Filler-matrix Interface Research Articles

Effect of Hygrothermal Aging on the Mechanical Properties of Fluorinated and Nonfluorinated Clay-Epoxy Nanocomposites.

Hydrophilic nature of epoxy polymers can lead to both reversible and irreversible/permanent changes in epoxy upon moisture absorption. The permanent changes leading to the degradation of mechanical properties due to combined effect of moisture and elevated temperature on EPON 862, Nanomer I.28E, and Somasif MAE clay-epoxy nanocomposites are investigated in this study. The extent of permanent degradation on fracture and flexural properties due to the hygrothermal aging is determined by drying the epoxy and their clay-epoxy nanocomposites after moisture absorption. Significant permanent damage is observed for fracture toughness and flexural modulus, while the extent of permanent damage is less significant for flexural strength. It is also observed that permanent degradation in Somasif MAE clay-epoxy nanocomposites is higher compared to Nanomer I.28E clay-epoxy nanocomposites. Fourier transform infrared (FTIR) spectroscopy revealed that both clays retained their original chemical structure after the absorption-desorption cycle without undergoing significant changes. Scanning electron microscopy (SEM) images of the fracture surfaces provide evidence that Somasif MAE clay particles offered very little resistance to crack propagation in case of redried specimens when compared to Nanomer I.28E counterpart. The reason for the observed higher extent of permanent degradation in Somasif MAE clay-epoxy system has been attributed to the weakening of the filler-matrix interface.

A systematic study on electrical properties of the BaTiO3–epoxy composite with different sized BaTiO3 as fillers

The epoxy composites consisting of BaTiO3 fillers with size of 100nm and 200nm were prepared, respectively. The thermo-mechanical and electrical properties of BaTiO3–epoxy composites were investigated using complex permittivity, electric modulus and ac electrical conductivity. The effects of filler size on the structure, filler–matrix interface and properties were discussed. The Havriliak–Negami model was used to analyze the dielectric relaxation of composites. The results show that larger filler tends to exhibit enhanced dynamic modulus and higher Tg because of the increase of effective volume fraction. The tetragonal phase formation in the BaTiO3 is important to achieve enhanced permittivity of the composite. The relaxation responses of composites revealed by electric modulus and complex permittivity show that the Maxwell–Wagner–Sillars (MWS) polarization and tetragonal phase transition of BaTiO3 affect the dielectric responses of composites. The conduction process is not only related with the barrier hopping but also tunneling effect of electrons. Based on the electrical properties of BaTiO3–epoxy composite with 100nm BaTiO3 at GHz, a compact band-pass filter with significant area reduction was designed and fabricated. The measured results show that the BaTiO3–epoxy composite is promising for advanced wireless device applications.

Effect of Carbon Nanotube Functionalization on Mechanical and Thermal Properties of Cross-Linked Epoxy–Carbon Nanotube Nanocomposites: Role of Strengthening the Interfacial Interactions

We have used amido-amine functionalized carbon nanotubes (CNTs) that form covalent bonds with cross-linked epoxy matrices to elucidate the role of the matrix-filler interphase in the enhancement of mechanical and thermal properties in these nanocomposites. For the base case of nanocomposites of cross-linked epoxy and pristine single-walled CNTs, our previous work (Khare, K. S.; Khare, R. J. Phys. Chem. B 2013, 117, 7444-7454) has shown that weak matrix-filler interactions cause the interphase region in the nanocomposite to be more compressible. Furthermore, because of the weak matrix-filler interactions, the nanocomposite containing dispersed pristine CNTs has a glass transition temperature (Tg) that is ∼66 K lower than the neat polymer. In this work, we demonstrate that in spite of the presence of stiff CNTs in the nanocomposite, the Young's modulus of the nanocomposite containing dispersed pristine CNTs is virtually unchanged compared to the neat cross-linked epoxy. This observation suggests that the compressibility of the matrix-filler interphase interferes with the ability of the CNTs to reinforce the matrix. Furthermore, when the compressibility of the interphase is reduced by the use of amido-amine functionalized CNTs, the mechanical reinforcement due to the filler is more effective, resulting in a ∼50% increase in the Young's modulus compared to the neat cross-linked epoxy. Correspondingly, the functionalization of the CNTs also led to a recovery in the Tg making it effectively the same as the neat polymer and also resulted in a ∼12% increase in the thermal conductivity of the nanocomposite containing functionalized CNTs compared to that containing pristine CNTs. These results demonstrate that the functionalization of the CNTs facilitates the transfer of both mechanical load and thermal energy across the matrix-filler interface.

