(713d) Characterization of Porosity and Water Content in Sponge-Like Hydrogels: USANS and Swelling Methods

Porous hydrogels have attracted attention as biomedical, biomimetic, super-absorbent, and stimuli-responsive materials. Characterization of pore size, pore phase volume fraction, and polymer phase water content is necessary to model the basic physics of these materials, and to optimize mass transport characteristics for intended applications. Swelling of the polymer matrix in response to environmental changes can impact both the pore size and volume fraction, quantities which are not readily accessible to traditional characterization techniques such as SEM (Fig. 1). By combining gravimetric swelling measurements with non-invasive ultra small-angle neutron scattering (USANS) methods, it is possible to extract detailed information regarding the structural characteristics of sponge-like gels having micrometer-scale porosity. The USANS and swelling techniques are applied to sponge-like poly(hydroxyethylmethacrylate) gels that contain 40 to 80 volume % pores and channels of typical diameter 1 μm to 30 μm. These gels are strongly opaque in the swollen state, but become transparent upon drying due to shrinkage or disappearance of the pores, indicating that the solvent content strongly affects the pore volume fraction. The equilibrium mass swelling ratio in water is related analytically to the volume fraction of pores in the dry and swollen states. The correlation length associated with porosity is measured by USANS by fitting to an appropriate scattering model. By combining gravimetric swelling measurements with USANS invariant analysis, it is possible to determine the equilibrium volume fraction of pores and the concentration of water in the gel phase. The USANS/swelling methods are broadly applicable to characterization of porous soft materials, provided they can be fairly treated as two-phase systems with homogeneous polymer phase composition.