Improved crack resistance and thermal conductivity of cubic zirconia containing graphene nanoplatelets

Composites of 8 mol.% yttria-stabilized zirconia (8YSZ) with graphene nanoplatelets (GNP) have been pointed as alternative interconnectors in SOFC due to their mixed ionic-electronic conduction. Here we show that GNP addition provides rising crack-resistance behavior, with long crack toughness up to 78% higher than that of 8YSZ, also improving its thermal conductivity (up to 6 times for the in-plane direction). Toughness versus crack length is measured for 7 and 11 vol.% of GNP using single edge V-notched beam technique and ultrashort pulsed laser notching; and thermal behavior is analyzed by the laser flash method. Materials also have highly anisotropic coefficient of thermal expansion. These properties contribute to enhance their performance under the harsh operating conditions of SOFC, as thermal residual stresses could be reduced while significantly improving the system mechanical stability. Moreover, the heat transfer may be enhanced especially along the interface direction which would increase the system efficiency.     

Compatibilization and morphology development of immiscible ternary polymer blends

We report a novel and effective strategy that compatibilizes three immiscible polymers, polyolefins, styrene polymers, and engineering plastics, achieved by using a polyolefin-based multi-phase compatibilizer. Compatibilizing effect and morphology development are investigated in a model ternary immiscible polymer blends consisting of polypropylene (PP)/polystyrene(PS)/polyamide(PA6) and a multi-phase compatibilizer (PP-g-(MAH-co-St) as prepared by maleic anhydride (MAH) and styrene (St) dual monomers melt grafting PP. Scanning electron microscopy (SEM) results indicate that, as a multi-phase compatibilizer, PP-g-(MAH-co-St) shows effective compatibilization in the PP/PS/PA6 blends. The particle size of both PS and PA6 is greatly decreased due to the addition of multi-phase compatibilizer, while the interfacial adhesion in immiscible pairs is increased. This good compatibilizing effect is promising for developing a new, technologically attractive method for achieving compatibilization of immiscible multi-component polymer blends as well as for recycling and reusing of such blends. For phase morphology development, the morphology of PP/PS/PA6 (70/15/15) uncompatibilized blend reveals that the blend is constituted from PP matrix in which are dispersed composite droplets of PA6 core encapsulated by PS phase. Whereas, the compatibilized blend shows the three components strongly interact with each other, i.e. multi-phase compatibilizer has good compatibilization between the various immiscible pairs. For the 40/30/30 blend, the morphology changed from a three-phase co-continuous morphology (uncompatibilized) to the dispersed droplets of PA6 and PS in the PP matrix (compatibilized).     

Structuration,selective dispersion and compatibilizing effect of (nano)fillers in polymer blends

Hybrid ternary blends comprising two polymers and one mineral (nano)filler are increasingly studied because they are starting to be widely used to respond to industrial issues. The objective of this review is to gather information on these particular systems. Concerning first thermodynamic effects of fillers on the phase separation of an immiscible polymer blend, Flory–Huggins theory demonstrate stabilization. This theory was particularly taken up and developed for the case of two polymers and one filler by Lipatov and Nesterov in the 90s. More recently, Ginzburg generalized this theory to the case of unfavorable enthalpic interactions between a particle and the two polymers. They showed that the amount of particles had to attain a certain threshold to stabilize the system and the lower the particle radius, the higher the stable zone area. Generally speaking, all the phenomena regarding the morphology of polymer blends are governed by thermodynamics and/or kinetic effects, as well as the localization of nanoparticles. The main discussed thermodynamically controlling parameter of the localization is the wetting parameter ω_AB. However, because of the viscosity of the system, the equilibrium dictated by ω_AB may never be reached. Hence, concerning the kinetic effects, the final localization of fillers in a polymer pair is guided by the sequence of mixing of the components, the viscosity ratio, the composition, the temperature, the shear rate and the time of mixing. When the particles are placed at the interface between two polymers, coalescence can be suppressed or/and interfacial tension can be reduced. In that case, particles are known to play the role of a compatibilizer. In a ternary system, (i) the shape of the particle (spheres, rods or “onions-shape”), (ii) the particle radius (R_p) versus the radius of gyration of the polymers (R_g) and (iii) the surface chemistry of the particles affect the final localization of the particles (thus, the compatibilizing effect) and the final properties of the material, such as mechanical, conductive, magnetic and thermal properties. This review details recent works for which those four above mentioned properties are improved by incorporating different kind of fillers in polymer blends.     

