Ductile-to-ductile transition in dispersely filled composites based on thermoplastic polymers

The fracture mechanism for rubber-filled composites based on gutta-percha, LDPE, medium-density PE, and rubber particles has been studied. An increase in the concentration of filler particles leads to a change in the stress-strain behavior of the composites from neck propagation to homogeneous plastic deformation. For the filled composites, the criterion for the ductile-to-ductile transition is the equality of yield and draw stresses. The critical concentration of rubber particles at the ductile-to-ductile transition is controlled by the ratio between the yield stress of matrix polymer and the neck propagation stress. Transition from neck propagation to homogeneous plastic flow of the material is accomplished under two conditions: the breaking strength of the polymer matrix should be higher than the yield stress, and stretching of the composite should not be accompanied by the formation of diamond cracks. The latter condition is fulfilled when the dimensions of rubber particles are below a certain critical value, which is determined by the ductility of the matrix.     

Mechanistic model for deformation of polymer nanocomposite melts under large amplitude shear

We report the mechanical response of a model nanocomposite system of poly(styrene) (PS)-silica to large-amplitude oscillatory shear deformations. Nonlinear behavior of PS nanocomposites is discussed with the changes in particle dispersion upon deformation to provide a complete physical picture of their mechanical properties. The elastic stresses for the particle and polymer are resolved by decomposing the total stress into its purely elastic and viscous components for composites at different strain levels within a cycle of deformation. We propose a mechanistic model which captures the deformation of particles and polymer networks at small and large strains, respectively. We show, for the first time, that chain stretching in a polymer nanocomposite obtained in large amplitude oscillatory deformation is in good agreement with the nonlinear chain deformation theory of polymeric networks. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

The effect of filler content on the lower yield stress of polymer composites

The effect of the concentration of the dispersed elastic filler on the lower yield stress of matrix composites based on plastic polymers is studied. As the matrix polymers, LDPE-HDPE and LDPE-(medium-density PE) are used. The elastic filler is rubber crumb prepared by roll grinding of worn tires or by deformation grinding of ethylene-propylene-diene rubber. Irrespective of the type of filler particles and their adhesion to the polymer matrix, the lower yield stress σ_d of the composite is described by the linear law σ_d = σ_dm(1 ? V _f ), where σ_dm is the lower yield stress of the polymer matrix and V _f is the volume content of the filler. Analysis of the published data shows that this relationship is quite general and describes the effect of rigid inorganic particles on the lower yield stress when adhesion between the filler particles and the matrix is poor.     

Analysis of the debonding process in polypropylene model composites

Polypropylene (PP) model composites were prepared using cross-linked PMMA particles with a very narrow particle size distribution as filler in order to study the micromechanical processes, which take place during deformation. Composites containing a commercial CaCO₃ filler with a broad particle size distribution were also prepared and studied for comparison. The filler loading of the composites was changed from 0 to 0.3 volume fraction in 0.05 volume fraction steps. Measurements of acoustic emission signals during the elongation of PP/PMMA model composites allowed us to assign the debonding process, including its initiation, unambiguously to a well-defined section of the stress vs. strain curve. The number and intensity of the acoustic signals detected during the deformation of the matrix polymer and the composite, respectively, differed considerably, which made possible the separation of the various micromechanical deformation processes occurring in them. At low extensions the composite is deformed elastically, then debonding takes place in a very narrow deformation range, followed by the plastic deformation of the matrix. At small particle content debonding occurs at relatively low stresses, which differ significantly from the yield stress. Considerable plastic deformation of the matrix begins at the yield point. At larger filler content debonding and shear yielding occur simultaneously. Micromechanical deformation processes cannot be separated as clearly in composites prepared from the commercial CaCO₃ filler with a broad particle size distribution. The debonding of particles with different size occurs in a wide deformation range because of the particle size dependence of debonding stress. The analysis of characteristic values derived from acoustic emission experiments proved that the interacting stress fields of neighboring particles influence the deformation process and that even large particles may aggregate or at least associate at large filler content.     

Evaluation and modeling of the mechanical properties of graphite nanoplatelets based rubber nanocomposites for pressure sensing applications

Acrylonitrile butadiene rubber (NBR) compounds filled with different concentrations of graphite nanoplatelets were experimentally investigated. The stress–strain curves of the nanocomposites were studied, which suggest good filler–matrix adhesion. The large reinforcement effect of the filler followed the Guth model for non‐spherical particles. The effect of graphite nanoplatelets on the cyclic fatigue and hysteresis was also examined. The loading and unloading stress–strain relationships for any cycle were described by applying Ogden's model for rubber nanocomposites. With this model for incompressible materials, expressions may be developed to predict the stress–strain relationship for any given cycle. The dissipated energy increased with graphite nanoplatelets concentrations and decrease with number of cycles. The rate of damage accumulation becomes marginal after first ten cycles. The rate of damage increases as the amount of graphite nanoplatelets increase into the rubber matrix. Copyright © 2011 John Wiley & Sons, Ltd.     

