Influence of constituent properties and microstructural parameters on the tensile modulus of a polymer/clay nanocomposite

A multi-scale model to predict the moduli of polymer–clay nanocomposites (PCNs) is presented. The model uses a locally orthotropic finite element model to develop constitutive equations to describe the stiffness properties of a group of aligned clay flakes with surrounding interphase suspended in a polymer matrix. The model then assembles a number of flake groups with varied orientations to predict the actual moduli seen in PCNs. The model is in good agreement with an experimentally obtained tensile modulus found in the literature. The model was also applied to estimate the relative influence of constituent properties and microstructural parameters on the anisotropic tensile modulus of the PCN.     

Thermomechanical modeling of polymer nanocomposites by the asymptotic homogenization method

There is much current interest to incorporate nano-scale fillers into polymer matrices to achieve potentially unique properties. Compared with traditional microcomposites, a nanocomposite has a significant large ratio of interface area to volume that results in improved thermomechanical properties. Desired thermomechanical properties of polymer nanocomposites, to achieve the ever-increasing performance requirements, can be obtained by tailoring their microstructures. To this end, computational analyses of the relations between the thermomechanical properties, e.g., Young’s modulus, shear modulus, Poisson’s ratio, yield strength, coefficient of thermal expansion and coefficient of thermal conductivity, in different directions and the microstructures of polymer nanocomposites are performed. The asymptotic homogenization method based on the finite element analysis is used to model the thermomechanical behaviors of different polymer nanocomposites with periodic microstructures. The effects of adding silica, rubber, and clay nanoparticles to epoxy resin as a polymer matrix are analyzed. Mixtures of the nano-particles which differ in volume fraction, material type, size and/or geometry are considered. Some predictions of the thermomechanical properties are compared with experimental data in order to verify the applied modeling technique as an effective design tool to tailor optimal microstructures of polymer nanocomposites.     

Prediction of elastic properties for polymer-particle nanocomposites exhibiting an interphase

Particle-polymer nanocomposites often exhibit mechanical properties described poorly by micromechanical models that include only the particle and matrix phases. Existence of an interfacial region between the particle and matrix, or interphase, has been posited and indirectly demonstrated to account for this effect. Here, we present a straightforward analytical approach to estimate effective elastic properties of composites comprising particles encapsulated by an interphase of finite thickness and distinct elastic properties. This explicit solution can treat nanocomposites that comprise either physically isolated nanoparticles or agglomerates of such nanoparticles; the same framework can also treat physically isolated nanoparticle aggregates or agglomerates of such aggregates. We find that the predicted elastic moduli agree with experiments for three types of particle-polymer nanocomposites, and that the predicted interphase thickness and stiffness of carbon black-rubber nanocomposites are consistent with measured values. Finally, we discuss the relative influence of the particle-polymer interphase thickness and stiffness to identify maximum possible changes in the macroscale elastic properties of such materials.     

Molecular interactions alter clay and polymer structure in polymer clay nanocomposites.

