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We use low-resolution proton NMR to probe the chain deformation in swollen and nonlinearly deformed vulcanized rubber and end-linked PDMS networks on a microscopic level, extending earlier work focusing on uniaxial stretching and isotropic dilation upon swelling toward biaxial deformation and deformation of swollen samples. Previous studies have revealed that chain deformation in bulk samples is best described by tube models, and that chains in swollen samples deform affinely after an initial desinterspersion stage, upon which entanglement-related packing effects are relieved. We test whether a subsequent deformation may also be closer to affine, and find that this is not the case. Unexpectedly, nonisotropic deformation of swollen samples also follows tube-model predictions, which is explained by a dominance of structural inhomogeneities and significant reorganization of the topological constraints active in the swollen and possibly even the bulk state., Partial funding was provided by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) in the framework of the projects SA982/6-1, SA982/9-1, and SA982/ 11-1. A.N. thanks the team at Bahıá Blanca for the generous support provided during her research stay. J.L.V. acknowledges the support provided by the MAT2014-52644-R, MAT2017- 87204-R, and 201860E045 projects.

The outstanding properties of carbon nanotubes (CNTs) present some limitations when introduced into rubber matrices, especially when these nano-particles are applied in high-performance tire tread compounds. Their tendency to agglomerate into bundles due to van der Waals interactions, the strong influence of CNT on the vulcanization process, and the adsorptive nature of filler¿rubber interactions contribute to increase the energy dissipation phenomena on rubber¿CNT compounds. Consequently, their expected performance in terms of rolling resistance is limited. To overcome these three important issues, the CNT have been surface-modified with oxygen-bearing groups and sulfur, resulting in an improvement in the key properties of these rubber compounds for their use in tire tread applications. A deep characterization of these new materials using functionalized CNT as filler was carried out by using a combination of mechanical, equilibrium swelling and low-field NMR experiments. The outcome of this research revealed that the formation of covalent bonds between the rubber matrix and the nano-particles by the introduction of sulfur at the CNT surface has positive effects on the viscoelastic behavior and the network structure of the rubber compounds, by a decrease of both the loss factor at 60 °C (rolling resistance) and the non-elastic defects, while increasing the crosslink density of the new compounds., his research was funded by Ministerio de Economía y Competitividad: MAT2014-52644-Rand MAT2017-87204-R; Ministerio de Ciencia, Innovación y Universidades: RTI2018-096636-J-I00;Consejo Superior de Investigaciones Científicas: 201860E045; Ministerio de Ciencia e Innovación:BES-2015-071516

The major controlling factors that determine the various mechanical properties of an elastomer system are type of chemical crosslinking and crosslink density of the polymer network. In this study, a catalytic amount of acrylonitrile butadiene copolymer (NBR) was used as a co-accelerator for the curing of polybutadiene (BR) elastomer. After the addition of this copolymer along with other conventional sulphur ingredients in polybutadiene compounds, a clear and distinct effect on the curing and other physical characteristics was noticed. The crosslinking density of BR was increased, as evidenced by rheometric properties, solid-state NMR and swelling studies. The vulcanization kinetics study revealed a substantial lowering of the activation energy of the sulphur crosslinking process when acrylonitrile butadiene copolymer was used in the formulation. The compounds were also prepared in the presence of carbon black and silica, and it was found that in the carbon black filled system the catalytic effect of the NBR was eminent. The effect was not only reflected in the mechanical performance but also the low-temperature crystallization behavior of BR systems was altered.Materials science; Materials chemistry; Crosslinking accelerator; Sulphur network; Solid state NMR; Curing kinetics; Activation energy; Acrylonitrile butadiene; Polybutadiene; Low-temperature; Crystallization., This work was partially funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - within the Research Training Group RTG 2430 "Interactive Fiber Elastomer Composites".

There are currently many well-established applications for recycled rubber from end-of-life tires (ELT), but it is essential to investigate and seek new approaches to enhance the value of these products. Recent developments in new technologies and innovative recycling and devulcanization processes have opened up new perspectives for ELT crumb rubber. To promote the use of these products in newly added value applications, it is essential to develop and optimize methods that allow the characterization of parameters related to the ultimate properties of potential final applications. In this respect, a novel characterization methodology based on advanced 1H double-quantum (DQ) nuclear magnetic resonance experiments has been applied for the first time to quantify the key parameters that characterize the structure of ELT crumb rubber after diverse recycling processes: from simple mechanical grinding to complex devulcanization methods. This experimental approach enables the quantification of parameters that define the network structure of rubber, such as the nonelastic network defects (sol fraction, dangling chain ends, loops), the cross-link density, and the heterogeneity of the network, directly from rubber granulate and powder (without any additional sample preparation steps), overcoming most of the drawbacks and uncertainties that limit the application of traditional rubber characterization methods (e.g., equilibrium swelling experiments). By applying this experimental approach, it is possible to identify and quantify the actual technical limits for a complete selective devulcanization process of ELT crumb rubber., The authors gratefully acknowledge the support given by Signus Ecovalor. We also thank Valoriza Medioambiente and Lehigh Technologies Inc. for the supplied crumb rubber samples. Financial support from Ministerio de Econom´ıa y Competitividad (MAT2017-87204-R), Ministerio de Ciencia, Innovacion y Universidades (RTI2018-096636-J-I00), and CSIC ´ (201860E045) is also gratefully acknowledged. Authors J.L.V., A.F.-T., P.P., R.H., F.M.S., and R.N. are members of the SusPlast platform from the Spanish National Research Council (CSIC)

