Residual Stress Behavior in Spin-cast Films of Soluble Polyimides

Using a bilayer bending technique, intrinsic stress and overall residual stress were measured for three soluble polyimides : poly(2,2???-bis(3-phenyl)hexafluoropropylene diphthallimide), poly(2,2???-bis(4-phenyl) hexafluoropropylene diphthallimide), and poly(4,4???-methylene-3,3???-dimethyldiphenylene benzophenone-tetracarboximide). For these polyimides, the intrinsic stress was 29-31 MPa at room temperature at which the films were prepared, indicating that the intrinsic stress is not sensitive to the type of backbone chemistry among these polymers. The measured intrinsic stress is not small enough to be neglected as one usually does. However, the intrinsic stress relaxed out with temperature in the first heating run and its relaxation behavior with temperature was strongly dependent upon the film properties, including mechanical properties, polymer chain stiffness, molecular order, and glass transition temperature. In the subsequent cooling run, the intrinsic stress was never recovered. That is, the intrinsic stress varied irreversibly with temperature. In contrast, once the polymer films were thermally treated, their overall stress was predominated by the thermal stress component which varies reversibly with temperature. In addition, the glass transition temperatures of the polymer films were estimated from the overall stress-temperature profiles and compared with those measured by dynamic mechanical thermal analysis