Nitrogen Sorption Porosimetry Research Articles

Porosity in Polysilsesquioxane Xerogels

ABSTRACTPolymerization 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.

Synthesis of poly(vinyl alcohol) / silica gel polymer hybrids by in-situ hydrolysis method

Homogeneous poly(vinyl alcohol) (PVA)–silica gel polymer hybrids were prepared by in-situ hydrolysis of poly(vinyl acetate) (PVAc) in a sol–gel reaction mixture with tetramethoxysilane (TMOS). The degree of hydrolysis was evaluated by FTIR and 13C CP/MAS NMR; it increased with an increase in the acid catalyst and reached 85% with 1.6 ml of 0.1 M HCl. The homogeneity of the polymer hybrids obtained was maintained when the reaction was performed at 60 °C. However, the polymer hybrid became turbid with an increase of the amount of catalyst present when the reaction was conducted at room temperature. The homogeneity of the polymer hybrids obtained was evaluated by nitrogen sorption porosimetry of a porous silica that was obtained by charring the PVA hybrid. The results confirmed the molecular-level dispersion of the PVA in the hybrid. © 1998 John Wiley & Sons, Ltd.

Bridged Bisimide Polysilsesquioxane Xerogels: New Hybrid Organic−Inorganic Materials

New hybrid organic−inorganic materials which incorporate the bisimide functionality were synthesized by sol−gel condensation of bisimide monomers containing pendant triethoxysilyl groups. Monomers were prepared by transimidation of bis (2-aminopyridyl)imides with (3-aminopropyl)triethoxysilane. This synthesis was applicable to a variety of commercially available bisanhydrides and did not result in the evolution of water which causes premature hydrolysis and condensation of the trialkoxysilyl group. Solid-state NMR (29Si, 13C, and 15N) was utilized to characterize the polysilsesquioxane bis-imides. Nitrogen sorption porosimetry was used to determine the surface area. The thermal and mechanical characteristics of the resultant materials were measured and compared to linear polyimides.

Synthesis and characterization of bis(trialkoxysilymethyl)arenes from related bis(trichlorosilylmethyl)arenes. Comparisons between some organosilicate xerogel materials derived from both

A series of bis(trialkoxysilylmethyl)aryl compounds1–17 {[(RO) 3 SiCH 2 ] 2 Ar, R=Me, Et, Pr , Bu, Ar=1,4-C 6 H 4 ; R=Et, Pr, Bu, Ar=1,4-C 6 H 2 Me 2 -2,6; R=Et, Pr, Bu, Ar=1,4-C 6 Me 4 -2,3,5,6 ; R=Et, Pr, Bu, Ar=1,3-C 6 H 4 ; R=Bu, Ar=1,2-C 6 H 4 ; R=Et, Pr, Bu, Ar=9,10-C 14 H 8 } prepared from the related bis(trichlorosilylmethyl)aryl compounds and characterised by 1 H, 13 C and 29 Si NMR and IR spectroscopy, and high resolution mass spectrometry. The X-ray crystal structures of two of the bis(trichlorosilylmethyl)aryl precursor compounds are reported. Organosilicate xerogels prepared from the precursors 2,7 and 12 are discussed. These have been studied by a combination of solid state NMR spectroscopy, scanning electron microscopy (one case) and nitrogen sorption porosimetry and their properties compared with those of xerogels derived from related bis(trichlorosilylmethyl)arenes.

Effects of Poly(Vinyl Alcohol) Content and Calcination Temperature on the Characteristics of Unsupported Alumina Membrane

Abstract Nonsupported alumina porous membranes without pinholes or cracks were prepared by the sol-gel process using aluminum sec-butoxide as the starting material. The effects of using different PVA contents at various calcination temperatures on the characteristics of the membrane were investigated by scanning electron microscopy and nitrogen sorption porosimetry. The results after calcining at 450°C for 5 hours showed that the range of pore size distributions increases with increasing PVA concentration. The active nucleus numbers of phase transition to α-alumina decreased as the content of PVA increased at about 1050°C. The morphology of unsupported alumina membranes was affected by the PVA concentration and calcination temperature.

Controlling Porosity in Bridged Polysilsesquioxanes through Elimination Reactions

The retro Diels-Alder reaction was used to modify porosity in hydrocarbon-bridged polysilsesquioxane gels. Microporous polysilsesquioxanes incorporating a thermally labile Diels-Alder adduct as the hydrocarbon bridging group were prepared by sol-gel polymerization of trans-2,3-bis(triethoxysilyl)norbornene. Upon heating the 2,3-norbornenylene-bridges polymers at temperatures above 250 C, the norbornenylene-bridging group underwent a retro Diels-Alder reaction losing cyclopentadiene and leaving behind a ethenylene-bridged polysilsesquioxane. Less than theoretical quantities of cyclopentadiene were volatilized indicating that some of the diene was either reacting with the silanol and olefinic rich material or undergoing oligomerization. Both scanning electron microscopy and nitrogen sorption porosimetry revealed net coarsening of pores (and reduction of surface area) in the materials with thermolysis.

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

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.

Welded tuff porosity characterization using mercury intrusion, nitrogen and ethylene glycol monoethyl ether sorption and epifluorescence microscopy

Porosity of welded tuff from Snowshoe Mountain, Colorado, was characterized by mercury intrusion porosimetry (MIP), nitrogen sorption porosimetry, ethylene glycol monoethyl ether (EGME) gas phase sorption and epifluorescence optical microscopy. Crushed tuff of two particle-size fractions (1-0.3 mm and less than 0.212 mm), sawed sections of whole rock and crushed tuff that had been reacted with 0.1 N hydrochloric acid were examined. Average MIP pore diameter values were in the range of 0.01–0.02μm. Intrusion volume was greatest for tuff reacted with 0.1 N hydrochloric acid and least for sawed tuff. Cut rock had the smallest porosity (4.72%) and crushed tuff reacted in hydrochloric acid had the largest porosity (6.56%). Mean pore diameters from nitrogen sorption measurements were 0.0075–0.0187 μm. Nitrogen adsorption pore volumes (from 0.005 to 0.013 cm 3/g) and porosity values (from 1.34 to 3.21%) were less than the corresponding values obtained by MIP. More than half of the total tuff pore volume was associated with pore diameters < 0.05μm. Vapor sorption of EGME demonstrated that tuff pores contain a clay-like material. Epifluorescence microscopy indicated that connected porosity is heterogeneously distributed within the tuff matix; mineral grains had little porosity. Tuff porosity may have important consequences for contaminant disposal in this host rock.