Amplified photo-responses in sequentially polymerized azobenzene-containing polymer networks: the role of isomer interconnection
We report a one-pot synthesis method based on two-step sequential thiol-ene reactions for the facial preparation of azobenzene-interconnected photoresponsive polymers (azo-ICPRPs). In the first step, block-length-controlled azobenzene prepolymers (azo-prepolymers) are prepared in situ in a molten reaction mixture by using the nucleophile-catalyzed chemoselective thiol-Michael reaction. In the second step, the formed azo-prepolymers are further crosslinked with ultraviolet (UV)-transparent monomers by the free radical-initiated thiol-ene “click” reaction, yielding a crosslinked polymer system with rubbery storage moduli (Er) from 3.6 to 6.7 MPa and glass transition temperatures (Tg) from 21 to 28 °C. The interconnection of azobenzene moieties in azo-ICPRPs is found to greatly amplify the photo-actuation performance. Compared to single-step polymerized polymers with the same monomer composition, azo-ICPRPs with long-block-length presented up to 7 times higher photostrains, with 50 times faster kinetics. Consequently, the specific actuation work and power output during each stroke were also greatly amplified by the interconnection of azobenzene. The maximum measured specific stroke work and power output under 15 mW cm-2 UV activation are 121.3 kJ m-3 and 34 W kg-1, which is comparable to skeletal muscles.