Influence of processing conditions and physicochemical interactions on morphology and fracture behavior of a clay/thermoplastic/thermosetting ternary blend

This study provides information on the mechanical behavior of epoxy‐poly(methyl methacrylate) (PMMA)‐clay ternary composites, which have been prepared using the phase separation phenomenon of PMMA and the introduction of organophilic‐modified montmorillonites (MMTs), the continuous matrix being the epoxy network. Two dispersion processing methods are used: a melt processing without any solvent and an ultrasonic technique with solvent and a high‐speed stirrer. TEM analysis shows that phase separation between PMMA and the epoxy network was obtained in the shape of spherical nodules in the presence of the clay in both process methods used. Nanoclay particles were finely dispersed inside thermosetting matrix predominantly delaminated when ultrasonic blending was used; whereas micrometer‐sized aggregates were formed when melt blending was used. The mechanical behavior of the ternary nanocomposites was characterized using three‐point bending test, dynamic mechanical analysis (DMA), and linear elastic fracture mechanics. The corresponding fracture surfaces were examined by scanning electron microscopy to identify the relevant fracture mechanisms involved. It was evidenced that the better dispersion does not give the highest toughness because ternary nanocomposites obtained by melt blending present the highest fracture parameters (K_Ic). Some remaining disordered clay tactoids seem necessary to promote some specific toughening mechanisms. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The effect of chemical modification on the fracture toughness of montmorillonite clay/epoxy nanocomposites

The change in fracture toughness and its dependence on the content of clay nanoplatelets and adhesion at the interface between clay nanoplatelets and anhydride-cured epoxy matrix are discussed. Three clay nanoplatelets with different chemical modifications were used in this investigation. To fabricate nanocomposites, the clay nanoplatelets were sonicated in acetone for 2 h. The role of the clay nanoplatelets in the mechanical/fracture properties was investigated by transmission electron microscopy (TEM). Bright-field TEM micrographs showed excellent dispersion of clay nanoplatelets in epoxy matrix. Both intercalation and exfoliation of clay nanoplatelets were observed depending on clay modification. Compact tension specimens were used for fracture testing. The fracture toughness increased with increasing clay content. The fracture toughness of clay/epoxy nanocomposites varied with the clay morphology in the epoxy matrix. Different morphologies of the fracture surfaces, highly dependent on the morphology of dispersed clay nanoplatelets, were observed using environmental scanning electron microscopy (ESEM). The fracture toughness was found to be correlated with the fracture surface roughness measured by confocal laser scanning microscopy (CLSM).     

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.     

Effects of clay orientation and aspect ratio on mechanical behavior of nylon-6 nanocomposite

The mechanical properties of nylon-6/clay nanocomposites with variations in clay aspect ratio and orientation were studied. A large-scale simple shear process was utilized to alter the clay aspect ratio and orientation within the reference nanocomposite. It was found that the modulus, strength, and heat distortion temperature of the nanocomposites decreased as the clay aspect ratio and degree of orientation were reduced. Furthermore, the reduction of clay aspect ratio and orientation led to an increase in fracture toughness and ductility. The Halpin-Tsai and Mori-Tanaka micromechanics-based models were implemented to gain a better understanding with regard to the dependence of clay structural parameters, i.e. aspect ratio and orientation, on the reinforcement effect of the nanofillers. The micromechanical models can accurately describe the relationship between clay structural parameters and the corresponding moduli for exfoliated nanocomposites.     

