Structure and properties of macromolecular surface phases

Abstract

The thickness of a surface phase is determined by the size of the molecules which constitute it and the range of the forces between them. In macromolecular surface phases, therefore, the thickness is very much larger than that of surface phases of low molecular weight materials of similar chemical constitution. Where the presence of the surface influences the macromolecular arrangement the thickness is determined by the resulting mean extension of the macromolecules in a direction normal to the surface. The free energy change from macromolecular distortion has to be balanced against the free energy change deriving from concentration changes and short range interactions with the solvent and the surface constituting material. In practically all such cases the resulting surface phase is many times thicker than the dimensions of the low molecular weight components involved. Hence only relatively mild concentration gradients are produced, as compared with the low molecular weight case and macromolecular surface phases are found to prosses unusual thermodynamic and rheological properties. Two functions are shown to be influenced by the presence of the interface. The distribution function of the segments within the individual polymer coil and the distribution function of centres of gravity of the macromolecules. The importance of the latter in determining interfacial thickness is stressed and discussed.