Alignment Of Platelets Research Articles

Facile Preparation of Polyacrylic Acid-Bentonite Nanocomposite Inks for Gas Barrier Thin-Films with Ultrasonic Treatment

The facile fabrication of an organic-inorganic hybrid nanocomposite film was investigated for a cost-effective gas barrier coating. Highly stable PAA/bentonite nanoink dispersion was achieved with high power ultrasonication treatment. The bar-coated PAA/bentonite film on PET substrate showed high barrier performance of oxygen transmission rate (OTR) value of 0.86 cc/m2·day at optimized nanoink concentration (5 wt%). In contrast, the OTR value was increased about 200 times with high ink concentraton (20 wt%). The detailed analysis of film morphology and microstructure revealed that the optimization of gas barrier properties have been achieved with proper clay platelet alignment and efficient water removal at interlayer. The gas barrier properties of one-step bar-coated barrier in this study provide industrially scalable simple gas barrier coating method for food and pharmaceutical packaging application.

Enhanced through-plane thermal conductivity of PTFE composites with hybrid fillers of hexagonal boron nitride platelets and aluminum nitride particles

Due to the strong horizontal orientation tendency of hexagonal boron nitride (hBN) platelets, hBN/polymer composites generally exhibit high in-plane thermal conductivity while low through-plane thermal conductivity. In this work, hBN platelets and aluminum nitride (AlN) particles are used as hybrid fillers to improve the thermal conductivity of polytetrafluoroethylene (PTFE). AlN particles effectively disrupt the alignment of hBN platelets along the lateral direction during the compression processing, which can be proved by the results of SEM and XRD. At 30 vol% filler loading, a high through-plane thermal conductivity as 1.04 W/mK is achieved, which is 3.8 times that of neat PTFE. The significantly increased thermal conductivity is related not only to the synergetic effect of hybrid fillers, but also the reduced in-plane orientation degree of hBN. Hashin-Shtrikman model is also used to quantify the filler connectivity within polymer matrix. The result suggests that filler morphology and orientation influence the filler connectivity and both control the effective thermal conductivity of composites.

Microlite orientation in obsidian flow measured by synchrotron X-ray diffraction

Clinopyroxene and plagioclase (andesine) microlites in an obsidian flow from Glass Mountain (NE California, USA) display strong alignment. Synchrotron X-ray diffraction, coupled with Rietveld analysis, was used to quantify crystallographic-preferred orientation (CPO). Clinopyroxene, with a rod-shaped morphology, shows a strong alignment of [001] in the flow direction and (010) aligned parallel to the inferred flow plane. Andesine, with a platy morphology, displays an alignment of (010) platelets in the flow plane. Some pole densities exceed 90 multiples of random distribution. Applying a model of rigid ellipsoidal inclusions in a viscous matrix, the local pure shear strains are between 2 and 3.

Hot-pressing induced alignment of boron nitride in polyurethane for composite films with thermal conductivity over 50 Wm−1 K−1

Flexible insulating composites with superior thermal conductivity (usually below 5 W m−1 K−1 for the previously-reported composites) are highly desirable for further miniaturization of high-power electronics and portable devices. Herein hexagonal boron nitride (hBN)/thermoplastic polyurethane (TPU) composites were readily fabricated by solution ball-milling of hBN and TPU in N,N′-dimethylformamide followed by precipitation in ethanol. The coagulated solids were then processed by hot-pressing to produce TPU composite films with well-dispersed hBN platelets. The amount of hBN in the TPU matrix was loaded as high as 95 wt% while retaining mechanical flexibility. The in-plane thermal conductivity variations' trend with hBN loadings exhibited a sudden jump happened with the loading level over 40 wt% in the polymer matrix. Intriguingly, the TPU composite film with 95 wt% hBN exhibits an in-plane thermal conductivity of up to 50.3 W m−1 K−1 (a 264-fold increase compared to pure TPU), due to the hot-compression induced alignment of hBN platelets and strong interactions in/with the TPU matrix. The corresponding out-of-plane thermal conductivity is also as high as 6.9 W m−1 K−1. The hBN/TPU composite films were further employed as heat spreaders for light-emitting diodes, resulting in a significant decrease by about 40 °C in the set-point and the saturation temperatures. Such superior thermal transport performance has rarely been reported in the polymer composites. The proposed method provides a facile and novel strategy for the scalable fabrication of high-performance polymer composites containing two dimensional layered materials.

Graphite nanoplatelet/rubbery epoxy composites as adhesives and pads for thermal interface applications

Composites of graphite nanoplatelets (GNPs) and rubbery epoxy (RE) resins as adhesives and pads are evaluated as thermal interface materials (TIM). GNP-15 and GNP-5 (15 and 5 µm across, respectively) were loaded in RE by 3-roll milling to produce GNP/RE composites. The role of composite processing techniques on the texture, thermal and electrical conductivities and compression properties of composites was studied and compared. Scanning electron microscopy revealed uniform dispersion of GNPs in RE, regardless of loading and X-ray diffraction texture measurement showed less platelet alignment in the composites at low loadings. Thermal conductivities of 20 wt% GNP-15/RE (3.29 W/m K) and 35 wt% GNP-5/RE composite (2.36 W/m K) were both significantly higher than pure RE (0.17 W/m K). GNP/RE retained good compliance, compressive moduli at 20 wt% loading being comparable to commercial BN/silicone TIM. Although thermal contact resistance of GNP/RE was higher than for commercial paste, its interfacial thermal transport outperformed GNP/silicone (due to RE’s strongly adhesive nature) and, across thick bond lines, outperformed reported GNP-pastes. The 20 wt% GNP-15/RE thermal pad had significantly lower thermal contact resistance than other GNP/RE pads. This decreased with increasing applied pressure, being comparable to commercial BN/silicone pad. GNP/RE composites are thus promising candidates for thermal interface adhesives and pads.

