Effect of Changing Molecular End Groups on Surface Properties:  Synthesis and Characterization of Poly(styrene-b-semifluorinated isoprene) Block Copolymers with −CF₂H End Groups

Poly(styrene- b -semifluorinated isoprene) block copolymers with −CF 2 H-terminated side groups were synthesized by the esterification reaction of a poly(styrene- b -hydroxylated 1,2-/3,4-isoprene) block copolymer with semifluorinated acid chlorides. Two distinct first-order transitions were observed by DSC measurements in the resulting block copolymers at temperatures below the glass transition of polystyrene. The transition temperatures of the longer fluorocarbon units (10 −CF 2 − units) were found to be higher than the shorter fluorocarbon units (eight −CF 2 − units). By comparing the block copolymers having −CF 3 end groups with those with −CF 2 H end groups, it was found that both the thermal behavior and surface properties are significantly affected by the terminal group of the semifluorinated side chains in the block copolymer. NEXAFS spectroscopy reveals that, although the surface coverage by semifluorinated groups is roughly the same regardless of whether the ends are −CF 2 H or −CF 3, the orientational order of the −CF 2 H surfaces is significantly poorer than that of the −CF 3 surfaces. After long-term water exposure the −CF 2 H surfaces were observed to reconstruct to more hydrophilic ones even though the analogous polymers with −CF 3 -terminated semifluorinated side chains were quite stable. The ease of surface reconstruction is probably related to the relatively poor orientation order of the polymers with −CF 2 H end groups as determined by NEXAFS.