Macromolecular motion is reduced in crowded polymer nanocomposites. Tracer diffusion is measured for deuterated polystyrene (dPS) into a polystyrene (PS):silica nanoparticle (NP) matrix using elastic recoil detection. This nanocomposite is ideal for studying diffusion in a crowded system because the interparticle distance (ID) that defines confinement can be varied from much greater than to much less than the size of the dPS chain, which is described by 2R g, the radius of gyration, and varies from 10 to 40 nm in this study. Diffusion is observed to be significantly slower than that predicted by the Maxwell model. The tracer diffusion coefficient of dPS in the nanocomposite relative to the pure PS matrix (D/D 0) plotted against the NP separation relative to probe size (i.e., ID/2R g) falls on a master curve, indicating that crowding is a property of both the dPS size and confinement in the nanocomposite. Moreover, the normalized diffusion coefficient decreases more rapidly when ID/2R g is less than ∼1, suggesting strong confinement conditions. The scaling of the diffusion coefficient with chain length is in excellent agreement with the entropic barrier model that accounts for the slowing down associated with the loss of chain entropy due to constrictive bottlenecks.

The tracer diffusion of deuterated polystyrene (dPS) is measured in a polystyrene nanocomposite containing silica nanoparticles (NPs), with number average diameters d n of 28.8 nm and 12.8 nm, using elastic recoil detection. The volume fractions of the large and small NPs (f NP ) range from 0 to 0.5, and 0 to 0.1, respectively. At the same volume fraction of NPs, the tracer diffusion of dPS is reduced as NP size decreases because the interparticle distance between NPs (ID) decreases. The reduced diffusion coefficient, defined as the tracer diffusion coefficient in the nanocomposite relative to pure PS (D/D 0 ), plotted against the confinement parameter, namely ID(d n ) relative to tracer size, ID(d n )/2R g , nearly collapses onto a master curve, although D/D 0 is slightly greater for the more polydisperse, smaller NPs. Using a log normal distribution of NP size from SAXS, the average ID of the smaller NPs is shown to increase by 25% at f NP ¼ 0.1 as polydispersity (s) increases from 1 to 1.39. By accounting for polydispersity, the confinement parameter better represents the effect of NP spacing on polymer diffusion. These experiments demonstrate that polymer tracer diffusion in polymer nanocomposites is empirically captured by the confinement parameter and that an increase in the average ID due to NP polydispersity has a secondary effect on model NP systems with a narrow distribution of sizes. However, for commercial systems, where polydispersity can be quite large, the effect of size distribution can significantly increase ID which in turn will influence polymer dynamics.1=3 À1 !(1)

The effect of the chemical structure of the dielectric layer in organic thin-film transistors was examined by evaporating pentacene onto five different styrenic polymer dielectrics: poly(styrene), poly(4-hydroxystyrene), poly(4-methylstyrene), poly(4-vinylpyridine), and poly(2-vinylnaphthalene). We find that the polymer has a significant effect, with measured field-effect mobilities ranging from between 0.1 and 1cm2∕Vs. This variation appears uncorrelated with either the polymer suface morphology or the observed pentacene crystallite size. The distribution of mobility, threshold voltage, on/off ratio, and subthreshold swing observed for each of the polymer dielectrics is presented.

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