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

Abstract

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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“…However, higher mobility than predicted in the regime II was found, indicating a less static tube constraint [61]. Regime 0 and I are instead usually investigated with fast-field cycling experiment [62][63][64].…”

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“…In addition to structural aspects being of course among the most relevant molecular features, the presence of a variety of molecular motions dictate many of the properties of these systems, and directly affect their function. This includes, for instance, transport [1] and optical properties [2] of semiconducting polymers, mechanical behavior of constructing and rubbery polymers [3,4], activity of proteins [5], the stability of pharmaceuticals [6], ionic transport in glasses [7], and many others. Among the techniques used to probe molecular dynamics, NMR is known to be highly site-and frequency-selective, providing methods capable of studying molecular motion in a broad range of frequencies in chemical detail.…”

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“…Likewise, Martín and coworkers [149] as well as Lagrené et al [203] did not detect any influence of confinement on molten PEO chains inside AAO templates. The entanglement network of molten high molecular weight polymers is likely to be more affected by the confinement than chain conformation [149,199,205]. Shin et al reported reduction of the viscosity of PS chains during capillary flow along AAO nanopores with dimensions smaller than that of the chain in bulk [199].…”