Molecular Ordering and the Direct Measurement of Weak Proton−Proton Dipolar Interactions in a Rubber Network

Abstract

We demonstrate a new NMR technique that is suitable for the measurement of weak proton dipolar interactions in fluid polymers where rapid segmental dynamics "preaverage" the rigid-lattice dipolar coupling. The method is especially applicable to polymer melts and networks, where such residual interactions can provide valuable insight regarding molecular order and reorientational dynamics. We use this method to study natural rubber under uniaxial stretching. By measuring the residual dipolar coupling and its orientation dependence, we are able to distinguish the influence of chain stretching from chain reorientation in the deformation process. The pulse sequence we employ directly generates a function β, which is zero in the absence of dipolar interactions, which is completely independent of all Zeeman dephasing associated with chemical shifts or magnetic inhomogeneity, and whose time dependence can yield both the magnitude and the fluctuation rate of the residual dipolar interaction.

Segmental Anisotropy in Strained Elastomers Detected with a Portable NMR Scanner

“…This second contribution represents the effect of the mean orientation field, characterized by the mean degree of order along the direction of the applied force. Besides 2 D NMR other NMR methods used for investigation of the residual dipolar couplings in stretched natural rubber bands are based on 1 H Hahn-and solid-echoes as well as double-quantum coherences and dipolar encoded longitudinal magnetization [23,24]. By measuring the residual dipolar couplings versus the extension ratio l and its dependence on the orientation angle Y, it is possible to distinguish the influences from chain stretching and from chain reorientation in the deformation process [23].…”

“…Besides 2 D NMR other NMR methods used for investigation of the residual dipolar couplings in stretched natural rubber bands are based on 1 H Hahn-and solid-echoes as well as double-quantum coherences and dipolar encoded longitudinal magnetization [23,24]. By measuring the residual dipolar couplings versus the extension ratio l and its dependence on the orientation angle Y, it is possible to distinguish the influences from chain stretching and from chain reorientation in the deformation process [23]. Moreover, two-dimensional 1 H maps of stress distributions were presented from a strained natural rubber band with a cut on one side [24].…”

Stress Relaxation in Entangled Polymer Melts

“…Forty years of research have led to a complex relaxation scenario based on a combination of local Rouse dynamics, reptation, contour length fluctuations, and constraint release [4]. The development and validation of a quantitative, microscopic theory crucially depends on the availability of experimental and simulation data for model systems.Entangled polymers are studied experimentally using rheology [1,2,7], dielectric spectroscopy [8], small-angle neutron scattering [9,10], and nuclear magnetic resonance [11,12]. Computer simulations [13][14][15][16] offer some advantages in the preparation of well-defined model systems and the simultaneous access to macroscopic behavior and microscopic structure and dynamics.…”

The use of NMR relaxation and NMR imaging in studying the aging of rubber

“…However, it is a hint of any kind of a distribution of decay processes. There are two basic concepts for an explanation, referred to as "distribution of interactions" (model 1) [11][12][13] and "distribution of the endto-end-vector" (model 2) 14) . A first point in both models is based on the existence of a very fast (s f f 10 -9 s), small (inside a monomeric unit) but anisotropic (due to the fixed chain geometry of larger chain parts) motion (Fig.…”