Exploring the response of photoswitchable polymer networks by atomistic simulations

Light-responsive polymers containing molecular photoswitches are promising candidates for the development of smart materials. Here, we investigate the precise nature of the response of a cross-linked azopolymer network depending on the length of spacer groups between the cross-linkers, the solvent, and the thickness of the polymer film. To this end, we develop an efficient and accurate atomistic force field that is able to describe not only the electronic ground state but also the photoisomerization of the azobenzene units via electronic excitation. Molecular dynamics simulations are performed by photoswitching different percentages of azo-units from trans to cis, yielding a reduction of polymer volume by up to 40 % for the initial photoirradiation. Repeated trans → cis and cis → trans photoswitching leads to a "collapsed" state of the polymer network that exhibits a weak and non-systematic photoresponse. This behavior is largely independent of solvent polarity, as it is seen to occur in water, ethanol, and benzene alike. The reversibility of the light-induced contraction/expansion of the polymer in polar solvents is considerably enhanced upon removing the alkyl spacer chains. Counterintuitively, however, the polymer volume in water is larger for the cis-state compared to the trans-state. For a densely stacked system of 2D polymer networks, no clear difference between the volumes in the two isomeric states is discernible. Increasing the stiffness of the torsional degree of freedom adjacent to the cross-linker produces the desired reversible contraction/expansion upon photoswitching.