For the first time since its formulation in 1986, the theoretical approach proposed by Helmis, Heinrich and Straube (HHS model), which considers the contribution of topological restrictions from entanglements to the swelling of polymer networks, is applied to experimental data. The main aspects and key equations are reviewed and their application is illustrated for unfilled rubber compounds. The HHS model is based on real networks and gives new perspectives to the interpretation of experimental swelling data for which the entanglement contributions are usually neglected by considering phantom network models. This investigation applies a reliable constrained-chain approach through a deformation-dependent tube model for defining the elastic contribution of swollen networks, which is one of the main limitations on the applicability of classical (affine) Flory-Rehner and (non-affine) phantom models. This short communication intends to provide a baseline for the application and validation of this modern approach for a broader class of rubber materials., The authors thank Goodyear S. A. for the permission to publish this paper. The present project was supported by the National Research Fund Luxembourg (project ref. 6916525), whose financial support is acknowledged. JLV thanks the Ministerio de Ciencia e Innovación (Spain) for his Ramon y Cajal contract.

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. Fil: Mansilla, Marcela Angela. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Instituto de Física de Materiales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Valentín, J. L.. Consejo Superior de Investigaciones Cientificas; España Fil: López Manchado, M. A.. Consejo Superior de Investigaciones Cientificas; España Fil: González Jiménez, A.. Consejo Superior de Investigaciones Cientificas; España Fil: Marzocca, Angel José. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina

A new coumarin diol was designed and used as initiator for the ring opening polymerization of e-caprolactone. A photo-reactive polyurethane based on the resulting polycaprolactone diol with pendant coumarin moieties was synthesized. Photo-dimerization/photocleavage reactions were studied by UV and Raman. The linear non-irradiated soft polyurethane transformed into a tough elastomeric crosslinked material. DQ-NMR measurements showed that conversion of the photo-reaction was not directly related to relative amount of crosslinking but provided complementary information. Crosslinking reduced the crystallinity of the polycaprolactone segments and the melting point of the crystals.

This study evaluates the formation of radical species in natural rubber (NR) and poly-isoprene rubber (IR) during the vulcanization process and the uniaxial deformation of the formed networks by means of Electron Spin Resonance (ESR). Vulcanization of NR and IR always shows a radical pathway, where the different vulcanization systems dictate the concentration of radical species in the course of this complex process. The greatest concentration of radicals were detected during the vulcanization with sulfur/accelerator based on efficient systems (EV), followed by conventional (CV) and sulfur donor systems, whereas azide and organic peroxide agents showed smaller concentration of radicals. Independently of the vulcanization system, certain amount of radicals was detected on the vulcanized samples after the end of the vulcanization process. Comparison between different matrices demonstrates that NR always shows higher concentration of radicals than IR in the vulcanization process as well as during uniaxial deformation, fact that could be associated to the presence of non- rubber components in NR.

The outstanding properties of carbon nanotubes (CNT) have generated a lot of interest at a scientist and industrial level. CNT have attracted great attention due to their exceptional mechanical, electrical and thermal properties and also for their large aspect ratio, that is, small size and higher surface area. On this regard, CNT-rubber composites have been prepared, using natural rubber (NR) and styrene butadiene rubber (SBR) as matrices, and varying the amount of filler from 0 to 20 phr. In order to understand and optimize the properties of these materials, a fundamental and systematic study has been done with the aim of quantifying the relative contribution of the different factors that determine the reinforcing effect of these nanoparticles. The results obtained from the different characterizations of the composites (study of the electrical and mechanical properties, Payne effect, dynamical mechanical analysis, low field NMR and swelling experiments) reveal a good filler-rubber interaction, a high reinforcement effect, and an increase on the electrical conductivity of the compounds. However, all these improvements are limited by the detrimental effect of the CNT on the vulcanization process., The authors thank Spanish Ministerio de Economia y Competitividad by the financial support of this work (project references MAT2014- 52644-R and MAT2017-87204-R). Pilar Bernal-Ortega thanks the Ministerio de Ciencia e Innovación (Spain) for his FPI contract (ref. BES- 2015-071516). Authors JLV, PP and RN are members of the SusPlast platform from the Spanish National Research Council.

