Structural and mechanical properties of advanced polymer gels with rigid side-chains using coarse-grained molecular dynamics

Computational modeling was utilized to design complex polymer networks and gels which display enhanced and tunable mechanical properties. Our approach focuses on overcoming traditional design limitations often encountered in the formulation of simple, single polymer networks. Here, we use a coarse-grained model to study an end-linked flexible polymer network diluted with branched polymer solvent chains, where the latter chains are composed of rigid side-chains or “spikes” attached to a flexible backbone. In order to reduce the entropy penalty of the flexible polymer chains these rigid “spikes” will aggregate into clusters, but the extent of aggregation was shown to depend on the size and distribution of the rigid side-chains. When the “spikes” are short, we observe a lower degree of aggregation, while long “spikes” will aggregate to form an additional secondary network. As a result, the tensile relaxation modulus of the latter system is considerably greater than the modulus of conventional gels and is approximately constant, forming an equilibrium zone for a broad range of time. In this system, the attached long “spikes” create a continuous phase that contributes to a simultaneous increase in tensile stress, relaxation modulus and fracture resistance. Elastic properties and deformation mechanisms of these branched polymers were also studied under tensile deformation at various strain rates. Through this study we show that the architecture of this branched polymer can be optimized and thus the elastic properties of these advanced polymer networks can be tuned for specific applications.     

Effect of sample thickness on the mechanical properties of injection-molded polyamide-6 and polyamide-6 clay nanocomposites

The effect of the thickness on the mechanical properties of injection-molded specimens of pure polyamide-6 (PA6) and polyamide-6 clay nanocomposites (PA6-NC) with 5 wt% of layered silicates was investigated. Plates of 0.5, 0.75, 1 and 2 mm thickness were characterized in the injection direction using Dynamic Mechanical Analysis under torsion and tension respectively, and tensile tests. The fracture surfaces were analyzed by Scanning Electron Microscopy. In contrast with PA6, PA6-NC showed thickness effect and clear differences in the mechanical and thermomechanical properties between skin and core, especially in the 2 mm thick samples. Increasing thickness in PA6-NC led to a reduction of tensile modulus and yield stress. In the fracture surface of the thicker tensile specimens the formation of a sheet-like structure was observed. Multiple voiding in the core causing initial failure in this region and a stiffer skin with a better orientation of the layered silicates in the injection direction are two important elements of a micromechanical model proposed in this paper to explain the fracture mechanism in PA6-NC.     

Calculating barrier properties of polymer/clay nanocomposites: Effects of clay layers

We analyzed the effects of clay layers on the barrier properties of polymer/clay nanocomposites containing impermeable and oriented clay layers. Using the relative permeability theory in combination with the detour theory, we obtained new relative permeability expressions that allow us to investigate the relative permeability R_p as a function of lateral separation b, layer thickness w, gallery height H, layer length L, and layer volume fraction Φ_s. It was found that intercalated and/or incomplete exfoliated structures and dispersed tactoids with several layers can effectively enhance the barrier properties of the materials. Furthermore, we developed the chain-segment immobility factor to briefly discuss the chain confinement from clay layers. The results showed that the chain confinement enhanced the barrier properties of the intercalated nanocomposites. Our model is better consistent with the experiments when Φ_s>0.01. The findings provide guidelines for tailoring clay layer length, volume fraction and dispersion for fabricating polymer-clay nanocomposite with the unique barrier properties.     

Multiscale micromechanical modeling of polymer/clay nanocomposites and the effective clay particle

