DIFFUSION OF POLYMERS AT INTERFACES - A SECONDARY ION MASS-SPECTROSCOPY STUDY

Secondary ion mass spectroscopy (SIMS) was used to measure depth profiles c(x,t) at symmetric deuterated (DPS)-protonated (HPS) polystyrene interfaces as a function of temperature T and molecular weight M. The self-diffusion coefficient D was obtained from c(x,t) with a depth resolution of ca. 130 angstrom, using the Matano-Boltzmann method. By using molecular weights with M = 10(6), it was possible to test scaling laws for interdiffusion predicted by the de Gennes reptation model at t < tau (reptation time) and distances x < R(G) (radius of gyration). The following results were obtained. At times t > tau, D is-proportional-to M-2; at t < tau, the apparent D value was much greater than the equilibrium long time value. This effect is due to Rouse segmental motion and not center of mass motion. At t < tau, the number of monomers crossing the interface N(t) and the average monomer interpenetration distance X(t) were determined from c(x,t) as N is-proportional-to t3/4 and X(t) is-proportional-to t1/4, in agreement with the reptation theory. At t > tau, both N(t) and X(t) increased with a t1/2 dependence; the crossover in scaling laws occurred approximately at tau. The temperature dependence of D was well described by the Vogel analysis. Thermodynamic slowing down of the diffusion coefficient due to the deuterium isotope effect was considered to be minimal in the distance range explored by the SIMS method, x < 500 angstrom. The SIMS results were in agreement with similar studies using neutron reflection.