Morphology study of immiscible polymer blends in a vane extruder

This study reports the morphology development of polymer blends in a novel vane extruder in which polymer mainly suffers from elongational deformation field. Rapidly cooled samples of polypropylene/polystyrene (PP/PS) are collected in the vane extruder after stable extrusion. Furthermore, the shape and size of the dispersed phase from initial to final stages are analyzed. In addition, in order to compare the final size of the dispersed phase, different immiscible blends, including polypropylene/polyamide and PP/PS, are prepared by vane extruder and twin‐screw extruder, respectively. The results show that the dispersed phase is made to change rapidly from stretched striations to droplets under the strong elongational deformation field in the vane extruder. Furthermore, the droplet size of dispersed phase of blends prepared by vane extruder is much smaller than that prepared by twin‐screw extruder, indicating that the vane extruder is more efficient in mixing for immiscible polymer blends. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Dielectric spectroscopy using dielectric probes: a new approach to study glass transition dynamics in immiscible apolar polymer blends

Dielectric relaxation spectroscopy using dielectric probes was applied to study the (glass transition) dynamics in binary blends of isotactic PP, PS and LDPE. The blends were prepared by melt-mixing and doped with 0.5% of the dielectric probe 4,4′-(N,N-dibutylamino)-(E)-nitrostilbene (DBANS) (van den Berg O, Sengers WGF, Jager WF, Picken SJ, Wübbenhorst M. Macromolecules 2004;37:2460. [17]). Due to the selective amplification of the dielectric relaxation processes related to the dynamic glass transition of the polymers, accurate relaxation data were obtained, even for the minor phases. No substantial influence of the blend composition and the blend morphology on the glass transition dynamics was found, indicating that both blend constituents behave like homogeneous bulk materials. The normalised relaxation strength of glass transition processes remained constant, regardless of the blend type and blend composition. This indicates that the probe molecule, DBANS, was equally distributed over the two blend components in all three polymer combinations PE-PP, PE-PS and PP-PS.     

A morphological study on the migration and selective localization of graphene in the PLA/PMMA blends

Nano‐filled polymer blends offer the opportunity to obtain materials with fine‐tuned properties. In this work, the dispersion and localization behavior of graphene nanoplate (GNP) and graphene oxide (GO) in solution mixed blends of polylactic acid (PLA) and polymethyl methacrylate (PMMA) were investigated. The blends were prepared by using different mixing sequences to investigate the effect of kinetics parameters and surface chemistry of filler as well as thermodynamics affinity on the localization of fillers. Field Emission Scanning Electron Microscopy (FESEM) and Rheometric Mechanical Spectroscopy (RMS) were employed. In addition, graphene materials were compared by Fourier transform infrared and Raman spectroscopy as well as elemental analysis characterization. Results showed that depending on the mixing sequence, the GNPs were localized in the both phases and interface through migration to reach thermodynamics equilibrium. However, GO localization was significantly affected by the mixing sequence due to better interaction with the polymer phases. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43799.     

The effect of graphene nanoplatelet thickness on the fracture toughness of Si3N4 composites

Si₃N₄ composites with 3 and 5?wt% of graphene nanoplatelet (GNP) additions were prepared by spark plasma sintering. We used both commercially available GNPs and thinner few-layer graphene nanoplatelets (FL-GNPs) prepared by further exfoliation through ball milling with melamine addition. We found that by employing thinner FL-GNPs as filler material a 100% increase in the fracture toughness of Si₃N₄/3?wt% FL-GNP composites (10.5?±?0.2?MPa?m^1/2) can be achieved as compared to the monolithic Si₃N₄ samples (5.1?±?0.3?MPa?m^1/2), and 60% increase compared to conventional Si₃N₄/3?wt% GNP composites (6.6?±?0.4?MPa?m^1/2). For 5?wt% filler content the increase of the fracture toughness was near 50% for both GNP and FL-GNP fillers. The hardness of the composites decreased with increasing GNP content. However, composites reinforced with 5?wt% of FL-GNPs displayed 30% higher Vickers hardness (12.8?±?0.2?GPa) than their counterparts comprising conventional GNP fillers (9.8?±?0.2?GPa). We attribute the enhanced mechanical properties obtained with thinner FL-GNPs to their higher aspect ratio leading to a more homogeneous dispersion, higher interface area, as well as smaller pores in the ceramic matrix.     

