(651a) Sorption and Diffusion of Alkyl Acetates in Polymethylacrylate/Silica Nanocomposites

Conventional theories for the effect of an inert filler on the sorption coefficient and diffusivity of small species in a composite[1] are inadequate when applied to polymer/nanoparticle composites (PNPCs)[2,3]. The abnormal behavior in PNPCs is attributed to significant modification of matrix properties in the immediate vicinity of the polymer/nanoparticle interface. Achieving a fundamental understanding of the matrix modification effect has great potential to allow design and optimization of mixed-media membranes for separation applications. We investigate this effect systematically for a model PNPC. We have made composites of spherical, sol-gel prepared silica nanoparticles (10-15 nm) with a bound protective layer of monodisperse poly(methyl acrylate) (PMA). The bound layer was made by controlled adsorption of monodisperse PMA onto silica in semidilute solution. Bound brush heights in solution were determined via dynamic light scattering (DLS) to be 2-3 nm. Absolute composition of PNPCs with various loadings of silica was determined by thermogravimetric analysis (TGA). Differential scanning calorimetery (DSC) measurements showed a Tg increase of 1°C over the range of compositions. Structural characterizations of the PNPCs were achieved by small angle and ultra small angle x-ray scattering (SAXS/USAXS) results. The dispersion of nanoparticles in the PNPCs were found to be dependant on sample annealing procedure and not random. Interval vapor sorption measurements of n-alkyl acetates (methyl, ethyl, n-propyl, and n-butyl) within the PNPC were made on a flow-through quartz crystal microbalance (QCM) apparatus enabling extraction of mutual diffusion coefficients and partition coefficients for the composite. Both properties decreased as the silica content was increased. In particular, the diffusion coefficients of composites were reduced much greater than predicted by conventional composite theory.

[1] R.M. Barrer, Diffusion and Permeation in Heterogeneous Media, in Crank, J.; Park, G.S. (Ed.), Diffusion in Polymers, Academic Press, London, 1968, pp 165-215

[2] R.M. Barrer, J.A. Barrie, N.K. Raman, Solution and diffusion in silicone rubber II -- the influence of fillers, Polymer. 3 (1962) 605-614.

[3] T.C. Merkel, B.D. Freeman, R.J. Spontak, Z. He, I. Pinnau, P. Meakin, A.J. Hill, Sorption, transport, and structural evidence for enhanced free volume in poly(4-methyl-2-pentyne)/fumed silica nanocomposite membranes, Chem. Mater. 15 (2003) 109-123.