Robust quantitative crosslink density characterization becomes necessary for the complete understanding of the structure and optimization of final properties of rubber compounds for industrial applications. A combination of different experimental techniques have been used to establish the quantitative consistency on the correlations between the results obtained by the individual methods within a reliable unique (physically based) platform reclined on the concept of rubber elasticity that considers the impact of entanglements in technical rubbers. The contribution of crosslinks and elastically active entanglements to mechanical properties has been quantified by the analysis of uniaxial stress-strain measurements by means of the extended tube model of rubber elasticity. In a complementary manner, rubber network structure has also been investigated by state-of-the-art multiple-quantum low-field NMR experiments and classical T1 and T2 relaxation measurements. In addition, equilibrium swelling data were analyzed by the classical phantom and Flory-Rehner limits, as well as by applying the theoretical approach proposed by Helmis, Heinrich, and Straube that takes into account topological constraints during swelling. Correlations among these complementary techniques have been reported, and the interpretation of the obtained differences is addressed. The baseline study focuses on unfilled NR, setting the basis for the investigation of unfilled SBR matrices and filled rubbers., The authors thank Goodyear S.A. for the permission to publish this paper. The present project is supported by the National Research Fund Luxembourg, whose fi nancial support is acknowledged. JLV thanks the Ministerio de Ciencia e Innovación (Spain) for his Ramon y Cajal contract.

The use of modern multiple-quantum proton NMR experiments for the determination of cross-link density requires precise knowledge of several model-dependent physical and structural quantities, like the dipolar static frequency or the definition of vector segmental order. In this paper, different models for describing segmental order of the polymer backbone, based on different assumptions about the contribution of crosslinks and entanglements, are critically reviewed and applied for the analysis of unfilled natural rubber samples with average mass between cross-links and entanglements determined from network tube model fittings of stress−strain data. A recent theoretical model developed by Lang and Sommer, which allows for the consideration of junction fluctuations, is adapted for the analysis of NMR experimental results. After having verified the correlation between the calculations with this enhanced theoretical treatment and the residual dipolar coupling from multiple-quantum NMR experiments carried out in a low-field spectrometer, a new simplified approach to determine the segmental order parameter is proposed for sulfur-cured rubbers, The authors thank Goodyear S. A. for the permission to publish this paper and Prof. J. U. Sommer and Dr. M. Lang (IPF Dresden) for fruitful discussions. The present project was supported by the National Research Fund Luxembourg (project reference 6916525), whose financial support is acknowledged. J.L.V. thanks the Ministerio de Ciencia e Innovación (Spain) for his Ramon y Cajal contract.

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.

Shape-memory elastomers based on a commercial rubber cross-linked by both ionic and covalent bonds have been developed. The elastomeric matrix was a carboxylated nitrile rubber (XNBR) vulcanized with magnesium oxide (MgO) providing ionic interactions that form hierarchical structures. The so-named ionic transition is used as the unique thermal transition responsible for the shape-memory effect (SME) in these elastomers. These ionic interactions fix the temporary shape due to their behavior as dynamic cross-links with temperature changes. Covalent cross-links were incorporated with the addition of different proportions of dicumyl peroxide (DCP) to the ionic elastomer to establish and recover the permanent shape. In this article, the SME was modulated by modifying the degree of covalent cross-linking, while keeping the ionic contribution constant. In addition, different programming parameters, such as deformation temperature, heating/cooling rate, loading/unloading rate and percentage of tensile strain, were evaluated for their effects on shape-memory behavior., The authors thank CICYT (MAT2011-23476 and MAT2014-52644-R projects) and Consejo Superior de Investigaciones Cientifı´cas, CSIC (201660E088 project) for the financial support of this work. MAM would also like to express her gratitude to CSIC for her fellowships Jae-Pre-088. AGJ would also like to express his gratitude to CSIC and MECD (Spain) for his Jae-Pre-CP and FPU fellowships, respectively. JLV thanks Ministerio de Ciencia e Innovacio´n (Spain) for his Ramo´n y Cajal contract. PTM thanks Syracuse University and the Syracuse Biomaterials Institute for facilities provided for MAM’s extended visit, as well as support from the National Science Foundation, NSF CMMI-1334658.

The network structure and chain dynamics of ionic elastomers based on carboxylated nitrile rubber (XNBR) cross-linked with different content of magnesium oxide (MgO) have been studied by different low-field time-domain NMR experiments. Ionic contacts created during the vulcanization tend to aggregate trapping some polymer segments that show restricted mobility as it was quantified by analyses of refocused free induction decays. Increasing the MgO content above the stoichiometric fraction has no effect on the amount of trapped polymer segments, but it increases the network cross-link density as measured by multiple-quantum (MQ) NMR experiments. The central finding of this work is that MgO addition above the stoichiometric content enhances the mechanical properties by creating a larger number of smaller ionic clusters, which act as dynamic cross-links, but are not readily seen by other techniques. Changes in the network structure and morphology of segregated thermolabile ionic domains have an impact on the ionic rearrangement dynamics and, in consequence, on the thermoplastic behavior of these materials at elevated temperatures.

