Residual dipolar couplings by 1H dipolar-encoded longitudinal magnetization, double- and triple-quantum nuclear magnetic resonance in cross-linked elastomers

Abstract

The measurements of residual dipolar couplings in elastomer system is desirable, because they reflect the hindrance to molecular motions by the cross-linking, topological constraints and the external factors like mechanical stress. Dipolar-encoded longitudinal magnetization nuclear magnetic resonance (NMR) decay curves, double-quantum and triple-quantum NMR buildup intensities for measuring the residual dipolar couplings, and the associated dynamic order parameters are introduced. It is shown that in the short excitation time regime the effective dipolar network is simplified. In the limit of this model based on localized dipolar couplings, the spin response to two-dimensional pulse sequences used to record multiple-quantum (MQ) NMR coherences was evaluated for longitudinal magnetization, double-, and triple-quantum coherences of methylene, and methyl protons in synthetic 1,4-cis-polyisoprene. The dynamic order parameters can be evaluated from this NMR response using a classical scale-invariant polymer model. These dynamic order parameters were measured for a cross-link series of synthetic polyisoprene and correlated with the cross-link density. The decay rates of the Hahn-echo amplitudes reflecting residual dipolar couplings as well as effects of molecular motion are also measured for the same cross-link series. The contribution of molecular motions to the transverse relaxation can be separated from the residual dipolar couplings using a train of magic echoes. The sensitivity of these transverse relaxation rates to the cross-link density is compared to that of residual dipolar couplings. The NMR time scale is shorter for the dipolar-encoded longitudinal magnetization and MQ experiments as compared to transverse relaxation experiments leading to an increased sensitivity to cross-link density of the former approaches.

Radiation-Induced Cross-Linking in a Silica-Filled Silicone Elastomer As Investigated by Multiple Quantum 1H NMR

“…Static 1 H Multiple Quantum NMR methods which measure the residual dipolar couplings, in specific, hold much promise for such efforts. [13][14][15][16][17][18][19][20]24 In elastomeric materials, the residual dipolar couplings have been shown to be the result of topological constraints interfering with fast reorientations on the NMR timescale that otherwise would be expected to average homonuclear dipolar couplings to zero. 9 The residual dipolar couplings, in fact, have been shown to be quite sensitive to network and morphological changes and have been used to test theories of polymer structure, ordering, and dynamics.…”

“…p. 4 More recently, it has been reported that the characterization of the growth of multiple quantum coherences can provide detailed insight into silicone network structure by increasing the selectivity of the NMR experiment to the structure and dynamics most connected to the topology of the polymer network, including chain ordering at the surface of inorganic filler particles. [13][14][15][16][17][18][19][20][21][22][23][24] In addition, it has been shown that echo based methods can over estimate the residual dipolar couplings due to the effects of magnetic susceptibilities and field gradients due to internal microscopic voids and the inorganic filler. 28 These effects have been observed to be large enough to reverse trends of <Ω d > vs crosslink density at different fields.…”

“…As a result, MQ-NMR has been shown to be more selective to local network structure and dynamics. [13][14][15][16][17][18][19][20] Typical curves for the various network models are shown in Figure 4. A network behaving as a single network with a single residual dipolar coupling shown in Figure 4A and described by equation {3}.…”

“…13 Normalization using the signal intensity after a single pulse, however, requires a more complex fitting scheme in which the long term decay must also be considered in addition to the initial short term growth. The method described by Schneider, et al, requires then approximately half the acquisition time for a complete data set.…”

“…The method described by Schneider, et al, requires then approximately half the acquisition time for a complete data set. 13 For the normalization method used here, semi-empirical mathematical descriptions of the MQ growth have been developed by Saalwächter. 17 In the case of spins characterized by a dominant residual dipolar coupling, and where the double quantum intensity dominates the higher order terms, the multiple quantum growth curve can then described by…”