Abstract
The 1H DQ Fourier and Laplace-like spectra for a series of cross-linked natural rubber (NR) samples naturally aged during six years are presented and characterized. The DQ build-up curves of these samples present two peaks which cannot be described by classical functions. The DQ Fourier spectra can be obtained after a numeric procedure which introduces a correction time which depends less on the chosen approximation, spin-½ and isolated CH2 and CH3 functional groups. The DQ Fourier spectra are well described by the distributions of the residual dipolar coupling correlated with the distribution of the end-to-end vector of the polymer network, and with the second and fourth van Vleck moments. The deconvolution of DQ Fourier spectra with a sum of four Gaussian variates show that the center and the width of Gaussian functions increase linearly with the increase in the cross-link density. The Laplace-like spectra for the natural aged NR DQ build-up curves are presented. The centers of four Gaussian distributions obtained via both methods are consistent. The differences between the Fourier and Laplace-like spectra consist mainly of the spectral resolution in the favor of Laplace-like spectra. The last one was used to discuss the effect of natural aging for cross-linked NR.
Highlights
Homonuclear and heteronuclear residual dipolar couplings (RDCs) or quadrupolar interactions in soft solids such as elastomers and biological tissues represent an important source of information about the structure and molecular dynamics [1,2,3,4,5]
Model free access to RDCs is given by the analysis of multiple-quantum (MQ) build-up [12,13,14,15,16,17,18] and decay [19] curves recorded in the initial regime of the excitation/reconversion periods of the experiment, as well as the accordion magic sandwich technique [20]
Site-selective RDCs can be elucidated by 2D NMR spectroscopy using, for instance, 13 C-1 H heteronuclear residual dipolar couplings and encoded spinning sideband patterns [21]
Summary
Homonuclear and heteronuclear residual dipolar couplings (RDCs) or quadrupolar interactions in soft solids such as elastomers and biological tissues represent an important source of information about the structure and molecular dynamics [1,2,3,4,5]. Residual dipolar couplings and its associated dynamic order parameter are dependent on the time scale of the NMR experiment [2] In such situations, the MQ buildup curves show a broad maximum from which the RDC distribution can be obtained by inverse Laplace transform by means of the Tikhonov regularization method [18], and references therein. Nie et al [34] have studied cross-linked (natural rubber) NR with and without carbon black (CB) aged thermally by hot air or physically by mechanical fatigue at different time intervals Their measurements are based on the nonlinear rheological parameter obtained from FT-Rheology and 1 H DQ-NMR techniques. Contrary to the classical methods which, in general, are using the data measured only in the initial time regime, the newest proposed procedures are applied on the entire DQ build-up curve
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