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 -CF2H-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 -CF2- units) were found to be higher than the shorter fluorocarbon units (eight -CF2- units). By comparing the block copolymers having -CF3 end groups with those with -CF2H 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 -CF2H or -CF3, the orientational order of the -CF2H surfaces is significantly poorer than that of the -CF3 surfaces. After long-term water exposure the -CF2H surfaces were observed to reconstruct to more hydrophilic ones even though the analogous polymers with -CF3-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 -CF2H end groups as determined by NEXAFS.