The payne effect for particle‐reinforced elastomers

The study deals with the Payne effect (a substantial decrease in the storage modulus of a particle‐reinforced elastomer with an increase in the amplitude of mechanical oscillations). The influence of temperature, concentration of filler and amplitude and frequency of strains is analyzed on the mechanical response of filled rubbery polymers. Constitutive equations are derived using the concept of two interpenetrating networks: one comprises semiflexible polymeric chains connected to temporary junctions, whereas the other is formed by filler clusters. Adjustable parameters are found by fitting experimental data for natural rubber, bromobutyl rubber and styrene‐butadiene rubber reinforced by carbon black and polymeric particles. The critical concentration of particles is determined that characterizes transition from an ensemble of disjoint clusters to the network of filler. The volume fraction of filler corresponding to this transition is found to be close to the theoretical predictions based on the percolation theory, as well as to experimental data for isolator—conductor transition.     

Relationship between thermoelasticity and reinforcement in elastomers: Silica filled silicone elastomers

Increases in modulus, tensile strength, and swelling caused by reinforcing silicone rubber with silica filler were correlated with the thermoelastic parameter, f_e/f. A new semiempirical equation of state, containing a generalized front factor, was derived to explain the experimental results. While the retractive force in pure gum elastomers is largely entropic in origin, reinforcement in silicone rubber-silica systems appears to arise by greatly augmenting the deformational free energy change stored in energetic modes.     

A hybrid model for mechanical spectra of filled and unfilled elastomers

The dynamic mechanical properties of elastomers are of vital importance in determining the product design/performance relationship. Unfortunately, the statistical theory of Gaussian networks, commonly used for the ideal rubbery state, cannot adequately model the moduli of elastomers in engineering applications. The WLF equation, although not originally designed to predict moduli, has a functional form that predicts moduli for the range from T_g to 100°K plus T_g. A hybrid equation which incorporates elements of the WLF equation and the statistical theory of Gaussian networks in an ideal rubbery state has been developed for explaining the mechanical spectrum of elastomeric materials. The new equation satisfactorily models the mechanical properties for both filled and unfilled elastomers. This model shows that filler loading tends to broaden the relaxation spectrum. This finding agrees with a previous study on the viscosity of uncured elastomer-filler systems.     

Analyzing the nanoindentation response of carbon black filled elastomers

Carbon black (CB) filled elastomers are structurally complex materials that offer unique properties at different length scales. They have tremendous potential applications in a number of fields including the automotive and aerospace industries and for designing innovative smart materials such as artificial muscles but their applications remain limited primarily due to inadequate understanding of their unique mechanical properties. Here, using the Berkovich technique to probe the surface mechanical properties at different scales the nanoindentation response of a series of composites made by homogeneously dispersed CB nanoparticles inside a semicrystalline copolymer matrix has been explored. While the measured loading part of the force–displacement curves is well described by Meyer's empirical power relation, the inverted methodology (IM) approach to deal with the unloading part has been considered and its outcome has been compared with that obtained from the standard Oliver–Pharr's method. The results were consistent with the observed increase of hardness when the applied displacement decreases for all composite samples over a large range of CB volume fraction. Zhang and Xu's model is demonstrated to produce experimentally consistent explanation of this indentation size effect. X-ray photoelectron spectroscopy (XPS) spectra also show composition gradients with depth up to 100 nm. Furthermore, the effect of CB content, surface features, and length scale-dependent deformation on the hardness–displacement behavior have been considered. These findings highlight the possibility of attaining a diverse set of mechanical properties by a better understanding of the nanoindentation response of CB filled elastomers which can be useful for material selection and design improvements in a number of practical applications.     

Segmental relaxations and other insights into filler-mediated interactions for carbon black-filled polybutadiene rubber

The role of interaction with filler particles in modifying segmental relaxations of elastomers is still not fully clear. This work examines the glass and melting transition of unvulcanized and vulcanized polybutadiene rubber filled with carbon black. It is found that for uncured rubbers, the calorimetric glass and melting transition temperatures are unaffected by the presence of carbon black, without invoking the concept of restricted chains near filler. In quenched vulcanizates, the increment in segmental relaxation with filler loading is detected, which is attributed to the reduction in chemical crosslink density caused by filler agglomeration. On the other hand, the crystalline part constrains the molecular motion in amorphous regions such that the glass transition temperature is raised when the crystallinity is large. This study reveals that the only effect of carbon black is to influence chemical crosslinks through filler networking and then change in segmental relaxations is detectable.     

