Introduction

Everyone is aware of one of the curious properties possessed by the surfaces of fluids; the ‘skin’ on the surface of water that allows one to ‘float’ a needle on the surface of pure water and which is used by insects to walk across the surface of ponds. This effect, termed surface tension, is a direct manifestation of the cohesive forces that hold liquids together. Polymer melts are no different from other, simpler liquids; they too exhibit surface tension and to understand and predict this surface tension we need to understand the way in which interatomic forces lead to cohesion in polymers.

The simplest picture of surface energy then, is a static one — near the surface of a fluid some of the contacts between molecules, which in the bulk are energetically favourable, are missing. The energy of these ‘missing bonds’ may be equated with the surface energy and from such simple arguments the surface energy may be directly related to the cohesive energy of the liquid. This essentially mechanical picture ignores the Brownian motion of the molecules, which is present at the interface as well as in the bulk. One can picture this Brownian motion producing fluctuations in the mean height of the surface or interface, which as a result becomes diffuse.