X‐ray studies on the deformation of an aromatic polyimide

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Abstract
The deformation of the aromatic polyimide poly{N,N′‐bis(phenoxyphenyl)‐pyromellitimide} (PMDA‐ODA) has been investigated by x‐ray scattering. PMDA‐ODA, which exhibits a smecticlike ordering in the bulk can be elongated up to 70% at room temperature without a distinct yield point. Under strain, the diffraction pattern of the intramolecular spacing occurring at a Bragg spacing of 15.4 Å is elliptical. The ellipticity shows that the molecule changes its repeat distance along the chain axis. This may result from bond angle and bond length distortions. Consistent with this is a rapid recovery well below the glass transition temperature where all but 20% strain is recovered. This localized extension, however, cannot account fully for the macroscopic deformation applied, and, as such, a global orientation of the chains occurs, as evidenced by the arcing of the diffraction profiles. The retention of the intrachain reflection indicates that bundles of chain segments ordered in a smectic fashion orient as a unit maintaining a lateral chain‐segment alignment. Finally, the scattering vector at which a small‐angle scattering reflection occurs was found to be correlated to the coherence length determined by a Scherrer analysis of the ca. 15.4 Å intramolecular reflection. As the strain increased, the intensity and spacing of the small‐angle scattering reflection increased parallel to the stretching direction and decreased perpendicular to this. This suggests that the origin of the small‐angle scattering reflection arises from electron density fluctuations along the chain axis, as is typically seen in semicrystalline polymers.

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