Fluoroazobenzene side groups were prepared with different lengths of -CF2- segments and then attached to the isoprene block of poly(styrene-b-isoprene) diblock copolymers which were synthesized via anionic polymerization. The surface properties and surface ordering organization of fluoroazobenzene side groups were studied by water contact angle measurements and NEXAFS analysis. The results indicate that the fluoroazobenzene side chains can organize into a well-ordered surface layer due to their combination of low surface energy and liquid crystallinity. The orientational order parameter of the fluoroazobenzene side chains increased with the length of -CF2- segment. We also investigated the effects of UV irradiation on surface properties and the reorientation of the fluorinated side chains as a result of the trans-cis photoisomerization of the azobenzene unit. NEXAFS experiments under in situ UV irradiation demonstrated that the UV irradiation caused the smectic surface layer to disorder and that the surface layer eventually regained its order after the UV irradiation was stopped. Contact angle measurements after UV irradiation showed an increase of surface energy, consistent with a photoisomerization process that causes an initial -CF3 group rich surface to disorder and become less -CF3 rich. These results also demonstrated that the surface photoisomerization process is reversible.
Pub Type: Journals
NEXAFS, orientation, photoresponsive, polymers, soft x-ray