Burkett, C. M.; Underwood, L. A., Volzer, R. S.; Baughman, J. A.; Edmiston, P. L. “Organic-Inorganic Hybrid Materials that Rapidly Swell in Non-Polar Liquids: Nanoscale Morphology and Swelling Mechanism.” Chemistry of Materials 20: 1312-1321 (2008).
A novel type of sol–gel derived zerogels that instantaneously swell greater than three times their dried volume in nonpolar solvents were investigated. Hybrid organic–inorganic materials that swell were only produced from a narrow set of precursors that possess an organic bridging group that contains an aromatic segment that is flexibly linked to the alkoxysilane polymerizable ends. Careful control over the processing conditions (catalyst, solvent, aging time) was necessary to yield animated zerogels. Various materials were studied by electron microscopy, infrared spectroscopy, nitrogen adsorption, and fluorescence spectroscopy using a covalently linked pyrene reporter. Collectively, the data support a model where swelling is derived from a morphology of interconnected organosilicate nanoparticles that are cross-linked to a particular extent during the gel state. Upon drying to the zerogel form, tensile forces generated by capillary-induced collapse of the polymeric matrix are released when interparticle interactions holding the dried material in the shrunken state are disrupted by a suitable solvent system. Swelling produced forces in excess of 50 N/g and is completely reversible. The molecular-scale organization of the nanoparticle structure seems critical for this swelling behavior. Further experiments indicate that the organosilicate materials can also swell in response to gas-phase organic molecules in a concentration dependent manner. These hybrid materials show promise for use in remediation technologies and chemical sensing.