Measurements of Polymer Diffusion Over Small Distances: A Check of Reptation Arguments

Abstract

We performed neutron reflectrometry (NR) and nuclear reaction analysis (NRA) measurements on interdiffusion of protonated and deuterated polystyrene films. Covering several orders of magnitude in time t and resolving interfacial widths w(t) between approximately 2 nm and 60 nm we found two successive time regimes with a crossover corresponding to the reptation time tau-d. Below tau-d we are able to approximate the interface profile quite well by two superposed error functions described by three parameters, sigma-c(t), sigma-t(t) and p(t). The two contributions are interpreted according to the Rouse and the reptation model. While the width sigma-c of the error function correlated to Rouse type motion stays almost constant, the width sigma-t of the second contribution increases with time. p(t) represents the percentage of the second contribution at the interface. It increases with time reaching 100 per cent at tau-d. An explanation is given in terms of the reptation model. The time behavior of sigma-c(t), sigma-t(t), p(t) and w(t) is discussed. tau-d and w(tau-d) are determined independently using three different methods. All results are in qualitative agreement with the reptation model.