Compensation of photoelastic birefringence of polymers by anisotropic molecules and analysis of compensation mechanism

We report the anisotropic molecule dopant method for compensating the photoelastic birefringence of polymers. In this method, a rodlike molecule that has a polarizability anisotropy was chosen and doped in a polymer. We demonstrated this method by compensating the negative photoelastic birefringence of poly(methyl methacrylate) and poly(2,2,2-trifluoroethyl methacrylate) at a wavelength of 633 nm. trans-Stilbene and fluorene were selected as the anisotropic molecules. Homogeneous doping with the molecules almost eliminated the photoelastic birefringence of the polymers. We found that the photoelastic birefringence of poly(methyl methacrylate) was compensated by the elastic motion of trans-stilbene in the polymer as determined by the analysis of the infrared absorption spectrum. Furthermore, the orientation distribution function of trans-stilbene in a uniaxially strained PMMA film in a solid glass state was estimated by the investigation of infrared dichroic ratios.