Itaru Mita

Photochemistry in polymer solids: 12. Effects of main-chain structures and formation of hydrogen bonds on photoisomerization of azobenzene in various polymer films

Abstract The free-volume distribution in various polymers and the effect of hydrogen bonds on the free-volume distribution were examined by using a photoisomerization of azobenzene as a photosensitive probe. Poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) and phenoxy resin films containing molecularly dispersed azobenzene were irradiated over a wide temperature range (77–340 K). The final cis fraction for the trans to cis photoisomerization of azobenzene decreases with decreasing temperature in PMMA, and smaller fractions are produced than in polycarbonate at the same temperature. These differences are due to the fact that the local free-volume fluctuation of the methylene main chains of PMMA is smaller than that of the aromatic chains of polycarbonate. Since the side chains of PMMA compared to the main chains are larger than those of polycarbonate, the local free volume around the main chains is filled up by the side chains. Hence, the final cis fraction of azobenzene in PMMA is small. Furthermore, the final cis fraction in poly(vinyl alcohol) is smaller than that in PMMA. Since the hydrogen bonds among the polymer chains in poly(vinyl alcohol) restrict the local molecular motion at low temperatures, the azobenzene molecule is tightly seized by the matrix polymer chains and the free-volume fluctuation is restricted in poly(vinyl alcohol). The kinetic model considering the local fluctuation of the free volume was also applied to the results for PMMA, poly(vinyl alcohol) and phenoxy resin, and the parameters for the model in these polymers are discussed.

Synthesis and properties of silethynylene-siloxane alternating copolymers

Abstract A series of novel silethynylene-siloxane alternating copolymers with variation of siloxane contents and substituents on the silicon atoms has been prepared. Reactions of bis(hydroxydiphenylsilyl)-ethyne with bis( N -methylacetamido)diphenylsilane or bis( N -ethylacetamido)dimethylsilane in THF gave polymers with molecular weights in the range of 5800 to 23,000. The polymers for which all substituents on the silicon atoms are phenyl groups have T m s at 155–174° and 10% weight loss temperatures at 503–521°.

PHOTOCHEMICAL HOLE BURNING OF TETRAPHENYLPORPHIN IN AN AROMATIC POLYIMIDE

Abstract Photochemical hole burning (PHB) was performed in free-base tetraphenylporphin (TPP) doped into an aromatic polyimide (PI). The microscopic ordered structure of PI, low energy excitation modes, and thermal stability of a hole burnt at 4.2 K were studied with the PHB measurements. Low energy excitation modes reflected microscopic ordered structure. Thermal stability of a hole showed little dependence on the ordered structure. It could be interpreted that the structural relaxation leading to the instability of a hole is a larger scale relaxation than the low energy excitation modes. This interpretation is consistent with the fact that the thermal energy for annealing temperature (30–80 K) is larger than the low energy excitation modes (∼ 10 cm−1).