On the surface properties of hyperbranched polymers

The melt surface tension for a series of hyperbranched polymers was measured using the micro-Wilhelmy wetting technique. The surface tension was found to be extremely high for the hydroxyl-terminated hyperbranched polymers and approached that of water extrapolated to high temperature. Substituting an alkane on the end groups reduced the surface tension to the point where a large degree of substitution produced a surface tension equivalent to that of pure alkane. Water contact angle measurements were used to show the solid surface tension (surface energy density) was similarly affected by alkane substitution. Liquids that predominantly interact with the surface via dispersion (nonpolar) forces allowed determination of the surface dispersive contribution to the surface tension. Extrapolation of the melt data to room temperature yielded the total surface tension, and the difference between the total and dispersive surface tensions gave the nondispersive contribution. This was found to be a linear function of the number of hydroxyl groups, for a given generation hyperbranched polymer, demonstrating the hydroxyl end groups each linearly contribute to the surface tension. We use this as partial proof that the end groups can significantly influence the surface properties. The water contact angle for a polyethylene film was not found to be significantly affected by addition of a small amount of alkane-terminated hyperbranched polymer.