In this work, using photoacoustic Fourier transform infrared spectroscopy (FTIR) we have studied the structural distortion of clay crystal structure in organically modified montmorillonite (OMMT) and polymer clay nanocomposites (PCN). To study the effect of organic modifiers on the distortion of crystal structure of clay, we have synthesized OMMTs and PCNs containing same polymer and clay but with three different organic modifiers (12-aminolauric acid, n-dodecylamine, and 1,12-diaminododecane), and conducted the FTIR study on these PCNs. Our previous molecular dynamics (MD) study on these PCNs reveals that significant nonbonded interactions (van der Waals, electrostatic interactions) exist between the different constituents (polymer, organic modifier, and clay) of nanocomposites. Previous work based on X-ray diffraction (XRD) and differential scanning calorimetry (DSC) on the same set of PCNs shows that crystallinity of polymer in PCNs have changed significantly in comparison to those in pristine polymer; and, the nonbonded interactions between different constituents of PCN are responsible for the change in crystal structure of polymer in PCN. In this work to evaluate the structural distortion of crystal structure of clay in OMMTs and PCNs, the positions of bands corresponding to different modes of vibration of Si-O bonds are determined from the deconvolution of broad Si-O bands in OMMTs and PCNs obtained from FTIR spectra. Intensity and area under the Si-O bands are indicative of orientation of clay crystal structures in OMMTs and PCNs. Significant changes in the Si-O bands are observed from each vibration mode in OMMTs and PCNs containing three different organic modifiers indicating that organic modifiers influence the structural orientation of silica tetrahedra in OMMTs and PCNs. Deconvolution of Si-O bands in OMMTs indicate a band at approximately 1200 cm(-1) that is orientation-dependent Si-O band. The specific changes in intensity and area under this band for OMMTs with three different organic modifiers further confirm the change in structural orientation of silica tetrahedra of OMMTs by organic modifiers. Thus, from our work it is evident that organic modifiers have significant influence on the structure of polymer and clay in PCNs. It appears that in nanocomposites, in addition to strong interactions at interfaces between constituents, the structure of different phases (clay and polymer) of PCN are also altered, which does not occur in conventional composite materials. Thus, the mechanisms governing composite action in nanocomposites are quite different from that of conventional macro composites.     

The role of thermal residual stress on the yielding behavior of carbon nanotube–aluminum nanocomposites

The initial yield envelopes of aluminum (Al) nanocomposites reinforced with carbon nanotubes (CNTs) subjected to biaxial loading are predicted in the presence of thermal residual stress (TRS) arising from the manufacturing process. Micromechanical model based on the unit cell method is presented to generate the yielding surfaces. The formation of the interphase caused by the interfacial reaction between the CNT and Al matrix is taken into account in the analysis. The effects of several important parameters, i.e. the change of temperature, CNT volume fraction, interphase thickness and Al material properties on the yielding onset of the CNT/Al nanocomposite are explored extensively. The results clearly reveal that the initial yield surfaces of nanocomposite are dependent on the TRS. Also, the interphase has a significant influence on the yielding behavior of Al nanocomposite in the presence of TRS. The results demonstrate that the size of initial yield surfaces become minimum with considering the coupled effects of TRS and interphase. With increasing the temperature variation, interphase thickness, elastic modulus and coefficient of thermal expansion of Al matrix, the size of initial yield surfaces reduces. The present study is consequential for understanding the key role of TRS on the initial damage of CNT/Al nanocomposites.     

Stress Distributions Around Rigid Nanoparticles

A closed form solution for the stress fields around a rigid nanoparticle under uniaxial tensile load is provided. The work explicitly accounts for the presence, around the nanoparticle, of an interphase of thickness comparable to the particle size and different elastic properties from those of the matrix. The solution allows one to determine, in closed form, the stress concentration around nanoparticles relevant for fracture and strength assessments of polymer nanocomposites.     

Micromechanical analysis of nanocomposites using 3D voxel based material model

A computational study on the effect of nanocomposite structures on the elastic properties is carried out with the use of the 3D voxel based model of materials and the combined Voigt–Reuss method. A hierarchical voxel based model of a material reinforced by an array of exfoliated and intercalated nanoclay platelets surrounded by interphase layers is developed. With this model, the elastic properties of the interphase layer are estimated using the inverse analysis. The effects of aspect ratio, intercalation and orientation of nanoparticles on the elastic properties of the nanocomposites are analyzed. For modeling the damage in nanocomposites with intercalated structures, “four phase” model is suggested, in which the strength of “intrastack interphase” is lower than that of “outer” interphase around the nanoplatelets. Analyzing the effect of nanoreinforcement in the matrix on the failure probability of glass fibers in hybrid (hierarchical) composites, using the micromechanical voxel-based model of nanocomposites, it was observed that the nanoreinforcement in the matrix leads to slightly lower fiber failure probability.     

