Solid-phase esterification between poly(vinyl alcohol) and malonic acid and its function in toughening hydrogels

Esterification reactions between carboxylic acids and alcohols usually occur with the assistance of a catalyst at an elevated temperature. Solid-phase esterification reactions between hydroxyl-containing polymers and small carboxylic acids without using any catalyst under ambient conditions have rarely been reported. Here, we report the occurrence of esterification reaction between poly(vinyl alcohol) (PVA) and malonic acid (MA) through a simple drying method. PVA-MA hydrogels are firstly prepared by a freezing-thawing method, and then they are dried under ambient conditions. The dried hydrogels cannot be dissolved in hot water even in the presence of a strong hydrogen bond breaking agent. Structural characterization of the PVA-MA hydrogels reveals the formation of ester bonds between the hydroxyl groups in PVA and carboxyl groups in MA. The occurrence of the esterification reaction is further confirmed by using ethylene glycol as a model compound to react with MA under ambient conditions. The strong H-bonding and chemical ester bonds formed during the drying process increase the cross-linking density of the hydrogels, and hence their mechanical properties are largely enhanced. The PVA-MA hydrogels after the drying-reswelling treatment possess excellent mechanical properties, showing high tensile strengths and moduli up to 11.19 and 10.66 MPa, respectively. When the physical cross-links in the hydrogels are broken by heating in water, the hydrogels mainly cross-linked by ester bonding show excellent elasticity and resilience. Moreover, the formation of chemical ester bonding by drying also endows the PVA-MA hydrogels with good healing ability. The esterification in the solid phase by drying provides a new strategy to introduce chemical cross-links into polymeric materials and hence endows them with enhanced mechanical properties and functional properties.