Enhancement of Mechanical Properties of Photo-Responsive Polymers

摘要

Photomechanical polymers are characterised by changes in their mechanical properties under light radiation. This process is driven by the reversible isomerisation of chromophores in a polymer matrix. Azobenzene-based materials can switch their isomeric structure from trans to cis by light of appropriate wavelength. When this class of materials is chemically connected to polymers as a side chain or backbone, the polymers show photo-induced mechanical property improvements. This paper reports on the photomechanical property changes in a series of copolymers of methyl methacrylate (MMA) and methacryloyloxy azobenzene (MOAB) with azobenzene in the side chains. The chromophore loading ranges between 10% and 65% mol. Nanoindentation was employed to quantify the relationship between the variation in the photo-driven mechanical properties and the amount of chromophore loading in the copolymer films. Results demonstrated that the stiffness of the polymer films increases up to 19% with an optimum loading of 30% mol azobezene. Above this level of azobenzene loading (30% mol) the stiffness decreases suggesting a maximum effect can be obtained with 30% mol loading in this copolymer matrix. The rate of isomerisation in thin films was established by monitoring the visible spectra before and after irradiation. The duration of trans to cis isomerisation increases with azobenzene loading. The cis to trans isomerisation under visible light is faster than the trans to cis conversion, due to the greater thermodynamic stability of the trans form.