Effect of Residual Dipolar Interactions on the NMR Relaxation in Cross-Linked Elastomers

Abstract

For elastomer networks above the glass transition temperature Tg, a unified approach is presented to relate the residual dipolar couplings in various independent NMR experiments to the crosslink density. This is demonstrated on a series of cross-linked poly(styrene-co-butadiene) elastomers. The presence of dynamic physical and permanent chemical cross-links leads to a nonzero average of the homonuclear and heteronuclear dipolar couplings, which results in a solid-like NMR relaxation behavior. The residual dipolar couplings are expressed as a function of the effective number of statistical segments Ne between the physical and N e X between the chemical cross-link points, using a simplified network model with Gaussian statistics. These effective numbers are extracted for each sample of the series from the 13 C-edited transverse 1 H magnetization relaxation of the CH group. It is shown that the respective Ne values can be used to scale the time domain of various NMR experiments such as (a) the free induction decay, (b) the 13 C-edited 1 H transverse magnetization relaxation, (c) the cross-polarization curves, and (d) the 1 H magnetization exchange between the CH and CH2 groups. This proves the validity of the unified view on the dipolar interactions in elastomer networks and provides a way to estimate the cross-link density.

Immobilized Polymer Fraction in Hyperbranched Polymer/Silica Nanocomposite Suspensions

“…NMR was applied to filled rubbers by studying the magnetization decay after a pulse sequence of a magnetic field. [30][31][32] The relaxation process showed a fast decaying component corresponding to unbound matrix and a slow decaying component corresponding to the bound matrix from which the immobilized layer thickness was estimated to be in the range 1-2 nm. 30 DSC was also used both in the step scan and in the modulated scan configurations to quantify the immobilized matrix fraction in nanocomposites.…”

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

“…One can obtain similar results using a distribution of the end-to-end vectors of the crosslinked polymer segments 14) . In this case no long correlation time s S (l1 ms) is used; the system is viewed as a rigid lattice for larger scales, really a more convincing assumption for strong cross-linked polymers.…”

“…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.…”

Aggregation behaviour of biohybrid microgels from elastin-like recombinamers

“…This indicates that the mobility contrast between hydrophobic and crosslinking domains reduces in ELR microgels which are more organized in terms of polypeptide composition compared to native a-elastin microgels. The increase in the crosslink density reduces the segmental motions of oligopeptides 49 and the size of the microgel. Hence, T 2 relaxation time becomes shorter as is shown in Fig.…”