The effect of interface debonding behaviors on the mechanical properties of microPCMs/epoxy composites

Microcapsules containing phase change materials (microPCMs) can be filled in polymeric matrix forming smart temperature‐controlling composites. The aim of this study was to investigate the relationship between the interface behaviors and the mechanical properties of methanol‐melamine‐formaldehyde shell microPCMs containing paraffin/epoxy matrix composites. The typical microPCMs with core/shell ratio of 2/1 shapely decreased their average diameter from 26.0 ± 1.5 to 13.8 ± 3.4 μm with the increasing of stirring speed from 1,000 to 3,000 r min⁻¹. But, both the thickness and yield point of shells had a little variation. Young's modulus of composites was almost independent on the particle size of microPCMs with lower volume fractions (5%–10%). For composites with higher microPCMs loading (20%–30%), there was a slight decrease in modulus with increasing particle size. The data of tensile strength decreased obviously with the increasing of the average diameter of microcapsules for composite samples with the same particle loading. A repeated heat‐transmission treatment was applied to enhance the interface debonding. The results show that the violent thermal transmission had decreased the mechanical properties of microPCMs/epoxy composites. The scanning electron microscopy morphologies had also proved that these phenomena attributed to the interphase separation and cracks. Moreover, a semiexperiential conclusion is that the increasing of interface area of composites will at the same time give more structure defects leading to poor mechanical properties. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

A new approach in modeling of mechanical properties of nanocomposites: effect of interface region and random orientation

The Young’s modulus of polymer nanocomposites is predicted using a numerical approximation system (NAS) model based on fully exfoliated nanoparticles, random orientation (with platelet and cylindrical forms), and nanoparticles of specific shapes (e.g., square platelets, nanotubes, and spherical). The thickness of interface between the polymer matrix and nanoparticles which plays an important role in reinforcing mechanism of nanocomposites is also employed in NAS model as a crucial parameter. The modulus of interface region on the surface of nanoparticle is another significant parameter which is taken into account through mathematical modeling procedure of NAS model as it may indicate the manner by which the polymer matrix bonds to the surface of nanoparticles. NAS model proposes a general formulation through which the Young’s modulus of a nanocomposite could be easily predicted, while the involving parameters change due to the shape of nanoparticle (e.g., platelet, cylindrical or spherical). The final predications of NAS model are validated by comparing them with the results of tensile tests for polyamide (PA)/Cloisite 30B nanocomposite system and the results reported in other similar studies on the mechanical properties of polymer nanocomposites.     

Multiscale modeling of size-dependent elastic properties of carbon nanotube/polymer nanocomposites with interfacial imperfections

We developed an efficient and extensible multiscale analysis to consider the carbon nanotube (CNT) size effect and weakened bonding effect at the interface on the effective elastic stiffness of CNT/polymer nanocomposites using molecular dynamics (MD) simulations and continuum micromechanics. Under the assumption that the CNT molecular structure is an equivalent solid cylinder, molecular mechanics calculation results for transversely isotropic elastic stiffness were found to decrease as the radius of the CNT increased. Similarly, the transversely isotropic elastic moduli of aligned pristine CNT-reinforced polypropylene composites obtained from molecular dynamics simulations exhibited the same CNT size dependency. However, a weakened interface effect was observed from the transverse Young’s modulus and two shear moduli. To account for the size effect and the weakened interface in the micromechanics-based multiscale model, a modified multi-inclusion model is derived with an effective particle scheme. Also, an effective matrix concept is suggested to account for the formation of an interphase near the surface of the CNT, and the elastic stiffness of the CNT and the effective matrix is defined as a function of the CNT radius to describe size-dependent elastic stiffness in the micromechanics regime.     

Temperature effect on mechanical strength and frictional properties of polytetrafluoroethylene-based core-shell nanocomposites

Polytetrafluoroethylene (PTFE) has shown an outstanding lubricity as a solid lubricant, but its application is limited due to its low-mechanical strength and high-wear rate. In this study, core-shell nanoparticles were synthesized using PTFE as the core and polymethylmethacrylate (PMMA) as the shell. The formed core-shell nanocomposites by leveraging the core-shell nanoparticles as basic structural units exhibit remarkable enhancement on uniformity, tensile strength, and wear resistance, compared to mechanically mixed composites with the same composition. Our experiments demonstrated the following results: (1) Owing to the excellent uniformity, the maximum tensile strength of core-shell nanocomposites was 62 MPa, three times higher than that of mechanically mixed composites. (2) The composite matrix formed by PMMA shell had better reinforcement and protection effect on inner PTFE phase, resulting in a reduced wear rate of 0.3 × 10⁻⁵ mm³/(N m), one order of magnitude lower than that of mechanically mixed composites. (3) The friction coefficient and interfacial mechanical properties of the core-shell nanocomposites at different temperatures have been systematically studied to get insights into lubrication mechanisms. It is proved that the temperature can decrease the modulus and increase the interfacial adhesion as well as the loss tangent of the core-shell nanocomposites, thus affecting the lubrication properties in multiple ways.     

