Observation of a characteristic length scale in the healing of glassy polymer interfaces

We examine the evolution ('healing') of the interface between two polymer films at various temperatures below the glass transition temperature, T-g. Specifically, neutron reflectometry is used to study the relaxation of the interface between both unentangled and entangled deuterated polystyrene (d-PS) and hydrogenated polystyrene (h-PS) films in the glass state where these bilayer films are supported on silicon substrates. We find that the initially sharp interface between the glassy polymer layers broadens with time (t), but the average interfacial thickness Delta sigma(t) between these layers then saturates after long time to a thickness (xi(p)) in a range between 1 nm and 3 nm after a long annealing time (>= 1 h) for the range of temperatures investigated. This characteristic scale, and the temperature dependence of the interfacial relaxation time tau, were unanticipated, and we thus investigated the dependence of xi(p) and tau on molecular mass (M), annealing temperature (T) and the thickness (h(f)) of the PS films. We find that xi(p) increases with h(f) at a fixed T and increases with T at fixed h(f). Our observation of a 'healing length' xi(p), regardless of whether the polymers are entangled or not, and the dependence of xi(p) on T rule out an interpretation of this parameter in terms of the reptation model. On the other hand, xi(p) has a scale comparable to the mobile interfacial layer thickness reported in both small molecule and polymeric materials in the glass state, suggesting that xi(p) is a well-defined dynamical length scale characterizing the interfacial properties of glassy materials. The existence of such an interfacial layer has numerous implications for the processing and scientific understanding of thin polymer films.