University of Connecticut |
Polymer Program SeminarAnomalous Diffusion in Glassy Copolymers, Blends and Polymer Nanocomposites as Observed by Pulse Field Gradient NMR Friday, February 27, 2004 11:00 am , IMS Room 20 The diffusion of molecules through heterogeneous media is often not simple Fickian diffusion on the length scale of the heterogeneities. Pulse field gradient NMR measurements of diffusion reflect this by yielding apparent diffusion constants which depend on the time, D, over which diffusion is observed. Such behavior is observed in porous systems and in the high permeability copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (PDD). The apparent diffusion constant and the dependence on D in this copolymer are altered by the addition of fumed silica nanoparticles. The self-diffusion constant of pentane and toluene in cast films of the pure polymer and nanocomposites containing 10, 20 and 30 wt% fumed silica were measured. Figure 1. Dapp for pentane versus D for different concentrations of fumed silica in the TFE/PDD copolymer. Pure polymer data are the half squares, 10 wt % silica are the diamonds, 20 wt % silica are the squares, 25 wt % are the circles and 30 wt % are the inverted triangles. Note the addition of 30 wt % fumed silica increased the self-diffusion constant by an order of magnitude in spite of the fact that the fumed silica is impenetrable. Besides the apparent diffusion constant dependence on D, the logarithm of the mean square displacement versus the logarithm of D can be used to characterize the diffusion process. For Fickian diffusion the plot would be linear with a slope of 1. The initial slope of such plots in the pure polymer give a slope from 0.5 to 0.6 and this is indicative of anomalous diffusion. At longer times the slope increases towards 1 as the diffusion becomes more Fickian in nature at longer length scales. Anomalous diffusion can arise from diffusion in a fractal space such as a random walk on a random walk. In the case of the copolymer, the space associated with diffusion process consists of high free volume regions in the glassy copolymer. Upon the addition of nanoparticles, the initial slope also increases towards 1. The nanoparticles appear to reduce the barriers to penetant diffusion by increasing the connections between the more porous, high free volume regions of the polymer. Blends of polyethylene oxide and poly(methyl methacrylate) also show anomalous diffusion because the PEO chains are very mobile below the glass transition while the PMMA chains are essentially rigid. Thus certain penetrant molecules can diffuse along the PEO backbone around regions of PMMA. The latter regions can only support slower diffusion. PFG NMR is one of the few tools available for characterizing the heterogeneous space affecting diffusion.
This seminar series is sponsored by generous grants from U.S. Surgical Corporation and Rogers Corporation. |