Cassava starch–kaolinite composite films. Thermal and mechanical properties related to filler–matrix interactions

The thermal and mechanical properties of cassava starch films are analyzed in relation to the starch–kaolinite interactions. The kaolinite filler induces a lowering of the crystallinity in accordance with an increase plasticization of the starch matrix. The increase of plasticization induces a decrease of the elastic modulus and an increase of the elongation at break. A retardant effect to the starch matrix decomposition is observed. The filler–matrix interface at which the plasticizer infiltration is enhanced and the reduction of starch chain–chain interactions appears as keys of the observed properties changes. POLYM. COMPOS., 36:184–191, 2015. © 2014 Society of Plastics Engineers

Effect of Al2O3 coverage on SiC particles for electrically insulated polymer composites with high thermal conductivity

Al2O3-covered SiC/epoxy composites were prepared using a simple sol–gel method. The results of FE-SEM, TGA, and XPS indicated that the surfaces of the SiC particles had a large, dense, and homogenous distribution of Al2O3. It was found that the introduction of Al2O3 on the SiC surface improved the interfacial adhesion between the epoxy matrix and SiC particles; this resulted in an increase in the thermal conductivity of the composites since the thermal boundary resistance at the filler–matrix interface was decreased. In addition, Al2O3-covered SiC composites showed decreased electrical conductivity owing to decreased electron tunneling compared with raw SiC composites. Thus, the Al2O3-covered SiC composites prepared in the present work could prove to be desirable polymer composites to be used as thermal interface materials that are employed in the electronics industry.

Surface modification of aluminum nitride by polysilazane and its polymer-derived amorphous silicon oxycarbide ceramic for the enhancement of thermal conductivity in silicone rubber composite

Polysilazane (PSZ) and its polymer-derived amorphous silicon oxycarbide (SiOC) ceramic were coated on aluminum nitride (AlN) by using a dip-coating method to allow moisture-crosslinking of PSZ on AlN, followed by heat treatment at 700°C in air to convert PSZ into SiOC on AlN. The results from FTIR, XPS and SEM indicated that the surface of AlN was successfully coated by PSZ and SiOC film. It was found that the introduction of PSZ and SiOC film help improve in the interfacial adhesion between the modified AlN (PSZ/AlN and SiOC/AlN) and silicone rubber lead to the increase in the thermal conductivity of the composites since the thermal boundary resistance at the filler–matrix interface was decreased. However, the introduction of SiOC as an intermediate layer between AlN and silicone rubber could help increase the thermal energy transport at the filler–matrix interface rather than using PSZ. This result was due to the decrease in the surface roughness and thickness of SiOC film after heat treatment at 700°C in air. Thus, in the present work, a SiOC ceramic coating could provide a new surface modification for the improvement of the interfacial adhesion between the thermally conductive filler and the matrix in which can enhance the thermal conductivity of the composites.

Enhanced thermal–mechanical properties of polymer composites with hybrid boron nitride nanofillers

The present work focuses on the investigation of the thermal–mechanical properties of the epoxy composites with hybrid boron nitride nanotubes (BNNTs) and boron nitride nanosheets (BNNSs). The stable dispersions of BNNTs–BNNSs were achieved by a noncovalent functionalization with pyrene carboxylic acid. The resulting epoxy/BNNTs–BNNSs composites exhibited homogeneously dispersed BNNTs–BNNSs and a strong filler–matrix interface interaction. The composites showed a 95 % increase in thermal conductivity and a 57 % improvement in Young’s modulus by addition of only 1 vol. % BNNTs–BNNSs. Meanwhile, the composites also retained a high electrical resistance of pure epoxy. Our study thus shows the potential for hybrid BNNTs–BNNSs to be successfully used as the nanofillers of polymer composites for applications in electrically insulating thermal interface materials.

