Mary Ann B. Meador

Tailoring Mechanical Properties of Aerogels for Aerospace Applications

Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liquid in silica gels under supercritical conditions. Several unique attributes such as extremely low thermal conductivity and low density make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions. However, native silica aerogels are fragile at relatively low stresses. More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers. This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels. Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy.

Iptycenes - Extended triptycenes

Abstract Triptycene is the first member of a large series of compounds for which we have coined the general term “iptycenes”. The prefix (tri, pent, etc.) indicates the number of separated arene planes. By fusing from one to six 9,10-anthradiyl moieties on the triptycene framework, one can derive a first generation of iptycenes (Table 1). Of these, only 3,4,8 and a substituted 2 are known; the remainder provide a synthetic challenge. Potentially interesting practical and theoretical properties of iptycenes and particular structural features of several (i.e. 15,16 and 24) are briefly discussed, as are certain extensions beyond the compounds in Table 1. Methods for preparing useful synthons 35–41 are described. Three new, much improved syntheses of triptycene 29, itself a useful iptycene synthon, are presented. In addition, improved syntheses of pentiptycenes 3 and 33 are described, as well as the first syntheses of pentiptycenes 32,34 and 52 and heptiptycene 54. The way is paved for future development of this mini-domain of unnatural products.

Tailoring Elastic Properties of Silica Aerogels Cross-Linked with Polystyrene

The effect of incorporating an organic linking group, 1,6-bis(trimethoxysilyl)hexane (BTMSH), into the underlying silica structure of a styrene cross-linked silica aerogel is examined. Vinyltrimethoxysilane (VTMS) is used to provide a reactive site on the silica backbone for styrene polymerization. Replacement of up to 88 mol % of the silicon from tetramethoxyorthosilicate with silicon derived from BTMSH and VTMS during the making of silica gels improves the elastic behavior in some formulations of the cross-linked aerogels, as evidenced by measurement of the recovered length after compression of samples to 25% strain. This is especially true for some higher density formulations, which recover nearly 100% of their length after compression to 25% strain twice. The compressive modulus of the more elastic monoliths ranged from 0.2 to 3 MPa. Although some of these monoliths had greatly reduced surface areas, changing the solvent used to produce the gels from methanol to ethanol increased the surface area in one instance from 6 to 220 m(2)/g with little affect on the modulus, elastic recovery, porosity, or density.

Flexible Nanofiber-Reinforced Aerogel (Xerogel) Synthesis, Manufacture, and Characterization

Silica aerogels are sol-gel-derived materials consisting of interconnected nanoparticle building blocks that form an open and highly porous three-dimensional silica network. Flexible aerogel films could have wide applications in various thermal insulation systems. However, aerogel thin films produced with a pure sol-gel process have inherent disadvantages, such as high fragility and moisture sensitivity, that hinder wider applications of these materials. We have developed synthesis and manufacturing methods to incorporate electrospun polyurethane nanofibers into the cast sol film prior to gelation of the silica-based gel in order to reinforce the structure and overcome disadvantages such as high fragility and poor mechanical strength. In this method, a two-stage sol-gel process was employed: (1) acid-catalyzed tetraethyl orthosilicate hydrolysis and (2) base-catalyzed gelation. By precisely controlling the sol gelation kinetics with the amount of base present in the formulation, nanofibers were electrospun into the sol before the onset of the gelation process and uniformly embedded in the silica network. Nanofiber reinforcement did not alter the thermal conductivity and rendered the final composite film bendable and flexible.

Hydrophobic monolithic aerogels by nanocasting polystyrene on amine-modified silica

We describe a three-dimensional core–shell structure where the core is the assembly of nanoparticles that comprises the skeletal framework of a typical silica aerogel, and the shell is polystyrene. Specifically, the mesoporous surfaces of silica were first modified with amines by co-gelation of tetramethylorthosilicate (TMOS) and 3-aminopropyltriethoxysilane (APTES). Next, styrene moieties were attached to the amines by reaction with p-chloromethylstyrene. Finally, dangling styrene moieties were crosslinked by a free-radical polymerization process initiated by AIBN and styrene, p-chloromethylstyrene or 2,3,4,5-pentafluorostyerene introduced in the mesopores. Polystyrene crosslinked aerogels are mechanically strong, lightweight (0.41–0.77 g cm−3), highly porous materials (they consist of ca. 63% empty space, with a BET surface areas in the range of 213–393 m2 g−1). Their thermal conductivity (0.041 W m−1 K−1) is comparable to that of glass wool. Hydrophobicity, however, is the property that sets the new material apart from analogous polyurea and epoxy crosslinked aerogels. The contact angles of water droplets on disks cut from larger monoliths are >120°. (By comparison, the contact angle with polyurea crosslinked aerogels is only ca. 60°.) Polystyrene crosslinked aerogel monoliths float on water indefinitely, while their polyurea counterparts absorb water and sink within minutes.

