Title: Brillouin Spectroscopy Studies of Polyacrylamide and Fmoc-FF Hydrogels to Determine the Role of Monomer and Solvent Concentrations on Longitudinal Moduli and Relaxation Processes

Britta Gorman

Biomimetic hydrogels are polymeric networks that contain a large amount of water. The viscoelastic properties of such hydrogels are important to both the field of applied polymers and to research industries which use them as substrates. Usual characterization studies on these materials include rheometry, atomic force microscopy, or infrared spectroscopy. Because a large fraction of the volume of these types of samples is water, it is essential to understand the contributions of such a critical component at various timescales. I have employed Brillouin spectroscopy to study hydrodynamics of polyacrylamide hydrogels and Fmoc-FF hydrogels and the effects of concentration and solvent composition on the viscoelastic properties at GHz frequency. I have determined that in both materials, the fluid portion of the sample dominates the storage and loss moduli at the sub-nanosecond timescale in the intermediate-high hydration regime. I have determined that the free water in polyacrylamide gels has a reduced longitudinal modulus and speed of sound from that of bulk water, and that the bounded water in the samples contributes a higher storage modulus than that of bulk water. I have found that the suprastructural ordering of Fmoc-FF in hydrogels does not affect the storage and loss moduli at the sub-nanosecond timescale but that DMSO and water complex in such a way to cause a much higher storage modulus than observed in the individual constituents. This information corroborates other studies of different timescales and gives insight into the special case of the effects of polyacrylamide on surrounding water, and shows a local phase-separation in DMSO and water mixtures at a lower DMSO concentration than previously shown.

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