Gregory M. Jamison

Alkylene-bridged polysilsesquioxane aerogels: highly porous hybrid organic-inorganic materials

Abstract Alkylene-bridged polysilsesquioxane gels were prepared by sol-gel polymerizations of α, ω-bis(triethoxysilyl)alkanes 1–5. The gels were extracted with supercritical carbon dioxide to afford a novel class of hybrid organic-inorganic aerogels. The effect of the length of the alkylene bridging group and catalyst (HCl and NaOH) on the structure was examined. The molecular structure was characterized by solid-state 13C and 29Si cross polarization magic angle spinning nuclear magnetic resonance spectroscopy. The alkylene bridging groups survived sol-gel polymerization to give materials with average degrees of condensation of 79 and 90% for the acid- and base-catalyzed aerogels, respectively. Scanning electron microscopy was used to examine the macroscopic structure of the gels and nitrogen sorption porosimetry was used to measure their surface areas and pore structures. Most of the alkylene-bridged aerogels were mesoporous, high-surface-area materials. As with alkylene-bridged polysilsesquioxane xerogels, the surface area decreased with increasing alkylene bridging group length. Only the base-catalyzed tetradecylene-bridged aerogel was found to be non-porous.

Photobleaching Comparison of Poly(methylphenylsilylene) and Poly(phenylsilyne)

The effect of dimensionality on the photobleaching characteristics of Si-based polymers with phenyl substituents was investigated by studying the change in absorption upon exposure to 248- and 337‐nm excitation. Poly(methylphenylsilylene) was chosen as an archetype of the linear-chain polysilanes, and poly(phenylsilyne) was used as the comparable network polysilyne. Differences in bleaching behavior of the two types of polymers are explained in terms of Si–Si bonding and electronic interactions between the phenyl groups and the silicon frameworks. Wavelength-dependent effects include the degree of photodegradation of the phenyl rings and changes in polymer configuration due to photoscission and crosslinking. Absorption changes were accompanied by changes in the refractive index of the exposed regions of the samples. The refractive index modification was used to write embedded strip waveguides in the photosensitive thin films.

Vacuum-ultraviolet spectroscopy measurement of poly(methylphenylsilylene) photosensitivity

Photoinduced optical absorption changes in the vacuum-ultraviolet (VUV) spectral range have been measured in poly(methylphenylsilylene) thin films under varied excitation photon energies and local atmospheric environments. Spectral changes in resonances associated with both the linear chain Si–Si backbone and the side groups of the hybrid structure are consistent with the photodisruption of backbone topology. These effects are more pronounced under a higher energy photon exposure (5.10eV) resonant with the fundamental π‐π* transition of the phenyl moiety. An aerobic environment also favors more dramatic bleaching of VUV absorption in these materials. Finally, the present study enables a Kramers-Kronig analysis of absorption change from the visible to the VUV. These results do not adequately describe the photoinduced refractive index changes measured at 632.8nm via ellipsometry, indicating the presence of other contributions to the index modifications observed.

Polysilane-based thin films with high photosensitivity

The present work investigates the intrinsic photosensitivity of a family of poly(alkyl)(aryl)silanes and poly(hydridophenyl)silane for use in the development of photoimprinted waveguide devices. Limited testing of passive optical behavior (e.g. absorption, refractive index) and photosensitive response was performed for these materials in thin film form. It was determined that the materials exhibited dramatic photobleaching under 248 nm (KrF excimer laser) exposure. Based on a Kramers-Kronig analysis of the absorption changes, refractive index changes on the order of - 0.1 are estimated. Confirmation of this calculation has been provided via ellipsometry which estimates refractive index changes at 632 nm of -0.14 ± 0.01. In addition, embedded strips have been photoimprinted into the material to confirm waveguiding capacity of the films. Possible sources of photosensitivity in this material and its potential for application in various device configurations will be discussed.

Photosensitive polysilane thin films for write-as-needed optical devices

The use of photosensitive materials for the development of integrated, refractive-index structures supporting telecom, remote sensing, and varied optical beam manipulation applications is well established. Our investigations of photosensitive phenomena in polysilanes, however, have been motivated by the desire to configure, or program, the photonic device function immediately prior to use. Such an operational mode imposes requirements on wavelength sensitivity, incident fluence and environmental conditions that are not typical of more conventional applications of photosensitive material. The present paper focuses on our efforts to understand and manipulate photosensitivity in polysilane thin films under different excitation wavelengths, local atmospheric compositions and thermal history in this context. We find that the photoresponse can be influenced through the control of such optical exposure conditions, thereby influencing the magnitude of the photoinduced refractive-index change attained.

Photo-control of nanointeractions.

The manipulation of physical interactions between structural moieties on the molecular scale is a fundamental hurdle in the realization and operation of nanostructured materials and high surface area microsystem architectures. These include such nano-interaction-based phenomena as self-assembly, fluid flow, and interfacial tribology. The proposed research utilizes photosensitive molecular structures to tune such interactions reversibly. This new material strategy provides optical actuation of nano-interactions impacting behavior on both the nano- and macroscales and with potential to impact directed nanostructure formation, microfluidic rheology, and tribological control.

LDRD final report on intelligent polymers for nanodevice performance control

A variety of organic and hybrid organic-inorganic polymer systems were prepared and evaluated for their bulk response to optical, thermal and chemical environmental changes. These included modeling studies of polyene-bridged metal porphyrin systems, metal-mediated oligomerization of phosphaalkynes as heteroatomic analogues to polyacetylene monomers, investigations of chemically amplified degradation of acid- and base-sensitive polymers and thermally responsive thermoplastic thermosets based on Diels-Alder cycloaddition chemistry. The latter class of materials was utilized to initiate work to develop a new technique for rapidly building a library of systems with varying depolymerization temperatures.

Hydrocarbon-bridged polysiloxane and polysilsesquioxane network materials

Hexylene and phenylene-bridged polymethylsiloxane xerogels X-2 and X-4, respectively, were prepared by the sol-gel hydrolysis and condensation of 1, 6-bis(diethoxymethylsilyl)hexane 1 and 1, 4-bis(diethoxymethylsilyl)benzene 2 under acidic and basic conditions. These polymerizations afforded network polymers in the form of wet gels within several hours. The gels were processed to afford xerogels whose characteristics (determined by solid state {sup 13}C and {sup 29}Si CP MAS NMR spectroscopy and nitrogen sorption porosimetry) were compared and contrasted with those of their analogous polysilsesquioxanes. {sup 29}Si CP MAS NMR indicates a high degree of hydrolysis and polycondensation; porosimetry measurements reveal that the materials have significant surface areas, save for the acid-catalyzed hexylene gels X-2.