Proton Multiple-quantum NMR Research Articles

Crosslinked Elastomers: Structure-Property Relationships and Stress-Optical Law.

We present a combination of independent techniques in order to characterize crosslinked elastomers. We combine well-established macroscopic methods, such as rheological and mechanical experiments and equilibrium swelling measurements, a more advanced technique such as proton multiple-quantum NMR, and a new method to measure stress-induced segmental orientation by in situ tensile X-ray scattering. All of these techniques give access to the response of the elastomer network in relation to the crosslinking of the systems. Based on entropic elasticity theory, all these quantities are related to segmental orientation effects through the so-called stress-optical law. By means of the combination of these techniques, we investigate a set of unfilled sulfur-vulcanized styrene butadiene rubber elastomers with different levels of crosslinking. We validate that the results of all methods correlate very well. The relevance of this approach is that it can be applied in any elastomer materials, including materials representative of various industrial application, without prerequisite as regards, e.g., optical transparency or simplified formulation. Moreover, the approach may be used to study reinforcement effects in filled elastomers with nanoparticles.

Interplay between Crystallization and Entanglements in the Amorphous Phase of the Crystal-Fixed Polymer Poly(ϵ-caprolactone)

This work focuses on the influence of amorphous-phase entanglements on the semicrystalline morphology of poly(ϵ-caprolactone). This polyester is classified as crystal-fixed; i.e., it displays no translational chain dynamics in the crystals. We study a wide range of well-entangled samples with molecular weights up to several million using polarization microscopy to assess the lamellar growth rate, small-angle X-ray scattering and proton time-domain NMR to characterize the morphology, and proton multiple-quantum NMR, auxiliary carbon-13 NMR, rheology, and tensile deformation to assess the entangled chain dynamics in the melt and semicrystalline states. We demonstrate a significant increase in the density of entanglements in the amorphous phase relative to the melt. The dependencies of our observables on crystallization temperature and molecular weight suggest that entanglements control the thickness of the amorphous layers. This is rationalized by an only slowly relaxing exclusion zone with enhanced entangl...

High-performance recyclable cross-linked polyurethane with orthogonal dynamic bonds: The molecular design, microstructures, and macroscopic properties

Polyurethane materials (PUs) have been widely used in industry and daily life due to the versatile chemistry. However, despite the rapid advance in synthetic chemistry, it still remains a significant challenge for the facile fabrication of PUs with a single cross-linked network embedded with excellent mechanical properties and recyclability. Herein, in this study, we proposed a simple strategy to fabricate a high-performance recyclable cross-linked PU and revealed the relationship between microscopic structure and macroscopic properties. The UPy (2-ureido-4-[1H]-pyrimidione) motifs were incorporated into the backbone chains of PUs, where the quadruple hydrogen bonding interactions between UPy dimers can significantly enhance the mechanical strength and toughness. Furthermore, a single small molecular Diels-Alder adduct was utilized as the chemical crosslinker, rendering the final cross-linked PUs healable and recyclable. The thermal reversibility of the Diels-Alder reaction was well verified by DSC and solid-state NMR spectroscopy. Notably, it was found that the incorporation of UPy motifs could enhance the strain-induced crystallization (SIC), leading to a large stress at break. The structural and dynamic changes induced by SIC were quantitatively addressed by proton multiple-quantum NMR spectroscopy and SAXS experiments, where SIC further imposed restrictions on the mobility of surrounding polymer chains in the soft domain and led to the change of microphase separated nanostructures. Overall, a simple strategy is proposed here for the facile fabrication of high performance recyclable PUs, and the detailed investigation here on the structure-property relationship may further provide insights into developing high performance polymeric materials.

