Optical and mechanical anisotropies of oriented poly(ethylene terephthalate) films

Reflectance difference spectroscopy (RDS) is used to characterize the optical anisotropy of thin, biaxially oriented poly(ethylene terephthalate) films. Molecular orientation in these films results in birefringence and thus gives rise to a RDS signal, which changes characteristically with external strain. A clear link between the optical and mechanical anisotropies can be established by comparing the azimuthal dependence of the elastic modulus with the RDS signal. Online measurements during tensile tests provide information on the evolution of the optical properties as a function of external strain, thus making RDS a highly sensitive probe for strain in oriented polymers.