RBS analysis of the diffusion of nano-size spheres in a polymer matrix

Rutherford Backscattering Spectrometry (RBS) with 1.5 MeV He ions was employed to measure the depth distribution of gold particles sandwiched between two poly (tert-butyl acrylate) layers following various isothermal annealing treatments; temperatures were kept below 170°C to prevent degradation of the polymer. Since PTBA also degrades when the ion beam impinges the sample, it was necessary to quantify the mass loss in order to obtain accurate diffusion coefficients. Although the total mass loss was substantial (40-60%), a simple expansion of the energy-to-depth scale was sufficient to account for the beam-induced shrinkage. This result is attributed to the fact that the mass loss occurs very rapidly, and is uniformly distributed through the depth of the sample. The data reveal that the diffusion of the nanometer-size gold particles is substantially slower than is predicted by a simple physical model (Stokes-Einstein) for non-interacting particles. This slowdown is attributed to polymer bridging interactions, in which polymer strands bind to individual metal particles, thereby retarding their diffusion.