A constitutive model for finite deformation response of layered polyurethane-montmorillonite nanocomposites

A constitutive model is developed to predict the finite deformation response of multilayered polyurethane (PU)-montmorillonite (MTM) nanocomposites. In PU-MTM nanocomposites, the PU matrix in the vicinity of the MTM nanoparticles is modified leading to an interphase region, and its effect on the finite deformation response of these nanocomposites is largely neglected in many existing models. In this work, the entire spatial volume is considered to be occupied by multi-layers of bulk PU and effective particles which consist of MTM nanoparticles and the modified PU interphase region. A Langevin chain based eight chain model is used to capture the large stretch hyperelastic behavior of bulk PU. The effective particle component of the model consists of a linear elastic spring to capture the initial elastic response, a non-linear viscoplastic dash-pot for the strain-rate dependent yield strength of nanocomposites, and a non-linear spring element in parallel to the dash-pot for the strain-hardening response. The model adopts the concept of amplified strain of the confined PU chains to accommodate the applied strain owing to the limited strain in the MTM nanoparticles. The constitutive model predicts all the major features of the stress-strain constitutive response of a family of PU-MTM nanocomposites including the initial linear elastic response, yield strength and post yield strain hardening for all volume fractions of MTM nanoparticles, thus confirming the efficacy of the proposed constitutive model.     

Melt mixed compatibilized polypropylene/clay nanocomposites: Part 1 – the effect of compatibilizers on optical transmittance and mechanical properties

Polypropylene (PP)/clay nanocomposites were prepared via a melt mixing technique. Two types of compatibilizers, polyolefin elastomer grafted maleic anhydride (POE-g-MA), and polypropylene grafted maleic anhydride (PP-g-MA) were incorporated to improve the dispersion of commercial organoclay (20A). With the introduction of PP-g-MA, the optical transmittance of the nanocomposites displayed higher transmittance than those of the POE-g-MA compatibilized case. However, POE-g-MA greatly increased the interlayer spacing of the clay compared with PP-g-MA. This interesting observation is pertinent to the complex morphology of compatibilized nanocomposites. The PP-g-MA compatibilized system conferred higher tensile strength, Young’s modulus, and cutting strength than the POE-g-MA compatibilized case. The high cutting strength of the PP/clay nanocomposites, with or without compatibilizers, signified the importance of crystalline yielding even in the nano-fracture zone of deformation. This finding has not been published in the literature of this field. Clay and its dispersion effect that conventionally claimed to enhance the tensile properties were rather insignificant under this condition of confined deformation of the cutting design. The current results suggest that a high extent of exfoliation may not guarantee high transparency or strength for nanocomposites. The matrix properties and interphase whose variations were caused by the additional compatibilizers to aid the clay dispersion were also crucial factors to the derived properties.     

A micromechanical model taking into account the contribution of α- and γ-crystalline phases in the stiffening of polyamide 6-clay nanocomposites: A closed-formulation including the crystal symmetry

A clay-induced crystal transformation has been widely pointed out in semi-crystalline polymer–clay nanocomposites, from the α-form to the γ-form in the particular case of polyamide 6 (PA6). The proposition of a predictive model taking explicitly into account the polymer crystalline structure is still needed for a reliable prediction of the structure–property relationship and for a better understanding of the reinforcement mechanism in such systems. The aim of this paper is to present an approach issued from the continuum-based micromechanical framework using self-consistency condition to predict the overall stiffness of PA6-clay nanocomposites. Besides the effect of clay particle characteristics, the micromechanical model introduces the contribution of α- and γ-form crystals in the overall stiffness by considering the PA6 matrix as a heterogeneous medium containing distinct amorphous and crystalline phases. A closed-formulation of the micromechanical model is derived using the Walpole spectral decomposition of the stiffness tensors for the two monoclinic crystalline phases. Two possible representations of the microstructure are considered: the first one considers that all the phases are independent and the second one that the γ-crystalline phase constitutes an interphase region around clay particles. The micromechanical model using the two morphological representations is used to predict the overall stiffness of PA6 nanocomposites reinforced with montmorillonite clay (adjusted from 1 wt% up to 20 wt%) for which the polymer crystalline structure was characterized by infrared spectroscopy and calorimetry. The respective role of clay particles and of two crystalline species in the stiffening of exfoliated and intercalated PA6-clay nanocomposites is discussed with respect to the micromechanical model.     