Energy balances and uniaxial damage of highly filled elastomers

A propellant sample undergoes internal damage while it is tested at finite deformation. The damage consists of broken molecular chains and void spaces which form around the filler particles and contribute to failure of the propellant. Attempts were made to determine the amount of this damage from the viewpoint of energy dissipations. Mechanical energy losses, called damage energy, were computed from energy changes during tension cycling experiments carried out at different temperatures and straining rates. Shift procedures were applied to the experimental results, and a double-reduced master curve for damage energies was obtained by using time–temperature and strain shift factors. The reduced master curve can be used to predict the extent of damage accumulated in a propellant sample during tensile tests at different straining rates and temperatures.     

Thermodynamics and kinetics insights into naphthalene hydrogenation over a Ni-Mo catalyst

Hydrocracking represents an important process in modern petroleum refining industry, whose performance mainly relies on the identity of catalyst. In this work, we perform a combined thermodynamics and kinetics study on the hydrogenation of naphthalene over a commercialized NiMo/HY catalyst. The reaction network is constructed for the respective production of decalin and methylindane via the intermediate product of tetralin, which could further undergo hydrogenation to butylbenzene, ethylbenzene, xylene, toluene, benzene, methylcyclohexane and cyclohexane. The thermodynamics analysis suggests the optimum operating conditions for the production of monoaromatics are 400℃, 8.0 MPa, and 4.0 hydrogen/naphthalene ratio. Based on these, the influences of reaction temperature, pressure, hydrogen/naphthalene ratio, and liquid hourly space velocity (LHSV) are investigated to fit the Langmuir-Hinshelwood model. It is found that the higher temperature and pressure while lower LHSV favors monoaromatics production, which is insensitive to the hydrogen/naphthalene ratio. Furthermore, the high consistence between the experimental and simulated data further validates the as-obtained kinetics model on the prediction of catalytic performance over this kind of catalyst.     

Influence of crosslinking and plasticizing on the viscoelasticity of highly filled elastomers

Solid propellants, like all highly filled elastomers, exhibit a complex nonlinear viscoelastic behavior. The aim of this study was to establish the relationships between the structure and properties, which is needed to construct a robust constitutive law for these materials. An extensive design of experiments approach allowed us to quantify the influence of the curing agents and plasticizer molecules on the microstructure of the propellant and its viscoelastic properties. Swelling and gel permeation chromatography measurements described the microstructure of the propellant and prestrained dynamic mechanical analysis (PDMA) characterized the viscoelastic behavior. The curing agents reacted with polymer chain ends participating in the network, in the sol fraction, or in filler–binder links. Consequently, the polymer network was incomplete even in stoichiometric conditions, and a minimum of 10% of the polymer was free in the microstructure. In addition, preswelling the polymer with plasticizer molecules before curing modified the obtained network by decreasing the crosslink density in the binder and increasing it in the vicinity of the filler surface. This study provided new insight into the local deformation mechanisms controlling nonlinearity as measured by PDMA. The nonlinear behavior appeared between 0 and 1.7% prestrain in both the elastic and viscous parts of the behavior. The network reached its maximum extensibility in the elastic part and constrained the sol fraction in this extended mesh for the viscous part. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40392.     

Influence of the filler type on the rupture behavior of filled elastomers

This work is devoted to the rupture behavior of elastomers filled with carbon black (CB) or silica. Two elastomers have been studied: one which crystallizes under strain, natural rubber (NR), and another one which does not crystallize, styrene butadiene rubber (SBR). The study of the crack propagation of Single Edge Notched specimen (SENT) during stretching at different speeds focuses on the crack initiation and crack deviation phenomenon. This deviation is of main importance in the materials crack resistance as it leads to a large increase in the energy needed for rupture. The deviation in filled or unfilled NR is controlled by crystallization, which is a slow process. In unfilled SBR, deviation is controlled by polymer chain orientation, which is hindered by relaxation mechanisms. The introduction of fillers promotes strain amplification, and strain anisotropy in the crack tip region of the notched samples, and therefore crack deviation. In term of energy density at break of the SBR composites, the SBR filled with silica treated with a covering agent is the most efficient. Thus, a weak interface between the silica and SBR promotes better rupture properties. When comparing Silica and CB filled NR, the highest strain energy to rupture is also obtained with silica. This might be due to the weaker filler‐matrix interface for silica. Thus, these results evidence the kinetic aspect of the rupture, and of the mechanisms it involves: the polymer relaxation, the crystallization (for NR), and the filler‐matrix interaction and decohesion, all of them being strongly interrelated. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

A relaxation model for property changes during the cure reaction of filled and unfilled silicone elastomers

A generalized kinetic model of cure, which is described by a distribution relaxation function, is used to predict physical and mechanical properties of silicone elastomers during isothermal and nonisothermal cure reactions. The model can also predict the effect of filler and cure behavior of filled elastomers. The structural relaxation times of the molecular state of uncured and cured elastomers is also discussed.     