Time-Domain NMR Observation of Entangled Polymer Dynamics: Focus on All Tube-Model Regimes, Chain Center, and Matrix Effects

Proton multiple-quantum time-domain NMR combined with time–temperature superposition is a powerful method to study entangled chain dynamics. Overcoming the previous limitation to regimes II–IV of the tube model, this study extends the method to regime I (localized Rouse motions) by use of a pulse sequence adapted to shorter times, thus covering all relevant regimes for the model case of poly(butadiene) with molecular weights (M) between 10 and 200 kDa. We determine a value for the entanglement time that is consistent with current rheological results and confirm a value below 1 for the time scaling exponent of the segmental orientation autocorrelation function (OACF) in regime I previously observed by other NMR techniques. The origins of deviations from tube-model predictions are assessed by focusing on the dynamics of the chain centers in end-chain deuterated triblock samples and by dilution of probe chains to 15% in a deuterated matrix with M of 2 MDa. The study is complemented by self-diffusion coeffici...

Viscoelasticity and Structures in Chemically and Physically Dual-Cross-Linked Hydrogels: Insights from Rheology and Proton Multiple-Quantum NMR Spectroscopy

Hydrogels have received considerable attention as an innovative material due to their widespread applications in various fields. As a soft and wet material, its mechanical behavior is best understood in terms of the viscoelastic response to the periodic deformation, which is closely related to the microscopic chemically/physically cross-linked structures. Herein, a dual-cross-linked (DC) hydrogel, where a physically cross-linked network by ionic coordination (Fe3+) is imposed on a chemically cross-linked poly(acrylamide-co-acrylic acid) network, was studied in detail by rheology and proton multiple-quantum (MQ) NMR spectroscopy. Rheology experiments revealed the diverse temperature- and strain-frequency-dependent viscoelastic behaviors for DC hydrogels induced by the dynamic Fe3+ coordination interactions, in contrast to the single chemically cross-linked (SC) hydrogels. During the shear experiment, the trivalent Fe3+ complex with moderate/weak binding strength might transform to those with strong binding...

Effect of entanglements in the microstructure of cured NR/SBR blends prepared by solution and mixing in a two-roll mill

The influence of preparation methodology of vulcanized elastomeric blends of natural rubber (NR) and styrene butadiene rubber (SBR) on the phase morphology and network structure has been studied. Two different compounding methodologies have been applied: (i) mechanical mixing in a two-roll mill and (ii) blending in solution, by dissolving the elastomers in an appropriated solvent.Samples with different NR/SBR ratios were vulcanized by using a conventional cure system based on sulfur and TBBS (n-t-butyl-2-benzothiazole sulfenamide) as accelerator. The phase distribution of rubber matrices in the blends was investigated by transmission electron microscopy (TEM), identifying differences in the compounds according to the preparation method.Weak residual dipolar couplings caused by the existence of cross-links and topological constraints in vulcanized samples were characterized by using proton multiple-quantum (MQ) NMR spectroscopy. The structural results obtained by NMR and equilibrium swelling experiments were compared and complemented with viscosity measurements.It was found that the preparation method has a strong influence on the phase morphology of blends and the molecular network structure of each rubber phase, with an important variation in the entanglements contribution, which is more pronounced when the prepared blends are richer in NR.

Epoxy resin curing reaction studied by proton multiple-quantum NMR

This study was supported by the Spanish Ministry of Science and Innovation (MICINN) under project MAT2011/23455 and MAT2013/48107-C3. M.M.G. and A.G.J. thank the Consejo Superior de Investigaciones Cientificas (CSIC) for their fellowships JAE-Pre and Jae-Pre-CP, respectively. J.L.V. thanks the Spanish Ministry of Science and Innovation (MICINN) for his Ramon y Cajal contract

Influence of the vulcanization system on the dynamics and structure of natural rubber: Comparative study by means of broadband dielectric spectroscopy and solid-state NMR spectroscopy

A study on the effects of the curing systems on the dynamics and network structure of Natural rubber (NR) is performed by using broadband dielectric spectroscopy (BDS) and proton multiple-quantum NMR. Sulfur-cured systems present a restricted segmental dynamics, whereas peroxide-cured systems have the fastest dynamics. Network structure also affects the segmental dynamics of NR. Peroxide-cured networks seem to have more heterogeneous spatial distribution of cross-links, while sulfur-cured networks have inhomogeneities at larger length scales associated to ZnO particles. These structural dissimilarities possibly explain the segmental dynamics behavior and fragility strength of NR. This investigation shows that BDS and NMR are complementary approaches for studying the dynamics and structure of NR. The correspondence between both techniques confirms that the segmental dynamics of vulcanized NR is not solely affected by the number of cross-links, but also by their spatial distribution and nature of these junctions, e.g. C–C or sulfidic bridges with different lengths.

