pH-dependent and self-healing properties of mussel modified poly(vinyl alcohol) hydrogels in a metal-free environment

3,4-Dihydroxyphenylalanine (DOPA)-based polymers are well-known to form functional hydrogels with self-healing properties by chelating metal ions. However, DOPA-based hydrogels with self-healing properties are difficult to obtain in the absence of the metal ions, as previously reported. Thus, the aim of this study is to prepare a self-healable DOPA-based hydrogel in the absence of metal ions. Firstly, poly(vinyl alcohol)-DOPA (PVA-DOPA) was synthesized by modifying PVA with DOPA through an esterification reaction. The composition of the PVA-DOPA polymer was determined by proton nuclear magnetic resonance (H-1 NMR) spectroscopy. Then, the PVA-DOPA hydrogel in a metal-free environment could be easily prepared by dissolving the polymer in buffer solution. Rheological analyses showed that the PVA-DOPA polymers had different dynamic moduli depending on the pH of the buffer solutions. The results from the FTIR and UV-vis spectra indicated that there were hydrogen bond interactions between the PVA-DOPA polymers under low pH conditions, while there were both hydrogen bond and covalent interactions under high pH conditions. The PVA-DOPA hydrogel could be rapidly self-healed within 270 s, which was much quicker than the hydrogel prepared in the presence of Fe3+ (about 600 s). The metal-free PVA-DOPA hydrogel has the potential for application in coating and biomedical fields.