On the overall elastic moduli of polymer-clay nanocomposite materials using a self-consistent approach. Part I: Theory

Although few investigations recently proposed to describe the overall elastic response of polymer-clay nanocomposite materials using micromechanical-based models, the applicability of such models for nanocomposites is far from being fully established. The main point of criticism to mention is the shelving of crucial physical phenomena, such as interactions and length scale effects, generally associated by material scientists, in addition to the nanofiller aspect ratio, to the remarkable mechanical property enhancement of polymer-clay nanocomposites. In this Part I of two-part paper, we present a micromechanical approach for the prediction of the overall moduli of polymer-clay nanocomposites using a self-consistent scheme based on the double-inclusion model. This approach is used to account for the inter-inclusion and inclusion-matrix interactions. Although neglected in the models presented in the literature, the active interaction between the nanofillers should play a key role in the reinforcing effect of nano-objects dispersed in a polymer matrix. The present micromechanical model incorporates the nanostructure of clay stacks, modeled as transversely isotropic spheroids, and the so-called constrained region, modeled as an interphase around reinforcements. This latter is linked to the interfacial interaction between matrix and reinforcements that forms a region where the polymer chain mobility is reduced. To account for length scale effects, interphase thickness and particle dimensions are taken as explicit model parameters. Instead of solving iteratively the basic homogenization equation of the self-consistent scheme, our formulation yields to a pair of equations that can be solved simultaneously for the overall elastic moduli of composite materials. When the interphase is disregarded for spheroids with zero aspect ratio, our formulation coincides with the Walpole solution (J Mech Phys Solids 1969;17:235-251). Using the proposed general form, a parametric study is presented to analyze the respective influence of aspect ratio, number of silicate layers, interlayer spacing and nanoscopic size of the transversely isotropic spheroids on the overall elastic moduli of nanocomposite materials.     

Simulation of interphase percolation and gradients in polymer nanocomposites

Experimental data suggests that well dispersed nanoparticles within a polymer matrix induce a significant interphase zone of altered polymer mobility surrounding each nanoparticle, which can lead to a percolating interphase network inside of the composite. To investigate this concept and the nature of the interphase, a two-dimensional finite element model is developed to study the impact of interphase zones on the overall properties of the composite. Thirty non-overlapping identical circular inclusions are randomly distributed in the matrix with layers of interphase surrounding the inclusions. The simulation results clearly show that the loss moduli of composites are either broadened or shifted corresponding to the absence or presence of a geometrically percolating interphase network. Our numerical study correlates well with experimental data showing broadening of loss peaks for unfunctionalized composites and a large shift of the loss modulus for functionalized nanotube polymer composites. Further, our results indicate the existence of a gradient in properties of the interphase layer and that incorporating this gradient into modeling is critical to reflect the behavior of polymer nanocomposites.     

Effective thermoelastic properties of random structure composites reinforced by the clusters of deterministic structure (application to clay nanocomposites)

Summary Polymer/clay nanocomposites consisting of an epoxy matrix reinforced by silicate clay plates have been observed to exhibit enhanced mechanical properties at low volume fraction of clay. The matrix and embedded nanoelements are modeled in the framework of continuum mechanics with known mechanical properties previously evaluated by, e.g., molecular dynamic simulation. Nanoclay composite is modeled by the aligned, uniformly distributed in the matrix stacks of parallel clay sheets separated from one another by interlayer matrix galleries of nanometer scale. Interaction of a finite number of oblate spheroidal inclusions modeling an individual stack inside the infinite matrix is carried by the multipole expansion technique. The obtained accurate numerical solution was incorporated into the multiparticle effective field method [5] for the estimation of effective thermoelastic properties. Detailed parametric analyses demonstrate the influence on the effective elastic moduli and stress concentrator factors of such key factors as the shape of nanoelements, interlayer distance, and the number of nanoelements in the stacks of deterministic structure.     

