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 (1H 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.