A molecular-level understanding of the strain response of elastomers is a key to connect microscopic dynamics to macroscopic properties. In this study we investigate the local strain response of vulcanized, natural rubber systems and the effect of nanometer-sized filler particles, which are known to lead to highly improved mechanical properties. A multiple-quantum NMR approach enables the separation of relatively low fractions of network defects and allows to quantitatively and selectively study the local deformation distribution in the strained networks matrix on the microscopic (molecular) scale. We find that the presence of nondeformable filler particles induces an enhanced local deformation of the matrix (commonly referred to as overstrain), a slightly increased local stress/strain heterogeneity, and a reduced anisotropy. Furthermore, a careful analysis of the small nonelastic defect fraction provides new evidence that previous NMR and scattering results of strained defect-rich elastomers cannot be in...

Thermoporosimetry is known as an analytical method to investigate network structures in swollen elastomers. Despite this interesting potential capability, only few investigations are published in literature which apply different solvents on differently crosslinked natural rubber samples. The choice of the solvent is critical as it strongly influences the separation of confined and bulk solvent transition signal, the confined solvent signal intensity and the peak width representing the pore size resolution. No critical comparison has been done in these investigations regarding the solvent choice related to peak separation, pore size resolution or signal intensity. Furthermore, no critical assessment is available relating thermoporosimetry results to solvent parameters to identify an optimal solvent regarding the mentioned criteria. This argument motivated the present investigation to identify the most appropriate solvents for the analysis of natural rubber compounds by thermoporosimetry. Different types of solvents (aliphatic, aromatic, non-polar, and polar) were selected and benchmarked against each other. It was concluded that n-heptane was identified as the best solvent for these investigations. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43998.

A step forward in the understanding of rubber vulcanization with organic peroxides is provided by combining a proper arrangement between polymer, vulcanizing agent and cure conditions. For this purpose, an ethylene–vinyl acetate copolymer with a high content of vinyl acetate (70 mol%) was used since a fully saturated polymer backbone allows its vulcanization via peroxide. For the range of conditions analysed here, it is shown that the predominant process taking place is crosslinking via radical recombination, minimizing or even avoiding undesirable secondary reactions such us polymer degradation. Once conditions had been optimized, peroxide vulcanization was analysed in more depth in the presence of 2,2,6,6-tetramethylpiperidinyloxyl, which is a mediating stable free radical commonly used in controlled radical polymerization. Consequently, it was possible to differentiate the termination reaction from the initiation and propagation steps, allowing the determination of the enthalpy of formed C–C crosslinks as measured using calorimetry. It was possible to isolate and determine the contribution of the crosslinking pathway from the global vulcanization reaction by means of calorimetric methods at optimum conditions. In fact, this simple methodology could be an important tool for understanding in detail the complex peroxide vulcanization of elastomers since reactions involved in this process determine the final network structure, and thus the final elastic properties of these compounds.

Twenty seven poly(ester–urethane–amide)s (PEUAs) were synthesized from the reaction of α,ω-telechelic poly(e-caprolactone) diols, 1,6-hexamethylene diisocyanate, and three different diamide–diol chain extenders derived from e-caprolactone and three aliphatic diamines with increasing length (n = 2, 4, and 6). The hard segment (HS) was designed to be crystalline and to introduce amide groups, more susceptible to hydrolysis than urethane or urea groups. Crystallization of the HS was achieved even at very low HS content, leading to a phase-separated material. Furthermore, HS crystallinity recovered from the material homogeneous melt on cooling. Thermal degradation was well above HS melting point, allowing for melt processing of these materials. HS melting point was affected by HS content, poly(e-caprolactone) diol length, and chain extender length. The length scale of the phase-separated morphology lied in between 11 and 13 nm up to ∼60% HS content and decreased at higher HS content values. HS content was the...

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) ...

Organic–inorganic hybrid nanoparticles of poly(styrene) (PSt) and fumed silica have been successfully synthesized via atom transfer radical polymerization (ATRP) and investigated as fillers for styrene–butadiene rubber (SBR). On the one hand, they were prepared by using the grafting from methodology, in which an ATRP initiator was covalently attached onto the silica and subsequently, PSt chains were grown directly from the surface. On the other hand, the grafting to methodology was used to attach onto the silica, a carboxylic acid end-functionalized PSt, previously synthesized by ATRP. Then, these hybrid nanoparticles could be easily incorporated as fillers into SBR elastomeric matrices by conventional methods employed in rubber technology. The properties of these composites, analyzed by strain sweep tests, scanning electron microscopy, dynamic mechanical measurements, 1H low field double quantum NMR and tensile strength tests, were compared with those obtained using unmodified silica as the filler. The presence of silica particles coated with PSt brushes deeply affects the dynamic and physical response of the elastomer composites prepared, due to the high dispersion and excellent compatibility between the SBR matrix and PSt coated silica. Moreover, the experimental results also indicated that hybrid nanoparticles synthesized by the grafting from methodology are much better for filler applications in the SBR matrix than those obtained by using the grafting to methodology.

