(222h) Bulk Property Changes in Glassy Materials Induced By Photoisomerization of Azobenzene
Changes induced by the photo-isomerization reaction for various azobenzene derivatives have been used in various applications, including light activated switches in polymer systems and surface modified materials. In recent years, studies have shown that the addition of azobenzene derivatives can allow for photo-induced changes in the properties of glassy materials. For instance, the photo-isomerization reaction within a glass matrix can cause the material to become malleable far below the glass transition (Tg) of the material. Glassy materials are of particular interest since they exhibit many mechanical properties similar to a crystalline solid, but the underlying structure is heavily dependent upon the process and conditions under which it was formed. Vapor deposited (VD) organic glasses have been of interest in recent years, exhibiting exceptionally enhanced properties depending on the temperature of the substrate it was deposited. Using a coarse-grained azobenzene model, we formed vapor deposited films at various substrate temperatures and investigated the dependence of photo-reactivity on substrate temperature. In addition to the other enhanced properties already observed in VD film, using a procedure that mimics the photo-excitation process of shining polarized light, we have shown that VD glasses are more photo-stable and exhibit a two-stage behavior on the way to the stationary state, which is not observed in the conventional glasses. Similar observations have been seen in vapor deposited films of an azobenzene derivative, disperse orange 37. During the initial stages of irradiation, the vapor deposited films change little in structure but a negative stress builds in the bulk of the film. The film then rapidly relaxes to the stationary state after enough stress has built up. There are some cases where this photo-induced malleability is of interest. For instance, we demonstrate that by using this process in a PEG-Azo-PLA polymersome system the glassy matrix of the PLA can be used as a gate for drug delivery. By plasticizing the PLA hydrophobic layer through the photo-isomerization reaction, the vesicles will release the drug and then reseal once the irradiation ceases and the glassy dynamics are restored.