Surface Properties and Metathesis Synthesis of Block Copolymers Including Perfluoroalkyl-Ended Polyethylenes

Abstract

Linear polyethylene with fluoroalkyl end groups has been prepared by ring-opening metathesis polymerization of cyclododecene in the presence of partially fluorinated acyclic olefins (chain transfer agents), followed by reduction of the double bonds in the backbone. Melt surface tensions (γ) were obtained for samples with one or both ends terminated by F(CF2)10 or F(CF2)4 groups at different molecular weights (MWs) of the hydrocarbon segments down to oligomeric sizes. The polyethylenes with two perfluorinated ends, i.e., PE(C10F21)2, were the most effective at lowering γ. At only 10% atomic F in the bulk, values of γ were obtained which were even lower than those for pure poly(tetrafluoroethylene). The surfaces of films of selected fluoroalkyl-terminated polymers were also characterized by advancing and receding contact angle measurements and angle dependent X-ray photoelectron spectroscopy (XPS). Advancing contact angles in hexadecane were as high as 70°, indicating a high surface concentration of perfluorinated species. Comparison with model systems indicate that much of the surface is covered by close-packed CF3 groups. XPS profiling quantified the ratio of fluorocarbon to hydrocarbon in the surface regions and also indicated a strong surface excess of oriented chain ends. The hexadecane contact angles of fluoroalkyl-ended metathesis polymers [PE(C10F21)2] decrease substantially as the molecular weight increases above ca. 40 000. In that range, the bulk concentration of chain ends becomes quite low and surface concentration of fluorocarbon chain ends at the surface is sharply reduced because of configurational entropic reasons. The surface properties of blends of a lower MW PE(C10F21)2 copolymer with a PE homopolymer were evaluated and compared to the series of pure PE(C10F21)2 copolymers where the high MW limit of the polyethylene center is approached. At similar bulk F levels in the blends and pure copolymer, the surface fluorocarbon content in the blends is significantly enhanced.