The effect of the temperature on the kinetics of vulcanization of natural rubber with the sulfur donor agent dipentamethylene thiuram tetrasulphide (DPTT) was investigated. The oscillatory disk rheometer and differential scanning calorimetry under dynamic and isothermal conditions were used. The Coran's model was applied to set the kinetic constants within each distinct step of the vulcanization process using the obtained values of the rheometer curves. During the curing period it was observed that there were two stages. Either the torque increment (ΔS) or the crosslink density arise a maximum value at 130°C. The dynamic and isothermal thermograms showed two overlapped peaks during the curing reaction. These results were in concordance with those obtained by curemeter testing. In addition, the activation energy of the vulcanization process was also calculated by means of both techniques. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

The cure of polybutadiene rubber (BR) with sulfur donor dipentamethylene thiuram tetrasulfide (DPTT) does not show reversion reaction, in contrast with similar natural rubber compounds. No polysulfure links are formed; whereas a great amount of -C-C- crosslinks are produced. The addition of tetramethyl thiuram monosulfidic (TMTM) strongly affects the crosslinking process. Mechanical properties of the cured compounds are poor. In DPTT, curing of BR the radical species produced through homolitic cleavage of DPTT molecule seems to have sufficient energy to produce crosslinking, via allylic ion, and also initiate the polymerization of double bonds producing areas of high crosslinking density that induce early material failure.

In order to analyze the effect of an epoxidized natural rubber (ENR) and filler treatment on the morphology and behavior of natural rubber (NR) nanocomposites, blends of these polymers have been prepared. The nature and extent of the clay dispersions in the filled samples were evaluated by X-ray diffraction. In the presence of ENR, an exfoliated structure was obtained which suggests that enough rubbery polymer was incorporated into the interlayer spacing. The effect of clay in rubber compounds was analyzed through rheological, mechanical and swelling characterization. A sensible improvement in the nanocomposite properties was observed by the addition of organoclay. It has been deduced that the properties of the compounds strongly depend on the extent of the silicate nanolayers dispersion into the rubber matrices as well as on the organoclay type and elastomer compatibility.

Acid activation of sepiolite removed the brucitic magnesium octahedral layers, while the tetrahedral sheets formed fibrous and amorphous silica. The coexistence of both structures, sepiolite and free silica after the acid treatment, causes changes in the structural behavior of the sepiolite at elevated temperatures. Whereas, the structure of pristine sepiolite is folded by the elimination of the coordinated water molecules, whereas acid pre-treated sepiolite, remains unfolded after the treatment at 550 °C. Therefore, a novel anhydrous unfolded sepiolite structure with empty and open channels was obtained and characterized by X-ray diffraction and 29 Si solid state NMR for the first time.

Polypropylene membranes modified with natural and organically modified montmorillonite clays were prepared. The permeability, diffusivity and solubility of helium, oxygen and nitrogen were determined for the unfilled and filled membranes over the temperature range 25–65 � C. Physical properties of polypropylene membranes were investigated using X-ray diffraction, thermogravimetric analyser, tensile testing and differential scanning calorimetry. The results showed that the filled membranes exhibit lower gas permeability compared to the unfilled polypropylene membrane. For helium, a reduced diffusivity is mainly responsible for the reduction in the permeability, in contrast, for nitrogen and oxygen, both diffusivity and solubility were reduced by the presence of fillers. The X-ray diffraction spectra showed that the incorporation of the unmodified and modified clay did not affect the crystallographic nature of polypropylene. � 2007 Elsevier Ltd. All rights reserved.

7 páginas, 5 figuras, 5 tablas., Novel millable polyurethane (PU)/organoclay nanocomposites have been successfully prepared by conventional transformation techniques. One natural (C6A) and two organically modified (C15A and C30B) montmorillonites have been used as clays for preparing PU nanocomposites. The optimum dispersion of nanofiller at a nanometer scale in PU matrix was confirmed by X-ray diffraction patterns and transmission electron microscopy. A substantial improvement of the PU properties by addition of only a small amount of organoclay was observed. It is worthy to note that the organoclays show a different interfacial interaction with the PU matrix, which was reflected in different macroscopic properties. Thus, C30B organoclay seems to react with PU chains to form covalent bonds, while C15Aonly interacts physically with PU chains. Mechanical and barrier properties are analyzed., The authors wish to thank the CICYT, MAT 2004-00825 and the Spanish Ministry of Science and Technology (MCYT), PPQ2003-03299, for financial support and Dr. López-Manchado for the concesion of a Ramón y Cajal contract (MCYT).

