Noncovalent Photochromic Polymer Adhesion

This paper explores whether the photochromic reaction of a molecule embedded in a polymer film can affect its surface adhesion properties, as measured by shear strength and delamination in water. The adherence of polystyrene (PS) to glass was chosen as a model system. Two commercially available photochromesa spiropyran derivative 1′,3′-dihydro-1′,3′,3′-trimethyl-6-nitrospiro­[2<i>H</i>-1-benzopyran-2,2′-(2<i>H</i>)-indole]) (SP) and a diarylethene derivative 1,2-bis­(2,4-dimethyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluoro-1-cyclopentene (DAE)are studied in detail. Both photochromic reactions can significantly enhance the adhesion of PS to a glass surface. The most dramatic results were obtained for PS/SP films, whose shear strength increased by a factor of 7 while the delamination rate was suppressed by at least 2 orders of magnitude after exposure to UV light. The enhanced polymer adhesion could only be partially reversed, even after extended exposure to visible light completely regenerated the UV-absorbing isomer. Nanoindentation and heating experiments suggest that the limited reversibility results from changes in polymer internal structure. We hypothesize that the adhesion changes arise from localized polymer and molecular motions that eliminate void spaces and surface gaps at the polymer–glass interface. The results show that adhesive forces between a prototypical polymer and an inorganic substrate can be modulated by photochromic reactions of embedded molecules