Ultrasound assisted synthesis of PMMA/clay nanocomposites: Study of oxygen permeation and flame retardant properties

PMMA/clay nanocomposites were synthesized by ultrasound assisted emulsifier-free emulsion polymerization technique. Ultrasound waves of different power and frequencies were applied to enhance the dispersion of the clay layers with polymer matrix. The structural information of the synthesized materials was studied by X-ray diffraction (XRD) and it was revealed that the interlayer spacing increased with clay loading. The magnitude of dispersion of the clay in the polymer matrix was detected by transmission electron microscopy (TEM). The Young’s modulus, breaking stress, elongation at break, toughness, yield stress and yield strain of the nanocomposites as a function of different clay concentrations and ultrasonic power were measured. Particle diameter of the nanocomposites was measured by laser diffraction technique. Oxygen permeability of the samples was studied and it was found that the oxygen flow rate was reduced by the combined effect of clay loading and ultrasound. The flame retardant property of the nanocomposites due to clay dispersion was investigated by measurement of limiting oxygen index (LOI).     

A molecular dynamics simulation study to investigate the effect of filler size on elastic properties of polymer nanocomposites

The influence of filler size on elastic properties of nanoparticle reinforced polymer composites is investigated using molecular dynamics (MD) simulations. Molecular models for a system of nanocomposites are developed by embedding a fullerene bucky-ball of various sizes into an amorphous polyethylene matrix. In all cases, bucky-balls are modeled as non-deformable solid inclusions and infused in the matrix with a fixed volume fraction. The interaction between polymer and the nanoparticle is prescribed by the Lennard-Jones non-bonded potential. The mechanical properties for neat polymer and nanocomposites are evaluated by simulating a series of unidirectional and hydrostatic tests, both in tension and compression. Simulation results show that the elastic properties of nanocomposites are significantly enhanced with the reduction of bucky-ball size. An examination at the atomic level reveals that densification of polymer matrix near the nanoparticle as well as the filler-matrix interaction energy play the major role in completing the size effect.     

Characterizing elastic properties of carbon nanotubes/polyimide nanocomposites using multi-scale simulation

This research is aimed at characterizing the elastic properties of carbon nanotubes (CNTs) reinforced polyimide nanocomposites using a multi-scale simulation approach. The hollow cylindrical molecular structures of CNTs were modeled as a transverse isotropic solid, the equivalent elastic properties of which were determined from the molecular mechanics calculations in conjunction with the energy equivalent concept. Subsequently, the molecular structures of the CNTs/polyimide nanocomposites were established through molecular dynamics (MD) simulation, from which the non-bonded gap as well as the non-bonded energy between the CNTs and the surrounding polyimide were evaluated. It was postulated that the normalized non-bonded energy (non-bonded energy divided by surface area of the CNTs) is correlated with the extent of the interfacial interaction. Afterwards, an effective interphase was introduced between the CNTs and polyimide polymer to characterize the degree of non-bonded interaction. The dimension of the interphase was assumed equal to the non-bonded gap, and the corresponding elastic stiffness was calculated from the normalized non-bonded energy. The elastic properties of the CNT nanocomposites were predicted by a three-phase micromechanical model in which the equivalent solid cylinder of CNTs, polyimide matrix, and the effective interphase were included. Results indicated that the longitudinal moduli of the nanocomposites obtained based on the three-phase model were in good agreement with those calculated from MD simulation. Moreover, they fit well with the conventional rule of mixture predictions. On the other hand, in the transverse direction, the three-phase model is superior to the conventional micromechanical model since it is capable of predicting the dependence of transverse modulus on the radii of nanotubes.     