Fracture behavior of thermoplastic polyolefin/clay nanocomposites

A thermoplastic polyolefin (TPO) containing 70 wt % styrene–ethylene–butadiene‐styrene‐g‐maleic anhydride and 30 wt % polypropylene and its nanocomposites reinforced with 0.3–1.5 wt % organoclay were prepared by melt mixing followed by injection molding. The mechanical and fracture behaviors of the TPO/clay nanocomposites were investigated. The essential work of fracture (EWF) approach was used to evaluate the tensile fracture behavior of the nanocomposites toughened with elastomer. Tensile tests showed that the stiffness and tensile strength of TPO was enhanced by the addition of low loading levels of organically modified montmorillonite. EWF measurements revealed that the fracture toughness of the TPO/clay nanocomposites increased with increasing clay content. The organoclay toughened the TPO matrix of the nanocomposites effectively. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Constitutive modeling for intercalated PMMA/clay nanocomposite foams

A constitutive model for tensile behavior of PMMA/clay nanocomposite foams was developed in this study. The elastic modulus of the nanocomposites is affected by the form of clays embedded in the polymer matrix. The reinforcing effect by intercalation of the clays and the detrimental effect by clay agglomeration were considered for the determination of the elastic modulus of the nanocomposites. A viscoelastic model was adapted for the tensile behavior of the material. The developed constitutive equation is expressed in terms of clay morphology and material properties. The aspect ratio of clays and the expansion of clay layer spacing in the intercalated clay clusters were proved to play a vital role in the reinforcing mechanism. For the verification of the constitutive model, Poly(methyl‐methacrylate) (PMMA)/clay nanocomposite foams were manufactured by batch process method and their uniaxial tensile test results were compared with theoretical predictions. Compared with the experimental results, the proposed constitutive equation showed agreement with the experimental test results. POLYM. ENG. SCI. 46:1787–1796, 2006. © 2006 Society of Plastics Engineers.     

Synthesis of exfoliated acrylonitrile-butadiene-styrene copolymer (ABS) clay nanocomposites: role of clay as a colloidal stabilizer

Acrylonitrile-butadiene-styrene copolymer (ABS) clay nanocomposites were synthesized using two clays (sodium montmorillonite, laponite). Both colloidal stability and mechanical properties of the nanocomposites were dependant on aspect ratios of clays. Laponite, a low aspect ratio clay, reduced particle sizes of ABS clay nanocomposite latexes, enhanced colloidal stabilities, and increased viscosity of the latexes. The colloidal stability of ABS clay latexes may result from four factors. Firstly, the electrostatic repulsion forces originated from surface charges of clays and anionic surfactant contribute to colloidal stability. Secondly, laponite layers separate sodium montmorillonite layers and polybudadiene latex particles preventing the coagulation. Thirdly, the laponite layers adsorbed on latexes act like steric barriers against coagulation. Fourthly, increased viscosity reduces latex mobility, lowering collision possibility among latex particles. Resultant ABS clay nanocomposites showed exfoliated structures, and their mechanical properties related to the relative weight ratio of sodium montmorillonite to laponite: as portions of sodium montmorillonite increased, dynamic moduli of the nanocomposites increased, because sodium montmorillonite has higher aspect (length/thickness) ratio than laponite.     

Surface morphology,thermomechanical and barrier properties of poly(ether sulfone)‐toughened epoxy clay ternary nanocomposites

Poly(ether sulfone) (PES)‐toughened epoxy clay ternary nanocomposites were prepared by melt blending of PES with diglycidyl ether of bisphenol A epoxy resin along with Cloisite 30B followed by curing with 4,4′‐diaminodiphenylsulfone. The effect of organoclay and thermoplastic on the fracture toughness, permeability, viscoelasticity and thermomechanical properties of the epoxy system was investigated. A significant improvement in fracture toughness and modulus with reduced coefficient of thermal expansion (CTE) and gas permeability were observed with the addition of thermoplastic and clay to the epoxy system. Scanning electron microscopy of fracture‐failed specimens revealed crack path deflection and ductile fracture without phase separation. Oxygen gas permeability was reduced by 57% and fracture toughness was increased by 66% with the incorporation of 5 phr clay and 5 phr thermoplastic into the epoxy system. Optical transparency was retained even with high clay content. The addition of thermoplastic and organoclay to the epoxy system had a synergic effect on fracture toughness, modulus, CTE and barrier properties. Planetary ball‐milled samples gave exfoliated morphology with better thermomechanical properties compared to ultrasonicated samples with intercalated morphology. Copyright © 2010 Society of Chemical Industry     

