Spreading of perfluoropolyalkylether films on amorphous carbon surfaces

Spreading of perfluoropolyalkylether thin films on amorphous carbon surfaces has been studied by scanning microellipsometry. Two types of perfluoropolyalkylethers with the same main-chain structure and various molecular weights (between 1000 and 6000 g/mol) were used: Zdol, with OH functional end groups, and Z, with nonfunctional CF3 groups. For Zdol, the thickness of the molecular layers in the spreading profile increase as Mn-n(0.6), where M-n is the mean molecular weight,with the second layer being nearly twice as thick as the first layer. This layered structure was not observed for Z in the molecular weight range under study. As expected, the thickness-dependent diffusion coefficient D(h) was found to decrease with increased molecular weight. Possible molecular conformations near the solid surface are discussed. The spreading of binary blends of Zdol-Zdol, Z-Z, and Zdol-Z were also studied. The results show that the spreading of the binary blend of the same kind of polymer with different molecular weight behaved like that of a lubricant with an intermediate molecular weight. The diffusion coefficient of a blend was found to obey the additivity of viscosity. For Zdol-Z blends, however, the faster moving Z molecules migrate through the network of the slower moving Zdol molecules, and form a monolayer ahead of Zdol. (C) 1999 American Institute of Physics. [S0021-9606(99)70506-4].