Polymer/clay nanocomposites have been observed to exhibit enhanced mechanical properties at low weight fractions (W_c) of clay. Continuum-based composite modeling reveals that the enhanced properties are strongly dependent on particular features of the second-phase ‘particles’; in particular, the particle volume fraction (f_p), the particle aspect ratio (L/t), and the ratio of particle mechanical properties to those of the matrix. These important aspects of as-processed nanoclay composites require consistent and accurate definition. A multiscale modeling strategy is employed to account for the hierarchical morphology of the nanocomposite: at a lengthscale of thousands of microns, the structure is one of high aspect ratio particles within a matrix; at the lengthscale of microns, the clay particle structure is either (a) exfoliated clay sheets of nanometer level thickness or (b) stacks of parallel clay sheets separated from one another by interlayer galleries of nanometer level height, and the matrix, if semi-crystalline, consists of fine lamella, oriented with respect to the polymer/nanoclay interfaces. Here, quantitative structural parameters extracted from XRD patterns and TEM micrographs (the number of silicate sheets in a clay stack, N, and the silicate sheet layer spacing, d₍₀₀₁₎) are used to determine geometric features of the as-processed clay ‘particles’, including L/t and the ratio of f_p to W_c. These geometric features, together with estimates of silica lamina stiffness obtained from molecular dynamics simulations, provide a basis for modeling effective mechanical properties of the clay particle. In the case of the semi-crystalline matrices (e.g. nylon 6), the transcrystallization behavior induced by the nanoclay is taken into account by modeling a layer of matrix surrounding the particle to be highly textured and therefore mechanically anisotropic. Micromechanical models (numerical as well as analytical) based on the ‘effective clay particle’ were employed to calculate the overall elastic modulus of the amorphous and semi-crystalline polymer-clay nanocomposites and to compute their dependence on the matrix and clay properties as well as internal clay structural parameters. The proposed modeling technique captures the strong modulus enhancements observed in elastomer/clay nanocomposites as compared with the moderate enhancements observed in glassy and semi-crystalline polymer/clay nanocomposites. For the case where the matrix is semi-crystalline, the proposed approach captures the effect of transcrystallized matrix layers in terms of composite modulus enhancement, however, this effect is found to be surprisingly minor in comparison with the ‘composite’-level effects of stiff particles in a matrix. The elastic moduli for MXD6-clay and nylon 6-clay nanocomposites predicted by the micromechanical models are in excellent agreement with experimental data. When the nanocomposite experiences a morphological transition from intercalated to completely exfoliated, only a moderate increase in the overall composite modulus, as opposed to the expected abrupt jump, was predicted.     

Insight into molecular interactions between constituents in polymer clay nanocomposites

The mechanisms responsible for the enhancement of physical properties of polymer clay nanocomposites (PCN) over pristine polymers are not well understood. This knowledge is important for obtaining a better control over the physical properties of PCN and designing PCN with tailored properties. The interactions among the different constituents of PCN may be a key factor for controlling physical properties of PCN. The interaction energy is an important measure of the interactions among different constituents of composites. Molecular dynamics (MD) is a useful tool for studying the nature and quantitative analysis of interaction energies of a molecular system. In this work, the interaction energies among different components of intercalated organically modified montmorillonite (OMMT) and PCN have been investigated. Here, the interaction of polymer or organic modifier with clay and polymer and modifier is studied. Also, the nature and quantitative contributions arising from functional groups or backbone chain have been assessed. This investigation provides important insight into the mechanism of intercalation, and specific information about the interactions of different constituents in the nanocomposites system. In this current work using MD, for the first time, we have provided a detailed quantitative picture of interactions among the different components of OMMT and PCN.     

TPO based nanocomposites. Part 1. Morphology and mechanical properties

The relationship between morphology and the mechanical properties of thermoplastic olefin (TPO) materials that are reinforced with organoclay fillers and prepared by melt processing is reported. Nanocomposites based on blends of polypropylene and elastomer and using an organoclay masterbatch were prepared in a twin-screw extruder. Transmission electron microscopy, atomic force microscopy and wide-angle X-ray scattering were employed to carry out a detailed particle analysis of the morphology of the dispersed clay and elastomer phases for these nanocomposites. The improvement in mechanical properties, e.g. stiffness enhancement as evaluated by stress-strain analysis and impact strength obtained from notched Izod impact tests, were successfully explained in terms of morphological changes induced by the presence of the clay and elastomer particles. Quantitative analyses of TEM micrographs and AFM images revealed a decrease in the aspect ratio of the clay particles and a reduction in the size of elastomer particles with increasing clay content. In addition, WAXD scans indicated a skin-core effect for the injection molded specimens in terms of both polypropylene crystal orientation and clay filler orientation. This information is essential for the understanding of the mechanism of mechanical property enhancement in nanocomposite materials.     

Large deformation mechanical behavior of flexible nanofiber filled polymer nanocomposites

In the present work, the large deformation behavior of high aspect ratio flexible nanofiber reinforced polymer composites is investigated. Simple or successive tensile tests are performed at room temperature, i.e. in the rubbery state. By studying two different types of fibers, namely cellulose nanofibrils and carbon nanotubes, with two processing routes, the role of entanglements and of interactions existing between fibers—within the nanofiber network that can be formed in the material—on the composite properties is highlighted. For cellulosic nanofillers, strong hydrogen bonds between fibers lead to a spectacular reinforcement effect combined with a decrease of the composite ultimate strain and an irreversible damage of composite properties after first deformation (rigid network). When such strong interactions between fillers are limited (soft entangled network or simple contacts between non-entangled fibers) the resulted reinforcement is less important and no decrease of the deformation at break is observed. For carbon nanotube fillers, the evolution of the filler network during tensile test is finally highlighted by in situ electrical measurements.     

