Birefringence of polymethylmethacrylate in a sinusoidal mechanical field

Abstract The method of birefringence in an oscillating mechanical field has been used to study the temperature dependence of the photo-elasticity coefficient e and the phase difference φ between the stress δ and the birefringence Δ n of PMMA. In the temperature interval of the region of inversion of the sign of Δ n of PMMA a break in the experimental dependence of φ on temperature is found. At temperatures close to that of the glass transition point of PMMA the maximum of the dependence of φ on temperature is found. The observed dependences are satisfactorily explained by assuming the existence of two mechanisms contributing to the total effetc of Δ n atomic and highly elastic. It is shown that the phase difference between δ and Δ n reflects not only the mechanical properties of PMMA but also the optical properties associated with the presence of several mechanisms characterized by optical anisotropy differing in value and sign of the corresponding molecular structures.