Dramatically Enhancing Mechanical Properties of Polyvinyl Alcohol Hydrogels by Simultaneously Introducing a Hydrogen Bonding Forming Polymer During Salting-Out Treatment

Salting-out method has been widely employed to enhance the mechanical properties of hydrogels. Unfortunately, the enhancement in mechanical properties is still unsatisfactory for many applications. Here we report a modified salting-out method for preparing polyvinyl alcohol (PVA) hydrogels with dramatically enhanced mechanical properties by soaking a weakly cross-linked PVA hydrogel in a mixed aqueous solution of sodium citrate (Na 3 Cit) and polyacrylic acid (PAA). Compared with the PVA hydrogel prepared with the common salting-out method, the PVA/Na 3 Cit/PAA hydrogels show dramatically enhanced tensile strengths, elastic moduli, and elongations at break up to 21.8 MPa, 4.6 MPa, and 1200%, respectively. The mechanical properties of the hydrogels can be adjusted by controlling the concentrations of Na 3 Cit and PAA. The use of a mixed solution of an inorganic salt and a polymer capable of forming hydrogen bonds enables the simultaneous introduction of additional components into the hydrogel network and the formation of more and stronger hydrogen bonds with PVA chains during the salting-out process, resulting in the increased cross-linking density and enhanced mechanical properties of the hydrogel. This work provides a simple yet novel method for improving the mechanical properties of hydrogels. Graphical Simultaneous introduction of a polymer capable of forming hydrogen bonds, e.g., polyacrylic acid (PAA), into a weakly cross-linked polyvinyl alcohol (PVA) hydrogel during the salting-out treatment leads to the formation of more and stronger hydrogen bonding interactions and increased cross-linking density of the hydrogel, producing hydrogels with dramatically enhanced mechanical properties.