Molecular dynamics in nanostructured polyimide-silica hybrid materials andtheir thermal stability

Molecular motion and thermal stability in two series of nanophase-separated polyimide-silica (PI-SiO2) hybrid networks with chemically bound component s were studied. The hybrids were prepared via a sol-gel process and differe d in PI structure and chain length, and in SiO2 content ranging from 10 to 50 wt.%. Differential scanning calorimetry, laser-interferometric creep rat e spectroscopy, dielectric relaxation spectroscopy, thermally stimulated de polarization current techniques, anp thermogravimetry were used covering, o n the whole, the ranges of 100-900 K and 10(-3)-10(9) Hz. Silica domains in fluenced PI dynamics in two opposite directions. Loosened segmental packing in chains confined to nanovolumes resulted mainly in rise of small-scale m otion below beta-relaxation region, while anchoring of chain ends to 'rigid walls' caused, contrarily, a partial or total suppression of segmental mot ion above T-beta, especially drastically at the temperatures close to and w ithin glass transition. The latter resulted in a large change in thermal st ability, e.g., 2.5-fold increasing of the apparent activation energy of the rmooxidative degradation, and more than 100(0) rise of predicted long-term thermal stability for the hybrids as compared to that for PI.