Elastic air: NMR of hydrogels

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  • Published: Aug 15, 2018
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Elastic air: NMR of hydrogels

Air today

The facile synthesis of doubly cross-linked PVPDMS-based aerogels via the free-radical polymerization/hydrolytic polycondensation method. Credit Kanamori et al, Angewandte Chemie/Wiley

Superflexible aerogels can be used as highly efficient absorbents, thermal insulators, and pressure sensors. Now, a team from Japan with a little help from nuclear magnetic resonance (NMR) spectroscopy have developed super-elastic aerogels that can be processed easily and at low cost.

Until now, fragility and brittleness have limited the practical applications of these delicate solids, which are essentially a solid foam comprising air-filled pores in an organic matrix. Writing in the journal Angewandte Chemie, a team from Japan explains how they have now developed extremely elastic aerogels that can be processed very easily. Secret to their success is the use of a doubly cross-linked organic-inorganic network structure that adjustable network density. NMR spectroscopy was used to investigate the chemical structure of the new materials.

Processor power

Aerogels can be made from a variety of materials, but they are inevitably very brittle by virtue of their structure. Processing them into different shapes using the conventional tools of component manufacture - cutting, drilling, and milling - are almost impossible. Moreover, the drying process needed to make them tenable is commonly rather expensive.

Kazuki Nakanishi and Kazuyoshi Kanamori at Kyoto University, Japan, working with Guoqing Zu have now developed a new class of unusually elastic aerogels. The new materials are based on the monomers vinyldimethylmethoxysilane and vinylmethyldimethoxysilane. These can be linked in an initial reaction to form polymer chains by a radical polymerization process that exploits the carbon double bonds in the monomers' vinyl groups. The polymer chains have organosilicon side chains (silanes), which – depending on the monomer – have one or two methoxy groups attached to the silicon atom. These groups then participate in the next step, a cross-linking reaction at one or two places along the silane side chain, once more dependent on the monomeric starting material. The ensuing polycondensation reaction forms siloxane bridges while the density of the resulting cross-linking of the polymers (polyvinylpolydimethylsiloxane-polyvinylpolymethylsiloxane copolymers) depends on the ratio in which the two monomers were mixed in the first place.

Wearables

The team explains that subsequent inexpensive air pressure or freeze drying then forms the requisite aerogels with tailored porosity. However, it is thanks to their flexible siloxane and hydrocarbon chains that the normally brittle and delicate structures inherit a highly elastic character. They can be bent, rolled, twisted, and cut into desired shapes. The team adds that the more densely cross-linked versions exhibit high thermal insulation, surpassing conventional materials such as the familiar polyurethane foam.

The team reports that their flexible aerogels show selective absorption in a mixture of hexane and water - they absorb the hexane exclusively. The organic solvent can then be removed by simply squeezing the material like a sponge or allowing it to dissipate through evaporation. Such a simple separation process can be repeated until all of the hexane has been extracted from the water. This might allow water contaminant to be easily removed from solvents including acetone and toluene too. Moreover, if oil or mineral oil has leaked into a body of water, the same process might be used to remove them from the water.

The team has made composites of their elastic aerogels with electrically conducting graphene nanoplatelets. Under pressure, the graphene platelets move closer together, reversibly increasing the conductivity. Such a composite might have applications in touch pads for electronic devices and wearable electronics.

Related Links

Angew Chem Int Edn Engl 2018, online: "Superflexible Multifunctional Polyvinylpolydimethylsiloxane-Based Aerogels as Efficient Absorbents, Thermal Superinsulators, and Strain Sensors"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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