Manufacturing polymer/clay nanocomposites through elongational flow technique

A novel processing technique that employs continuous elongational flow to fabricate polymer/clay nanocomposites has been developed and evaluated in this work. A self-made vane mixer has been used to supply the continuous elongational flow, while high-density polyethylene (HDPE) and organic montmorillonite (O-MMT) were used as the polymer matrix and clay, respectively. The morphology of resultant nanocomposites has been carefully revealed and studied by examining wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM). Intercalation effect and dispersion of O-MMT layers have been investigated by the morphology study. The results indicate that the elongational flow has a great potential in melt intercalation of O-MMT, and can lead to an orderly O-MMT layers’ distribution. Thermal properties of as-mixed nanocomposites that prepared under elongational flow have been determined by the differential scanning calorimetry (DSC), which demonstrates that the introduction of O-MMT nano-sheets is bad for the crystal of HDPE matrix. The universal tensile test shows how O-MMT layers affect the mechanical properties of nanocomposites, including the tensile strength and elongation at break. The strain–stress relationship reveals that with continually adding O-MMT layers, the tensile strength increases at first, and then decreases. While the elongation at break shows the same trend.     

尼龙6/POE/粘土纳米复合材料的形态、结构及性能研究

采用熔融方法制备了PA 6/POE/粘土纳米复合材料.PA 6/POE/粘土纳米复合材料具有良好的力学性能,其缺口冲击强度比PA 6有显著提高.研究结果表明,有机粘土剥离于PA 6基体中,POE相以岛状结构均匀分散于PA 6基体中;PA 6/POE/粘土纳米复合材料的储能模量比PA 6的低,并且随着有机粘土含量的增加而降低.PA 6/POE/粘土纳米复合材料的玻璃化转变温度(Tg)随着粘土含量的增加而升高,具有良好的热稳定性.     

Effect of organoclay addition on the two-body abrasive wear characteristics of polyamide 6 nanocomposites

Polymer clay nanocomposites (PCN) exhibit improved mechanical properties due to nanolevel dispersion of clay in the polymer matrix. They also exhibit good tribological performance under dry sliding conditions. Abrasive wear behaviour of these materials would be different from dry sliding behaviour as the mechanisms of the both are entirely different. Hence the abrasive wear behaviour of these materials needs to be investigated. The abrasive wear characteristics of polyamide 6 nanocomposites, with 1, 3 and 5% (wt.) clay prepared by melt intercalation technique, under two-body abrasive wear conditions have been reported. Abrasive wear tests were conducted using a pin-on-disc tribometer containing an abrasive counterface. All the polyamide nanocomposites investigated exhibited a low abrasive wear resistance compared with pristine Nylon. The wear performance of the nanocomposites was correlated with the mechanical properties. Dominant ploughing and cutting wear were observed in polymer clay nanocomposites. The amount of clay present alters the wear mechanism.     

Effect of organic modification of sepiolite for PA 6 polymer/organoclay nanocomposites

Polyamide 6 nanocomposites based on sepiolite needle-like clay were prepared via melt extrusion. Sepiolite was organomodified with trimethyl hydrogenated tallow quaternary ammonium (3MTH) by using different amounts of modifier respect to the sepiolite. The effect of modifier/sepiolite ratio on the final nanocomposite properties and the catalytic effect of the sepiolite on the polymeric matrix were evaluated. The presence of organomodified sepiolite on the polymer matrix favoured the crystallinity of the PA 6. The catalytic effect of the sepiolite was reduced as the modifier amount increased. The elastic modulus and Heat Deflection Temperature (HDT) in PA 6/organosepiolite nanocomposites increased ∼2.5 times respect to the neat PA 6 matrix. The higher the modification grade the better the dispersion and orientation of needle-like sepiolite clay were attained. This effect supported the reinforcement efficiency of organosepiolites with high modifier content.