Photoswitchable Liquid-to-Solid Transition of Azobenzene-Decorated Polysiloxanes

摘要

Having external control over fundamental properties of polymers, such astheir physical state, is a crucial yet challenging design criterion for smartmaterials. Liquifying polymers through photochemical events has significantlyadvanced various research lines. However, the opposite process of solidifyinga polymer that is intrinsically in a liquid state reversibly with light isunattained. Herein, the light-controlled liquid-to-solid transition ofpolysiloxanes is reported, which are decorated with a small number ofazobenzene-functionalized ureidopyrimidinone (Azo-UPy) pendants. The UPymoieties toggle between intra- and intermolecular hydrogen bonding viatrans→cis photoisomerization of the azobenzene. This transformation on themolecular level leads to the formation of strong supramolecular cross-links,which, in turn, results in the macroscopic solidification of the material. Thephotoswitching event enables the post-synthetic tailoring of the polymers’mechanical properties, thus providing an alternative to the addition ofplasticizers or hardeners. Moreover, the adhesion strength of thephotochromic material increases by a factor of 6 upon exposure to UV light. Insitu illumination during rheological measurements reveals the delicateinterplay between wavelength dependent penetration depth andphotoswitching efficiency. This conceptually new (de)bonding on demandstrategy paves the way for creating light-responsive materials with excitingapplications in temporal adhesion, recycling, lithography, and materialprocessing.