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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 Tg 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 Tg close to free or nonabsorbing interfaces.1−8 One may expect that potential changes in Tg, being related to the time scale of segmental relaxation, should directly influence also larger scale relaxations. However, even slow mechanical relaxation around Tg and aging processes below Tg appear to bear no simple relation to the observed Tg changes,9,10 and the reason may be sought in a modified monomer packing persisting over length scales much beyond the radius of gyration (Rg), 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,11−15 with most studies concluding no deviation from Gaussian behavior. Nevertheless, the self-concentration 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.15−18 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 role.7,18 Note that in all cases the effects were observed in a range up to or even beyond Rg.   In this Communication, we present first results on the degree of anisotropy of segmental orientation fluctuations in entangled and geometrically confined chains in two different types of well-defined model nanocomposites with weakly interacting interfaces. We address temperatures far above Tg and time scales beyond the bulk entanglement time τe, providing a molecular-basis view of the discussed phenomena. We use a recently established multiple-quantum (MQ) NMR technique,23−26 which probes the time-dependent degree of local chain orientation via the monomer-averaged apparent residual dipolar coupling constant, Dres. At a given time and temperature, the experimental intensity at short pulse-sequence times τDQ depends on the square of this quantity; more precisely, it is directly proportional to the orientation autocorrelation function      where θ is the segmental orientation relative to the external magnetic field. In polymer melts, the decay of C(t) reflects chain motions in the tube-model regimes II−IV,23,26 and its magnitude C(τe)1/2 ∼ 1/Ne reflects the macroscopic elasticity in terms of the number of segments in an entangled strand (tube diameter). In networks, C(t→∞)1/2 ∼ 1/Nc exhibits a long-time plateau that at high cross-link density is directly proportional to the cross-link density.(25)

Confinement Effects on Chain Dynamics and Local Chain Order in Entangled Polymer Melts