Importance of structural order for the low surface energy of perfluoroalkyl substituted polymethacrylates

Low surface energy coatings or anti-wetting agents play an essential role i n microelectronics, anti-fogging and anti-fouling applications, and even ha ve promising medical applications. Currently, perfluoroalkyl substituted po lymethacrylates (PFPMs) are widely in use, and the anti-wetting proper-ties of these and related polymers are believed to arise from the segregation o f CF, groups to the surface. However, proof of a direct correlation between surface energies and surface structure, and the importance of the order in the underlying bulk is still lacking. Here we report near edge X-ray absor ption fine structure (NEXAFS) results which establish such a correlation. O ur studies of three PFPMs with, respectively, isotropic, nematic, and smect ic A bulk phases at room temperature reveal in all cases a greater order at the surface than in the bulk. The surface order parameter is found to corr elate with the surface energy, which we find indeed to be lowered by segreg ation of CF, groups to the film surface. Most importantly, temperature depe ndent NEXAFS measurements in the 20-140 degreesC range, covering bulk phase transitions from smectic A to nematic to isotropic, show that it is the bu lk order (phase) which ultimately limits the achievable surface order, and hence the surface properties. (C) 2001 Elsevier Science BY. All rights rese rved.