INTERDIFFUSION OF POLYMERS ACROSS INTERFACES

Neutron Reflection (NR) and Dynamic Secondary Ion Mass Spectroscopy (D SIMS) experiments were conducted on symmetrically deuterated polystyre ne triblock bilayers (HDH/ DHD) which directly probed the interdiffusi on dynamics of the chains during welding. The HDH chains had their cen ters deuterated 50%, the DHD chains had their ends deuterated (25% at each end) such that each chain contained approximately 50% D. During w elding, anisotropic motion of the chains produces a time-dependent osc illation (ripple) in the H and D concentration at the interface, which bears the characteristic signature of the polymer dynamics. These osc illations were compared with those predicted by Rouse, polymer mode co upling (PMC), and reptation dynamics. The following conclusions can be made from this study. (a) During the interdiffusion of high molecular weight HDH/DHD pairs, higher mobility of the chain ends caused a conc entration oscillation which increased to a maximum amplitude, and even tually vanished at times, t > tau(D). The amplitude, or excess enrichm ent found, was appreciably more than that predicted by Rouse and PMC s imulations, and was only slightly less than that predicted from reptat ion simulations. (b) The oscillations were completely missing in the 3 0 and 50K HDH/DHD polymers, which are only weakly entangled. The lack of oscillations for the 30 and 50K pairs may be due to a combination o f surface roughness and fluctuations of order 30 Angstrom. (c) It was found that the position of the maximum in this ripple stayed at the in terface during its growth. This is also consistent with reptation and has not been explained by other theories. (d) All dynamics models for linear polymers produce ripples, many of which are qualitatively simil ar to that predicted for reptation However, each ripple bears the fing erprint of the dynamics in terms of its time-dependent shape, position , and magnitude, and the models are clearly distinguishable. Our resul ts, in summary, support reptation as a candidate mechanism of interdif fusion at polymer-polymer interfaces and its uniqueness is being furth er pursued. (C) 1996 John Wiley & Sons, Inc.