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

• Published in: Journal of polymer science. Part B, Polymer physics Polymer physics, 2002-05, Vol.40 (10), p.1056-1069
• Main Authors: Bershtein, V. A., Egorova, L. M., Yakushev, P. N., Pissis, P., Sysel, P., Brozova, L.
• Format: Article
• Language: English
• Subjects: glass transition; molecular dynamics; nanocomposites; polyimides; thermal properties
• ISSN: 0887-6266; 1099-0488
• DOI: 10.1002/polb.10162

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 β‐relaxation region, occurred. However, a partial or total suppression of segmental motion could be observed above the β‐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 °C rise in long‐term thermal stability could be predicted for some hybrids with respect to pure PI. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1056–1069, 2002