Wool The question of whether polymer molecules reptate or not remains as one of the most important and controversial problems in polymer physics. The reptation model was proposed by P.-G. de Gennes (1971) to describe the dynamics of linear polymer chains in the melt. We have designed an experiment which probes the essential features of this model, i.e., the snakelike motion of chains due to their confinement by the other chains, and related scaling laws. We explore the interdiffusion of two sets of matching triblock chains, HDH/DHD, where the chain centers are deuterated on one side (HDH) and the chain ends (DHD) are deuterated on the other side of the interface. If the chains reptate, a characteristic "ripple" develops in the concentration profile CD(x, t) which grows to a maximum and then decays. The ripple is due to the anisotropic motion of the chains. Our initial Ripple experiments Nature, 365, 235, Sept 1993 using SIMS and Neutron Reflection analysis of matching polystyrene triblocks provide strong support for reptation. Several critical experiments remain to complete the investigation of the reptation model using matching triblocks. These include (a) the effects of molecular weight at M >> Mc, (b) the effects of Rouse dynamics at M < Mc (critical entanglement molecular weight), (c) location of chain ends, (d) effects of chemistry, (e) scaling behavior with time and molecular weight, (f) quantitative comparision between computer simulation and experiment, and others. These experiments should provide a definitive statement on the validity of the reptation model. %%% Reptation is a key concept in the motions of linear polymer chains. A validation, or negation, of this model would have important consequences in such areas as the healing of cracks in rubber or plastics.
