Roger A. Assink

Sol-gel transition in simple silicates II☆

Abstract Silicate gels were prepared under a range of conditions in which the rate of hydrolysis was varied from fast to slow with respect to the rate of condensation. When hydrolysis was fast, larger, more highly condensed polymers were formed during gelation. Conversely, for slow hydrolysis, smaller, less highly condensed polymers were formed. These gels dried to low density coarse textured and high density fine textured gels, respectively. High temperatures, (>800°C) were required to densify the coarse gels by viscous sintering. Lower temperatures were sufficient to densify fine gels by a process which was postulated to consist of polymer relaxation followed by condensation and pore collapse.

Morphology and toughness characterization of epoxy resins modified with amine and carboxyl terminated rubbers

Abstract The morphology (dispersed phase composition, size distribution, and particle/matrix interface shapes) of epoxy resins modified with 5–15 parts by weight (pbw) of carboxyl (CTBN) and amine (ATBN) terminated butadiene acrylonitrile rubber have been characterized. The characterization techniques were transmission electron microscopy coupled with energy dispersive X-ray analysis, differential scanning calorimetry and proton and 13 C nuclear magnetic resonance. ATBN modified epoxies have a diffuse-appearing interface between the dispersed rubber phase and the epoxy matrix, in contrast to the sharp boundaries of CTBN particle interfaces. Interface mixing of epoxy and rubber, hypothesized initially to explain the interface diffuseness in ATBN modified epoxy, was not found in either CTBN or ATBN modified epoxy. The difference in interface appearance is attributed to ATBN particles having highly irregular shapes compared to the nearly spherical CTBN particles. Bimodal particle size distributions are observed with both modifiers. Both rubber modifiers also produce essentially identical toughness values which do not increase with rubber content in the range 5–15 pbw despite a commensurate increase in the population of large particles.

Sol-gel kinetics I. Functional group kinetics

Abstract 1 H and 29 Si NMR were used to measure the concentrations of the various functional groups (alkoxy, silanol and SiOSi bond) in an acid-catalyzed Si(OCH 3 ) 4 (TMOS):CH 3 OH:H 2 O sol-gel solution. There are three kinds of functional group reactions: hydrolysis, water producing condensation and alcohol producing condensation. The hydrolysis rate constant was found to be greater than 0.2 1/(mol min). The water-producing condensation and alcohol-producing condensation rate constants are 0.006 and 0.001 1/(mol min), respectively. The rate of hydrolysis is much faster than either rate of condensation. Both condensation pathways are significant for typical sol-gel reaction conditions.

Intercalation of molecular species into the interstitial sites of fullerene

Molecular species were found to diffuse readily into the octahedral interstitial sites of the fcc lattice of C{sub 60}. The {sup 13}C NMR spectrum of C{sub 60} under magic angle spinning (MAS) conditions consisted of a primary resonance at 143.7 ppm and a minor peak shifted 0.7 ppm downfield. The downfield shift obeys Curies law and is attributed to the Fermi-contact interaction between paramagnetic oxygen molecules and all 60 carbon atoms of rapidly rotating adjacent C{sub 60} molecules. Exposure of C{sub 60} to 1 kbar oxygen for 1.75 h at room temperature resulted in a spectrum of seven evenly spaced resonances corresponding to the filling of 0 to 6 of the adjacent octahedral interstitial sites with oxygen molecules. The distribution of site occupancies about a C{sub 60} molecule provided evidence that the intercalation process is controlled by diffusion kinetics. Exposure to 0.14 kbar hydrogen gas at room temperature for 16 h filled a substantial fraction of the interstitial sites of C{sub 60} and C{sub 70} with hydrogen molecules.

Sol-gel kinetics: II. Chemical speciation modeling☆

Abstract We present an exact theoretical kinetic formalism which specifically treats the temporal evolution of the various chemical functional groups about a specific silicon atom undergoing concurrent hydrolysis and condensation reactions. This formalism is applied to the chemical kinetics of a typical sol-gel reaction system. We also present a simplified statistical reaction model which reduces the number of chemically distinct rate coefficients necessary to describe the kinetics from 165 to 3. Experimental results which support this statistical model are also presented.

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.