Multiscale characterization of filler dispersion and origins of mechanical reinforcement in model nanocomposites

We report on the influence of parameters controlling filler dispersion and mechanical reinforcement in model nanocomposites. We elaborate a series of nanocomposites and present a structural characterization of silica dispersion in polymer matrix for several particle sizes and polymer matrices, at all relevant scales, by coupling Small Angle X-ray Scattering and Transmission Electronic Microscopy. The mechanical properties are investigated in the linear regime by coupling Dynamical Mechanical Analysis and plate/plate rheology. The results show that: (i) for all filler sizes and matrices, a structural transition is observed from non-connected fractal aggregates at low silica concentration to connected network at high particle content. (ii) In the dilute regime, the reinforcement implies a polymer chain contribution with different possible origins: increase of entanglements density for PS and increase of friction coefficient for PMMA. (iii) In the concentrated regime, for a given polymer, the reinforcement amplitude can be tuned by the rigidity of the filler network, which directly depends on the particle–particle interaction.     

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.     

聚丙烯基纳米复合材料的制备方法与力学性能

综述了聚丙烯(PP)基纳米复合材料的制备方法和力学性能的研究进展，介绍了目前国内外研究的以PP为基体与粘土层状物、无机、金属纳米粒子复合制备的复合材料的表面处理、制备方法与材料力学性能的关系。用传统的表面处理方法可改善纳米粒子的分散性与力学性能，少量纳米粒子可使PP同时获得增强增韧。     

The effect of moisture on the dynamical mechanical properties of bacterial cellulose/glucuronoxylan nanocomposites

The plant cell wall possesses unique material properties due to its hierarchical organisation. In order to biomimic a native structure like a plant cell wall, a model system consisting of microfibrillar cellulose, produced by the gram-negative bacteria Acetobacter xylinum, and a glucuronoxylan matrix derived from aspen holocellulose was constructed. The glucuronoxylan was extracted from delignified aspen (Populus tremula) wood chips using DMSO to preserve its native chemical composition. Dynamic mechanical analysis (DMA) measurements performed with moisture scans showed a moisture-induced softening of delignified aspen wood fibres due to the plasticization of glucuronoxylan. A similar result was observed for the model system. However, the softening behaviour of the delignified aspen fibre and the model system was not identical, most probably due to differences in spatial organisation of the components. Dynamic FTIR-studies indicated that interactions between the cellulose and the glucuronoxylan exist in the aspen holocellulose while the components in the nanocomposite appear to be more isolated.     

Organofunctionalized montmorillonite/epoxy nanocomposites: The effect of interlayer cation distribution on mechanical properties

The present study indicates that a distribution of alkylammonium chain lengths in the interlayer of montmorillonite (MM) results in a greater degree of dispersion into an epoxy matrix (or possibly different polymer matrices) and improved ultimate tensile strengths. This hypothesis is supported by x‐ray diffraction (XRD) data from the modified MM clays, which clearly shows that an increase in d‐spacing does not result in an improvement in tensile strengths. Additional support is obtained from DSC analyses of the first and second heats of the treated clays which indicate that the C6 alkyl amine in a deposited (C18 + C6) alkyl ammonium mixture results in intercalation pathways for the epoxy resin. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Analysis of the intragranular microstructure in ceramic nanocomposites and their effect on the mechanical properties

The formation of intragranular microstructure in Al₂O₃/ZrO₂ and Si₂N₂O/Si₃N₄ nanocomposites was analyzed, and the effect of intragranular microstructure on the mechanical properties of nanocomposites was investigated. Results suggest 3 requisite conditions for the formation of intragranular microstructure and the role of intracrystalline glass phase and scar microstructure. In case of Al₂O₃/ZrO₂, the intragranular microstructure leads to the formation of transgranular fracture, which in turn improves the mechanical properties via strengthening and toughening. On the other hand, in case of Si₃N₄/Si₂N₂O nanocomposites, intragranular microstructure reduces the possibility of bridging, pulling out, and crack deflection, thereby leading to the deterioration of strength and toughness. Based on these results, we can conclude that the formation of intragranular microstructure does not necessarily improve the mechanical properties in all kinds of materials. Rather, the effect of intragranular microstructure on the mechanical properties of nanocomposites is related to the strengthening and toughing mechanism of matrix materials.     

