Chain order in filled SBR elastomers: a proton multiple-quantum NMR study (original) (raw)
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Effect of Residual Dipolar Interactions on the NMR Relaxation in Cross-Linked Elastomers
Macromolecules, 1996
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.
Journal of Magnetic Resonance, 2004
The homonuclear and heteronuclear residual dipolar couplings in elastomers reflect changes in the cross-link density, temperature, the uniaxial and biaxial extension or compression as well as the presence of penetrant molecules. It is shown theoretically that for an isolated methyl group the relative changes in the intensity of the homonuclear double-quantum buildup curves in the initial time regime due to variation of the residual dipolar coupling strength is less sensitive than the changes in the triple-quantum filtered NMR signal when considering the same excitation/reconversion time. For a quadrupolar nucleus with spin I=2 the sensitivity enhancement was simulated for four-quantum, triple-quantum, and double-quantum buildup curves. In this case the four-quantum build-up curve shows the highest sensitivity to changes of spin couplings. This enhanced sensitivity to the residual dipolar couplings was tested experimentally by measuring 1H double-quantum, triple-quantum, and four-quantum buildup curves of differently cross-linked natural rubber samples. In the initial excitation/reconversion time regime, where the residual dipolar couplings can be measured model free, the relative changes in the intensity of the four-quantum buildup curves are about five times higher than those of the double-quantum coherences. For the first time proton four-quantum coherences were recorded for cross-linked elastomers.
Acta Physica Polonica A, 2005
Uniaxial stretching and swelling are considered as two limiting cases of deformations of elastomers. Under both conditions, the molecular dynamics is changed with respect to the behavior that describes the undisturbed, equilibrium elastomer. Particularly the spectrum of segmental motions, which reveals itself in the frequency-dependence of the longitudinal NMR relaxation time, is discussed in this study, but also order effects expressed via the dipolar coupling strength are investigated. For stretched elastomers, a significant change of the relaxation dispersion is found for three different types of rubber; it is a consequence of a change of the mode spectrum of segmental motions that becomes obvious at low frequencies (below 1 MHz at room temperature). In swollen elastomers, on the other hand, a cross-over towards a behavior expected for semi-dilute solutions is found, and a comparison to solutions of uncross-linked polymers reveals a significant effect of the cross-links only in the kHz range. A much more pronounced difference between elastomers and polymer solutions, however, is found from double-quantum encoded NMR measurements where the residual order introduced by the presence of permanent cross-links is maintained even in the presence of solvent.
NMR relaxation dispersion of vulcanized natural rubber
Solid State Nuclear Magnetic Resonance, 2004
The dependence of the 1 H spin-lattice relaxation time on the magnetic field strength has been determined for linear and crosslinked polyisoprene for Larmor frequencies between 5 kHz and 20 MHz: Universal power-law relations are found for all temperatures and cross-link densities under investigation and are compared to published results of rotating-frame experiments on similar natural rubber samples. The shape of the individual dispersion functions can be superposed into a master curve using appropriate shift factors. While addition of filler particles even at large weight fractions has only a minor effect on the relaxation times, uniaxial deformation and swelling are demonstrated to alter the molecular dynamics significantly.
Determination of the Local Cross-Link Density in Polyisobutylene-Based Elastomers by NMR Imaging
Gravimetrical Flory-Rehner experiments are combined with MRI experiments to study the spatially dependent degree of cross-linking in unfilled, 1,6-hexamethylenediamine-cured poly-(isobutylene-p-methylstyrene-p-bromomethylstyrene) terpolymers. MRI relaxometry reveals two proton T 2 relaxation decay times in CCl4 swollen specimens: a fast decaying component reflecting the constrained chain segments near cross-links and entanglements and a slow decaying component originating from less constrained, remote chains. On the basis of a linear relation between the bulk M n,eff (number-average molecular weight between effective cross-links) and the volume-averaged T2 decay times, MRI allows the determination of the local cross-link density in inhomogeneously cross-linked specimens by means of the spatially dependent T 2 decay times. MRI solvent diffusion experiments reveal that the ingress kinetics of cyclohexane in PIB terpolymers is Fickian. It is shown that the diffusion coefficient varies linearly with the cure state and the volume-averaged T 2 decay times.