Properties of ultrahighly filled composites based on polymers and ground rubber

The stress-strain and strength properties of ultrahighly filled composites based on thermoplastic polymers and ground rubber wastes are studied. The content of the elastic filler is higher than 70 wt%. As is shown, introduction of minor amounts of the plastic polymer, which serves as the binder for the filler particles, makes it possible to improve the strength properties of ultrahighly filled composites and to prepare materials of a desired thickness. A correlation between the stress-strain properties of the plastic polymer-rubber systems and the effective viscosity of the matrix polymer is established. When a polymer with homogeneous deformation and good adhesion to the elastic filler is used as the matrix, the resultant composites are characterized by properties close to those of vulcanized rubbers. A new method is proposed for processing of ground rubber wastes and preparation of materials that are similar to hard rubbers.     

Structure and dielectric properties of a thermoplastic blend containing dispersed metal

In the present work broadband dielectric relaxation spectroscopy measurements were employed to investigate the dielectric properties of polymer composites. A polyethylene/polyoxymethylene (PE/POM) thermoplastic blend was used as a matrix, while the inclusions were iron (Fe) particles. For comparison, the two pure polymers- PE and POM- were used as a matrix, too. In the PE/POM-Fe composites, the polymer matrix is two-phase and the filler particles are localized only in the POM phase, resulting in an ordered distribution of the dispersed filler particles within the blend. In PE-Fe and POM-Fe composites, the filler spatial distribution is random. The behaviour of all the composites studied is described in terms of the percolation theory. The PE/POM-Fe composites, based on the PE/POM blend, demonstrate different electrical behaviour compared to that of POM-Fe and PE-Fe systems. The percolation threshold value of the PE/POM-Fe composites was found much lower than that of the other two systems. The results were related to the microstructure of the composites. A schematic model for the morphology of the composites studied has been proposed. This model explains the peculiar behaviour of the PE/POM-Fe composites by taking into account the ordered distribution of the filler particles in a binary polymer matrix. Optical microscopy photographs confirm this model.     

Tuning the mechanical properties in model nanocomposites: Influence of the polymer‐filler interfacial interactions

This article presents a study of the polymer‐filler interfacial effects on filler dispersion and mechanical reinforcement in Polystyrene (PS)/silica nanocomposites by direct comparison of two model systems: ungrafted and PS‐grafted silica dispersed in PS matrix. The structure of nanoparticles has been investigated by combining small angle neutron scattering measurements and transmission electronic microscopic images. The mechanical properties were studied over a wide range of deformation by plate–plate rheology and uni‐axial stretching. At low silica volume fraction, the particles arrange, for both systems, in small finite size nonconnected aggregates and the materials exhibit a solid‐like behavior independent of the local polymer‐fillers interactions suggesting that reinforcement is dominated by additional long range effects. At high silica volume fraction, a continuous connected network is created leading to a fast increase of reinforcement whose amplitude is then directly dependent on the strength of the local particle–particle interactions and lower with grafting likely due to deformation of grafted polymer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Calorimetric studies of poly(ethylene terephthalate) stretching over a wide temperature range

Heat effects and structural transformations in amorphous crystallizable poly(ethylene terephthalate) (PET) during uniaxial stretching accompanied by neck formation, have been investigated by calorimetric and x-ray methods over a wide range of temperatures and deformation rates. At small deformation (not exceeding 1–2%) and at temperatures below the glass transition temperature of the polymer, PET behaves as an elastic body. Upon stretching at a constant rate, constant heat power is absorbed, heat effects during loading and unloading coincide completely, and no hysteresis is observed. At large deformations (of the order of 50%), cold drawing develops in this temperature range. The internal energy change in cold drawing is zero within experimental error. A periodic heat release during the self-oscillation regime of drawing PET corresponds to periodic changes in stress, in the rate of the neck formation, and in the appearance of the sample. The temperature limits of the region where crystallization resulting from an uniaxial drawing of the polymer is possible, have been determined, and the heat effect of this phase transition has been measured. Orientation crystallization develops only from 70 to 94°C. These limits are insensitive to changes in deformation rate within one decimal order. The structure of PET in this temperature range has been investigated. The heat of phase transition of orientation crystallization of PET has been determined from the relationship between the measured values of the internal energy change during this process and the limiting degree of crystallinity for the stretched samples. This heat proves to be 5.5 ± 0.1 cal/g.     

