Effect of mechanical stress on optical properties of polydimethylsiloxane

•We report astrong influence of deformation on absorption and refractive index of PDMS.•Changes of optical properties are due to a change of density of the material.•Experimentally obtained data prove validity of ‘generalized Poisson’s relation’. In this paper we present results of our investigation of the effect of mechanical stress on the coefficient of absorption and the refractive index of polydimethylsiloxane (PDMS) in the NIR region. We study optical transmittance and optical paths of PDMS samples compressed up to a length expressed by the value of the relative stress −0.5. The experimentally obtained results imply that the stress-induced changes of the absorption coefficient and the refractive index of PDMS are due to a change of the density of dimethyl-siloxane groups rather than a change of the PDMS’s molecular structure. Since we performed measurements for high stresses, we modified the Poisson’s relation to obtain its generalized form that holds for any stress and that we named generalized Poisson’s relation. To obtain this relation we assumed that an elementary increase of each of the two lateral dimensions of a sample is determined by the Poisson’s constant and the actual longitudinal dimension of the sample which is subject to change during the process of deformation. Then to express the change of the density of the electric dipoles in a PDMS sample we used this relation. The realized measurements indicate that the deformation dependent changes of the optical transmittances and the optical paths of the samples are in a good agreement with the values calculated using the assumption of the dominant influence of the density of the elementary dipoles and the validity of the generalized Poisson’s relation. The deformation affects the optical transmittance and the refractive index so strongly that it is necessary to take it into account when the PDMS is used as a medium for optical applications. The transmittance at the wavelength of 1700nm and for the relative stress −0.5 decreases about three times and the change of the refractive index at the wavelength of around 1500nm reaches a value of about 0.05. These values imply that it is possible to utilize the PDMS for constructing optical sensors that could be used for measuring stress or mechanical displacement.