Effect of homogenization treatment on the fracture behaviour of silicon nitride/graphene nanoplatelets composites

Si₃N₄ based composites with 7 wt.% of graphene nanoplatelets (GNPs) were prepared using different homogenization methods. Si₃N₄/GNPs powder mixtures were dispersed in isopropanol and homogenized by attritor milling, ball milling or planetary ball milling. The ball milling technique was also used for the homogenization of Si₃N₄/GNPs mixture in dry state. Fractography analysis was carried out in order to assess the individual homogenization treatment. Depending on the homogenization methods, the size of the processing flaws varied from 20 μm up to 400 μm. The agglomeration of the GNPs and the residual porosity were found as the most frequently observed types of the critical flaws. The planetary ball milling with previous ultrasonication of GNPs in isopropanol was found to be the most promising homogenization technique, resulting in the composites with the highest bending strength (average value is 740 MPa) and the lowest average size of the processing flaws (around 20 μm).     

A review on synthesis and properties of polymer functionalized graphene

This review highlights the functionalization chemistry of graphene with polymers by both covalent and non-covalent approaches. Due to the strong cohesive interactions graphene platelets agglomerate, causing difficulty to attain its optimum properties. The covalent functionalization is illuminated both from ‘grafting to’ and ‘grafting from’ techniques discussing the merits and demerits of the processes. The controlled free radical polymerization techniques used for this purpose e.g. ATRP, SET–LRP and RAFT etc. are discussed along with the conventional free radical polymerization. We have also noted the various approaches used in non-covalent functionalization e.g. π–π, H-bonding and hydrophobic interactions. These functionalized graphenes show good and stable dispersion facilitating composite formation with commodity plastics enhancing it's mechanical, thermal and conductivity properties. The optoelectronic properties of these functionalized graphene are interesting to fabricate sensors, photovoltaics, supercapacitors etc. A short account of the properties of these modified graphenes is also embodied with an emphasis on different area where future developments are expected.     

Electrical conductivity of the graphene nanoplatelets coated natural and synthetic fibres using electrophoretic deposition technique

Herein, electrically conductive natural and synthetic yarns through electrophoretic deposition (EPD) technique were fabricated. A parametric study on the conductivity enhancement of the yarns is carried out by Taguchi method. Using this method, the desirable conditions are determined by studying the effects of important parameters on the electrical conductivity of the yarns in the EPD coating process. Based on the L₁₈ design of experiments table, the preferred combination of factors to obtain the highest electrical conductivity of the yarns is found by Taguchi analysis. In addition, the Pareto ANOVA analysis is conducted to identify the major contributing factors on the electrical conductivity of the yarns. Characterisation techniques, such as scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy (FTIR) in attenuated total reflectance (ATR) mode, and thermogravimetric analysis (TGA) are utilised for better understanding the microstructure and physical properties. When powered by only 3 V, the maximum temperature of a Joule heated conductive sample based on natural fibre yarns reached 102°C in less than 25 s.     

Relaxation behavior of polymer blends with complex morphologies: Palierne emulsion model for uncompatibilized and compatibilized PP/PA6 blends

This paper deals with the dynamic rheological behavior of polypropylene/polyamide6 (PP/PA6) uncompatibilized blends and those compatibilized with a maleic anhydride grafted PP (PP/PP-g-MAH/PA6). The terminal relaxation times of the blends predicted by the Palierne emulsion model were compared with those obtained from experimental relaxation time spectra. The Palierne model succeeded well in describing PP/PA6 uncompatibilized blends with relatively low dispersed phase contents (10 wt%) and failed doing so for those of which the dispersed contents were high (30 wt%). It also failed for the compatibilized ones, irrespective of the dispersed phase content (10 or 30 wt%) and whether or not interface relaxation was taken into consideration. In the case of the uncompatibilized blend with high dispersed-phase content, interconnections among inclusions of the dispersed phase were responsible for the failure of the Palierne model. As for the compatiblized blends, in addition to particle interconnections, the existence of emulsion-in-emulsion (EE) structures was another factor responsible for the failure of Palierne model. A methodology was developed to use Palierne emulsion model upon taking into account the effects of the EE structure on the viscosity of the continuous phase and the effective volume fraction of the dispersed phase.     

A rheological model for polymer blends based on the concentric multilayer approach. I. Homogeneous polymer blends

The melt rheological behavior of polymer blends was investigated by means of a capillary rheometer. The systems chosen for study were blends of polystyrene (PS) with different molecular weights and blends of polymethylmethacrylate (PMMA) with different molecular weights. A modified concentric multilayer model was proposed to correlate the rheological properties of the polymer blends with the composition and shear rate. The agreement between the calculated values and the measured ones is satisfactory.     

