Nanostructure and molecular dynamics in rodlike polyimide/flexible-chain polyimide molecular composites

A complete study of structure and molecular dynamics was performed for a series of rigid rodlike polyimide (PIR)/flexible-chain polyimide (PIF) molecular composites, with 0, 20, 40, 50, 60, 80, and 100 wt% of the rigid component. The PIR and PIF were synthesized from 3,3′4,4′-biphenyl-tetracarboxylic dianhydride and 1,4-phenylenediamine or 4,4′-oxydianiline, respectively. Small-angle x-ray scattering, differential scanning calorimetry, dynamic mechanical analysis, laser-interferometric creep rate spectroscopy, dielectric relaxation spectroscopy, and thermally stimulated depolarization currents techniques were used for overall characterization of nanostructure, glass transition, and sub-T g relaxations. The experiments were carried out, on the whole, at temperatures ranging from 100 to 660K and frequencies ranging from 10−2 to 106 Hz. All the experiments indicated pronounced deviations from additivity in both nanostructure and dynamics of the molecular composites. Mixing of the PIR and PIF components led, in particular, to a smaller nanostructure, down to formation of the nanoscale-homogeneous 20PIR/80PIF composite. Changes of the glass transition characteristics in two opposite directions and the arising of large dynamic heterogeneity around T g were observed. The results were readily treated in terms of nanostructural changes, loosening of molecular packing due to confinement of PIF chains between PIR chains as the “rigid walls,” and the constraining influence of the latter on segmental motion in PIF chains adjoining the “rigid walls.”