Fumito Nishida

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.

Polarization anisotropies in the electric-field-dependent diffraction efficiency in Azo dye doped photoconducting electro-optic polymer

The steady-state and transient behavior of the photoconducting electro-optic polymer, PVK:TNF:DR1:ECz, was analyzed with various writing and reading beam polarization configurations to elucidate the contribution of various effects including the photoisomerization process on the diffraction phenomenon at the wavelength of 632.8 nm. The effect of the applied field on the diffraction efficiencies was observed to be strongly dependent on the polarization states of the writing and reading beams. It was found that the electro-optic contribution dominated the diffraction phenomenon for the p-polarized writing and reading beams while the molecular reorientation effect dominated for the s-polarized writing and reading beams and when the writing beams are cross polarized. The diffraction grating formed via the photoisomerization process was confirmed to be refractive index grating instead of an absorption grating.

Silicone polymers for optical films and devices

Silicones are among the most suitable materials for optical telecommunication devices due to their tolerance to high optical flux and their thermo-mechanical and environmental stability; they also have excellent processability. This work focuses on utilizing silicon-based branched resins and linear polymers for optical waveguides and switches where both refractive index and thermo-optic coefficient need to be controlled to the requirements of specific applications. Materials were synthesized with high optical transmission bands between 1.3 and 1.6 μm by varying the amount of aliphatic and aromatic C-H in the material. At the same time, the ratio of methyl to phenyl groups also controls the refractive index in the range of nD = 1.4 ... 1.6 precisely enough that both core and cladding components (Δn < 0.5%) can be obtained. Films of 5 to 20 μm thickness prepared on silicon substrates by spin-coating from solution were evaluated by measuring refractive index, thermo-optic coefficient, optical loss, and film uniformity both before and after exposure to high temperature and humidity. These films can be patterned through a number of techniques to form the required features. The resinous materials show very low birefringence and excellent resistance to heat and moisture.

FASTER GRATING BUILDUP CHARACTERISTICS IN LOW SILANOL-CONTAINING POLYSILANE-BASED PHOTOREFRACTIVE POLYMER

The effect of silanol content on the grating buildup dynamics was examined by preparing two different lots of polymethylphenylsilane (PMPS). While the high silanol-containing PMPS showed very sluggish grating formation time, the lower silanol-containing PMPS exhibited very rapid grating formation in a diffraction efficiency measurement. The diffraction efficiency, on the other hand, was found to be unaffected. The results suggest that the silanol is acting as hole trap and a reduction of its content is a key to improving the response characteristics of polysilane-based photoconducting electro-optic polymer composite for photorefractive applications.