Effect of modified carbon nanotube on the properties of aromatic polyester nanocomposites

Aromatic polyester nanocomposites based on poly(ethylene 2,6-naphthalate) (PEN) and carbon nanotube (CNT) were prepared by melt blending using a twin-screw extruder. Modification of CNT to introduce carboxylic acid groups on the surface was performed to enhance intermolecular interactions between CNT and the PEN matrix through hydrogen bonding formation. Morphological observations revealed that the modified CNT was uniformly dispersed in the PEN matrix and increased interfacial adhesion between the nanotubes and the PEN, as compared to the untreated CNT. Furthermore, a very small quantity of the modified CNT substantially improved thermal stability and tensile strength/modulus of the PEN nanocomposites. This study demonstrates that the thermal, mechanical, and rheological properties of the PEN nanocomposites are strongly dependent on the uniform dispersion of CNT and the interactions between CNT and PEN, which can be enhanced by slight chemical modification of CNT, providing a design guide of CNT-reinforced PEN nanocomposites with a great potential for industrial uses.     

Effect of silane structure on the properties of silanized multiwalled carbon nanotube-epoxy nanocomposites

Multiwalled carbon nanotubes (MWCNTs) were functionalized with aminosilanes via an aqueous deposition route. The size and morphology of siloxane oligomers grafted to the MWCNTs was tuned by varying the silane functionality and concentration and their effect on the properties of a filled epoxy system was investigated. The siloxane structure was found to profoundly affect the thermo-mechanical behavior of composites reinforced with the silanized MWCNTs. Well-defined siloxane brushes increased the epoxy T_g by up to 19 °C and significantly altered the network relaxation dynamics, while irregular, siloxane networks grafted to the MWCNTs had little effect. The addition of both types of silanized MWCNTs elicited improvements in the strength of the nanocomposites, but only the well-defined siloxane brushes engendered dramatic improvements in toughness. Because the silanization reaction is simple, rapid, and performed under aqueous conditions, it is also an industrially attractive functionalization route.     

Effect of particle size on the mechanical properties of polystyrene and poly(butyl acrylate) core/shell polymers

The effects of particle size and polymer location (core or shell) on the mechanical properties of core/shell materials composed of polystyrene (PST) and poly(butyl acrylate) (PBA) made by a two-stage emulsion or microemulsion polymerization process are reported. Low-seed content (LSC) latexes were made by batch polymerization in microemulsions stabilized with DTAB in the presence of an organic salt (dibutyl phosphite). High-seed content (HSC) latexes were produced by microemulsion or emulsion polymerization in semi-continuous process. These latexes were subsequently used to form core/shell particles of PST/PBA or PBA/PST and their mechanical properties were examined and compared. Our results indicate that core/shell particle size and the location of the polymers have important effects on the mechanical properties.     

Mapping the real micro/nanostructures for the prediction of elastic moduli of polypropylene/clay nanocomposites

An understanding of the overall nanocomposite behaviour is developed through the combination of real data from micro/nanostructures and the fundamental material characteristics of the constitutive phases. Captured morphological images, using either scanning electron microscopy (SEM) or transmission electron microscopy (TEM), are utilised to generate the geometric information regarding mapping the micro/nanostructures. The material properties, on the other hand, are obtained from conducted tests and previous literature. The numerical results predicting the elastic moduli of polypropylene (PP)/clay nanocomposites are compared with the experimental data and available composites theoretical models. Very good agreement has been shown, establishing the viability of this kind of morphology-based numerical approach.     

Effect of glass fiber surface chemistry on the mechanical properties of glass fiber reinforced, rubber-toughened nylon 6

The mechanical properties of nylon 6 and its blends with maleated ethylene-propylene rubber (EPR-g-MA) plus glass fibers were examined as a function of the chemical functionality of the silane surface treatment applied to the glass fibers. Three reactive silane coupling agents, with anhydride, epoxy, or amine functionality, were used and found to have little effect on the mechanical properties when no EPR-g-MA is present. When 20 wt% EPR-g-MA is used as a rubber toughener, however, the yield strength and Izod impact strength were lowest for the amine functional silane and highest for the anhydride silane, while the epoxy silane fell in-between. These results were attributed to the differences in reactivity of the three reactive silanes. An unreactive silane (octyl groups) was used as a release agent on the glass fibers and compared with the anhydride functional silane. The octyl silane did not improve the ductility of the composite, as may have been speculated, and had poor yield strength and impact resistance when compared to the anhydride silane. Both octyl and anhydride treated glass fibers improve the heat distortion temperature such that most of the high temperature stiffness that is lost on addition of EPR-g-MA is regained by adding glass fibers.     