Fracture behavior of core‐shell rubber–modified clay‐epoxy nanocomposites

Morphology and fracture mechanisms in two nanoclay‐filled epoxy systems were investigated using both microscopy and spectroscopy tools. Clay exfoliation was achieved using a series of sample preparation steps, and confirmed using wide angle X‐ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Significant improvement in modulus was obtained when clay exfoliation was achieved. Incorporation of core‐shell rubber (CSR) in both caly‐filled epoxy systems leads to greatly enhanced fracture toughness. Optical microscopy and TEM observations of the CSR‐modified nanocomposites suggest that CSR cavitation. shear yielding of the matrix, clay layer delamination. CSR bridging, crack bifurcation. and crack deflection are among the operative toughening mechanisms observed, depending on the nature of the epoxy matrix utilized.     

Toughening epoxies with halloysite nanotubes

An experimental attempt was made to characterize the fracture behaviour of epoxies modified by halloysite nanotubes and to investigate toughening mechanisms with nanoparticles other than carbon nanotubes (CNTs) and montmorillonite particles (MMTs). Halloysite-epoxy nanocomposites were prepared by mixing epoxy resin with halloysite particles (5 wt% and 10 wt%, respectively). It was found that halloysite nanoparticles, mainly nanotubes, are effective additives in increasing the fracture toughness of epoxy resins without sacrificing other properties such as strength, modulus and glass transition temperature. Indeed, there were also noticeable enhancements in strength and modulus for halloysite-epoxy nanocomposites because of the reinforcing effect of the halloysite nanotubes due to their large aspect ratios. Fracture toughness of the halloysite particle modified epoxies was markedly increased with the greatest improvement up to 50% in K_IC and 127% in G_IC. Increases in fracture toughness are mainly due to mechanisms such as crack bridging, crack deflection and plastic deformation of the epoxy around the halloysite particle clusters. Halloysite particle clusters can interact with cracks at the crack front, resisting the advance of the crack and resulting in an increase in fracture toughness.     

Effects of carbon nanofiller functionalization and distribution on interlaminar fracture toughness of multi-scale reinforced polymer composites

Carbon nanofillers with different surface functional groups and aspect ratios, including carboxyl carbon nanotubes, un-functionalized carbon nanofibers (CNFs), glycidyloxypropyl-trimethoxysilane carbon nanotubes (GPS-CNTs) and nanofibers were evaluated for their potential for increasing the interlaminar fracture toughness of an S2-glass fiber/epoxy composite. The fillers were added in the matrix of the fiber reinforced plies, in the resin interlayer between plies, or in both regions. Comparisons were made based on mode I and mode II interlaminar fracture toughness. For composites made with CNTs dispersed in the matrix, fracture toughness was largely unaffected except for a slight increase seen with long GPS-CNTs. However, adding a CNF or CNT modified resin interlayer significantly increased the fracture toughness, with the highest improvement over the baseline material achieved by adding long GPS-CNTs in the interlayer (79% and 91% for mode I and mode II onset toughness, respectively). Important material parameters identified for improving interlaminar fracture toughness are the nanofiller aspect ratio and concentration at the fracture plane. Based on microscopic evaluations of the fracture surfaces, a high density of high aspect ratio nanofillers causes the best entanglement between the filler and glass fibers and effectively obstructs interlaminar crack propagation.     