The mechanism and a slip model for the initial plastic deformation of amorphous polyethylene under uniaxial tension

The mechanical behaviors of a polyethylene (PE) bulk consisting of amorphous molecular chains under uniaxial tension have been explored using molecular simulations. The stress–strain relationship and the plastic deformations of the PE bulk have been analyzed. Two deformation stages were found in the stress–strain curve, the elastic stage with a straight linear part of the curve and the plastic stage with a flat sawtooth‐like part. The Young's modulus calculated from the elastic part is in good agreement with experimental results. Some key parameters such as the energy variations in different terms reveal that the interchain slip should be chiefly responsible for the initial plastic deformations of amorphous PE under uniaxial tension. In order to address how this slip influences the plastic deformations, the mechanical details of a single chain have been elucidated when it was pulled out from two PE clusters consisting of regular and amorphous chains, respectively. The interchain slip, found as the basic movement style, is responsible for the movement of the stretched chain. Both the critical slip force and the critical slip length have been found in these two cases. For the straight chain pulled out from the cluster with regular chains, the critical slip force is about 1.81 nN and the critical slip length is about 40 polymerization degrees. While for the chain in the amorphous cluster, the critical force is about 0.86 nN and the critical length is almost the same. Based on the simulation results, a meso slip model has been deduced to explain the behaviors of the amorphous PE bulk under uniaxial tension. With reference to the slip model of single crystals and polycrystals a constitutive relation was obtained by considering the Young's modulus, the equivalent slip stress and the average orientation parameters of each chain. The comparison of the results from the constitutive relation and the simulations proves that this model does well in predicting the mechanical behaviors of amorphous PE under uniaxial tension in general. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 986–998     

The effect of the physicomechanical characteristics of the polymer matrix and structure of a filler on the stress-strain behavior of polymer-montmorrilonite nanocomposites

The stress-strain behavior of PE-Na⁺-montmorillonite nanocomposites is studied. The stress-strain characteristics of the composites are shown to be controlled by the structure of their nanofiller, which is formed upon melt mixing of the polymer and layered silicate (intercalated or exfoliated), the profile of the stress-strain curve of PE matrix, and the fracture mechanism (either adhesive or cohesive). In nanocomposites with a strong adhesive bonding between matrix and clay particles (under cohesive fracture), both the modulus and yield point are found to be markedly increased. In the case of adhesive fracture, mechanical characteristics are less improved due to debonding between matrix and filler results. For nanocomposites, experimental stress-strain characteristics are compared with theoretical estimates calculated according to the models proposed for predicting the characteristics of filled thermoplastic polymers. In some cases, experimental values of modulus and elongation at break appear to differ appreciably from theoretical estimates. The applied models should take into account the orientation of anisodiametric inclusions in a polymer matrix and the character of separation in the composite (adhesive or cohesive).     

Effect of matrix elasticity on affinity binding and release of bioparticles. Elution of bound cells by temperature-induced shrinkage of the smart macroporous hydrogel

The first step of bacterial or viral invasion is affinity and presumably multisite binding of bioparticles to an elastic matrix like a living tissue. We have demonstrated that model bioparticles such as inclusion bodies (spheres of about 1 microm in size) Escherichia coli cells (rods 1 x 3 microm), yeast cells (8 microm spheres), and synthetic microgel particles (0.4 microm spheres) are binding via different affinity interactions (IgG antibody-protein A, sugar-lectin, and metal ion-chelate) to a macroporous hydrogel (MH) matrix bearing appropriate ligands. The elastic deformation of the MH results in the detachment of affinity bound bioparticles. The particle detachment on elastic deformation is believed to be due to multipoint attachment of the particles to affinity matrix and the disturbance of the distance between affinity ligands when the matrix is deformed. No release of affinity bound protein occurred on elastic deformation. The efficiency of the particle release by the elastic deformation depends on the density of the ligands at the particle surface as well as on the elasticity of the matrix for relatively large particles. The release of the particles occurred irrespectively of whether the deformation was caused by external forces (mechanical deformation) or internal forces (the shrinkage of thermosensitive macroporous poly-N-isopropylacrylamide hydrogel on increase in temperature).     

A novel gel deformation technique for fabrication of ellipsoidal and discoidal polymeric microparticles

A simple fabrication technique for anisotropic particles of ellipsoidal/discoidal shape has been developed, based on stretching/compressing of oil-in-water emulsion templates embedded into an elastic aqueous gel; a range of solid anisotropic microparticles have been fabricated by polymerising of the deformed oil drops in the elastic gel matrix and their shape and aspect ratios have been studied as a function of the gel deformation.     

