ELLIPSOMETRIC STUDIES ON MUTUAL DIFFUSION AND ADHESION DEVELOPMENT AT POLYMER POLYMER INTERFACES

Ellipsometry was used to estimate the interface thickness-lambda between two bulk layers of dissimilar polymers, poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN), as a function of annealing time t above the glass transition temperature T(g). The value of lambda was estimated to be around 20 nm at an early stage of annealing and increased with time, e.g. up to 60 nm at 130-degrees-C after 12 h for miscible pair such as PMMA and SAN-25 (AN content = 25 wt%), while it remained constant for an immiscible system such as PMMA/SAN-5, even after long annealing for more than 12 h. For miscible systems, the mutual diffusion coefficient D was obtained from the slope of the lambda vs. t1/2 plot according to the Brochard theory: lambda = 2(Dt)1/2. The D value was found to be roughly proportional to the thermodynamic driving force \chi - chi(c)\, as has also been predicted by Brochard (chi and chi(c) being the Flory interaction parameter at the annealing temperature and at the critical point, respectively). The molecular-weight dependence of D was found to follow the 'slow theory'. Further, to analyse the adhesion development, the results for lambda(t) were combined with the time variation of adhesive strength-sigma(t) observed by Fowler et al. It was found that the late stage could be described well by the reptation theory, sigma is-proportional-to lambda(1/2), while the adhesive strength-sigma of a thin interface at an early stage was much higher than the theoretical value.