Effects of silane functionalization of alumina whiskers on high-density polyethylene composites

New polyethylene matrix and alumina whiskers composites have been designed in order to combine the processability of common thermoplastics with improved physical properties. This work analyzes the influence of the composite formulation on the morphological, rheological and thermal properties of the new materials. Concerning rheological properties, a significant increase in viscosity and storage modulus is observed for high alumina whiskers content. Furthermore, the whiskers were functionalized with silane coupling agent in order to improve compatibility with the matrix. Two surface treatments were used for comparison purposes, and Fourier transform infrared spectroscopy was applied for evaluating the chemical changes on the surface of whiskers. Pre-treatment with the silane coupling agent brought about beneficial changes in morphology and rheology, related with improved dispersion of whiskers and increased filler–matrix interface. Finally, the inclusion of only 5 wt.% filler, functionalized with 100 wt.% silane, increased the thermal stability of matrix around 37%.

Abrasive wear of transparent polymer coatings: Considered in terms of morphology and surface modification of nanoparticles

Silica nanoparticles having different morphology and surface modification were used to prepare transparent polymeric coating samples. The dispersion level of nanoparticles, filler–matrix interface and abrasive wear tests were studied. Compared with the neat coating, all the nanocoatings exhibited significantly improved abrasive wear resistance at various testing conditions. By analyzing worn surfaces and profiles of the samples, such improvement was ascribed to the enhanced load-bearing ability and crack resistance of the nanocoatings. The morphology and surface modification of nanoparticles affected the wear resistance also. The pyrogenic nanoparticle-filled coatings were superior to colloidal ones in wear resistance, probably due to the fact that the aggregates had floc-like morphology, which may offer higher load-carrying ability, interlocking with matrix. When comparing the nanoparticles having the similar morphology, the filler–matrix interface will play a key role in wear resistance; the stronger interface corresponded to higher wear resistance.

Overview of strength, crack propagation and fracture of nuclear reactor moderator graphite

Nuclear reactor moderator graphite is an aggregate of needle coke filler particles within a matrix of fine coke flour particles mixed with pitch binder. Following extrusion in green condition, impregnation with liquid pitch binder and graphitisation, a polygranular aggregate with orthotropic properties is produced. Its mechanical properties under several different loading conditions and associated cracking behaviour were examined to establish crack initiation and propagation behaviour. Both virgin and radiolytically oxidised material were examined using optical and electron optical microscopy, focused ion beam microscope and digital image correlation. The appearance of force vs. displacement curves varied with type of loading. Mostly linear elastic traces occurred in uniaxial tensile and flexural tests. Large departures from linear elastic behaviour were observed in standard uniaxial and diametral compression testing. Digital image correlation has shown that the initiation of cracking involves formation of a process zone which grows to a critical size of approximately 3–5mm before a macro-crack is initiated. Cracks straddle a torturous path which zigzags between the filler particles through the matrix consistent with crack propagation along the filler matrix interface. This paper provides an overview of strength, crack propagation and fracture of nuclear reactor moderator graphite. It reviews the physical processes and mathematical approaches that have been adopted to describe the behaviour of brittle materials and then considers if they apply to reactor core graphites.

Imaging after Local Tumor Therapies: Kidney and Liver

Nuclear reactor moderator graphite is an aggregate of needle coke filler particles within a matrix of fine coke flour particles mixed with pitch binder. Following extrusion in green condition, impregnation with liquid pitch binder and graphitisation, a polygranular aggregate with orthotropic properties is produced. Its mechanical properties under several different loading conditions and associated cracking behaviour were examined to establish crack initiation and propagation behaviour. Both virgin and radiolytically oxidised material were examined using optical and electron optical microscopy, focused ion beam microscope and digital image correlation. The appearance of force vs. displacement curves varied with type of loading. Mostly linear elastic traces occurred in uniaxial tensile and flexural tests. Large departures from linear elastic behaviour were observed in standard uniaxial and diametral compression testing. Digital image correlation has shown that the initiation of cracking involves formation of a process zone which grows to a critical size of approximately 3–5 mm before a macro-crack is initiated. Cracks straddle a torturous path which zigzags between the filler particles through the matrix consistent with crack propagation along the filler matrix interface. This paper provides an overview of strength, crack propagation and fracture of nuclear reactor moderator graphite. It reviews the physical processes and mathematical approaches that have been adopted to describe the behaviour of brittle materials and then considers if they apply to reactor core graphites.

