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

Molecular motion and thermal stability in two series of nanophase-separated polyimide-silica (PI-SiO2) hybrid materials with chemically bound components were studied. The hybrids were synthesized from p-aminophenyltrimethoxysilane-terminated poly(amic acid)s as PI precursors and tetramethoxysilane as a silica precursor via a sol-gel process. The hybrids differed in their PI chemical structure and chain length (number-average molecular weight = 5.000, 7.500, or 10.000) and in their SiO2 content, which ranged from 0 to 50 wt %. Differential scanning calorimetry, laser-interferometric creep rate spectroscopy, and thermally stimulated depolarization current techniques were used for studying the dynamics from 100 to 650 K and from 10(-3) to 10(-2) Hz. Comparative thermogravimetric measurements were also carried out from 300 to 900 K. Silica nano- or submicrodomains that formed affected PI dynamics in two opposite directions. Because of the loosening of the molecular packing of PI chains confined to nanometer-scale spaces between silica constraints, an enhancement of small-scale motion, mostly at temperatures below the beta-relaxation region, occurred. However, a partial or total suppression of segmental motion could be observed above the beta-relaxation temperature, drastically so for the shortest PI chains at elevated silica contents and within or close to the glass-transition range, because of the covalent anchoring of chain ends to silica domains. Large changes in thermal stability, including a 2.5-fold increase in the apparent activation energy of degradation, were observed in the hybrids studied. A greater than 100 degreesC rise in long-term thermal stability could be predicted for some hybrids with respect to pure PI. (C) 2002 Wiley Periodicals, Inc.