariscopes are not suitable for detecting all material birefringence in normally illuminated plates because the birefringence in question coincides with the wave normal of the propagating light. The present paper describes phenomenologically the presence of the material birefringence in an arbitrarily selected group of unstressed cross-linked polymers and presents means of their determination. The material-birefringence information obtained can be used as a means of taking proper precautions for conducting faultless scattered-light photoelastic analyses. Octagonally shaped plates were machined to permit data acquisition using four different light-propagation directions. The orientation and diffusion of molecular chains appear to be the major source of the material birefringence observed.