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.

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To mimic the uniformly elongated endothelium in natural linear vessels, bovine aortic endothelial cells (BAECs) are cultured on micro- to nanogrooved, model poly(dimethylsiloxane) (PDMS) substrates preadsorbed with about 300 ng/cm(2) of fibronectin. BAEC alignment, elongation, and projected area were investigated for channel depths of 200 nm, 500 nm, 1 microm, and 5 microm, as well as smooth surfaces. Except for the 5 microm case, the ridge and channel widths were held nearly constant about 3.5 microm. With increasing channel depth, the percentage of aligned BAECs increased by factors of 2, 2, 1.8, and 1.7 for 1, 4, 24, and 48 h. Maximum alignment, about 90%, was observed for 1 microm deep channels at 1 h. The alignment of BAECs on grooved PDMS was maintained at least until cells reached near confluence. F-actin and vinculin at focal adhesions also aligned with channel direction. Analysis of confocal microscopy images showed that focal adhesions localized at corners and along the sidewalls of 1-microm deep channels. In contrast, focal adhesions could not form on the bottom of the 5-microm deep channels. Cell proliferation was similar on grooved and smooth substrates. In summary, PDMS substrates engraved with micro- and nanochannels provide a powerful method for investigating the interplay between topography and cell/cytoskeletal alignment.

Diffusion of deuterated poly(methyl methacrylate) (dPMMA) is slowed down in a PMMA matrix filled with hydroxyl-capped spherical silica nanoparticles, from 13 to 50 nm in diameter and at loadings up to 40 vol %. At constant T − T g = 75 K, the normalized diffusion coefficients (D/D 0 ) collapse onto a master curve, when plotted against the confinement parameter, ID/2R g , where ID is interparticle distance and 2R g is probe size. This result suggests that the confinement parameter captures the effect of nanoparticle size, size polydispersity, and volume fraction on polymer dynamics for the PMMA composite. For ID < 2R g , the master curve exhibits a strongly confined region where D/D 0 decreases by up to 80%, whereas for ID > 2R g , the curve falls in a weakly confined region where D/D 0 decreases only moderately by up to 15%. Surprisingly, D/D 0 is reduced even when ID is 8 times larger than 2R g . A comparison between the master curves for PMMA and polystyrene nanocomposites indicates that attractive interactions in the PMMA system do not significantly alter the centerof-mass diffusion of macromolecules in polymer nanocomposites.

The glass transition (Tg) of thin polystyrene films (ca. 3000 A˚) cast on silicon wafers was determined by a new technique. An ellipsometer was used to determine the refractive index and thickness of the polystyrene films. Tg was determined by measuring the temperature dependence of the refractive index. The change in thickness with temperature was used to calculate the linear and bulk thermal expansion coefficients of the material. A significant shift in Tg, possibly due to strains induced in the cooled films, was observed between heating and cooling for polystyrene films. © 1993 John Wiley & Sons, Inc.