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

Abstract

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

Polymer Dynamics of Polybutadiene in Nanoscopic Confinement As Revealed by Field Cycling 1H NMR

“…It appears that the confinement effect becomes smaller for lower temperatures, which is in agreement with our previous DQ NMR data. 11 For bulk PB at 293 K the three power-law regimes for T 1 (ν) described by Kimmich and Fatkullin 1 (and marked in Figure 2 by solid black lines) well reproduce our data. In addition, we show in Figure 2 the prediction of the tube-reptation model (regime II) which according to the corset effect is assumed to hold in confinement.…”

“…The present contribution addresses this problem directly, using the same method but for another well-defined model nanocomposite as in ref 11.…”

“…Polybutadiene (PB) with molecular weight M = 87 kDa (polydispersity PD = 1.05), which is above M e = 2 kDa, and with a radius of gyration R g = 11 nm, which is well below the pore diameters of the AAO membranes, was infiltrated into the AAO. 11 The samples used here were prepared as in the case of those used in ref 11. We stress that strong efforts were undertaken to ensure that the pores of the AAO were completely filled with equilibrated PB, as controlled by weightuptake experiments.…”

Reduced-mobility layers with high internal mobility in poly(ethylene oxide)–silica nanocomposites

“…14,15 Since the truncated-mode fraction found by NSE for PEO on silica 14,15 pertains to the NMR-detected mobile majority fraction, we should mention that more in-depth studies of this fraction by NMR require a more advanced (multiplequantum) method. 33,34 For the system with larger particles and longer chains, this method has provided the quantification of a fraction of entropically active bridging chains between the particles. 13 Thus, in-depth comparisons of neutron-scattering and NMR data on the same samples will help to develop an improved understanding of confinement effects.…”

Molecular Dynamics in Polymers

“…The general tendency in such layered confinement structures is that confinement leads to a more anisotropic character of segment reorientation so that the orientation correlation function decays more slowly at least as concerns its long-time tail [102]. Polybutadiene confined in anodic aluminum oxide matrices (20 and 60 nm) and polydimethylsiloxane restrained to emulsion droplets (200-1,000 nm) were studied with the aid of multiple-quantum build-up NMR spectroscopy [103]. Evidence for geometrical confinement effects on polymer chain dynamics independent of the chain length was also observed with the aid of dielectric relaxation spectroscopy [104,105].…”