Understanding the Swelling Mechanism of Stimuli Responsive Organically-Modified Silica

Mattaeus Klonowski


Stimuli-responsive polymers (SRPs) are materials which exhibit a change in physical properties upon a change in local environment. Some SRPs can expand in volume upon absorption of liquid, and are referred to as swellable materials. The two swellable materials studied in this project were sodium polyacrylate (SPA) and swellable organically-modified silica (SOMS). SPA swells only upon absorption of water, whereas SOMS can swell upon the absorption of a variety of organic solvents. Both SPA and SOMS generate a pressure when swollen within a confined volume. Although SPA generates greater swelling pressure than SOMS, SOMS takes significantly less time to reach maximum swelling pressure. SOMS exhibits immediate pressure generation upon contact with solvent, whereas SPA has an initially slow buildup in pressure generation. The different behaviors of SPA and SOMS are due to the different solvent-network interactions driving the swelling of each material. SOMS was swollen with different solvents, and maximum swelling pressure was found to have minimal dependence on absorbed solvent. However, the time it took for SOMS to reach maximum swelling pressure varied significantly with absorbed solvent, and may correlate to the viscosity of the solvent. Modeling SOMS as an exponential spring revealed a difference in the elasticity of SOMS for different amounts of absorbed solvent. Swelling mechanisms governing the pressure generation of SOMS remain unknown. However, progress was made by fitting relevant models to the swelling of SOMS. The next step in determining the swelling mechanism of SOMSwill be correlating physical processes to the models used.