Flexible, low-density polymer crosslinked silica aerogels

doi:10.1016/j.polymer.2006.05.073

Polymer

Volume 47, Issue 16, 26 July 2006, Pages 5754-5761

https://doi.org/10.1016/j.polymer.2006.05.073 Get rights and content

Abstract

Polymerization of a di-isocyanate with the amine-modified surface of a sol–gel derived mesoporous silica network crosslinks the nanoparticles of the silica skeleton, and reinforces the otherwise fragile framework. Systematically adjusting the processing variables affecting density produce aerogels whose macroscopic properties could be controlled, and are attributed to changing nanoscale morphology. Aerogels crosslinked using the smallest amount of silica studied exhibit as much as a 40-fold increase in strength over the corresponding non-crosslinked framework, and are flexible.

Introduction

Because of their low density and high mesoporosity, sol–gel derived silica aerogels are attractive candidates for many unique thermal, optical, catalytic, and chemical applications [1], [2], [3]. However, their inherent fragility and environmental sensitivity have restricted the use of monolithic aerogels to, for example, extreme applications such as insulating the batteries on the Mars Sojourner Rover where weight is at a premium and the temperature is −40 °C [4]. Future space exploration missions as well as advanced aeropropulsion systems demand lighter weight, robust, dual purpose materials for insulation, radiation protection and/or structural elements of habitats, rovers, astronaut suits, and cryotanks. Reducing fragility by crosslinking the mesoporous silica structure of an aerogel with a polymer [5], [6], [7] has the potential to foster their use as lightweight structural and insulating materials critical in aeronautics, space and commercial applications.

The mesoporous structure of silica aerogels results from a sol–gel process using tetramethoxysilane (TMOS) and a base catalyst followed by supercritical fluid extraction [4]. The mesoporous structure consists of fully dense 1–2 nm amorphous silica particles which assemble into ball-like secondary particles (5–10 nm). These secondary particles connect together through neck regions fashioned by dissolution and re-precipitation of the silica gel during aging, forming a pearl necklace-like structure containing large voids [8]. Such empty space between entangled nanoparticle strands is responsible for the low-density and low thermal conductivity of the aerogels [9], [10]. When stress incident on the material causes failure, fracture occurs at the interface of secondary particles (necks), while primary particles remain intact [11]. The incorporation of a nanocast polymer coating covalently bonded to the surface of the silica framework before supercritical drying provides reinforcement by widening the interparticle necks while minimally reducing porosity [12].

We have previously reported crosslinking with isocyanate-derived chemistry utilizing surface silanols, which comprise reaction sites for the di-isocyanate [5], [7]. Polymerization of the di-isocyanate oligomers, however, is not only dependent on surface –OH reactivity but also in fact builds on amines generated in situ when water adsorbed on the mesoporous surfaces reacts with the isocyanate moieties [7]. To create additional reactive sites and further increase strength without perhaps the addition of more weight, we have modified the underlying aerogel to include amine functionality by co-polymerization of TMOS with aminopropyltriethoxy silane (APTES) as previously reported for use with epoxy crosslinkers [6] and more recently with isocyanates [12]. For steric reasons and since hydrolysis of APTES is slower than hydrolysis of TMOS [13], the amine functionality is mostly positioned on the surface of the secondary particles where it is readily available for crosslinking [14]. Through reaction with amines, we expect a conformal polymer coating to be attached to the surface through urea linkages as shown in Scheme 1.

We have previously explored di-isocyanate crosslinked aerogels with density ranges of 0.2–0.5 g/cm³ and have shown that these materials have very high specific strength compared to native (non-crosslinked) aerogels with a similar silica framework [7], [12]. However, aerogels have been reported with densities as low as 0.003 g/cm³ [15]. These extremely low-density materials have extraordinary insulating properties but their fragility limits their utility. Herein, we report a study centered on the improvements in mechanical properties obtained when building polymer crosslinking on a lower density silica framework.

Section snippets

Materials

Raw materials tetramethoxysilane (TMOS), 3-aminopropyl triethoxysilane (APTES), and high-purity acetonitrile (CH₃CN) were purchased from Sigma–Aldrich and used as received. Deionized water was obtained from a Milli-Q water purification system (Millipore). The di-isocyanate crosslinker Desmodur N3200 (average molecular weight 500) was obtained by the courtesy of Bayer Corporation and also used as received. For ease of study, the sols were poured into 5 mL polypropylene molds (Wheaton

Results and discussion

Twenty-eight different aerogel monoliths plus two repeats were prepared as summarized in Table 1. In these experiments, three preparation conditions have been systematically varied: silane concentration (s) at four different levels, di-isocyanate concentration (d) also at four levels and polymerization temperature (t) at two levels. The initial silane concentration (total APTES plus TMOS in a 1–3 v/v ratio) which was varied from 4.1% to 30% by volume in acetonitrile (CH₃CN) should determine the

Conclusions

Flexibility is a property that aerogels rarely exhibit and thus it is indicative that at very low silica content, the aerogel abandons the properties of the silica template and takes on more properties of the polymer crosslink. Naturally, the use of other types of polymers (toughened epoxies, rubbers, etc.) as crosslinker may introduce even more flexibility into the system. Kramer et al. have demonstrated a similar notion in 1996, with a highly porous ormosil made by supercritically drying a

Acknowledgements

Financial support from the NASA GRC IR&D Fund, NASA's Low Emission Aircraft Program (LEAP) and Advanced Extravehicular Activities Program (AEVA) is gratefully acknowledged. We also thank Bayer Corporation for providing samples of Desmodur N3200, Ms. Linda McCorkle, Ohio Aerospace Institute for the SEM micrographs, Ms. Anna Palczer for surface analysis and Dr. Amala Dass of the University of Missouri-Rolla for assistance with the three-point flexural bending tests.

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¹: NASA Undergraduate Summer Research Program.

²: Employed by the Ohio Aerospace Institute.

³: Present address: Department of Chemistry, University of Missouri-Rolla, Rolla, MO 65409, United States.

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Flexible, low-density polymer crosslinked silica aerogels

Abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Acknowledgements

Catal Today

J Non-Cryst Solids

J Non-Cryst Solids

J Non-Cryst Solids

J Non-Cryst Solids

Prog Polym Sci

Science

Chem Rev

Nano Lett