Effect of mechanical stress on optical properties of polydimethylsiloxane II - Birefringence

In the paper we present the results of an experimental study of photoelasticity of polydimethylsiloxane (PDMS) in its deformation in compression with relative shortening in the range in which the dependence of the mechanical stress is not a linear function of strain (up to epsilon = --0.45). We observed nonlinearity of the dependence of the refractive index difference between beams polarized parallel and perpendicular to the direction in which the sample is compressed on the deformation which is significantly lower than the nonlinearity of the stress-strain dependence measured at the same sample. This fact can be explained by the assumption that the birefringence involves two mechanisms: (i) a change in polarizability of atoms, which is proportional to stress and, (ii) a change in structure of the environment, which we assume to be a linear function of strain. Appropriate choice of the impact ratio of these mechanisms gives a good match between experimentally observed dependence of birefringence on deformation and dependence arising from the above mentioned assumption. The contribution of the individual effects to the observed photoelasticity we investigated within the wavelength range (400-1800) nm. We have found that: (i) the effect of the polarizability of the environment is dominant for strain larger than epsilon = -03; (ii) the difference in the refractive indices of the beams with different orientation of polarization slightly decreases with increasing wavelength and, (iii) there are visible less-pronounced local extremes in the vicinity of the absorption lines of PDMS in the near-infrared. (C) 2014 Elsevier B.V. All rights reserved.