Network model for the viscoelastic behavior of polymer nanocomposites

A theoretical network model reproducing some significant features of the viscoelastic behavior of unentangled polymer melts reinforced with well dispersed non-agglomerated nanoparticles is presented. Nanocomposites with low filler volume fraction (∼10%) and strong polymer-filler interactions are considered. The model is calibrated based on results obtained from discrete simulations of the equilibrium molecular structure of the material. This analysis provides the statistics of the network of chains connecting fillers, of dangling strands having one end adsorbed onto fillers, and that of the population of loops surrounding each nanoparticle. The network kinetics depends on the attachment-detachment dynamics of grafted chains of various types and is modeled by using a set of convection equations for the probability distribution functions. The overall viscoelastic response depends strongly on the lifetime of the polymer-filler junctions. The largest reinforcement is observed at low strain rates and low frequency oscillations. A solid like behavior is predicted for systems in which the polymer molecules interact strongly with the nanoparticles, effect which is associated with the behavior of the network of bridging segments.     

Influence of organo-montomorillonite on mechanical and rheological properties of polyamide nanocomposites

The effect of organically modified montmorillonite (OMMT) on polyamide nanocomposites was studied. OMMT/polyamide nanocomposites were prepared through direct melt compounding on a conventional twin screw extruder. With increasing the loading of OMMT, the Young modulus, elongation at break and tensile strength increased. 1 mass% loading of OMMT/polyamide resulted in 11% increase of the elongation at break compared to virgin polymer, while 4% loading showed 13%. Rheological data like torque, fusion time, viscosity and shear rate were also recorded on Brabender Plasticorder and were correlated with M = CS^a and τ = K(γ)ⁿ. The value n < 1 indicated pseudo-plastic nature of the polyamide/OMMT. The torque decreased with increased loading due to soft nature of OMMT, which acts as a lubricating agent. This improvement in mechanical properties with increase in amount of OMMT loading was also indicated by the reduction in shear viscosity and torque.     

Mechanical and optical properties of clay-based nanocomposites coatings for wood flooring

Clay-based nanocomposites coatings cured by UV light were prepared by four different types of dispersion equipment: a high-speed mixer, a ball mill, a bead mill and a three-roll mill. For each treatment, formulations were prepared at 1, 3 and 10 wt% of clay. A previous study had established the dispersion efficiency of each treatment and the effect of clay dispersion on UV curing. The present study relates to the effect of clay dispersion on mechanical and optical properties. The mechanical properties studied included abrasion resistance, direct and reverse impact resistance as well as Persoz hardness. Optical properties, such as haze, gloss, color and optical clarity were assessed. An analysis of variance (ANOVA) was conducted to evaluate the effect of clay loading and process type on mechanical and optical properties. A statistical analysis revealed that each property varied with the percentage of clay loading in every treatment. It was found that the quality of the clay dispersion strongly affected both mechanical and optical properties.     

Visualization of nanomechanical mapping on polymer nanocomposites by AFM force measurement

Nanocomposite based on an elastomer, natural rubber (NR), and pristine multi-walled carbon nanotubes (MWCNT) was prepared using a two-roll mill mixer. The high shearing stress induced homogeneous dispersion of 5 phr. MWCNTs in NR matrix. A procedure based on combination of Johnson-Kendall-Robert (JKR) contact mechanics and “two-point method” together with AFM force measurements, was successfully used to visualize nanomechanical mapping on the resulting nanocomposites. Topography, elastic modulus, and adhesive energy distribution maps were obtained at the same point and at the same time in a single scan. Such maps were successfully used to identify and characterize CNTs and NR regions in nanocomposites. The intermediate modulus region formed around CNTs was investigated on the quantitative evaluation in real space and demonstrated the existence of interaction between CNTs and NR matrix.     

