Maki Itoh

Photo-probe study of siloxane polymers. I. Local free volume of an MQ-type silicone resin containing crosslinked nanoparticles probed by photoisomerization of azobenzene

Abstract We have investigated the local free volume of an MQ-type silicone resin consisting only of M (R 3 SiO 1/2 ) and Q (SiO 4/2 ) units by using photoisomerization of azobenzene molecularly dispersed in the matrices. Photoisomerization profiles in the uncured and cured MQ resin over a wide temperature range (4–300 K) are discussed based on three parameters: first-order plots, final cis fraction and quantum yields for trans-to-cis photoisomerization of azobenzene. For uncured MQ resins or prepolymers, for which the structure was proposed to be an assembly of crosslinked nanoparticles, a large fluctuation of local free volume was observed between room temperature and 20 K. At 4 K, a drastic decrease in the final cis fraction was observed in both the uncured prepolymer mixture and the cured MQ resin, implying the complete suppression of the free-volume fluctuation at 4 K. The formation of heterogeneous sites after curing is shown by the deviation from first-order kinetics for the cured MQ resins. Data for the cured MQ resin are analyzed quantitatively with the three-component approximation and the validity of a free-volume fluctuation model is verified. Comparison of the results for linear polysiloxanes with those for poly(methyl methacrylate) shows that siloxane chains are very flexible at low temperatures down to 20 K.

Characterization and Some Insights into the Reaction Chemistry of Polymethylsilsesquioxane or Methyl Silicone Resins

Structural characterization of a polymethylsilsesquioxane (PMSQ) and a DT-type methyl silicone resin (MeDT) has been carried out by various instrumental analyses including GPC, NMR, gas chromatography, and gas chromatography-mass spectrometry. Although the PMSQ had a Mw around 5000, the resin contained a significant amount of low molecular weight species consisting of T2 [MeSi(OH)O2/2] and T3 [MeSiO3/2] units, ranging from to including many isomers. One isomer of was isolated of which structure was determined as a cage structure. The species are supposed to consist mainly of cyclotetra- and cyclopentasiloxanes, but presence of strained rings such as cyclotrisiloxane rings also was suggested. In MeDT, species in which the T2 units in the molecules from PMSQ is replaced with D2 [Me2SiO2/2] were found, for example, , suggesting that general silicone resins consist of similar structures as silsesquioxanes. The Mark-Houwink exponent for these methyl resins was ~0.3, indicating the molecular shape to be compact. Investigation on the formation chemistry of the cubic octamers indicates that siloxane bond rearrangement is an important mechanism in the molecule build-up process.

Silsesquioxanes: Recent Advancement and Novel Applications

1Department of Chemistry, Biotechnology, and Chemical Engineering, Graduate School of Science and Engineering, Kagoshima University, Kagoshima, Kagoshima 890-0065, Japan 2Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, MA 01003, USA 3Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan 4Electronics Solutions S&T, Dow Corning, Ichihara, Chiba 299-0108, Japan 5Electronic Material Research Division, Osaka Municipal Technical Research Institute, Osaka, Osaka 536-8553, Japan 6Department of Chemistry and Materials Technology, Graduate School of Science and Technology, Kyoto Institute of Technology, Kyoto, Kyoto 606-8585, Japan

Photo-probe study of siloxane polymers: low-temperature structural relaxation in siloxane polymers probed by persistent spectral hole burning

Abstract We have investigated low-temperature structural relaxation in polysiloxane matrices by using persistent spectral hole burning (PSHB). The irreversible increase in hole width and the irreversible decrease in hole area were evaluated in terms of PSHB temperature-cycling experiments starting at 4 or 20 K. MQ-type silicone resins containing cross-linked nanoparticles (an uncured prepolymer mixture and cured MQ resins) and poly(methylphenylsiloxane) (PMPS) were used. The increase in hole width was observed at 15–20 K for all measured polysiloxanes. The increase is attributed to the local relaxation originating from the siloxane chains. The local structural relaxation of siloxane chains in this temperature range is confirmed for the first time. The local structural relaxation observed in the PSHB can be related to the rapid decrease in the final cis fraction between 4 and 20 K observed in the previous azobenzene photo-isomerization study. The curing effects of the MQ resin on the local relaxation are also discussed.