Molecular dynamics in nanostructured polyimide-silica hybrid materials and their thermal stability

摘要

Molecular motion and thermal stability in two series of nanophase-separated polyimide-silica (PI-SiO_2) hybrid networks with chemically bound components were studied. The hybrids were prepared via a sol-gel process and differed in PI structure and chain length, and in SiO_2 content ranging from 10 to 50 wt.%. Differential scanning calorimetry, laser-interferometric creep rate spectroscopy, dielectric relaxation spectroscopy, thermally stimulated depolarization current techniques, and thermogravimetry were used covering, on the whole, the ranges of 100-900 K and 10~(-3)-10~9 Hz. Silica domains influenced PI dynamics in two opposite directions. Loosened segmental packing in chains confined to nanovolumes resulted mainly in rise of small-scale motion below #beta#-relaxation region, while anchoring of chain ends to 'rigid walls' caused, contrarily, a partial or total suppression of segmental motion above T_(#beta#), especially drastically at the temperatures close to and within glass transition. The latter resulted in a large change in thermal stability, e.g., 2.5-fold increasing of the apparent activation energy of thermooxidative degradation, and more than 100~0 rise of predicted long-term thermal stability for the hybrids as compared to that for PI.