Impact of molecular structure on water vapour sorption properties in nanostructured polymeric films

[摘要] ENGLISH ABSTRACT: In this study, the use of surfactants, plate-like clays, organophilic molecules and side-chaincrystallinity was investigated for their impact on the transport mechanisms of water vapourthrough polymer films.A model polymer latex, poly(styrene-co-butyl acrylate), was prepared using miniemulsionpolymerization. Three different types of surfactants, sodium dodecyl benzene sulfonate (SDBS, ananionic surfactant), octyl phenol ethoxylate (OPE, a nonionic surfactant) and dodecyl ammonium-3-butenoate (DA3B, a reactive surfactant) were used to stabilize the latex. Films were preparedfrom the resultant latices and their water vapour sorption behaviour determined across a watervapour partial pressure range of 0.1 to 0.9. Sigmoidal kinetic behaviour was seen for all threefilms, with the DA3B stabilized film exhibiting high diffusion coefficients compared to filmsstabilized with SDBS and OPE. The thermodynamic behaviour of the films differed and wasdependent on the reactivity of the surfactant. SDBS and OPE stabilized films exhibited Flory-Huggins behaviour, while the DA3B stabilized film followed Henry's Law. Despite significantdifferences in terms of these properties, the permeability coefficients were similar for the threefilms across the water vapour partial pressure range evaluated.The impact of sodium montmorillonite (Na-MMT) clay and an organophilic modifier, 2-acrylamido-2-methylpropanesulfonic acid (AMPS), on the water vapour sorption properties ofpoly(styrene-co-butyl acrylate) was evaluated. These polymer clay nanocomposites (PCNs) weresynthesized using miniemulsion polymerization. The resultant latex films were characterized andused for water vapour sorption analyses. It was shown that complete exfoliation of the Na-MMTwas necessary to minimize the equilibrium water vapour uptake. Even when Na-MMT wascompletely exfoliated, the amount of water vapour sorbed by the PCN was high and this wasattributed to the hydrophilic nature of the clay. Using a least squares regression fit, goodcorrelation was obtained between the experimental isotherms and the sorption behaviourpredicted by the Dual Mode Sorption model which was originally developed for polymers in theirglassy state.The impact of side chain crystallinity on the water vapour sorption properties ofpoly(methyl methacrylate-co-octadecyl acrylate) was evaluated. These randomcopolymers containing increasing amounts of octadecyl acrylate, and therefore increasingdegrees of crystallinity, were synthesized using solvent polymerization. Although it couldbe expected that side chain crystallinity would be the main contributing factor resulting ina reduction in the diffusion coefficient, it was shown that the methyl group on the á-carbon of the vinyl group in the methacrylate reduced the diffusion to a greater extentthrough the increased stiffness of the polymer backbone. This was also reflected inpoly(methyl methacrylate-co-octadecyl acrylate) having a greater activation energy fordiffusion compared to polyoctadecyl acrylate.

[发布机构] Stellenbosch University