Highly transparent thermoresponsive surfaces based on tea-stain-inspired chemistry

A highly transparent thermoresponsive surface that could switch its wettability at different temperatures was constructed via tea-stain-inspired chemistry. The pristine tannic acid was modified by alkyl bromide with a substitution degree of 1.7 alkyl bromide units per tannic acid molecule. A coating of the alkyl bromine modified tannic acid with a thickness of 22 ± 3 nm was deposited onto the surface of glass via auto-oxidation. A poly(N-isopropylacrylamide) (PNIPAAm) brush was grafted from the alkyl bromide initiator via surface initiation atom transfer radical polymerization with a polymer grafting density of 8.6 × 10⁻³mg/cm². Due to the low thickness of the tannic acid and PNIPAAm coating, the transparency of this thermoresponsive surface remained constant at 94.3% even when the temperature was changed from 20 to 40 °C, but the water contact angle of this surface increased rapidly when the temperature was elevated from 25 to 35 °C. Due to the inevitable hydrolysis and deprotonation, this tea-stain-inspired chemistry-based coating was stable in aqueous solution with a pH of 7 or isopropanol for soaking times of up to 24 h. The coating reported here may have various potential applications such as surfaces for cell culture media, food storage, or self-cleaning materials.