UC Riverside

Photochromic behavior is not a new concept in chemistry and was mentioned for the first time in literature more than a century ago. This dissertation focused on using photochromic molecules to innovate and develop methods to remotely modify adhesion of thin films to substrates. We explored both photo-induced covalent and non-covalent adhesion.

To induce non-covalent adhesion, we hypothesized that if photochromic molecules embedded in a polymer were exposed to light, this could change the properties of the molecule which might lead to the change of polymer adhesion to substrate. We investigated the effect of ultraviolet (UV) and visible light exposure on the adhesion of polystyrene (PS) for a range of photochromic molecules such as, donor-acceptor Stenhouse adducts (DASA), diarylethene derivatives (DAE) and spiropyrans (SP) to glass substrate. We utilized adhesion measurement methods like water detachment, pull-off and the lap shear tests, which were used to detect the increase and decrease of adhesion to the glass surface after light exposure. We also investigated the effects of photochrome concentration, light wavelength, substrate properties, and polymer structure on adhesion and de-adhesion.

The innovative part of our final project was to engineer multi-functional nanoparticles and substrates with light-switchable covalent adhesion. Nanoparticles with surface-bound anthracene (AN) were able to undergo [4+4] photocycloaddition reactions to form covalent adhesion. In theory this reaction could help nanoparticles to selectively assemble and make a predesigned structure after being activated by light. Our results demonstrated that under ultraviolet (365 nm) illumination, both unimolecular and bimolecular photochemical reactions led to the loss of surface-bound AN absorbance. These competing reaction pathways decreased the efficiency of the cross-linking dimerization reaction.