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Polymer gels have potential use for a wide variety of applications, primarily due to the ability to tailor the gel properties by varying several material parameters. While substantial attention has focused on water-based hydrogels, the use of these materials is limited due to a narrow operational temperature range. This report describes a nonaqueous polymer gel, composed of a cross-linked polybutadiene network swollen with low volatility polymer plasticizers. Thermal, mechanical, and adhesive characterization illustrated that the gels exhibit performance over an extremely broad temperature range (−60–70 °C). Solvent quality and loading played a critical role in the operational temperature window with small solvent solubility parameter deviations dramatically reducing the operational temperature range. In addition, the processing conditions had a large impact on the gel mechanical properties. As a result, it is important to consider the influence of processing conditions and solvent quality when tailoring polymer gels for practical applications.
The introduction of organic substituents into sol-gel materials can often result in networks that collapse during drying to afford non-porous xerogels. This can prove useful if non-porous coatings or membranes are the ultimate objectives. Collapse of porosity is also manifested in bridged polysilsesquioxanes with flexible bridging groups. Alkylene-bridged polysilsesquioxanes are hybrid xerogels whose organic bridging group is an integral constituent of the network polymer that can be systematically varied to probe the influence of its length on the xerogels' porosity and morphology. Our previous studies have shown that hexylene-bridged polysilsesquioxane xerogels prepared from 1, 6-bis(triethoxysilyl)hexane under acidic conditions are nonporous while the pentylene-bridged polysilsesquioxanes prepared under the same conditions are porous. We also discovered that the more reactive 1, 6-bis(trimethoxysilyl)hexane monomer could polymerize under acidic conditions to afford porous xerogels. Here, we have extended our study of bis(trimethoxysilyl)alkanes to include the heptylene (C7), octylene (C8), nonylene(C9) and decylene (C10) bridges so as to ascertain at what bridging group length the porosity collapses. The morphology of the resulting xerogels was characterized by nitrogen sorption porosimetry and electron microscopy. Solid state NMR was used to structurally characterize the materials.
Aging of silica gels before drying is known to result in significant changes in xerogel morphology, porosity and properties. In this study, the influence of aging gels on the porosity and morphology of alkylene-bridged polysilsesquioxane xerogels was examined. Gels of hexylene-, heptylene, octylene, nonylene, and decylene-bridged polysilsesquioxanes were prepared by the sol-gel polymerization of the respective bis(trimethoxysilyl)alkane monomers under acidic or basic conditions in methanol and in tetrahydrofuran. The gels were aged 3, 7, 14, 28, 35, 42, 49, and 56 days before drying to afford xerogels. The xerogels were characterized by nitrogen sorption porosimetry. Xerogels prepared in THF were non-porous. Those prepared and aged under basic conditions in methanol or tetrahydrofuran exhibited coarsening of porosity with aging time. With the exception of the hexylene-bridged gels, those prepared and aged in acidic methanol showed little change with aging. The surface area of the hexylene-bridged xerogels nearly tripled with aging times of up to several weeks, then decreased, for the gels aged for more than two weeks, to around 100 meters squared per gram.
Hydrolysis and condensation of trialkoxysilanes, HSi(OMe)3 and HSi(OEt)3, has been used to prepare polyhydridosilsesquioxaes for dielectric applications. In this study we examined the ability of trimethoxysilane (TMS) and triethoxysilane (TES) to undergo sol-gel polymerization to afford gels. Sol-gel polymerization experiments were conducted under acidic (HCl), basic (NaOH), and neutral conditions in methanol or ethanol. Gels prepared with basic catalysts were exothermic with the evolution of hydrogen gas. Gel times are compared with silica gels prepared from tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). Gels were worked up under aqueous conditions to afford xerogels. Surface area analyses by nitrogen sorption porosimetery revealed that the materials were mostly mesoporous materials with surface areas in the hundreds of square meters per gram. Solid state 29Si CP MAS NMR was used to determine the amount of hydrido group remaining in the xerogels. Gels prepared under acidic conditions were essentially polysilsesquioxanes with very little loss of hydride functionalities. In gels prepared under basic conditions the hydride groups were completely gone leaving silica gels. Gels prepared with neutral water lost approximately 66% of the hydride groups.
Polymerization of organotrialkoxysilanes is a convenient method for introducing organic functionality into hybrid organic-inorganic materials. However, not much is known about the effects of the organic substituent on the porosity of the resulting xerogels. In this study, we prepared a series of polysilsesquioxane xerogels from organotrialkoxysilanes, RSi(OR′)3, with different organic groups (R = H, Me, Et, dodecyl, hexadecyl, octadecyl, vinyl, chloromethyl, cyanoethyl). Polymerizations of the monomers were carried out under a variety of conditions, varying monomer concentration, type of catalyst, and alkoxide substituent. The effect of the organic substituent on the sol-gel process was often dramatic. In many cases, gels were formed only at very high monomer concentration and/or with only one type of catalyst. All of the gels were processed as xerogels and characterized by scanning electron microscopy and nitrogen sorption porosimetry to evaluate their pore structure.
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