The effect of shear forces on the microstructure and mechanical properties of epoxy–clay nanocomposites

The objective of this work is to understand the effect of shear force on the properties of epoxy–clay nanocomposites. The shear force was controlled by changing the revolutions per minute on a mechanical mixer. Differences in the aspect ratio of clay layers and differences of clay particle distribution in the epoxy matrix were caused by shear force. Shear force mechanism on epoxy–clay nanocomposites' intercalation/exfoliation were compared with the other mechanism already suggested. X‐ray diffraction, transmission electron microscopy, and scanning electron microscopy were utilized to investigate the degree of exfoliation and morphology. The mechanical and thermal properties were also studied to demonstrate the effect of shear force. This study revealed that appropriate shear force and mixing time on nanocomposite preparation was required to achieve the desired properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3465–3473, 2006     

Functionalisation of polypropylene by solid phase graft polymerisation and its effect on mechanical properties of silica nanocomposites

Abstract

To prepare macromolecular compatibiliser for grafted nano-SiO₂ /polypropylene (PP) composites, solid phase graft copolymers of PP with styrene and ethyl acrylate were synthesised, respectively. It was found that both per cent grafting and grafting efficiency can be adjusted by changing initiator concentration, reaction temperature and reaction time. As a result of partial chain scission and deterioration of ordered structure of PP during the graft polymerisation, the grafted PP exhibits poorer thermal stability and crystallisability than the unmodified PP. Mechanical tests of grafted nano-SiO₂ /PP composites indicated that the addition of PP copolymer with the same species of grafting polymer as that on the nanoparticles further improves the ductility of the composites. Molecular rigidity of the grafting polymers, presence of the homopolymer produced during the graft polymerisation, and strain rate of the load applied have an important influence on the toughening effect of the functionalised PP.     

Effect of nanoplatelet aspect ratio on mechanical properties of epoxy nanocomposites

To study the effect of the aspect ratio of nanoplatelets on the mechanical properties of polymer nanocomposites, epoxy/α-zirconium phosphate nanocomposites with two distinctive aspect ratios at ca. 100 and 1000 have been prepared and characterized. Scanning electron microscopy and transmission electron microscopy were utilized to confirm the two different sizes and aspect ratios of nanoplatelets in epoxy. As expected, it is found that mechanical properties of the nanocomposite are affected by the aspect ratio of nanoplatelets in epoxy. That is, a higher aspect ratio renders a better improvement in modulus. It is also found that the interfacial characteristics between the nanoplatelets and polymer matrix are most critical in affecting the strength and ductility of the polymer. The operative fracture mechanisms depend strongly on the aspect ratio of the nanoplatelets incorporated. The crack deflection mechanism, which leads to a tortuous path crack growth, is only observed for the high aspect ratio system. The implication of the present findings for structural applications of polymer nanocomposites is discussed.     

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.     

Multiscale computer simulation of polymer nanocomposites based on thermoplastics

A literature review is presented on a multiscale approach to the simulation of nanocomposites based on thermoplastic polymers that includes calculations using quantum-chemical methods and molecular dynamics simulations with the use of full-atomic and mesoscopic models. Common problems arising during the multiscale simulation of thermoplastic nanocomposites and the ways to solve them are discussed. The results of studies of the structural, thermal, and mechanical properties of thermoplastic nanocomposites obtained via the simulation with consideration for the detailed chemical structures of components are given.     

Morphology and mechanical properties of polypropylene/organoclay nanocomposites

Polypropylene (PP) nanocomposites were prepared by melt intercalation in an intermeshing corotating twin‐screw extruder. The effect of molecular weight of PP‐MA (maleic anhydride‐ modified polypropylene) on clay dispersion and mechanical properties of nanocomposites was investigated. After injection molding, the tensile properties and impact strength were measured. The best overall mechanical properties were found for composites containing PP‐MA having the highest molecular weight. The basal spacing of clay in the composites was measured by X‐ray diffraction (XRD). Nanoscale morphology of the samples was observed by transmission electron microscopy (TEM). The crystallization kinetics was measured by differential scanning calorimetry (DSC) and optical microscopy at a fixed crystallization temperature. Increasing the clay content in PP‐ MA330k/clay, a well‐dispersed two‐component system, caused the impact strength to decrease while the crystallization kinetics and the spherulite size remained almost the same. On the other hand, PP/PP‐MA330k/clay, an intercalated three‐component system containing some dispersed clay as well as the clay tactoids, showed a much smaller size of spherulites and a slight increase in impact strength with increasing the clay content. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1562–1570, 2002     

