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Introduction. Effects of nanometric confinement on polymer structure and dynamics have been in the focus for more than 15 years, primarily with the aim of providing the physical understanding for nanoscale polymer applications such as in composite materials. Changes in T g are probably the most researched, but still controversial issue, with clear and intuitively expected indications of slowed-down segmental dynamics for the case of strongly absorbing surfaces but sometimes conflicting trends and a lack of understanding of the physical basis for the often decreased T g close to free or nonabsorbing interfaces. [1][2][3][4][5][6][7][8] One may expect that potential changes in T g , being related to the time scale of segmental relaxation, should directly influence also larger scale relaxations. However, even slow mechanical relaxation around T g and aging processes below T g appear to bear no simple relation to the observed T g changes, 9,10 and the reason may be sought in a modified monomer packing persisting over length scales much beyond the radius of gyration (R g ), as seen in computer simulations. 2 At larger length scales, changes in the whole-chain conformation, as again seen in simulations, 2,4 have been assessed by scattering techniques, [11][12][13][14][15] with most studies concluding no deviation from Gaussian behavior. Nevertheless, the selfconcentration is expected to be increased locally since the random walk of a chain close to a neutral interface is reflected onto itself. Consequently, a reduced interchain entanglement density has been discussed as the reason for enhanced flow of confined chains. [15][16][17][18] In contrast, a "sticky" surface or local orientation effects and thus anisotropic dynamics of the segments close to an interface are thought to be the reason for enhanced elasticity 19 or reduced diffusion coefficients. 20,21 Interestingly, a slowdown of diffusion has also been reported for chains close to a free surface, 22 in apparent contrast to the (sometimes only transiently) reduced viscosity observed by others. 15,17,18 Annealing and nonequilibrium effects may thus play an important

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Changes in large-scale polymer diffusivity along interfaces, arising from transient surface contacts at the nanometer scale, are not well understood. Using proton pulsed-gradient NMR, we here study the equilibrium micrometer-scale self-diffusion of poly(butadiene) chains along ∼100 μm long, 20 and 60 nm wide channels in alumina, which is a system without confinement-related changes in segmental relaxation time. Unlike previous reports on nonequilibrium start-up diffusion normal to an interface or into particulate nanocomposites, we find a reduction of the diffusivity that appears to depend only upon the pore diameter but not on the molecular weight in a range between 2 and 24 kg/mol. We rationalize this by a simple volume-average model for the monomeric friction coefficient, which suggests a 10-fold surfaceenhanced friction on the scale of a single molecular layer. Further support is provided by applying our model to the analysis of published data on large-scale diffusion in thin films.

Characterization and modeling of the molecular-level behavior of simple hydrocarbon gases, such as methane, in the presence of both nonporous and nanoporous mineral matrices allows for predictive understanding of important processes in engineered and natural systems. In this study, changes in local electromagnetic environments of the carbon atoms in methane under conditions of high pressure (up to 130 bar) and moderate temperature (up to 346 K) were observed with C magic-angle spinning (MAS) NMR spectroscopy while the methane gas was mixed with two model solid substrates: a fumed nonporous, 12 nm particle size silica and a mesoporous silica with 200 nm particle size and 4 nm average pore diameter. Examination of the interactions between methane and the silica systems over temperatures and pressures that include the supercritical regime was allowed by a novel high pressure MAS sample containment system, which provided high resolution spectra collected under in situ conditions. For pure methane, no significant thermal effects were found for the observedC chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values ofC nuclides in methane change measurably as a function of pressure both in the pure state and in mixtures with both silica matrices, with a more pronounced shift when meso-porous silica is present. Molecular-level simulations utilizing GCMC, MD, and DFT confirm qualitatively that the experimentally measured changes are attributed to interactions of methane with the hydroxylated silica surfaces as well as densification of methane within nanopores and on pore surfaces.

INTRODUCTIONApplying electronic (fast) field cycling (FC) NMR, Kimmich and co-workers have performed extensive studies on the collective dynamics in (bulk) polymer melts. 1 As a result, a generic dispersion behavior has been found for the dipolar fluctuations of polymer segments. The NMR relaxation dispersion exhibits characteristic power-law regimes and shows a crossover from a dispersion predicted by the Rouse model for short polymer chains to that of entangled polymer dynamics which can be described by the renormalized Rouse formalism. According to these works, the relaxation power laws observed for entangled polymers do not follow those expected from the tube-reptation model, introduced first by de Gennes 2 and further developed by Doi and Edwards. 3 This is a challenging statement because currently the well-established reptation model offers the most successful concept of polymer dynamics.Kimmich, Fatkullin, and co-workers 4À6 have also reported FC NMR results on polymers confined in nanoporous solid-state matrices supporting the idea that the tube-reptation model is indeed applicable for polymers in confinement. Independently of the polymer chain length, i.e., below as well as above M e (the entanglement molecular weight), the NMR relaxation dispersion shows the characteristic power-law behavior of the tube-reptation model. More precisely, the regime II of the tube-reptation model has been identified. Surprisingly, confinement effects have been observed for confinement sizes in the range of 5À1000 nm, i.e., for sizes in most cases much larger than the size of a single polymer chain. The phenomenon has been called "corset effect" and explained as a finite size phenomenon which leads to reptation in a tight effective tube of diameter corresponding to the nearest-neighbor distance (which is much smaller than the typical size of the effective tube of the tube-reptation model in the bulk melt). Recently, the results have been questioned by neutron scattering (NS) experiments. 7,8 Studying polymers in 40 nm confinement, the authors have reported a slowing down of the dynamics in the Rouse regime whereas no change has been observed for the glassy ("local") dynamics, and a strong corset effect has been ruled out. In response, Kimmich and Fatkullin pointed out the important difference in observing potential confinement effects in terms of dynamic structure factor (NS) vs orientation autocorrelation functions (NMR), 9,10 claiming a higher sensitivity to confinement effects of the latter. Applying static 1 H double-quantum (DQ) NMR probing reorientation correlations at long times, Ok et al. 11 have found a relatively weak confinement effect when investigating the dynamics for polybutadiene (PB) in self-ordered anodic aluminum oxide matrices (AAO). A 2À3 nm layer in proximity to the neutral confining wall has been identified, within which the isotropization of chain motion due to reptation appears to be suppressed, or at least much protracted.In a recent series of papers of the Bayreuth group, polymer dynamics in the ...

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Hydrocarbon Behavior at Nanoscales Interfaces 497 of experimental and computational efforts relevant to the behavior of Earth materials (defined as gases, solutions, and solids) include the study of CO 2 in thin pores (Belonoshko 1989), water structure and dynamics in clays (