Reflection anisotropy spectra of polydimethylsiloxane under a range of mechanically applied stress

Reflection anisotropy spectra of transparent elastomeric polydimethylsiloxane (PDMS) have been determined over the spectral range 1.5 to 5.0 eV for a range of tensile loads. Using a slight wedge shaped sample the spectra from the incident air/PDMS surface are shown to be weak and largely featureless at all loads. By contrast the spectra from the light transmitted through the sample and reflected at the other interface are large in magnitude and exhibit features which arise from the small stress birefringence of PDMS.The birefringent dominated spectra can readily be simulated using expressions derived from a Stokes–Mueller treatment of the optical system and this enabled the stress-optical coefficient to be determined at any energy within the spectral range covered. It is shown that there is a small angle between the optical axes and the mechanical axes in the sample and that this is consistent with the model of an elastomer which comprises a network of soft linear chains cross linked with stiff bridges.