ANOMALOUS DIFFUSION - SURFACE PLASMON RESONANCE MEASUREMENTS AS PROBES OF NANOMETER-SCALE FILM-SWELLING DYNAMICS FOR CH3OH IN POLY(METHYL METHACRYLATE)

摘要

Surface plasmon (SP) resonance position measurements were used to study CH3OH permeant-induced interfacial swelling processes in ultrathin (d less than or equal to 35 nm) poly(methyl methacrylate) (PMMA) films, which are the key molecular events in the anomalous case II diffusion process. An instrumental system was developed for precision control of sample thermal history with a feedback temperature regulation system based on a digital velocity proportional integral controller control algorithm to address the extreme temperature sensitivity of diffusion processes in glassy polymers. Excellent agreement between measured and calculated SP reflectivity curves was obtained after inclusion of a small interfacial Ag/dielectric roughness layer, the properties of which were calculated from Maxwell-Garnett theory. Experiments with bare Ag films demonstrated that the change in optical response of the Ag-PMMA composite structure was dominated by the change in the optical properties of the PMMA upon CH3OH uptake. Thus, changes in the resonance position of SP curves were used to measure the film swelling, Delta V, and changes in the average optical frequency dielectric constant of the swollen film, epsilon, from which the permeant volume fraction, phi, was calculated. These curves were based on Fresnel reflectivity relationships for four- and five-layer systems exhibiting one-dimensional swelling. Changes in film thicknesses were observable with this method with a resolution of less than 1 Angstrom. Permeant volume fractions corresponding to resonance shifts were also calculated and found to be identical for films in the 100-300-Angstrom-thickness regime.