The Effect of Crossunking on Toughness of Composite Ldpe/Silica

Abstract

The effect of crosslinking on the toughness of LDPE filled with two different grades of silica was investigated. An elastic plastic fracture mechanism based on the J integral has been used to evaluate the results of notch impact resistance. Crosslinking of the matrix in PE/silica composites leads to improved toughness when compared to uncrosslinked composites. The increase of toughness results mainly from an increase in the amount of plastic deformation and, consequently, higher ultimate deformation at fracture. A positive effect of crosslinking on the development of plastic deformation was also demonstrated by SEM, showing that the fracture is entirely cohesive.     

On the fracture of composites based on ultrahigh-molecular-weight polyethylene and fine aluminum particles

Mechanical characteristics of polymerization filled composite materials based on ultrahigh-molecular-weight polyethylene and fine aluminum particles are studied. The prepared composites preserve their ability for high plastic deformations even when the volume filler content is φ = 0.57. For the tensile drawing of the composite material with randomly distributed particles, an equation describing the dependence of breaking stress on the volume filler content is derived. For the model of the composite with regularly ordered particles, the Nielsen equation is the approximation of the equation proposed in this work for a material with randomly distributed particles.     

Structure and properties of linear polyethylene-montmorillonite nanocomposites

Nanocomposites based on PE and three types of montmorillonite are prepared by melt blending on a Brabender plastograph. The structure of hybrid materials is studied with the use of X-ray diffraction analysis and transmission electron microscopy. The type of modifier and the polarity of the polyolefin matrix affect the character of the filler structure (intercalated and exfoliated) formed during blending with the polymer. A correlation is found between the parameters of structural organization and the mechanical and barrier properties of nanocomposites. The tensile modulus of the nanocomposites containing 3?C5 wt % modified montmorillonite is 75% higher than that of the unfilled PE, whereas the time of combustion and the rate of flame propagation are 20 and 10% smaller, respectively.     

A theory for the drawing of oriented crystalline polymers

The load-elongation behavior during the postneck drawing stage in the deformation of crystalline polymers is shown to be modeled quantitatively by an aligned short-fiber composite in which crystalline fibrils form the reinforcing phase in a matrix of less well-ordered material. Three modes of deformation are distinguished in the model and are shown to correspond to the observed loadelongation relations in polyethylene and polypropylene. The regions are (I) elastic–plastic crystals in an elastic matrix, (II) elastic–plastic crystals in an elastic–plastic matrix, (III) elastic crystals in an elastic–plastic matrix. A requirement of the theory is that the flow stress in the crystals is little affected by temperature whereas that in the matrix falls as the temperature rises. Expressions are given for stress in terms of the applied strain and the relevant parameters of the system: concentration of fibrils, length and diameter of fibrils, and elastic and yield properties of fibrils and matrix.     

Aging properties of styrene-butadiene rubber nanocomposites filled with carbon black and rectorite

To better understand the effect of rectorite and carbon black (CB) on the aging performance of styrene-butadiene rubber (SBR), SBR/CB, SBR/CB/rectorite and SBR/rectorite nanocomposites with the same total filler loading were prepared. The microstructure of the three SBR nanocomposites was characterized by XRD, TEM and SEM. After thermal aging, oxygen-containing molecules were found to be formed in the SBR nanocomposites, as verified by FTIR analysis. The SBR/rectorite nanocomposite showed the highest aging coefficient and the lowest change rate of tensile strength and stress at 100% strain among the three SBR nanocomposites, indicating that the introduction of nano-dispersed rectorite layers can enhance the thermal aging resistance of the nanocomposites. For the SBR/CB/rectorite nanocomposite, the addition of CB helped to improve the interfacial compatibility between the filler and matrix, resulting in the best crack resistance as the aged SBR/CB/rectorite nanocomposite always demonstrated the least cracks on the surface during either stretching or bending experiments.     

Mechanical properties of clay‐reinforced polyamide

Intercalated and exfoliated nanocomposites were prepared by extrusion and injection of polyamide‐6 and highly swollen or slightly swollen montmorillonite, respectively. The microstructure of the nanocomposites has been studied previously. In this article, we investigated the influence of the preferential orientation of the montmorillonite sheets on the mechanical properties of the nanocomposites. Dynamic mechanical analysis and tensile tests showed that the elastic modulus depends mainly on the filler loading. A parallel coupling could well account for the behavior of the nanocomposites. The calculated elastic and storage moduli of montmorillonite were set to 140 and 40 GPa, respectively. Compression tests were performed to study the anisotropy of the mechanical properties. The elastic modulus and flow strain were sensitive to the filler orientation. A Tandon–Weng approach was applied to consider the geometry of the filler. In all low‐deformation tests, no significant difference between intercalated and exfoliated systems was observed. Finally, the influence of the dispersion and exfoliation state of the filler on the ultimate properties of the nanocomposites (tensile tests) is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 272–283, 2002