Structure−Property Relationships in Porous 3D Nanostructures: Epoxy-Cross-Linked Silica Aerogels Produced Using Ethanol as the Solvent

Cross-linking silica aerogels with organic groups has been shown to improve the strength over un-cross-linked aerogels by as much as 2 orders of magnitude. Previous cross-linking chemistry has been developed using solvents specifically chosen to dissolve the monomers and accommodate the reaction temperature. Because the process of making the aerogels requires so much solvent, it is of interest to consider less toxic solvents such as ethanol to increase safety and enhance scale up. To this end, two different epoxy precursors with suitable solubility in ethanol were evaluated as cross-linkers for silica gels prepared from (3-aminopropyl)triethoxysilane and tetraethylorthosilicate. In addition, 1,6-bis(trimethoxysilyl)hexane (BTMSH) was used as an additive in the underlying silica structure to add flexibility to the aerogels. It was found that the ethanol-derived aerogels exhibited more shrinkage than those prepared from other solvents but that including BTMSH in the aerogels significantly reduced this shrinkage. Inclusion of BTMSH also imparted the ability of the aerogel monoliths to recover elastically when compressed up to 50% strain. In addition, optimized cross-linked aerogels prepared in this study have mechanical properties comparable to those using other more undesirable solvents and cross-linkers.

Reinforcing polymer cross-linked aerogels with carbon nanofibers

We have previously reported cross-linking the mesoporous silica structure of aerogels with di-isocyanates, styrenes or epoxies reacted with amine decorated silica surfaces. These approaches have been shown to significantly increase the strength of aerogels with only a small effect on density or porosity. Herein, we examine the effect of including up to 5% (w/w) carbon nanofibers in the silica backbone before cross-linking. The addition of 5% carbon nanofibers to the lowest density aerogels studied triples the compressive modulus and the tensile stress at break is increased five-fold with no density penalty. The carbon fiber also improves the strength of the initial hydrogels before cross-linking, which may have implications in manufacturing.

Elastic Behavior of Methyltrimethoxysilane Based Aerogels Reinforced with Tri-Isocyanate

The elastic properties and/or flexibility of polymer reinforced silica aerogels having methyltrimethoxysilane (MTMS) and bis(trimethoxysilylpropyl)amine (BTMSPA) making up the silica structure are examined. The dipropylamine spacer from BTMSPA is used both to provide a flexible linking group in the silica structure, and as a reactive site via its secondary amine for reaction with a tri-isocyanate, Desmodur N3300A. The tri-isocyanate provides an extended degree of branching or reinforcement, resulting in increased compressive strength of the aerogel monoliths while the overall flexibility arising from the underlying silica structure is maintained. The compressive moduli of the reinforced aerogel monoliths in this study range from 0.001 to 158 MPa. Interestingly, formulations across this entire range of modulus recover nearly all of their length after two compressions to 25% strain. Differences in pore structure of the aerogels due to processing conditions and solvent are also discussed.

On the Oxidative Degradation of Nadic Endcapped Polyimides: I. Effect of Thermocycling on Weight Loss and Crack Formation

The effects of thermocycle frequency, aging temperature and post-cure conditions on weight loss, microhardness and crack formation of PMR-15 neat resins were investigated. Crack formation was monitored by metallography. The molecular level changes, occurring under the same conditions, were monitored by microscopic FT-IR in reflectance mode on the same samples. FT-IR analysis was carried out in zones 100 μm in diameter, allowing examination of different regions of the aged samples. It was found that weight loss, crack formation and microhardness were highly dependent on aging time and temperature, but not thermocycling frequency. Molecular level changes were also highly correlated to time and temperature of aging. Both the physical effects of aging and the chemical effects were isolated to a thin surface layer. No changes whatsoever to the interiors of the samples were evident by FT-IR, microhardness or microscopy.

Elastic low density aerogels derived from bis[3-(triethoxysilyl)propyl]disulfide, tetramethylorthosilicate and vinyltrimethoxysilane via a two-step process

A series of low density, porous structures were prepared using bis[3-(triethoxysilyl)propyl]disulfide (BTSPD), tetramethylorthosilicate (TMOS) and vinyltrimethoxysilane (VTMS) as precursors via a two-step (acid–base) sol-gel process followed by supercritical CO2 extraction. Using statistical experimental design methodology and empirical modelling, the concentrations of BTSPD, TMOS and VTMS were varied in the production of the monoliths and found to have a significant effect on their bulk density, porosity, BET surface areas, hydrophobicity and mechanical properties. Increasing the TMOS concentration significantly increases the surface area and Youngs modulus while higher VTMS concentration improves hydrophobicity and higher BTSPD concentration leads to increased elastic recovery after compression. Optimized aerogels produced in the study have a combination of high Youngs modulus, good hydrophobicity and near complete recovery after compression in agreement with model predictions.