Time and strain dependent thermal expansion of carbon-black filled SBR elastomers

Length-temperature measurements were made on unfilled and carbon-black filled vulcanizates of styrene-butadiene rubber. As in earlier experiments on polybutadiene, transients in thermal expansion behavior were observed following abrupt changes in the temperature. The nature of this time dependence was not dramatically affected by either the level of equilibrium strain or the structure and loading of the carbon-black filler. Both the magnitude of the linear coefficient of thermal expansion and the point of thermoelastic inversion were sensitive to the presence of carbon-black. A model based on considerations of occluded rubber and strain amplification in the free rubbery matrix was used successfully to rationalize the observed behavior of the filled compounds.     

Influence of fillers and bonding agents on the viscoelasticity of highly filled elastomers

Highly filled elastomers such as solid propellants exhibit a complex nonlinear viscoelastic behavior. This work aimed at determining the influence of binder–filler and filler–filler interactions on the microstructure and the viscoelastic properties of the propellant using a design of experiments method. The influences of the filler fraction and of the filler–binder bonding agents (FBBA) were measured by swelling experiments and prestrained dynamic mechanical analyses. The results showed that FBBA react on the filler surface and concentrate the curing agents in the vicinity of the fillers. The nonlinearity of the viscoelastic behavior originated from filler–filler interactions that created high stress zones between fillers and therefore constrained the movements of the macromolecules of the binder. Filler–binder interactions induced by the FBBA increased the filler effective volume as well as the heterogeneous stress distribution in the microstructure. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40664.     

Preparation and characterization of hemp hurd powder filled SBR and EPDM elastomers

The effect of HP loading on the curing characteristics and mechanical properties of filled SBR and EPDM composites was investigated using bis-(3-triethoxysilylpropyl) tetrasulfide (Si69) as coupling agent. For all composites, 20 phr (part per one hundred parts of rubber) silica was used. The addition of HP enhances the vulcanization process of composites filled with silica. The hybrid reinforcement of HP and silica imparts good stiffness and toughness to filled rubber composites. An excess of HP will tend to form agglomerates in the rubber matrix, which adversely affects the silica-rubber matrix interfacial interaction, and consequently lowers the overall mechanical properties. The HP distribution and filler-rubber matrix interaction, which were analyzed by scanning electron microscopy and equilibrium swelling, explained well the changes in mechanical properties of composites filled with hybrid fillers. Dynamic mechanical analysis indicated that the composites exhibited higher Payne effect and storage modulus, and lower tanδ_max value with an increase of HP loading.     

The effects of degree of mixing on the properties of filled elastomers

A new technique has been established for improving the degree of mixing of the filler in an elastomer matrix. This technique includes a unique heating and colling mixing procedure in conjunction with surface modification of the carbon black and proper choices of polymers. The heating cycle is for decreasing the microvoid concentration and for enhancing polymer–filler adhesion. The cooling cycle is for improving the degree of mixing of the filler. Experimental data obtained from several polychloroprene systems strongly substantiate the new mixing technique. The results clearly show that a good degree of mixing obtained using the foregoing mixing technique can indeed enhance the mechanical and permeability properties of filled elastomers.     

Synthesis and nonisothermal crystallization kinetics of thermoplastic polyamide-6 elastomers

Three kinds of thermoplastic polyamide-6 elastomers (TPAEs) with varying polytetramethyleneglycol (PTMG) contents of 10, 20, and 30 wt % were prepared via a one-pot polymerization synthetic route and named as TPAE1, TPAE2, and TPAE3, respectively. First, their structures were investigated by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, scanning electron microscope, and X-ray diffraction. The obtained results confirmed that targeted TPAEs were successfully synthesized and the unit cell of crystallization in TPAEs with α form was confirmed. Subsequently, the thermal properties of prepared TPAEs were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements, respectively. DSC curves showed that melting points of synthesized TPAEs were in the range of 209.2–215.9 °C. Moreover, TGA results showed TPAEs possess good thermal stability and cannot be decomposed under 300 °C. Additionally, the modified Avrami's equation, Ozawa's theory, and Mo's method were employed to investigate the nonisothermal crystallization kinetics of prepared TPAEs. It is found that the Mo's method exhibited great advantages in treating the nonisothermal crystallization kinetics of prepared TPAEs. Meanwhile, the crystallization kinetics and halftime are influenced by the contents of PTMG and follows a nonlinear fashion in agreement with the trend inactivation energies calculated by the Kissinger method. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47388.     