Filler geometry and interface resistance of carbon nanofibres: Key parameters in thermally conductive polymer composites

The thermal conductivity of polymer composites is measured for several tubular carbon nanofillers (nanotubes, fibres, and whiskers). The highest enhancement in the thermal conductivity is observed for functionalized multiwalled carbon nanotubes (90% enhancement for 1 vol. %) and Pyrograf carbon fibres (80%). We model the experimental data using an effective thermal medium theory and determine the thermal interface resistance (RK) at the filler-matrix interface. Our results show that the geometry of the nanofibres and the interface resistance are two key factors in engineering heat transport in a composite.

Role of nanoparticle dispersion and filler-matrix interface on the matrix dominated failure of rigid C60-PE nanocomposites: A molecular dynamics simulation study

In this study, using classical molecular dynamics simulation approach, we have investigated the effects of nanoparticle dispersion and the intensity of filler and matrix interface attractions on the mechanical properties of buckyball reinforced polyethylene nanocomposites. The dispersion state of nanoparticles was studied by modeling nanoparticles either in a homogenously distributed form or in a clustered form. The quality of polymer–matrix interface was varied by altering the van der Waals interaction energy potential between the polymer and nanoparticle. Mechanical properties of the neat and nanocomposite systems were then evaluated using simulated hydrostatic tension tests. We have shown that the quality of nanoparticle dispersion can modify matrix morphology differently, which ultimately can alter the deformation and failure of the matrix material. We have demonstrated that the quality of nanoparticle–polymer interface affects the matrix morphology and the mechanical properties of nanocomposites. We have also shown that the enhancement in mechanical properties of nanocomposites depend solely on the extent of increase in overall matrix density, regardless the condition of the interface.

Surface modification of cotton nanocrystals with a silane agent

The research herewith aims at obtaining cellulose nanocrystals with a reduced hydrophilic surface character using a silane with isocyanate groups (isocyanatepropyltriethoxysilane), which are very reactive to hydroxyl groups and thus, are readily able to react with the low quantity of free hydroxyl groups present in the cellulose nanocrystal surfaces, therefore, promoting surface modification. Cellulose nanocrystals were obtained by hydrochloric acid hydrolysis of cotton fiber and were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and solid state 29Si nuclear magnetic resonance (NMR) and their morphologies were investigated by scanning and transmission electron microscopy techniques. The nanocrystals presented a needle-like geometry with a 10 nm approximate diameter and a 166 nm average length. FTIR, 29Si NMR and silicon mapping images showed that nanocrystal surface chemical modification was successfully achieved. Also, the results confirm that the chemical modification occurred mainly at the nanocrystal surface, keeping the morphological integrity of the nanocrystals. The applied methodology for surface modification of the cellulose nanocrystals provided nanofillers with more appropriate surface characteristics that allow the dispersion in polymeric matrices and the adhesion at filler-matrix interface to be obtained. This may result in a better performance of these nanocrystals as reinforcing agents of hydrophobic polymer matrices.

Thermal and structural characterization of biodegradable blends filled with halloysite nanotubes

Fully biodegradable composites based on a commercial PHBV/PBAT blend with varying concentrations of neat or silanized clay nanotubes were prepared by twin‐screw extrusion and filmed by melt blowing process. Thermal analysis suggested the simultaneous occurrence for the PHBV phase of both homogeneous crystallization and heterogeneous nucleation promoted by the included fillers. These phenomena show effects of mutual entities depending on the quality of the filler–matrix interface and the applied cooling rate. On the contrary, for the PBAT phase, although the filler may alter kinetics and mechanisms of crystallizations, no effect is reported in terms of its degree of crystallinity. All these evidences have been supported by structural investigations. Moreover, X‐ray diffraction analysis indicated that the process of film blowing enables the formation of more perfect crystals and, as expected, favors the orientation of the included nanotubes of clay. POLYM. COMPOS., 34:1460–1470, 2013. © 2013 Society of Plastics Engineers

Effect of Curing and Functionalization on the Interface Thermal Conductance in Carbon Nanotube–Epoxy Composites