Microscopic Study of Chain Deformation and Orientation in Uniaxially Strained Polymer Networks: NMR Results versus Different Network Models

The complex pathway of transmitted stress and strain within elastic networks affects the conformations of single network chains in a complex and inhomogeneous manner. For a novel experimental assessment, we investigated moderately to highly cross-linked vulcanized natural rubber networks with proton multiple-quantum NMR and studied the correlation between macroscopic uniaxial elongation and local effects of strain and/or confinement on the level of elastically active subchains. Our method and the data treatment allow for a separate analysis of chain deformation and orientation, which in turn allows for critical comparisons of our experimental results with predictions from various network models. Specifically, comparisons with the affine fixed-junction model, the phantom model, and different tube models allowed us to draw conclusions on the presence of possibly tube-related local confinement effects. As none of the models alone was able to provide a comprehensive, quantitative description of all experimental data, we performed numerical calculations inspired by earlier simulation results, which match the data and support the presence of two distinct populations of network chains that are either more or less stretched as well as oriented.

Reinforcement in Natural Rubber Elastomer Nanocomposites: Breakdown of Entropic Elasticity

We propose an approach based on the combination of different techniques in order to discriminate various reinforcement effects in vulcanized natural rubber elastomers with various cross-link densities, filled with carbon black or silica: mechanical response, independent measurements of the cross-link density by proton multiple-quantum NMR, and measurements of chain segment orientation under stretching by X-ray scattering. We show that, while the modulus measured in dynamical mechanical measurements decreases as the strain amplitude increases (Payne effect), the response of the elastomer matrix in terms of average chain segment orientation under stretching measured by X-ray scattering stays constant. The amplification of average chain segment orientation is comparable to the amplification of modulus measured at medium/large strain amplitude. By analyzing the deviations with respect to the behavior of the pure unfilled elastomer matrix, we show that the contribution due to strain amplification effects in the elastomer matrix can be selectively distinguished. We show that the mechanical response at medium/large strains is essentially driven by strain amplification effects, while, in the linear regime, there is a strong additional reinforcement which is not related to the properties of the elastomer matrix. Hypothesis on the origin of this additional reinforcement are suggested and discussed.

Sulfur-Cured Natural Rubber Elastomer Networks: Correlating Cross-Link Density, Chain Orientation, and Mechanical Response by Combined Techniques

We present a combination of independent techniques in order to characterize vulcanized natural rubber elastomer networks. We combine solid state proton multiple-quantum NMR, equilibrium swelling, mechanical experiments, and in-situ tensile X-ray scattering measurements, all of them giving access to the segmental orientation effects in relation to the cross-linking of the systems. By means of the combination of these techniques, we investigate a set of unfilled natural rubber networks with different levels of cross-linking. The relevance of this work is the application of this approach in order to study the reinforcement effect in filled elastomers with nanoparticles in a following work.

Chain Dynamics and Strain-Induced Crystallization of Pre- and Postvulcanized Natural Rubber Latex Using Proton Multiple Quantum NMR and Uniaxial Deformation by in Situ Synchrotron X-ray Diffraction

The structural development and morphology in unvulcanized and vulcanized (both pre- and postvulcanized) natural rubber latex were studied in a relaxed state and under deformation by multiple-quantum (MQ) NMR and in situ wide-angle X-ray diffraction (WAXD), respectively. Vulcanization was carried out using both sulfur and peroxide, showing important differences on the spatial distribution of cross-links according to the source of vulcanizing agents. Sulfur prevulcanization promotes the formation of highly homogeneous networks in the dispersed rubber particles, whereas peroxide vulcanization makes broader spatial cross-link distributions. The latter is compatible with the formation of core–shell network structures. Molecular orientation and strain-induced crystallization were analyzed by both stress–strain relations and WAXD. An increase in the vulcanizing agent concentration led to an increase in modulus and crystalline fractions. For sulfur vulcanization, the additional heat treatment (postvulcanization) ...

