Anomalous Diffusion in Poly(vinyl alcohol)−Poly(acrylic acid) Thin Films

Abstract

Dynamic in situ spectroscopic ellipsometry studies of poly(vinyl alcohol)−poly(acrylic acid) (PVA−PAA) composite films are presented. Thin films immobilized on glass substrates were conditioned over time in aqueous media. Dynamic measurements consisted of thickness (swelling ratio) and optical constant changes in time. The films displayed anomalous diffusion that was characterized by fast initial swelling followed by a much slower thickness reduction. A simple mechanism based on polymer network relaxation was proposed as the likely explanation for thickness reduction. A diffusion/relaxation formalism was used to quantitatively model measured data. From this modeling the diffusion coefficient of water and the network relaxation rate constants were estimated for three different types of composite films that differed in the degree of cross-linking. The diffusion Deborah number was estimated to be a high value ((DEB)D ≈ 5 × 103) which suggested elastic diffusion and non-Fickian behavior. Slow polymer network relaxation was the likely cause of a considerable amount of water expelled from the films. Alternative explanations for such anomalous diffusion were tested, and it was concluded that they did not play a significant role in the dynamic properties of the composites. It was proposed that the existence of the two relaxation rate constants (k r 2 ≈ 8 × 10-3 min-1, k r 1 = variable) could be related to mutually independent PAA and cross-linked PVA backbone relaxation, respectively.

Dynamics of Rhodamine 6G Sorption into a Porous Tri-block Copolymer Thin Film

“…The positions of the Gaussian oscillators were also fixed, but their width and amplitude terms were variables in the fit. The oscillators were fitted to the experimental spectrum acquired for the film equilibrated in R6G + solution under the conditions of a static spectroscopic ellipsometric scan 21,22 to determine the film extinction coefficient, k(λ). It was found that only one combination of the monomer extinction and dimer oscillator functions was able to adequately reproduce the experimental extinction spectrum.…”

“…Concurrent Spectroscopic Ellipsometry and Quartz Crystal Microgravimetry. A method for modeling mass transport in thin polymer films has been previously described. , It is based on the assumption that the proportionality between ellipsometry determined film thickness and film mass has the form …”

“…In certain cases, the substrates were functionalized with the bifunctional linker APTS to improve film adhesion. , For this specific procedure, clean glass substrates were soaked overnight in 2 M NaOH to activate the surface. Quartz crystals for QCM measurements were activated for a shorter period of time by dipping to prevent the damage of the crystal.…”

“…A method for modeling mass transport in thin polymer films has been previously described. 21,22 It is based on the assumption that the proportionality between ellipsometry determined film thickness and film mass has the form In this strategy, film thicknesses measured under dynamic and static experimental conditions were directly input for modeling film mass transport. The measurement of film thickness in ellipsometry is through indirect determination based on changes of the light polarization upon reflection from the surface expressed as angles Ψ and ∆.…”

“…We are aware of only one report that treats aggregation of R6G + in bulk polymer material 11 and one in relatively thin polymer films . We have developed an in situ optical method based on spectroscopic ellipsometry for thin film dynamic studies that can also be used for studies of dye aggregation in thin films. − Spectroscopic ellipsometry has important advantages over other optical methods because both film physical changes (thickness) and optical properties of the dye and the film itself are measured in time in a single experiment. The method is sensitive enough to identify aggregation in very thin films and of molecules characterized by weak extinction.…”