This study investigates the interface thermal conductance in a functionalized carbon nanotube (CNT)–epoxy composite system and how it is modified when the surrounding matrix is cured. We have used nonequilibrium molecular dynamics simulations to study the interface thermal conductance in both cured and uncured matrices, based on diglycidyl ether of bisphenol F (EPON-862) and diethylenetoluenediamine. The functionalization is modeled using a dynamic crosslinking algorithm and represents a realistic model of the matrix-filler interface. The thermal interface conductance increases linearly with the degree of functionalization up to the studied 2.5% due to stronger thermal coupling between functionalized CNT and the matrix. In addition, it was observed that curing of the matrix increases the interface conductance by 20% relative to the uncured matrix. This increase is attributed to an increase in thermal conductivity of cured epoxy resulting from relative enhancement in nonbonded interactions (originating from volume reduction) and structural rigidity during curing. Our results suggest that the interface conductance can be strongly influenced by the thermal properties of the bulk matrix as well as the interface chemistry of the additives such as CNTs.

Rheology and thermal behavior of polyamide reinforced with alumina whiskers

Alumina whiskers were melt-mixed with polyamide 12 to obtain new composites with high performance properties and similar processability of neat polyamide. Three kinds of alumina whiskers were used to prepare composites (purified fibers, ribbon-shaped fibers and silica-coated fibers). The rheology, morphology and thermal behavior of composites were evaluated as function of the type of whisker and its amount in the samples. In terms of rheological properties, modulus and viscosity were enhanced as function of the amount of filler added. Good dispersion of alumina whiskers in PA12 and an optimal filler-matrix interface were observed in morphological tests independently of the kind of fiber used. Studies on crystallization behavior indicated the nucleation action of whiskers, although the most important changes in crystalline state of PA12 were observed with ribbon-shaped and silica-coated fibers. Whiskers improved significantly the thermal stability of polyamide. But kinetics of degradation process studied using Coats-Redfern method indicates that the best behavior wasobtained principally with ribbon-shaped and silica-coated whiskers. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers

The role of solubility partition coefficient at the mixed matrix interface in the performance of mixed matrix membranes

Abstract The impact of the solubility partition coefficient at the matrix–filler interface on the effective diffusivity of mixed matrix systems has been calculated using effective medium theory. These predictions are able to explain trends in mixed matrix membrane performance reported in the literature. In particular, partition coefficients which favor sorption of the penetrants in the matrix phase over the dispersed phase, will lead to an increase in the selectivity of the MMM compared to that of the matrix, but will decrease the effective diffusion of the penetrant. On the other hand, partition coefficients, which favor sorption in the dispersed phase have the potential to enhance both selectivity and diffusion, although, to observe this particular effect, the volume fraction of the dispersed phase in the MMM needs to be above a certain threshold.

Vibration analysis techniques for detecting filler-matrix decohesion in composites

This study proposes a comparison between two methods for the identification of dynamic mechanical properties of a composite material model charged with glass beads. We also quantify the influence of the quality of filler-matrix interface on the dynamic mechanical properties of the charged polymer. The elastic and dissipative behavior parameters are identified from non-destructive tests. The vibration techniques used are modal analysis and dynamic mechanical analysis. We used these techniques in order to cover a very large wavelength spectrum for the dynamic loading. MOTS-CLES: Materiau composite, endommagement, vibrations, analyse modale, amortissement, facteur de perte, methode de caracterisation non destructive.

Effect of maleic anhydride grafted polyethylene on engineering properties and morphology of fumed silica filled polyethylene blown films

Blown films of high-density polyethylene (HDPE)/fumed silica (FS) composites were prepared by melt-mixing in twin-screw extruder. Maleic anhydride grafted polyethylene (PE-g-MA) was introduced to improve the state of filler dispersion during melt processing. The blown films were tested for thermal, mechanical, morphological, barrier, and optical properties. The Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR) study indicated that interaction between FS and HDPE was slightly enhanced by PE-g-MA. No change in percent crystallinity was observed but mechanical properties improved after modification. Morphological analyses showed large FS clusters at higher loading which decreased after modification. Both oxygen and water vapor transmission rates of the films increased in composites as compared to HDPE, plausibly due to formation of nanogaps in blown film and/or increased solubility of penetrant molecules at filler–matrix interface.