Effect of organoclay chemistry and morphology on properties of poly(lactic acid) nanocomposites

Poly(lactic acid) (PLA) nanocomposites with different layered organoclays (variation in the surface treatment of silicate) and one special nanofiller (mixed mineral thixotrope) were melt-compounded using a semi-industrial co-rotating twin-screw extruder. Effects of the silicate surface treatment and shape on the structure as well on processing and utility properties in PLA matrix were investigated. The structural changes in polymer matrix were evaluated from dynamic experiments in the shear flow using low-amplitude oscillatory measurements. Moreover, new approach for morphological investigation of nanocomposites using small-angle X-ray scattering was presented. Concerning utility properties, tests of mechanical and barrier properties were performed to compare enhancement of PLA matrix due to incorporation of different nanoparticles. Surprisingly, filling the PLA matrix with mixed mineral thixotrope resulted into very high material performance (in particular, significant improvement in barrier properties) compared to filling with commercial layered silicates. In this way, new type of nanofiller for PLA applications has been successfully tested.     

Effect of nanosized mesoporous MCM-41 (with template) material on properties of natural rubber nanocomposites

Abstract

Mesoporous mobil composition of matter 41 (MCM-41) (with template) was used directly as a new filler for naural rubber (NR). Inside the pore chanels, and on the outer surface of the MCM-41 particle, were cationic surfactant CTAB and Pluronic F₁₂₇ (molecular weight = 11 500) mixture. Results showed that the tensile properties and the thermal stability of NR/mesoporous MCM-41 (with template) nanocomposite were improved at low filler loading as compared with those of NR compound. Scanning electron microscopy observations revealed that enhancement of the interface was obtained by adding MCM-41 (with template).     

Sub-micron dispersed-phase particle size in polymer blends: overcoming the Taylor limit via solid-state shear pulverization

A comparison was made of the fineness of dispersion in immiscible polymer blends achieved by a continuous mechanical alloying technique, solid-state shear pulverization, relative to that achieved by melt mixing. Two polymer blend systems were investigated. A polystyrene (PS)/polyethylene (PE) wax blend was studied because, based on a classic analysis by G.I. Taylor, melt mixing was expected to yield a number-average dispersed-phase domain size, D_n, well above 1 μm. A PS/high density polyethylene (HDPE) blend was also studied because it was known to produce a sub-micron number-average dispersed-phase particle size when mixed by twin-screw extrusion. In the case of the PS/PE wax blend at compositions ranging from 1 to 15 wt% polyethylene wax, pulverization resulted in nearly identical D_n values (typical value of 0.7 μm) independent of minor-phase content; these D_n values were an order of magnitude smaller than the anticipated Taylor limit for melt-mixed blends. In contrast, PS/PE wax blends made by batch, intensive melt mixing yielded D_n values between ∼3 μm at both 1 and 5 wt% minor-phase content and 17.5 μm at 15 wt% minor-phase content. The increase in D_n with increasing dispersed-phase content in the melt-mixed blend is a consequence of coalescence present during melt processing; such effects are disallowed in the pulverization process occurring in the solid state. Scanning electron microscopy of a 95/5 wt% PS/HDPE blend provided D_n values of 500 and 270 nm in the twin-screw extruded and pulverized samples, respectively. Fractionated crystallization studies further corroborated the ability of pulverization to result in a finer, nanoscopic dispersion of the minor phase as compared to extrusion.     

Effect of the ratio of maleated polypropylene to organoclay on the structure and properties of TPO-based nanocomposites. Part I: Morphology and mechanical properties

The structure-property relationships of thermoplastic olefin (TPO)-based nanocomposites prepared by melt processing are reported with a main focus on the ratio of maleic anhydride-grafted polypropylene (PP-g-MA) to organoclay. The morphological observations by transmission electron microscopy, atomic force microscopy, and X-ray diffraction are presented in conjunction with the mechanical and rheological properties of these nanocomposites. Detailed quantitative analyses of the dispersed clay particles revealed that the aspect ratio of clay particles decreased as clay content increased but increased as the amount of PP-g-MA increased. Analysis of the elastomer phase revealed that the aspect ratio of the elastomer phase increased in both cases. The presence of clay causes the elastomer particles to become highly elongated in shape and retards the coalescence of the elastomer particles. The modulus and yield strength are enhanced by increasing the PP-g-MA/organoclay ratios. High levels of toughness of the TPO can be maintained when moderate levels of (organoclay) MMT and PP-g-MA are used. The rheological properties suggested that the addition of clay particles and PP-g-MA has a profound influence on the long time stress relaxation of the TPO nanocomposites. Based on these analyses, it is clear that it is important to optimize the ratio of PP-g-MA and organoclay to obtain the desired balance of mechanical properties and processing characteristics for TPO nanocomposites.