On the response of infrared absorption spectra of polymers to large inelastic deformations

Abstract

It is well known that the refractive index of a material and its coefficient of absorption are coupled quantities. Within the theory of light propagation they appear as a single complex eigenvalue. Thus, birefringence and in particular induced birefringence as the physical base of stress optics, have their counterpart in the absorption spectrum. This phenomenon is called dichroism or induced dichroism. Dichroism is recording absorption spectra using light with two different planes of polarisation (generally, the orientation of the planes is perpendicular to each other) resulting in two different absorption spectra of the same specimen at the same amount of stretch. It can be shown that the dependence of absorption spectra on the orientation of the polarisation plane of the light comes out of two roots. A (small) detuning off the resonance frequency of the molecular oscillators of the material and a (dramatic) change of absorption, the latter being a consequence of change of orientation of the molecular polarisation vectors. Both, the detuning and the orientation effects, can be related to the macrodeformation of the specimen. Some possible applications of these effects in experimental mechanic are discussed.