Measurements of equi-biaxial stress in adhered polyimide films by tilted beam polarized light microscopy

Abstract

The equi-biaxial tension, developed during the drying process of the adhered polyimide films on a rigid substrate, was determined by measuring the out-of-plane birefringence of the dried films. Using a polarized light microscope and a tilting sample stage, the out-of-plane birefringence of a linear polyimide film that was dried during a given thermal process was determined to be (7.0 ± 0.5) × 10 −3. The equibiaxial tension was calculated by using the stress-optical law to be around 60 MPa, in agreement with that by the commonly used bending beam method. Due to the additional spatial resolution of stress distribution and the higher accuracy for measuring soft polymer coatings, the birefringence method described here is considered to be a useful alternative to the bending beam technique, which measures only the macroscopically averaged stress and its accuracy is generally limited by the huge modulus ratio between the rigid substrate and the polymer.