Pore structure evolution in silica gel during aging/drying. II: Effect of pore fluids

A two-step acid-base-catalyzed silica gel has been aged in a series of alcohol and water baths and some chemical and physical structures of the gel were measured using several techniques (low-field NMR, 29Si and 13C MAS-NMR, IR, Raman, nitrogen adsorption) on gels in wet and dry states. When the gel was placed in alcohol, the surface area increased as a result of esterification and depolymerization to 1500–2000 m2/g in the wet state which then decreased to 1000 m2/g after drying. In water, hydrolysis and condensation decreased surface area to 1000 m2/g (wet) and 500 m2/g (dry). This process was reversible. Alcohol-aging led to small pore size distributions (in much shorter times than previous studies of aging in the gels mother liquor, ∼ 90% ethanol, 10% H2O). Surface areas calculated from 29Si MAS-NMR-derived Q distributions were in good agreement with adsorption-derived values for the samples dried from alcohol but water-dried samples appeared to contain some surface area which was inaccessible to adsorbing nitrogen after drying. These results indicate that the pore structure of silica gels may be dramatically altered during the aging process and that their wet gel features may be preserved (at least partially) upon drying.

Preparation of oxidatively stable cation-exchange membranes by the elimination of tertiary hydrogens

Abstract Model compounds and radiatively grafted membranes were prepared in order to study the effect of tertiary hydrogens on the stability of membranes used in oxidatively aggressive environments. Sulfonated isopropylbenzene, which contains a tertiary hydrogen, yielded substantial amounts of degradation products when exposed to a strong oxidizing solution. Sulfonated t-butylbenzene, which is identical to sulfonated isopropylbenzene except that the tertiary hydrogen is replaced by a methyl group, yielded only minute amounts of degradation products when exposed to the same oxidizing solution. Styrene (contains a tertiary hydrogen) and α-methylstyrene (tertiary hydrogen has been replaced by a methyl group) were radiatively grafted onto poly (tetrafluoroethylene) films and sulfonated to form cationic membranes. The styrene-prepared membrane exhibited several times more weight loss during exposure to an oxidizing aging solution than did the α-methylstyrene prepared membrane. The energy efficiencies of styrene-grafted membranes decreased rapidly to less than 60% after 60 or 80 cycles in a cycling test cell. The energy efficiencies of α-methylstyrene grafted membranes were 82.6% and 83.0% for 700 test cycles and showed no effects of chemical degradation during this period.

Sol-gel kinetics III. Test of the statistical reaction model☆

Abstract The distribution of functional groups, -OCH3, -OH, -OSi, about a silicon atom, undergoing concurrent hydrolysis and condensation in an acid catalyzed Si(OH3)4:CH3OH:H2O sol-gel, were measured by 29Si NMR. These NMR results were compared to predictions based on a statistical reaction model which assumes that all the hydrolysis and condensation rate constant depend only on the functional group reactivity and not on the local silicon environment. The hydrolysis and condesation reactions of monomeric species were successfully predicted by the model. The greater than predicted population of silicon species bonded to one other silicon via an SiOSi bond and the less than expected population of silicon species bonded to two other silicons via SiOSi bonds indicates that the condensation of partially condensed species is slower than the condensation of monomeric species.

Solution chemistry effects in Pb(Zr, Ti)O3 thin film processing

Ferroelectric thin films in the PZT compositional family were prepared by two different solution deposition methodologies. The approaches were based on the use of carboxylate and alkoxide precursors, and acetic acid. Solution aging and mixing order effects on thin film microstructure and ferroelectric properties were studied. Films prepared from 15 day old solutions had a lower remanent polarization (1.4 μC/cm2) and higher coercive field (256 kV/cm) than films prepared from 1 day old solutions (Pr = 18.7 μC/cm2; Ec = 55.2 kV/cm). Raman, FTIR, and NMR spectroscopies were employed to confirm the role of acetic acid in the process, and to begin to understand why changes in solution chemistry can so dramatically affect the resulting film microstructure and properties. FTIR spectroscopy indicated that in addition to acting as a solvent, acetic acid also serves as a chemical modifier in the two processes. That is, it reacts with the alkoxide precursors on a molecular level, replacing the alkoxy ligands. Ester f...