Effect of processing and thickness on the structure in a polyimide film of poly(pyromellitic dianhydride‐oxydianiline)

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Abstract
It is well known that poly(pyromellitic dianhydride‐oxydianiline) (PMDA‐ODA), orders at curing temperature well below its Tg with chains oriented parallel to the film plane. Structure analysis using X‐ray scattering reveals two structural features: (a) The ordered structure range from biaxial‐nematic‐like to crystalline; (b) the symmetry of orientation of the ordered regions change from a fiber, to planar to isotropic as a function of depth. Both of these structural features are attributed to surface effects and solvent retention during curing. The solvent profile is estimated during the imidization process using a theoretical model. The model considers the change in temperature, imidization rate, diffusivity, and film thickness with time during the curing. The model indicates that solvent content after most of the imidization is complete is crucial in determining the order and texture in the film. At the air/film interface, a thin skin formation due to solvent depletion and surface ordering is suggested from the model. Depending on the process condition the overall structure of the film can vary from crystalline to mesomorphic. The biaxial nematic and smectic like structure are probably not equilibrium structures but are frustrated order due to limited solvent‐induced mobility. The process condition affect the quality and quantity of order, and the texture due to chain's anisotropic cross‐section is affected primarily by film thickness. By replacing a single component precursor with a blend of precursors of the same polyimide, both higher isotropy and order can be obtained in thin PMDA‐ODA films. The order and extended chain conformation is better than that observed in the top 10 nm nearsurface‐structure.