Liquid crystalline, semifluorinated side group block copolymers with stable low energy surfaces: Synthesis, liquid crystalline structure, and critical surface tension

Monodisperse poly(styrene-b-semifluorinated side chain) block copolymers were synthesized by anionic polymerization of poly(styrene-b-1,2/3,4-isoprene) followed by the corresponding polymer analogous reactions. By controlling the block copolymer composition and the relative lengths of the fluorocarbon and hydrocarbon units in the side group, the effect of chemical structure on surface properties and the influence of liquid crystalline structure of the semifluorinated side chain on the surface behavior were evaluated. The composition of side groups does not greatly affect the as-prepared sample surface tension, but influences instead the transition temperatures of the room temperature liquid crystal phase. It was observed that the shorter fluorocarbon units (six -CF2- units) form a smectic A phase at room temperature. The critical surface tension of the SA phase is 10.8 mN/m, and the polymer surface undergoes significant reconstruction when immersed in water. However, when the fluorocarbon side chain contains more than eight -CF2- units, the resulting surface possesses a lower critical surface tension (ca. 8 mN/ m) and exhibits negligible surface reconstruction. We believe the stability results from the highly ordered packing of the room temperature smectic B phase. This mesophase resists the reconstruction of the surface, since to do so would require loss of the enthalpies of transition. The estimated activation energy to destroy the smectic B phase is about 3-10 times higher than that of smectic A phase. This phase forms a uniform, hexagonally packed -CF3 terminated surface with a low critical surface tension similar to that of fluorocarbon-based Langmuir-Blodgett films. The self-assembly of these liquid crystalline block copolymers at both the molecular and microstructural level provides a valuable approach to creating stable, low surface energy materials.