Ultrasound-Induced Cross-Linking and Formation of Macroscopic Covalent Hydrogels in Aqueous Polymer and Monomer Solutions

A new sonochemical method of synthesizing macroscopic permanent hydrogels, that is, water-swellable 3D networks consisting of covalently linked polymer chains, is presented. Subjecting Ar-saturated aqueous solutions of bifunctional monomers, polyethylene glycol diacrylate (PEGDA 700), polyethylene glycol dimethacrylate (PEGDMA 800), and a mixture of PEGDA and vinylpyrrolidone (VP), containing neither initiators nor any other additives to the action of ultrasound leads to the formation of permanent continuous hydrogel filling all of the space previously occupied by monomer solution. For 10% PEGDA solution at an ultrasound frequency of 622 kHz, the reaction time is as short as 30 s. By selecting the appropriate sonication time, frequency of ultrasound, and concentration of substrate, one can control the monomer conversion degree and cross-link density of the resulting gel. Hydrogels can also be obtained by using typical ultrasonic cleaning bath (35 kHz), albeit a higher dose of ultrasound energy (i.e., longer time of expose to ultrasound) is required. In separate experiments, we demonstrate the ultrasound-induced increase in average molecular weight of a polymer (Pluronic F127) in monomer- and initiator-free aqueous solutions, which indicates the prevalence of free-radical cross-linking reactions over degradation. Results are interpreted in terms of ultrasound-induced cavitation leading to the generation of OH radicals, which in turn initiate the monomer/polymer reactions and to a momentary local temperature increase at the surface of cavitation bubbles while undergoing quasi-adiabatic compression.