The electron beam irradiation of a fully aliphatic thermoplastic polyetherurethane was carried out in air at doses ranging from 50 to 4000kGy at room temperature and 100°C and a dose rate of 5.1kGymin-1. The effects of the irradiation on the chemical structure and the physical properties were studied. Gel was obtained already at the lower dose, increasing to almost 100% gel at the higher dose, with cross-linking predominating over chain scission. FTIR and DSC measurements proved that both, soft segment and hard segment, were affected by irradiation. Mechanical properties were adversely affected by irradiation mainly due to the cross-linking produced in the amorphous part of the material. Irradiation temperature did not have any significant effect except at the highest dose of 4000kGy.

This study demonstrates that the curing reaction of thermosetting polymers, in particular, epoxy resins cured with aliphatic amines, can be characterized by different multiple-quantum (MQ) NMR experiments performed in low-field spectrometers. Measurements of the curing reaction at 40 C, 70 C, and 100 C are carried out to obtain the kinetic parameters like induction time, vitrification time, polymerization rate, and activation energy being consistent with data obtained by other traditional and complementary techniques. Besides, it is demonstrated that 1H MQ-NMR spectroscopy is a successful approach to overcome the experimental challenge arising from the characterization of high dipolar-coupled polymers to study the network structure and segmental dynamics of thermosetting polymer networks., 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 Cientifıcas (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

The scope of this study is to continue our earlier studies on the peroxide curing of diene elastomers. The effect of peroxide content and temperature on the curing and mechanical properties of NBR and H-NBR rubbers with different acrilonitrile content was evaluated. Experimental evidence indicates that saturated rubbers behave as expected whereas in unsaturated nitrile rubbers abstraction of allylic hydrogen and addition to the double bonds can act as mechanism of crosslinking, the weight of each mechanism is dependent of DCP content and curing temperature. The addition mechanism produces densely crosslinked zones or clusters, generating a heterogeneous network with effect on the vulcanizate properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3377–3382, 2007

The effect of oleyl amine on processing and physical properties of SBR compounds filled with silane–silica particles has been evaluated. Two different types of silane molecules have been used as coupling agents to minimize the silica–silica interactions and reduce the formation of secondary structures of silica, in addition with an increment in the filler–rubber interactions. Significant differences between those compounds have been found according with the silane structure. However, in both cases, the dynamic, rheological, and mechanical properties of the filled rubber compounds were improved after the incorporation of oleyl amine molecules. This fact could be related with the capacity of the amine molecules to interact with the free silanol groups of silane-modified silica forming an amine-modified silica complex, which reduces the hydrophilic nature of the silica surface. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1806–1814, 2007

The effects of silica and silane modified silica fillers on the pervaporation properties of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) dense membranes have been studied. Crystallinity, thermal and mechanical properties of unfilled and filled PPO membranes with silica and silane modified silica nanoparticles were investigated. The surface energy together with the solubility parameters of the membranes and the nanoparticles were determined. Pervaporation separation of methanol/methyl tert butyl ether (MTBE) mixtures over the entire range of concentration were carried out using both filled and unfilled membranes. The results are discussed in terms of the solubility and the diffusivity of each liquid component in the membranes. Flory‐Huggins theory was used to predict the sorption methanol selectivity. Compared to the unfilled PPO membrane, the filled PPO membranes exhibit higher methanol selectivity and lower permeability. For methanol concentration in liquid feed mixture lower than 50 wt%, methanol selectivity of the filled PPO membranes with silane modified silica is better than that of the silica filled and unfilled PPO membranes. q 2005 Elsevier Ltd. All rights reserved.

The scope of this article is the study of peroxide curing of two nitrile rubbers with low and high nitrile content. The peroxide efficiency can be much higher than one, and the polymer structure determines the mechanism of cross-linking. In the rubber with low nitrile content, the peroxide radical may give rise to a polymerization reaction between adjacent double bonds generating a heterogeneous network with a negative effect on the vulcanizate properties. On the contrary, in the nitrile rubber with high nitrile content, this negative effect it is not present or is present to a lesser extent, and their vulcanizates show good physical properties. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1–5, 2005

A novel experimental approach based on the freezing-point depression of a solvent in a swollen gel has been developed to characterize the structure of nanocomposites. A higher depression in the freezing point has been related with an exfoliated nanocomposite. This increased depression not only depends on the formation of a tighter network but also on the decrease of the size of the solvent cages where the nucleation takes place.

