Abstract
Effect of temperature on nanophase separated structure of epoxy-based hydrogels containing polyoxyethylene (POE), polyoxypropylene (POP) and diglycidyl ether of Bisphenol A propoxylate (PDGEBA) was studied using small-angle neutron scattering (SANS). At the macroscopic level, increase of temperature causes an expulsion of water from the hydrogels.
At the microscopic level, the expulsion of water is accompanied by redistribution of water and POE inside the hydrogels. Two kinds of structure have been revealed in the hydrogels by SANS.
The structure with shorter characteristic length measured by Bragg's distance (ca 70-80 angstrom) is observed in all hydrogels and at all investigated temperatures (5-80 degrees C). It is attributed to a distribution of water into water-rich and water-poor domains caused by nanophase separation of the crosslinked system due to differences in interaction of individual network building blocks with water.
The water-rich domains consist of a part of POE blocks mixed with water. The water-poor domains consist of POP and PDGEBA blocks mixed with a small amount of water and remaining POE blocks.
The domains are separated by a diffuse interface with effective thickness ca 7-10 angstrom as estimated from deviations from Porod's law. Formation of a new structure with longer characteristic length (ca 240 angstrom) is observed in the highly non-stoichiometric hydrogels.
Its contribution to SANS becomes more pronounced with increasing temperature. This contribution is attributed to the formation of regions of pure water (water pools) as a new hydrogel phase that is enabled by low network density and high amount of pending chains in the highly non-stoichiometric epoxy networks subject to swelling.