An instrument for simultaneous kinetic measurements of microscopic infrared dichroism and stress of inhomogeneous polymer thin films at constant elongation rate

A new instrument has been built for a study on deformation mechanisms of inhomogeneous polymer thin films from simultaneous in situ kinetic measurements of microscopic infrared dichroism and stress at constant elongation rate. During elongation, the center position of the film is slightly adjusted manually by direct observation of the sample using the microscope so as to measure IR interferogram from the same rectangular area with a side from 20 to 600 μm. The average orientation angle θ(t) of the main chain to the draw direction is estimated from the function A(t) = (A_∥ - A_⊥)/(A_∥ + A_⊥) using a photoelastic modulator. Here A_∥ and A_⊥ are the absorbances measured with radiation polarized parallel and perpendicular to the draw direction, respectively. True stress is calculated from nominal stress by correcting changes in film thickness as well as width accompanied by large deformation. Capability and limit of the instrument are examined in detail using two polypropylene films with different morphologies, one consisting of crystallites showing no gross morphology such as spherluites and another including only a huge spherulite embedded in the matrix.