The influence of tetramethylthiuram monosulfide (TMTM) in the vulcanization process of natural rubber and butadiene rubber with dipentamethylene thiuram tetrasulfide (DPTT) was investigated. Vulcanization parameters as reaction rate and reversion varied when TMTM was added in NR-DPTT formulations. Crosslinking density and proportion of monosulfidic crosslinks increased with TMTM content increase. However, a drop in physical properties was observed at high TMTM content and at long times of curing well over t 97 . Formation of densely crosslinked areas due to polymerization reaction was postulated as the origin of these variations. In BR-DPTT compounds, torque and monosulfidic crosslink proportions increased when TMTM was added. Polymerization reaction takes place in BR systems too. However, in BR vulcanizates, C-C bonds decreased with TMTM proportion, whereas in NR vulcanizates increased.

We study the impact of excluded volume interactions on the orientation statistics of chain segments in polymer gels, and show that nuclear magnetic resonance (NMR) experiments provide a direct and unique measure of excluded-volume effects on the chain statistics. In particular we consider the tensor order parameter, which can be expressed as the second Legendre polynomial of the segment orientation with respect to a fixed end-to-end distance vector and which is directly related to the residual coupling constant obtained in NMR experiments. We provide analytical results for the case of single chains in a good solvent and for semidilute solutions. Computer simulations using the bond fluctuation model are applied to compare with the analytical predictions. Considering polymer gels at the equilibrium state of swelling we predict a unique relation between the tensor order parameter and the correlation length (blob size) of the gel. Experiments applying multiple-quantum NMR methods to both end-linked and randomly cross-linked polymer networks are in excellent agreement with this prediction. The initial decay of the tensor order parameter as observed in experiments at low and intermediate degrees of swelling can be explained as a solvent effect without making additional assumptions about constraint release processes during swelling.

13 páginas, 11 figuras, 4 tablas., An experimental approach based on the freezing-point depression of a solvent in a swollen gel has been developed to characterize the structure of rubber networks. This property depends on the conditions required for the formation of crystalline nuclei, which are limited by the elastomer network restrictions. Information about the functionality, spatial distribution, and number of crosslinks can be obtained by the use of this easy and ready experimental method. Application of the tube model of rubber elasticity in the uniaxial stress–strain experiments of natural rubber samples vulcanized with dicumyl peroxide yields the characteristic parameters of the rubber networks, which are in concordance with the network structures predicted by the freezing point method. Finally influence of vulcanization conditions in network structure and its relationship with the mechanical properties was evaluated.

11 páginas, 10 figuras, 2 tablas., The mechanism of the reaction between a silica sample coming from acid treatment of sepiolite (denominated Silsep) and an organosilane, namely bis(triethoxysilylpropyl)tetrasulfane (TESPT), has been evaluated by solid state NMR spectroscopy, being compared with the silanization reaction of a commercial silica. The effect of the silane concentration and temperature on the course of the reaction was considered. Experimental results indicate that the silanization reaction is more effective in the case of Silsep, favoring both the reaction of silane molecules with the filler surface and the reaction between neighboring silane molecules. This different behavior is attributed to structural factors, moisture, and number of acid centers on silica surface. Environmental scanning electron microscopy (ESEM) was used to deposit micrometric water drops on the surface of these samples and to evaluate the proportion and distribution of the organophylization process., The authors thank the CICYT, MAT 2001-1634, for financial support and Dr. Lopez-Manchado for the concession of a Ramón y Cajal contract (MCyT).

8 páginas, 7 figuras, 6 tablas., The use of fatty amines, obtained from natural fatty acids, in rubber compounds to improve silica dispersion was evaluated. Fatty amines can interact with the free silanol groups of the bis(3-triethoxysilyl-propyl) tetrasulfide (TESPT) silane-modified silica forming an amine-modified silica complex, which reduces the hydrophilic nature of the silica surface, minimizing the silica–silica interactions, and reducing the formation of secondary structures of silica. The effect of the addition of different amines and/or polyethylenglycol to SBR compounds loaded with silane-modified silica was evaluated. Finally, incorporation of fatty amines improved the processability and traction properties of rubber compounds loaded with modified silica., Contract grant sponsor: CICYT; contract grant number: MAT 2001/1634.

14 páginas, 10 figuras, 7 tablas., Non-toxic biodegradable linear poly(ester-urethane-urea)s with different hydrophilic character were synthesized from poly(ε-caprolactone) as macrodiol, l-lysine diisocyanate (LDI) and ethyl ester l-lysine or l-ornithine as chain extenders. Linear segmented polymers were synthesized by the prepolymer method with a tertiary amine playing a critical role in the chain extension in heterogeneous conditions. The prepared segmented polymers were fully chemically and physically characterized, including water uptake and hydrolytic stability measurements. Depending on the poly(ε-caprolactone) length, the segmented polymers were amorphous or semicrystalline. The crystallinity degree strongly affected the mechanical properties and water uptake behaviour., Partial financial support of this work by CICYT (MAT2004- 01654), CSIC and CONICET (2004AR0061), Fundación ANTORCHAS and ANPCyT is gratefully acknowledged. The donation of LDI by Kyowa Hakko Kogyo Co., Ltd (Japan) is acknowledged.

