Synthesis and surface properties of films based on solventless liquid fluorinated oligoester

Abstract

Two kinds of solventless liquid oligoesters, linear and three-armed, were synthesized by direct esterification of a mixture of diacids and diol or triol. The molecular weights of such oligoesters are in the range 800-1000. Linear oligoesters have a single melting point of between -5 and 0 °C while only one glass transition temperature at about -60 °C was found for three-armed oligoesters. The oligoesters were partially end-capped with a perfluoroalkyl group by reacting with perfluoroalkyl acid chloride. A polymeric film was prepared from the homogeneous mixture of normal and fluorinated oligoesters and a liquid polyisocyanate cross-linker and cured at elevated temperatures. As the fluorine content in the films increases from 0 to about 1.4 wt %, the surface tension of the top surface decreases as much as 20 mN/m, indicating that the top surface is mainly occupied by low surface energy fluorine-containing species. The surface enrichment of fluorine-containing species is confirmed by X-ray photoelectron spectroscopy (XPS) investigations. The fluorine level at about 5 nm depth is about 20-80-fold above its stoichiometric level, whereas the fluorine level at the surface becomes almost saturated at the fluorine content of about 1 wt %. Thus, a fluorine-rich surface with low surface energy can be readily produced based on solventless liquid oligoester systems. The driving force for the surface segregation (self-stratification) of fluorine-containing species comes apparently from the big difference of surface energy between oligoesters and their fluorinated counterparts.