Connectivity and Structural Defects in Model Hydrogels: A Combined Proton NMR and Monte Carlo Simulation Study

We present a study of the structure of Tetra-PEG model networks, using proton multiple-quantum NMR at low field in combination with computer simulations. Tetra-PEG is a novel high-performance hydrogel designed by combination of two symmetric tetra-arm macromonomers. In contrast to conventional hydrogels, which are highly heterogeneous due to fixed concentration fluctuations, Tetra-PEG exhibits a much less heterogeneous microstructure as indicated by previous light and small-angle neutron scattering studies. Here, the local-scale connectivity inhomogeneities, i.e., the sol and dangling polymer chains as well as the typical network connectivity defects resulting from the A–B reaction of four-arm macromonomers, are quantified experimentally for the first time, studying as-prepared Tetra-PEG hydrogels, cross-linked at different polymer concentrations and stoichiometries. To this end, we developed a novel approach for the analysis of double-quantum buildup curves consisting of well-distinguishable components with different segmental dynamic order parameters, benefitting from the superb large-scale homogeneity of the samples. As a model for each component we suggest different connectivity modes between the macromonomers—mainly regular single links and double links between individual stars as well as other network defects with lower order parameters. To support the model, we report results from computer simulations with the bond-fluctuation model, which confirm the concentration-dependent trends of the network and double-link fractions.

Time-Domain NMR Observation of Entangled Polymer Dynamics: Universal Behavior of Flexible Homopolymers and Applicability of the Tube Model

Using a benchtop NMR spectrometer, we performed proton multiple-quantum NMR experiments to study the dynamics of well-entangled linear poly(butadiene), poly(isoprene), and poly(dimethylsiloxane) in a broad molecular weight range. The method provides a direct time-domain measurement of the segmental orientation autocorrelation function over many decades, based on time−temperature superposition. The function can be directly compared to theoretical predictions based on the tube model, and the Rouse and the disentanglement times can be evaluated relative to the known entanglement time. We obtain universal results for all three polymers when plotted against the molecular weight normalized by the entanglement molecular weight, with the familiar mass scaling exponents of 2.0 ± 0.2 and 3.2 ± 0.2, respectively. However, the time scaling exponent derived for motions in the constrained-Rouse regime II is not constant at the predicted value of −1/4 but is always larger and also a universal function of molecular weigh...

Precise dipolar coupling constant distribution analysis in proton multiple-quantum NMR of elastomers

In this work we present an improved approach for the analysis of (1)H double-quantum nuclear magnetic resonance build-up data, mainly for the determination of residual dipolar coupling constants and distributions thereof in polymer gels and elastomers, yielding information on crosslink density and potential spatial inhomogeneities. We introduce a new generic build-up function, for use as component fitting function in linear superpositions, or as kernel function in fast Tikhonov regularization (ftikreg). As opposed to the previously used inverted Gaussian build-up function based on a second-moment approximation, this method yields faithful coupling constant distributions, as limitations on the fitting limit are now lifted. A robust method for the proper estimation of the error parameter used for the regularization is established, and the approach is demonstrated for different inhomogeneous elastomers with coupling constant distributions.