5 páginas, 4 figuras, 3 tablas., The influence of network topology on tensile strength at elevated temperature of natural rubber compounds vulcanized with different systems was studied. The tensile strength behavior of natural rubber at high strain is attributed to the capacity to crystallize on stretching. Variation of this property with the temperature and/or crosslink density was related, not only with the nature of the crosslinks, but also with their spatial distribution. In this sense, measurements of the freezing point depression of cyclohexane imbibed in the rubber matrix were used to evaluate the spatial distribution of the crosslinks., The authors thank the CICYT for partial support of this research (MAT 2001/1634).

An ethylene-propylene copolymer (EPR) was functionalized in the melt by grafting with itaconic acid and its mono- and di-methyl esters by using dicumyl peroxide as radical initiator. Grafting reactions were carried out in a Brabender-Plasticorder at 180 °C under a nitrogen flow at a speed of 75 rpm for 6 min. Fourier transform infrared spectroscopy (FTIR) was used to confirm the existence of grafted monomers. The absorption bands characteristic of each monomer grafted were observed in the FTIR spectrum of the modified EPR and were taken as evidence of grafting. The percentage of grafting attained was also followed by FTIR analysis, where the amount of monomer incorporated in EPR was estimated from a calibration curve established for the monomers used in this work. The degree of grafting obtained was 1.1% by weight of EPR for itaconic acid, 1.2% for monomethyl itaconate and 2.5% for dimethyl itaconate. Rheological and contact angle measurements indicate that the polarity of EPR is increased by grafting reaction.

The effect of forming additional covalent cross-links on the thermo-reversible network structure of ionic elastomers was evaluated. Ionic elastomers are characterized by a strong physically cross-linked network resulting from a phase separation of ionic-rich nano-domains. The ionic domains are formed by the association of ionic groups that act as cross-links, promoting the elastic behaviour of these polymers. In addition, the trapped glassy rubber around the ionic associations improves the physical properties of these materials acting as reinforcing points of the soft rubbery matrix. These materials are considered as thermoplastic elastomers because of the thermo-labile nature of the ionic associations; however, this property limits their potential applications at elevated temperatures. In order to overcome this disadvantage some permanent covalent cross-links were formed in the ionic structure, improving their properties at high temperature but without altering their reversible thermoplastic nature. The formation of covalent crosslinks increases the number of ionic nano-domains that present a smaller and more homogeneous size distribution. This variation in the network structure is closely related to the enhanced properties shown by these materials. Therefore the proper combination of covalent and ionic cross-links allows the control of the network structure of ionic elastomers in order to obtain tunable properties for these materials.

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.

Elastomeric composites have been prepared using pre-treated fumed silica as a reinforcing system. The processing disadvantages of fumed silica were overcome by a novel one-pot modification method based on silanization treatment of silica particles after an ultra-high energy sonication process that breaks down the micro-aggregates/agglomerates of pristine silica into nanoparticles. Grafted organosilane molecules on the nanoparticle surfaces avoid the natural tendency for silica particles to re-agglomerate obtaining a stable dispersion of organo nanosilica particles that are compatible with the rubber matrix. The modified nanosilica can be easily incorporated into the elastomer matrix by conventional methods employed in rubber technology, contrary to pristine fumed silica which shows significant compounding disadvantages leading to high viscosity compounds. The improved dispersion and enhanced interface significantly enhance the final properties of the composites. Finally, the filler–rubber interface was characterized combining 1H low field Double Quantum (DQ) NMR spectroscopy and equilibrium swelling experiments. This novel experimental methodology demonstrates the existence of improved interactions in the interface between silica particles and rubber macromolecules.

The combination of reinforcing fillers is one of the most promising tendencies in elastomer reinforcement, where a synergic effect between them is desired. Up to now, the use of mixed fillers has been focused on the preparation of dual fillers based on a complex synthesis process where silica particles are in situ generated in a carbon black matrix. In this paper, mixed filler systems are prepared following a straightforward methodology, where carbon black is gradually and partially substituted by modified fumed silica. The presence of reinforcing fillers with different surface energies significantly decreases filler networking, reducing the non-linear viscoelastic response of the composites. The hybrid reinforcing systems prepared were easily incorporated into the elastomer matrix by conventional methods in rubber technology. Physical and dynamic responses of the elastomeric composites were characterized in order to find the optimum mixed system combination. Filler–rubber interface was characterized combining 1H low field DQ (Double Quantum) NMR spectroscopy and equilibrium swelling experiments.