An investigation of the effects of curing conditions on the residual stress and dimensional stability in polyimide films
A number of aspects on the processing of PMDA-ODA polyimides have been investigated. These aspects included the effects of curing procedure on residual stress development in polyimide films, characterization of stresses arising in films during processing on tenter frames and the effects of stress history on irreversible shrinkage behavior.
Chemical curing using a gel technique was found to reduce the development of residual stresses in PMDA-ODA films during cooling. Chemically cured polyimide films made from a gelatin technique possessed consistently lower in-plane thermal expansion coefficients than thermally cured films. This was due to increased in-plane orientation development in the chemically cured films during drying.
An analysis of stresses arising during tenter frame processing of polymeric films indicates the presence of large in-plane shear stresses as a result of stress gradients along the machine direction. These shear stresses vary linearly across the width of the line and are responsible for creating orientational anisotropy and non-uniformity profiles across the width as well. The principal directions of stress tend to lie at $\pm 45\sp\circ$ to the machine direction but may be altered by machine and transverse drawing by the frame. Methods to eliminate or at least reduce in-plane anisotropy were discussed as well.
An investigation of strain storage behavior revealed a strong relationship between irreversible shrinkage and stress history in several different polymeric materials. Irreversible shrinkage in polyimide films was found to be largely due to the cooling of the materials under stress. Shrinkage behavior in PMDA-ODA and Upilex-R$\sp\circler$ films could be described in a quantitative manner using linear viscoelasticity.
Two forms of highly crystalline PMDA-ODA based polyimide were formed by a high pressure imidization process. Polyimide powder with a density of 1.46 $\pm$ 0.01 g/cm$\sp3$ was formed by curing a polyamic acid solution in dimethylacetamide at 200$\sp\circ$C and 1.38 MPa. Heat treatment of chemically cured gel films under the same conditions resulted in films with a density of 1.44 $\pm$ 0.01 g/cm$\sp3$.