Assessing robustness of hyperelastic models for describing nonlinearity of the mechanical response for pristine and swollen carbon black filled elastomers

We consider the hyperelastic response of semi-crystalline ethylene–co-butyl acrylate (EBA) samples filled with carbon black (CB) particles. Such material is structurally complex with its microstructure being characterized by many structural parameters including crosslink density, filler/matrix interfaces, crystallinity, filler network, and chain entanglement which have different degrees of influence on the effective mechanical properties. We evaluate the ability of a number of analytical models to correctly reproduce the non-linear elastic mechanical response of these samples. We do this by considering either dry samples, or samples which are swollen by a non-polar solvent (toluene) at equilibrium, and subjected to uniaxial tension at room temperature. As test cases, we focus on six physical models for the purpose of analyzing the stress–strain curves of samples with different cross-linking densities. Among these frameworks, we show that the Mooney–Rivlin (MR), Ogden, and eight-chain models accurately describe the stress–strain curves of both dry and swollen CB-EBA samples. These findings highlight the possibility of attaining a diverse set of mechanical properties of filled polymer samples by tailoring their structural parameters.     

Effects of polymer-filler adhesion on the properties of polychloroprene elastomers filled with surface-treated fillers

A new analytical method has been employed to determine values for the surface-energy properties, such as the solid/vapor surface energy and the equilibrium work of adhesion, of several polychloroprene compositions filled with organosilane-treated wollastonite fillers. The equilibrium work of adhesion, which represents the amount of energy stored in the interface formed between polymer and filler, has been used as a key material parameter to correlate changes in the mechanical and rheological properties of wollastonite-filled polychloroprene compositions. Experimental results show that some properties such as tear strength, tensile modulus, shear viscosity, and compression set, which depend on polymer-filler adhesion to varying degrees, increase as the equilibrium work of adhesion increases. On the other hand, properties such as tensile strength and ultimate elongation, which largely depend on the degree of mixing of filler particles and such defect structures as microvoids, decrease with the increase of the equilibrium work of adhesion. A power-law relationship between the tensile modulus and the equilibrium work of adhesion has also been established. This relationship can be used for selecting organosilane-treated fillers in order to achieve optimum properties.     

Reinforcement of model filled elastomers: synthesis and characterization of the dispersion state by SANS measurements

This work is the first part of a study devoted to the understanding and the determination of the molecular mechanisms that are at the origin of the specific properties shown by reinforced elastomers. Different model filled elastomers composed of cross-linked polyethylacrylate chains reinforced with grafted silica nanoparticles were prepared varying the reactivity of the coupling agent with the ethylacrylate monomers. They were synthetized applying and adapting the method developed by Ford et al. [11] which consists to polymerize a colloidal suspension of grafted silica particles in acrylate monomers. In this paper we will present how filled elastomers having different dispersion states can be prepared whilst keeping the same interactions between the particles and the polymer chains. The dispersion states were characterized by Small Angle Neutron Scattering. We found that there are two opposite effects which control the final dispersion state of these filled elastomers during the polymerization. The first one is a depletion mechanism favoring the formation of aggregates. The second one is a repulsive steric interaction due to the growth of polymer chains from the particle surfaces avoiding contacts between the silica inclusions. Using these results we can prepare sets of samples having the same particle/matrix interface but different dispersions states. By comparing their mechanical properties we should to able to estimate the relative weight of the dispersion state quality and the one of the particle/matrix interface on the mechanical behavior of these filled elastomers.     

Effect of the sol fraction and hydrostatic deformation on the viscoelastic behavior of prestrained highly filled elastomers

This study focuses on the relations between the microstructure and the viscoelastic behavior of an industrial solid propellant belonging to the class of highly filled elastomers. Precisely, the study aims at determining the impact on the viscoelastic behavior of the presence of the sol fraction inside the polymer network. The sol fraction is the part of the binder that a good solvent can extract. The solid propellant is swollen to various extents by solutions of plasticizer and polymer molecules. This swelling leads to a hydrostatic deformation of the polymer network, corresponding to an extension or contraction loading for each specimen. Prestrained dynamic mechanical analysis tests, superimposing a small oscillating strain on a prestrain, characterize the viscoelastic behavior. The degree of swelling of the network and the effective filler fraction drive the viscoelastic response. In addition, the mechanical behavior does not depend on the chemical nature of the introduced sol fraction. Moreover, a nonlinear behavior, i.e., an increase in both storage and loss moduli with increasing prestrain, is initiated at low prestrain. This nonlinearity depends on the contraction or extension of the network and could result from particles aligning with prestrain, which is expected in such highly filled materials. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013