A simple NMR method to measure crosslink density of natural rubber composite
Polymer Testing, 2010
The crosslink density of carbon black-reinforced natural rubber (NR) was correlated with 1 H chemical shift by liquid-state 1 H NMR spectroscopy. Instead of observing the rubber directly with the solid-state NMR technique, we used toluene molecules in the liquid state as probes to explore the inner cavities of the rubber sample. Four NR composites with different carbon black contents of 20, 40, 60, and 80 phr were tested. The toluene signals were split into two: one from the bulk solvent and the other from inside the rubber. The chemical shift difference between those two split signals was observed to be in linear relationship with the carbon black content. The samples thermally aged at 70 C for 100 days showed larger chemical shift differences than the unaged sample. Furthermore, the chemical shift difference was directly proportional to the apparent crosslink density obtained independently by the conventional swelling method. This result offers by far the simplest method to measure the apparent crosslink density in rubber.
Critical Analysis of Network Defects in Cross-Linked Isobutylene-Based Elastomers by NMR Imaging
Macromolecules, 1999
The analysis of chemical and physical network heterogeneities has been carried out for synthetic isobutylene-based elastomers using 1 H spin echo NMR imaging. Spatial homogeneity was compared for isobutylene-p-methylstyrene-p-bromomethylstyrene (PIB-PMS/BrPMS) terpolymers cured with inorganic versus organic curatives and unfilled materials versus those filled with carbon black. Surprising differences in the network structure were found for materials cured with an inorganic curative (ZnO) relative to those cured with an organic curative (1,6-hexamethylenediamine). Voids and void distributions were found to be an important performance variable on the basis of comparisons between the NMR imaging data and mechanical testing. Systematic comparison of solvent images and polymer images revealed the optimum experimental conditions for enhancing network density contrast or detecting microvoids. For the first time, direct 1 H images of polymer spins in a fully compounded, commercial polyisobutylene-based elastomer were acquired in the absence of any swelling solvent. Multislice imaging experiments were investigated as a method to construct three-dimensional void densities in elastomer compounds. Chemical-shift selective polymer imaging was used to selectively obtain network density information for either component of a two-phase blend containing the PIB-PMS/BrPMS terpolymer and polybutadiene. The ability of NMR imaging to provide data over statistically relevant sampling areas (hundreds of mm 3 ) for both neat polymers and fully compounded commercial materials (containing carbon black) represents a unique advantage over microscopic imaging methods for material property analyses.
Multispin moments edited by multiple-quantum NMR: application to elastomers
Journal of Magnetic Resonance, 2005
The spin system response to the five-pulse sequence used for measurements of double-quantum and triple-quantum buildup curves is evaluated in the initial excitation/reconversion regime. The multispin dipolar network that is present also in many soft solids like elastomers was considered. It is proved rigorously that the relevant quantity for analysis of double-quantum build-up curves in the initial regime is the second van Vleck moment. The higher-order moments edited by double-quantum as well as higher-order coherences in the multiple-quantum build-up experiments are different from van Vleck moments. These results can be applied to compare 1 H residual moments edited by double-quantum and triple-quantum experiments with those measured by other NMR methods. The sensitivity of multiple-quantum coherences to the changes in the values of residual dipolar couplings for crosslinked natural rubber under uniaxial elongation is also discussed. Under such conditions 1 H second van Vleck moments were measured for different elongation ratios of a cross-linked natural rubber. Moreover, 1 H triple-quantum edited moments were also measured for the same sample under uniaxial compression. The dependence of the second van Vleck moment and the time of the maximum of the double-quantum buildup curve on the cross-link density of natural rubber measured at low magnetic field was also investigated.