Open questions about polymer interfacial diffusion

Abstract

The goal of understanding diffusion in bulk has been a large part of the agenda in polymer physics from the start of our discipline. It seemed obvious to researchers, many years ago, that this and other aspects of dynamics at the level of individual molecules surely underpinned the macroscopic properties, such as viscoelasticity and other properties discussed in this viewpoint. Great names are associated with the seminal understanding of diffusion in the bulk: we have Rouse, Zimm, Doi, Edwards, and de Gennes on the theoretical side, but on the experimental side, important work has been done by so many researchers that to single out individual names would not be meaningful. Nowadays, the study of the mechanisms and timescales of polymer diffusion in solution and in melts is a developed field of study that is summarized in textbooks. Interesting modern work does continue, for example, in understanding diffusion in blends and microstructured block copolymers. Although we should never claim that our understanding is definitive, polymer science has developed a rather mature understanding of how the dynamics depend on the chain length, chain architecture, and concentration. Why is the same not so for polymer dynamics at surfaces? Ultrahigh-vacuum surface science has studied surface diffusion for many years; polymer science has not. One basic limitation has been that the needed experimental tools have not been available. It is true that methods of dynamic light scattering have given insight into the global dynamics of polymer brushes. It is also true that in the field of polymer adsorption, work has approached this question from the perspective of understanding surface on–off (adsorption–desorption) kinetics. At the same time, it is also desirable to seek an understanding of in-plane polymer diffusion at surfaces, which is a distinctly different problem. In the past, techniques imported from other fields of science have enabled important advances. Secondaryion mass spectrometry and related methods from ultrahigh-vacuum surface science have been used to measure concentration profiles in multilayer thin films spin-coated onto solid supports, from which diffusion has been cleverly inferred. Neutron reflectometry has been used to study interdiffusion in thin films. Specular X-ray scattering has allowed us to study capillary fluctuations in thin films. X-ray scattering has even been used to assess segmental equilibration. These methods refer to thin melt films on solid supports. Atomic force microscopy and surface force apparatus are much used to measure how friction and surface forces are perturbed when polymer films are very thin. Correspondence to: S. Granick (E-mail: sgranick@uiuc. edu)