Fracture of unsaturated polyester and the limitation of layered silicates

In this work, we explain why the incorporation of organically modified nano‐clay into unsaturated polyester resins, unlike epoxy, does not improve their fracture toughness despite continuing aggressive research activities based on this approach. The mechanism behind this phenomenon is explored by studying the effect of mixing method on improving the degree of exfoliation in simple nanocomposites and its final effect on fracture behaviour. Rheometry and X‐ray diffraction show that the two mixing methods lead to different degrees of exfoliation. The mechanical properties primarily depend on clay content and are less sensitive to degree of exfoliation. In the case of toughness, there is no observable effect of degree of exfoliation. This despite the increased fracture surface area evident in SEM images of the sample with finer exfoliation as compared with those of the sample with a lower degree of exfoliation. Dispersed silicate layers influence the toughness by increasing the tortuosity of the crack path locally while micron scale intercalated tactoids can result in crack deflection. Both of these mechanisms depend on localized plasticity for significant energy dissipation. Since unsaturated polyester has very low localized plasticity below ～90°C, one cannot significantly improve its room temperature toughness by manipulating the micro‐/nanostructure of the nanocomposite the nanocomposite without incorporating another material. This new understanding of the fracture behavior of unsaturated polyesters and their nanocomposites allows for the development of more complex toughened systems. POLYM. ENG. SCI., 55:1303–1309, 2015. © 2015 Society of Plastics Engineers     

Effect of clay type on morphology and thermal stability of PMMA-clay nanocomposites prepared by heterocoagulation method

Poly(methyl methacrylate) (PMMA)-clay nanocomposites were prepared by a heterocoagulation method. A cationic PMMA emulsion was prepared by emulsion polymerization using a cationic initiator in the presence of free surfactant, cetyl trimethylammonium bromide (CTABr), followed by mixing with an aqueous clay slurry. Clays used in present research included montmorillonite (MMT), synthetic hectorites and fluorohectorites (with two different sizes). WAXD results and TEM images indicate that the morphologies of these nanocomposites depend on clay colloid stability as well as clay loadings. WAXD and TEM results also indicate the good morphology preservation of the nanocomposites during solution and melt processing. Thermal stability of these nanocomposites was studied by TG-DTG analyses; the mechanism of thermal stability improvement is discussed based on experimental results.     

Facile preparation of nylon 6 nanocomposites based on clay reinforcement and core‐shell latex toughening: Morphology,properties, and impact fracture behavior

This work is focused on a facile route to prepare a new type of nylon 6‐based nanocomposites with both high fracture toughness and high strength. A series of nylon 6‐matrix blends were prepared via melting extrusion by compounding with poly (methyl methacrylate‐co‐butadiene‐co‐styrene) (MBS) or poly(methyl methacrylate‐co‐methylphenyl siloxane‐co‐styrene) (MSIS) latices as impact modifier and diglycidyl ether of bisphenol‐A (DGEBA) as compatibilizer. Layered organic clay was also incorporated into above nylon 6 blends for the reinforcement of materials. Morphology study suggests that the MBS or MSIS latex particles could achieve a mono‐dispersion in nylon 6 matrix with the aid of DGEBA, which improves the compatibilization and an interfacial adhesion between the matrix and the shell of MBS or MSIS. High impact toughness was also obtained but with a corresponding reduction in tensile strength and stiffness. A moderate amount of organic clay as reinforcing agent could gain a desirable balance between the strength, stiffness and toughness of the materials, and tensile strength and stiffness could achieve an improvement. This suggests that the combination of organic clay and core‐shell latex particles is a useful strategy to optimize and enhance the properties of nylon 6. Morphology observation indicates that the layered organic clay was completely exfoliated within nylon 6 matrix. It is found that the core‐shell latex particles and the clay platelets were dispersed individually in nylon 6 matrix, and no clay platelets were present in MBS or MSIS latex particles. So the presence of the clay in nylon 6 matrix does not disturb the latex particles to promote high fracture toughness via particle cavitation and subsequent matrix shear yielding, and therefore, provides maximum reinforcement to the polymer. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Preparation of intercalating agent-free epoxy/clay nanocomposites

A simple intercalating agent-free approach to prepare epoxy/montmorillonite (MMT) clay nanocomposite is reported. Through this new approach, no organic modifiers are needed. Thus, the cost for preparing polymer nanocomposites can be significantly reduced. The extent of dispersion and exfoliation of clay in epoxy was characterized by X-ray diffraction and transmission electron microscopy observations. It is found that the MMT clay is well-dispersed in epoxy matrix. The clay platelets in epoxy show a stacked structure with dimensions of about 1–2 μm in length and about 20 nm in thickness. At 4.5 wt% of clay loading level, the flexural modulus of the epoxy nanocomposite is increased by about 35%. No reduction in fracture toughness or glass-transition temperature is observed. The implication of the present finding for commercialization of polymer nanocomposites is discussed. POLYM. ENG. SCI., 47:1708–1714, 2007. © 2007 Society of Plastics Engineers     

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.     

