A. E IOFFE PHYSICAL-TECHNICAL INSTITUTE OF THE RUSSIAN ACADEMY OF SCIENCES, ST. PETERSBURG, 194021, RUSSIA DSC was used to study the influence of fast electron irradiation on the molecular mobility and melting of semicrystalline polymers and copolymers of ethylene and of fluorocopolymer. The heat eapacity-temporature dependences obtained in the range from 100 K to 500 K revealed the specific features of the irradiation effect on four relaxational transitions associated with the appearance of segmental motion in different molecular elements of the disordered regions in the polymer. The pronounced dependence of the radiation stability of crystallites on the thermal prehistory of the object under study was found. The results were interpreted on the basis of the decisive role of the molecular mobility and free volume for the predominance of the radiation cross-linking of the molecules. Correlations were established between the characteristics of the thermal transitions and the mechanical properties of the irradiated polymers.

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

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Molecular motion and thermal stability in two series of nanophase‐separated polyimide‐silica (PI‐SiO2) 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 SiO2 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 10−3‐109 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 β‐relaxation region, while anchoring of chain ends to ‘rigid walls’ caused, contrarily, a partial or total suppression of segmental motion above Tβ, 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° rise of predicted long‐term thermal stability for the hybrids as compared to that for PI.