Influence of the viscosity ratio in PC/SAN blends filled with MWCNTs on the morphological,electrical, and melt rheological properties

Co-continuous polycarbonate (PC)/poly(styrene-acrylonitrile) (SAN) = 60/40 wt.% blends were filled with 1 wt.% multi-walled carbon nanotubes (MWCNTs), which selectively localized within the PC component. To study the influence of the viscosity ratio, PCs with different viscosities were selected resulting in PC/SAN viscosity ratios (at 100 rad/s) between 1.2 and 4.5. With increasing viscosity ratio, smaller blend structures were observed. Furthermore, optical microscopy revealed that the filler dispersion was improved with decreasing PC viscosity. The highest electrical conductivity was achieved for the blend composite with the coarsest morphology, containing the low viscosity PC and having the lowest PC/SAN viscosity ratio. Transmission electron microscopy analysis indicated that for the composite prepared with high viscosity PC, not all of the incorporated MWCNTs were able to localize completely into the PC component. Instead, some MWCNTs were found to be stacked at the interface of the two polymers, indicating that the high PC melt viscosity had a restricting effect on the movement of the MWCNTs. Moreover, with electrical conductive atomic force microscopy, it was proven that small, spherical PC particles, even if filled with CNTs, do not take part in the conductive network of the blend composites. Rheological analyses showed a correlation with the morphological analysis and the electrical conductive behavior of the blend composites. In summary, a lower viscosity ratio between the blend components, in which upon addition due to thermodynamic reasons the CNTs localize (here PC), and the other component (here SAN) is favorable for high electrical conductivity values.     

Conjugated effects of the compatibilization and the dynamic vulcanization on the phase inversion behavior in poly(butylene terephthalate)/epoxide-containing rubber reactive polymer blends

The conjugated effects of both reactive compatibilization and dynamic vulcanization on the phase inversion behavior of poly(butylene terephthalate) (PBT)/epoxide-containing rubber blends have been studied in detail. Pure ethylene-methyl acrylate random copolymer (E-MA) and ethylene-methyl acrylate-glycidyl methacrylate random terpolymer (E-MA-GMA) were used as non reactive or reactive rubber phase, respectively. Location of the phase inversion region was studied using several techniques, including transmission electron microscopy (TEM) and dynamical mechanical thermal analysis (DMTA). To evaluate the relative influence of the blend compatibilization and the dynamic vulcanization on the phase inversion behavior, the relative kinetics of the two reactions were modified using different PBT and E-MA-GMA grades. The obtained results show unambiguously that the position and the width of the phase inversion region is essentially governed by the kinetic of the dynamic vulcanization process. The effect of the blend compatibilization remains quite limited even in the case of fast interfacial reaction. The crosslinking of the rubber phase induces an important shift of the phase inversion composition to higher rubber content. For blends containing low molecular weight PBT, up to 60 wt% of rubber can be homogeneously dispersed in the PBT matrix at long mixing time. In this case, development of high performance PBT based thermoplastic vulcanisates can be envisioned.     

Influence of nanoparticle-polymer interactions on the apparent migration behaviour of carbon nanotubes in an immiscible polymer blend

We investigate the influence of nanoparticle-polymer interactions on the apparent migration behavior of multiwall carbon nanotubes (CNTs) in an immiscible polymer blend of ethylene-acrylate copolymer (EA) and polyamide 12 (PA). The polymer-CNTs interaction is tuned by using different surface modification strategies, comprising grafting and coating. Poly(methyl methacrylate) (PMMA) and polystyrene (PS) are chosen as surface modifiers. The nanocomposite materials are prepared by melt-blending polymer-modified-CNTs in EA and PA. Polymer-grafted-CNTs tend to concentrate at the PA/EA interface, even if predispersed in PA, as opposed to pristine CNTs, which stay inside PA under the same circumstances. This new behavior is consistent with the morphology of PA/EA/(PMMA or PS) ternary blends and suggest a dominance of interfacial thermodynamics on CNTs localization. If we use polymer-coated-CNTs instead, the behavior depends on molar mass of the coating polymer. For low molar mass, it is similar to that of pristine CNTs and indicates desorption of the coating, owing to the weak interaction with the CNTs surface. Interestingly, we observe that long PS chains do not desorb and can drive the CNTs to the interface of the PA/EA blend. Moreover, the influence of kinetics is clearly observed through the dependence of CNTs interfacial confinement on dispersed droplet size.     