Vulcanization behavior and mechanical properties of organoclay fluoroelastomer nanocomposites

The vulcanization behavior and mechanical properties of clay/fluoroelastomer nanocomposites produced by melt‐mixing of Dyneon FPO 3741 (a terpolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene) with 10 phr of unmodified montmorillonite (CloisiteNA) or di(hydrogenated tallow‐alkyl) dimethyl ammonium‐modified montmorillonites (Cloisite15A and Cloisite20A) were studied. The properties of clay/FKM nanocomposites were compared with composites prepared using 10 and 30 phr of carbon black. The effects of clay surfactant and surfactant concentration on the vulcanization behavior, mechanical, and dynamical properties of peroxide cured composites were studied. XRD results of cured composites showed a decrease in d‐spacing and indicated deintercalation of the clays after the vulcanization process. It was also found that organoclays retard the FKM peroxide vulcanization process. Significantly, higher maximum torque on vulcanization was obtained with organoclays versus unmodified clay and carbon black. Although the morphologies of organoclay/FKM nanocomposites studied by XRD and TEM suggest similar intercalated/exfoliated structures, the organoclay with the lowest concentration of surfactant (95 meq/100 g clay) resulted in the highest increase in torque, modulus, hardness, and tear strength in the clay/FKM nanocomposites. It was also found that organoclays can increase both the hydrodynamic reinforcement and hysteresis loss of FKM nanocomposites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Preparation and mechanical properties of epoxy/diamond nanocomposites

Nanodiamond (ND) has recently attracted much attention for its outstanding mechanical and other interesting properties. Surface functionalization of ND is necessary for applications in polymers. In this study, ND particles were functionalized with amine by covalent linking of triethylene tetramine, and further grafted with epoxy which was cured with amine curing agent. The particle dispersion and mechanical properties of epoxy/ND nanocomposites were evaluated. Both fracture toughness and storage modulus of epoxy resin were significantly improved with a low loading of ND‐NH₂ particles. The morphological structure of the epoxy/ND nanocomposites was examined, and toughening mechanism was explored. POLYM. COMPOS., 35:2144–2149, 2014. © 2014 Society of Plastics Engineers     

Synthesis and mechanical properties of unsaturated polyester based nanocomposites

Two classes of nanocomposites were synthesized using an unsaturated polyester resin as the matrix and sodium montmorillonite as well as an organically modified montmorillonite as the reinforcing agents. X‐ray diffraction pattern of the composites showed that the interlayer spacing of the modified montmorillonite expanded from 1.25 nm to 4.5 nm, indicating intercalation. Glass transition values of these composites increased from 72°C, in the unfilled unsaturated polyester, to 86°C in the composite with 10% organically modified montmorillonite. From Scanning Electron Microscopy, it is seen that the degree of intercalation/exfoliation of the modified montmorillonite is higher than in the unmodified one. The mechanical properties also supported these findings, since in general, the tensile modulus, tensile strength, flexural modulus, flexural strength and impact strength of the composites with modified montmorillonite were higher than the corresponding properties of the composites with unmodified montmorillonite. The tensile modulus, tensile strength, flexural modulus and flexural strength values showed a maximum, whereas the impact strength exhibited a minimum at approximately 3–5 wt% modified montmorillonite content. These results imply that the level of exfoliation may also exhibit a maximum with respect to the modified montmorillonite content. The level of improvement in the mechanical properties was substantial. Adding only 3 wt% organically modified clay improved the flexural modulus of unsaturated polyester by 35%. The tensile modulus of unsaturated polyester was also improved by 17% at 5 wt% of organically modified clay loading.     

Influence of variation in the local interface fracture properties on shear debonding of CFRP composite from concrete

Abstract

The debonding mode of failure, which is observed in concrete beams strengthened using externally attached CFRP composite sheets, is investigated using the direct shear test. The Mode II, cohesive stress-crack relative slip relationship is established using full-field displacements obtained from digital image correlation. The interface crack is associated with a cohesive stress-transfer zone of fixed length. The load capacity of the CFRP composite bonded to concrete is attained when the cohesive crack is fully established. The acoustic emission monitored during the interface fracture initiation and propagation indicates that microcracking events accumulate at a constant rate up to failure. The variations in the local fracture parameters are quantified and are adequately represented using the normal probability distribution. A numerical analysis of the direct-shear debonding response of CFRP composite attached to a concrete substrate is performed to study the influence of the variability of the local fracture parameters on the load-carrying capacity and the ultimate failure. An instability associated with a snapback in the load response resulting from a decrease in both load and displacement, is predicted close to failure. The variation in the local fracture properties does not influence the load-carrying capacity or the intensity of snapback instability at ultimate failure.