Uncertainties in the Determination of Cross-Link Density by Equilibrium Swelling Experiments in Natural Rubber

Equilibrium swelling is a feasible and simple experiment to determine the cross-link density of networks. It is the most popular and useful approach; however, in most of the cases, the given values are highly uncertain if not erroneous. The description of the complex thermodynamics of swollen polymer networks is usually based on the Flory−Rehner model. However, experimental evidence has shown that both the mixing term described by the Flory−Huggins expression and the elastic component derived from the affine model are only approximations that fail in the description and prediction of the rubber network behavior. This means that the Flory−Rehner treatment can only give a qualitative evaluation of cross-link density because of its strong dependence on the thermodynamic model. In this work, the uncertainties in the determination of the cross-link density in rubber materials by swelling experiments based on this model are reviewed. The implications and the validity of some of the used approximations as well as their influence in the relationship of the cross-link densities derived from swelling experiments are discussed. Importantly, swelling results are compared with results of a completely independent determination of the cross-link density by proton multiple-quantum NMR, and the correlation observed between the two methods can help to validate the thermodynamic model.

Gelation as Studied by Proton Multiple-Quantum NMR

A novel NMR approach for the study of gelation in flexible polymer systems is presented, and the data are compared to results from rheological and light scattering experiments. Proton multiple-quantum NMR, also applicable at low field and amenable to high-throughput experimentation, represents a simple and robust yet powerful method that allows one to detect the topological gel point, quantify the gelation kinetics, and characterize composition, microstructure, and dynamics of the forming and the final gel. Applications to end-linked and statistically cross-linked poly(dimethylsiloxane) chains in the bulk, as well as statistically cross-linked poly- (styrene) chains in solution are presented, and good agreement as well as complementarity between the results from the different methods is found.

Chain Dynamics in Elastomers As Investigated by Proton Multiple-Quantum NMR

The influence of segmental and cooperative dynamics in elastomers on the time evolution in NMR experiments is considered using static proton multiple-quantum experiments. A novel experimental strategy as well as Monte Carlo simulations of a simple rotational diffusion model is used to investigate the applicability of the Andersen−Weiss approximation, upon which our analytical solutions are based. On the basis of temperature-dependent experiments, the validity of one of the most popular models used so far for the interpretation of NMR experiments is disproved. This model, which is based on slow rotational diffusion of a preaveraged residual coupling tensor, is refined by explicit consideration of the time scale of fast segmental processes which average the intrasegmental dipolar coupling toward a plateau value that is related to the chain order parameter associated with topological restrictions by chemical and physical cross-links. Models based on exponential or power-law loss of correlation are shown to provide physically reasonable representations of the data. As opposed to various earlier approaches, multiple-quantum NMR is unique in that it can be used to verify or falsify the different models. Data measured on permanently cross-linked systems are only weakly influenced by slow, cooperative processes, while reptation dynamics in corresponding linear-chain melts effectively prevents the observation of a well-defined order parameter.

Chain Order and Cross-Link Density of Elastomers As Investigated by Proton Multiple-Quantum NMR

We present a quantitative study of local segmental order in vulcanized natural and butadiene rubber far above the glass transition. Network chain order is dependent on the density of cross-links an...

Swelling Heterogeneities in End-Linked Model Networks: A Combined Proton Multiple-Quantum NMR and Computer Simulation Study

The segmental order of monomodal and bimodal polymer networks is investigated by proton multiple-quantum NMR spectroscopy as applied to poly(dimethylsiloxane) model systems and by Monte Carlo simulations using the bond fluctuation model. The multiple-quantum method is sensitive to chain order parameter distributions and therefore probes heterogeneities in the extent of fast motional averaging of individual network strands subject to topological constraints. These data are in qualitative agreement with the simulations. We find a broadening of the chain order parameter distribution upon swelling accompanied by a nonaffine change of this distribution, indicating a heterogeneous swelling process. Comparing the simulated tensor order parameter with the autocorrelation function of segments, we observe major deviations in the swollen state but also for the long-chain fraction of the dry, bimodal network. These effects are attributed to the fluctuation dependence of the tensor order parameter which comprises information about both orientational order as well as fluctuation properties of fast segment reorientation processes. In general, our results indicate that segmental order is not simply related to the cross-link density. Fluctuations on larger scales such as reorientation of chain clusters, dynamical correlations between segments in the entangled state, and heterogeneities in the segmental fluctuations have an essential influence on the tensorial order. The spatial length scale of the heterogeneities is also investigated by self-diffusion measurements of the solvent molecules.