Multiazobenzene compounds, hexakis-O-[4-(phenylazo)phenoxyalkylcarboxyl]-D-mannitols and hexakis-O-[4-(4-hexylphenylazo)phenoxyalkylcarboxyl]-D-mannitols, exhibit photochemically reversible liquefaction and solidification at room temperature. Their photochemical and thermal phase transitions were investigated in detail through thermal analysis, absorption spectroscopy, and dynamic viscoelasticity measurements, and were compared with those of other sugar-alcohol derivatives. Tensile shear strength tests were performed to determine the adhesions of the compounds sandwiched between two glass slides to determine whether the compounds were suitable for application as adhesives. The adhesions were varied by alternately irradiating the compounds with ultraviolet and visible light to photoinduce phase transitions. The azobenzene hexyl tails, lengths of the methylene spacers, and differences in the sugar-alcohol structures affected the photoresponsive properties of the compounds.
Reworkable adhesion of glass plates was demonstrated using colourless adhesives. This was achieved using anthracene-terminated liquid compounds with hexamer structures (2500 g mol −1) as adhesives, which are photo-crosslinkable and thermo-degradable materials. It was found that the substitution position at the anthracene was important to obtain liquid-phase compounds at room temperature. However, although the compounds were liquid, they were too viscous to be used as adhesive. Therefore, they were mixed with 25 wt.% of dibutyl phthalate to increase fluidity. The liquid mixture could then be applied to the glass substrates without further modification, and was curable via photodimerisation of anthracene moieties to tightly attach the glass substrates together. The cured material was stable up to 120°C, but it returned to the original liquid state at room temperature after heating at 180°C, resulting in the detachment of the substrates without applying any strong force. The cycles of the curing and liquefying processes was reproducible for at least five consecutive times.
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