The characterization of the structural and dynamic properties of rubber networks is of fundamental importance in rubber science and technology to design materials with optimized mechanical properties. In this work, natural and isoprene rubber networks obtained by curing at three different temperatures (140, 150, and 170 °C) and three different sulfur contents (1, 2, and 3 phr) in the presence of a 3 phr accelerator were studied using a combination of low-field time-domain NMR (TD-NMR) techniques, including ¹H multiple-quantum experiments for the measurement of residual dipolar couplings (D_res), the application of the Carr–Purcell–Meiboom–Gill pulse sequence for the measurement of the transverse magnetization decay and the extraction of ¹H T₂ relaxation times, and the use of field cycling NMR relaxometry for the determination of T₁ relaxation times. The microscopic properties determined by TD-NMR experiments were discussed in comparison with the macroscopic properties obtained using equilibrium swelling, moving die rheometer, and calorimetric techniques. The obtained correlations between NMR observables, crosslink density values, maximum torque values, and glass transition temperatures provided insights into the effects of the vulcanization temperature and accelerator/sulfur ratio on the structure of the polymer networks, as well as on the effects of crosslinking on the segmental dynamics of elastomers. D_res and T₂ were found to show linear correlations with the crosslink density determined by equilibrium swelling, while T₁ depends on the local dynamics of polymer segments related to the glass transition, which is also affected by chemical modifications of the polymer chains occurring during vulcanization.

A molecular-level understanding of the structure of the polymeric network formed upon the curing of air-drying artists’ oil paints still represents a challenge. In this study we used a set of analytical methodologies classically employed for the characterisation of a paint film—based on infrared spectroscopy and mass spectrometry—in combination with solid state NMR (SSNMR), to characterise model paint layers which present different behaviours towards surface cleaning with water, a commonly applied procedure in art conservation. The study demonstrates, with the fundamental contribution of SSNMR, a relationship between the painting stability and the chemical structure of the polymeric network. In particular, it is demonstrated for the first time that a low degree of cross-linking in combination with a high degree of oxidation of the polymeric network render the oil paint layer sensitive to water.

In this paper, different aspects concerning anisotropy in Nuclear Magnetic Resonance (NMR) spectroscopy have been reviewed. In particular, the relevant theory has been presented, showing how anisotropy stems from the dependence of internal nuclear spin interactions on the molecular orientation with respect to the external magnetic field direction. The consequences of anisotropy in the use of NMR spectroscopy have been critically discussed: on one side, the availability of very detailed structural and dynamic information, and on the other side, the loss of spectral resolution. The experiments used to measure the anisotropic properties in solid and soft materials, where, in contrast to liquids, such properties are not averaged out by the molecular tumbling, have been described. Such experiments can be based either on static low-resolution techniques or on one- and two-dimensional pulse sequences exploiting Magic Angle Spinning (MAS). Examples of applications of NMR spectroscopy have been shown, which exploit anisotropy to obtain important physico-chemical information on several categories of systems, including pharmaceuticals, inorganic materials, polymers, liquid crystals, and self-assembling amphiphiles in water. Solid-state NMR spectroscopy can be considered, nowadays, one of the most powerful characterization techniques for all kinds of solid, either amorphous or crystalline, and semi-solid systems for the obtainment of both structural and dynamic properties on a molecular and supra-molecular scale. Graphic

¹H spin lattice relaxation rate (R₁) dispersions were acquired by field-cycling (FC) NMR relaxometry between 0.01 and 35 MHz over a wide temperature range on polyisoprene (IR), polybutadiene (BR), and poly(styrene-co-butadiene) (SBR) rubbers, obtained by vulcanization under different conditions, and on the corresponding uncured elastomers. By exploiting the frequency–temperature superposition principle, χ″(ωτ_s) master curves were constructed by shifting the total FC NMR susceptibility, χ″(ω) = ωR₁(ω), curves along the frequency axis by the correlation times for glassy dynamics, τ_s. Longer τ_s values and, correspondingly, higher glass transition temperatures were determined for the sulfur-cured elastomers with respect to the uncured ones, which increased by increasing the cross-link density, whereas no significant changes were found for fragility. The contribution of polymer dynamics, χ_pol^″(ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χ_glass^″(ω), well represented using a Cole–Davidson spectral density. For all elastomers, χ_pol^″(ω) was found to represent a small fraction, on the order of 0.05–0.14, of the total χ″(ω), which did not show a significant dependence on cross-link density. In the investigated temperature and frequency ranges, polymer dynamics was found to encompass regimes I (Rouse dynamics) and II (constrained Rouse dynamics) of the tube reptation model for the uncured elastomers and only regime I for the vulcanized ones. This is clear evidence that chemical cross-links impose constraints on chain dynamics on a larger space and time scale than free Rouse modes.