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 in microelectronics, anti-fogging and anti-fouling applications, and even have promising medical applications. Currently, perfluoroalkyl substituted polymethacrylates (PFPMs) are widely in use, and the anti-wetting properties of these and related polymers are believed to arise from the segregation of CF groups to the surface. However, proof of a direct correlation between surface energies and surface structure, and the 3 importance of the order in the underlying bulk is still lacking. Here we report near edge X-ray absorption fine structure (NEXAFS) results which establish such a correlation. Our studies of three PFPMs with, respectively, isotropic, nematic, and smectic 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 correlate with the surface energy, which we find indeed to be lowered by segregation of CF3 groups to the film surface. Most importantly, temperature dependent NEXAFS measurements in the 20–140 8C range, covering bulk phase transitions from smectic A to nematic to isotropic, show that it is the bulk order (phase) which ultimately limits the achievable surface order, and hence the surface properties.  2001 Elsevier Science B.V. All rights