Changes in the interfacial properties of PC/SAN blends with compatibilizer

Block copolymers of polycarbonate‐b‐poly(methyl methacrylate) (PC‐b‐PMMA) and tetramethyl poly(carbonate)‐b‐poly(methyl methacrylate) (TMPC‐b‐PMMA) were examined as compatibilizers for blends of polycarbonate (PC) with styrene‐co‐acrylonitrile (SAN) copolymer. To explore the effects of block copolymers on the compatibility of PC/SAN blends, the average diameter of the dispersed particles in the blend was measured with an image analyzer, and the interfacial properties of the blends were analyzed with an imbedded fiber retraction (IFR) technique and an asymmetric double cantilever beam fracture test. The average diameter of dispersed particles and interfacial tension of the PC/SAN blends were reduced by adding compatibilizer to the PC/SAN blends. Fracture toughness of the blends was also improved by enhancing interfacial adhesion with compatibilizer. TMPC‐b‐PMMA copolymer was more effective than PC‐b‐PMMA copolymer as a compatibilizer for the PC/SAN blends. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2649–2656, 2003     

Microstructural,mechanical and marine water tribological properties of plasma-sprayed graphene nanoplatelets reinforced Al2O3- 40 wt% TiO2 coating

Graphene nanoplatelets (GNPs) as reinforcement in the ceramic matrix is rising continuously due to their outstanding mechanical and lubricative properties. Herein, different compositions of GNPs (0.5–2 wt%) reinforced alumina-titania coatings were prepared using atmospheric plasma spraying. The relative density of AT coating increased from 83% to 94% with just (1.5 wt%) addition of GNP. Consequently, mechanical properties i.e. hardness and elastic modulus were improved by ~77% and ~69% respectively. Fracture toughness also increased from 2.65 ± 0.95 MPa.m^1/2 to 5.85 ± 1.07 MPa.m^1/2. Furthermore, the seawater wear test, using a ball-on-disc tribometer revealed that the wear rate of AT coating decreased from ~11 × 10^?14 m³/Nm to ~4 × 10^?14 m³/Nm, whereas the coefficient of friction reduced from 0.33 ± 0.05–0.16 ± 0.03. The mechanisms involved to improve these properties, viz. GNP sandwiching, crack bridging, crack arrest, etc. GNP’s multi-layers facilitated long-term lubricity and enhanced the wear resistance properties of the coatings.     

The effect of graphene nanoplatelets on the thermal and electrical properties of aluminum nitride ceramics

The thermal conductivity (κ) of AlN (2.9 wt.% of Y₂O₃) is studied as a function of the addition of multilayer graphene (from 0 to 10 vol.%). The κ values of these composites, fabricated by spark plasma sintering (SPS), are independently analyzed for the two characteristic directions defined by the GNPs orientation within the ceramic matrix; that is to say, perpendicular and parallel to the SPS pressing axis. Conversely to other ceramic/graphene systems, AlN composites experience a reduction of κ with the graphene addition for both orientations; actually the decrease of κ for the in-plane graphene orientation results rather unusual. This behavior is conveniently reproduced when an interface thermal resistance is introduced in effective media thermal conductivity models. Also remarkable is the change in the electrical properties of AlN becoming an electrical conductor (200 S m⁻¹) for graphene contents above 5 vol.%.     

The effect of the embedded interface dynamics on mass transport in immiscible polymer blends

Mass transport of solvents into immiscible blends may exhibit a non-Fickian behavior due to the deformation of the embedded interface that couples with diffusion. We introduce an interface area covariant tensor N as a structural state variable and derive a set of thermodynamically-consistent PDEs and ODEs transport equations for the bulk and time-dependent boundaries. The proposed model, which is a reformulation of that derived by El Afif (2008) and El Afif et al. (2003), improves both mathematically and numerically the investigation of the diffusion-interface coupling and provides reasonable predictions of the sorption-permeation one dimensional treatment affording good agreement with experimental data. The tensor N englobes, into a single morphological quantity, all information regarding diffusion-induced changes in the size and shape anisotropy of the interface area. Predicted results include concentration, components of N , residual stresses, mass-uptake, and swelling. Scaling leads to three relevant dimensionless parameters: a mixing-interface coupling constant and bulk and boundary diffusion Deborah numbers.     

Effect of sample size on solvent extraction for detecting cocontinuity in polymer blends

The effect of sample size on the results of solvent extraction measurements for detecting cocontinuity in polymer blends was investigated. Poly(ethylene oxide)/polystyrene (PEO/PS) blend samples of several thicknesses were analyzed by removing the PEO phase using water extraction. The experimental degree of continuity was shown to have a linear dependence on the reciprocal of sample thickness. A model is proposed to explain this dependence and to allow the bulk or true degree of continuity to be determined. Measurement of the bulk degree of continuity is useful for understanding properties of cocontinuous polymer blends such as electrical conductivity, impact strength, or tensile strength.