Effect of clay with different cation exchange capacity on the morphology and properties of poly(methyl methacrylate)/clay nanocomposites

PMMA/clay nanocomposites were successfully prepared by in situ free‐radical polymerization with the organic modified MMT‐clay using methyl methacrylate monomer and benzoyl peroxide initiator. Two clays with different cation exchange capacity have been used to prepare and compare the several properties. The clays have been modified using Amphoterge K2 by ion exchange reaction to increase the compatibility between the clay and polymer matrices. The modified clays have been characterized by wide‐angle X‐ray diffraction pattern, Fourier transform infrared spectroscopy, and thermogravimetric analysis (TGA). The powdered X‐ray diffraction and transmission electron microscopy techniques were employed to study the morphology of the PMMA/clay nanocomposites which indicate that the modified clays are dispersed in PMMA matrix to form both exfoliated and intercalated PMMA/modified clay nanocomposites. The thermomechanical properties were examined by TGA, differential scanning calorimetry, and dynamic mechanical analysis. Gas permeability analyzer shows the excellent gas barrier property of the nanocomposites, which is in good agreement with the morphology. The optical property was measured by UV–vis spectroscopy which shows that these materials have good optical clarity and UV resistance. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Parameters determining the strength and toughness of particulate-filled epoxy resins

The effects of such parameters as the filler volume fraction, particle size, aspect ratio, modulus and strength of filler, resin-filler adhesion, and toughness of the matrix on the stiffness, strength, and toughness of particulate-filled epoxy resins were evaluated. The mechanisms of crack initiation and subsequent crack propagation in these multiphase materials are discussed and illustrated by scanning electron microscopy of fracture surfaces.     

On the importance of specific interface area in clay nanocomposites of PMMA filled with synthetic nano-mica

In clay nanocomposites, the specific interface area is the key factor determining potential improvements of properties. Nevertheless, in most systematic studies of nanocomposites little emphasis is put on assuring and characterizing dispersion quality. To probe the influence of dispersion quality, we compare nanocomposites filled with two layered silicates which were made by melt compounding and solution blending, respectively. Poly(methyl methacrylate) (PMMA) is chosen here as a thermoplastic model matrix which was compounded with a synthetic nano-mica (O-hect) and commercial Bentone with typical diameters of 5–7 μm and <300 nm, respectively.     

Structure and properties of strain‐induced crystallization rubber–clay nanocomposites by co‐coagulating the rubber latex and clay aqueous suspension

Natural rubber (NR)–clay (clay is montmorillonite) and chloroprene rubber (CR)–clay nanocomposites were prepared by co‐coagulating the rubber latex and clay aqueous suspension. Transmission electron microscopy showed that the layers of clay were dispersed in the NR matrix at a nano level, and the aspect ratio (width/thickness) of the platelet inclusions was reduced and clay layers aligned more orderly during the compounding operation on an open mill. However, X‐ray diffraction indicated that there were some nonexfoliated clay layers in the NR matrix. Stress–strain curves showed that the moduli of NR were significantly improved with the increase of the amount of clay. At the same time, the clay layers inhibited the crystallization of NR on stretch, especially clay content of more than 10 phr. Compared with the carbon‐black‐filled NR composites, NR–clay nanocomposites exhibited high hardness, high modulus, high tear strength, and excellent antiaging and gas barrier properties. Similar to NR–clay nanocomposites, CR–clay nanocomposites also exhibited high hardness, high modulus, and high tear strength. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 318–323, 2005