Water activated healing of thiolene boronic ester coatings

Abstract

A vinyl phenyl boronic ester (VPBE) was synthesised and cured with pentaerythritol tetrakis(3-mercaptoptopionate) (PTMP) and 3,6-dioxa-1,8-octanedithiol (DODT) to produce boronic ester polymer networks. The reversible nature of the boronic ester linkage in water provides an opportunity to investigate their intrinsic healing for a variety of crosslink densities and healing conditions. The boronic ester networks prepared here, all had sub-ambient glass transition temperatures (Tg) and therefore enhanced molecular mobility at room temperature and above. Intrinsic healing of the different networks is investigated after making scratches on the polymer surface and observing behaviour in humid environments, underwater and a hot dry environment. Underwater healing, as measured by the reduction in the size of the scratch, is extemely efficient and completely healed any surface scratches. This is compared with room temperature healing in moist environments where healing efficiency is more than 60% after 6 h. In a hot dry environment, no healing is observed. The healing mechanism is investigated using 3D profilometry, optical microscopy and fourier transform infrared spectroscopy (FTIR) which revealed that scratches become wider at the scratch rim, but more shallow over time. It is propsed that the presence of moisture at the base of the scratch facilitates a zipping up effect as the surface comes together to re-esterify.