Fabrication of bioinspired structured glass–ceramics with enhanced fracture toughness

Nature has the amazing ability to evolve materials with combined strength and toughness that far exceed those of their individual constituents. This study aims to develop a bioceramic that mimics the hierarchical organisation of the structure of nacre on multiple scale levels by using a widespread ceramics processing technique and bioactive components. We synthesised hexagonal-shaped platelets of β-tricalcium phosphate (β-TCP) with high aspect ratio (diagonal length of ~800 nm and thickness of ~150 nm) to mimic the aragonite (CaCO3) platelets in nacre. Bioactive glass nanoparticles (BGn, median size of 50 nm) with composition (mol%) of: B2O3 9.98, SiO2 49.02, CaO 39.64, P2O5 1.54 were fabricated to mimic bridges and asperities on the surface of platelets in nacre. Hierarchically organised structures were achieved by freeze-casting of a suspension containing β-TCP platelets and various concentrations of BGn in a custom-designed mould by adjusting the casting parameters. Results demonstrated successful self-organisation of all structural features during the freezing stage, including local alignment of platelets and entrapment of BGn between the platelets. The most significant mechanical results were obtained for nacre-structured β-TCP/5 wt% BGn sintered at an unconventionally low temperature (900 °C), which showed an indentation fracture toughness of 1.6 MPa m1/2, compared to reported values of 0.3–1.1 MPa m1/2 for β-TCP sintered at 1100–1300 °C, as well as other conventional bioceramics and glasses.

Modulation of light localization in the iridophores of the deep-sea highlight hatchetfish Sternoptyx pseudobscura under magnetic field

Efficient light control by certain tissue in bioluminescent fish offers potential for new optical devices. In the world’s deep-seas, there are numerous fish species with such bioluminescence yet to be discovered. One such species, the deep-sea highlight hatchetfish Sternoptyx pseudobscura, which was found at depths of ca. 800–900 m in the neighboring waters off the Amami Islands, Japan, exhibits a new form of light-reflection produced in guanine crystal platelets present in the skin Strongly reflecting platelets in the dorsal and anterior-ventral region enhance its reflection in the presence of magnetic fields either in parallel or perpendicular to the incident light. A diamagnetic ordering of the platelet alignment is suggested in producing a small angular rotation of the platelets when placed in a magnetic field. The synchronized ordering of these platelets in parallel possibly modulates the localization of light in a multilayer structure inside the iridophore. The developed method should be useful for investigating the bio-ethological meaning of guanine crystal platelet arrangements in the deep sea fish reflecting light.

Using anisotropic effective medium theories to quantify elastic properties of sandstone-shale laminated rocks

We have developed a new rock-physics model based on a combination of effective medium theories to reconstruct rock fabric, and we have estimated the elastic properties of laminated hydrocarbon-bearing shales. This innovative model is based on the separate treatment of isotropic sandstone and vertical transverse isotropic (VTI) shale volumes of each layer; the model reproduces many typical properties of laminated shales, such as alignment of cracks, alignment of clay platelets, distinction between isolated and connected porosity, and presence of fractures. Moreover, the model enables the simulation of Stoneley velocity based on the estimated stiffness of the rock in addition to compressional and shear velocities commonly calculated with rock-physics models. Finally, it is a joint model that also calculates vertical and horizontal electrical resistivity while keeping a consistent microstructure for the elastic and resistivity simulations, thereby decreasing nonuniqueness of the solution. Nonuniqueness is an inherent flaw of effective medium modeling and has rarely been addressed in previous studies. Our simulations assumed VTI elastic behavior and suggested the presence of compliant horizontal pores and natural fractures. Our model was then validated by comparing computed properties against compressional and shear velocity and, when available, Stoneley velocity and resistivity logs. Our simulation method is implemented in two wells in the Haynesville Shale and in one well in the Barnett Shale, giving rise to accurate estimates of wave velocities, with errors lower than 5.9% for wells in the Haynesville Shale and 3.8% difference for the well in the Barnett Shale. Identification of suitable depth zones to drill a horizontal well or to initiate fractures is then based on the inferred elastic properties of the rocks.

Thermal conductivity enhancement of platelets aligned composites with volume fraction from 10% to 20%

Hexagonal boron nitride (hBN)-filled composites are widely used in electronics for thermal management. In order to enhance the materials heat transport capability, the hBN platelets are expected to be assembled into well-ordered structure. Such structure has been achieved in practice by the magnetic alignment approach. However, this approach is limited to the composites loaded with low volume fraction of platelets (<10%). In this paper, we report the use of combined mechanical and magnetic stimuli to fabricate the well-aligned composites at the volume fraction from 10% to 20%. The platelets in the resulting composites exhibit a high degree of alignment. For instance, in the 10vol.% composite, the angle of 95.3% of platelets is greater than 45°, only ∼5% of platelets falls into the horizontal direction. Thermal conductivity of the composites was investigated experimentally. It exhibited strong correlation with the platelets alignment. The measured thermal conductivity of 10vol.% aligned composite is 74% higher than that of unaligned composite. Thermal conductivity were also analyzed by a theoretical model. Thermal boundary resistance (Rb), arising at the platelets–matrix interface, was extracted by fitting the measured thermal conductivity to model prediction. Rb is found to decrease with the increase of alignment degree. This study suggests that assembling the platelets into well-ordered structure can greatly enhance the heat transport capability due to the formation of conductive networks and the reduction of Rb.

Strain-controlled fluorescence polarization in a CdSe nanoplatelet-block copolymer composite.

By dispersing semi-conducting CdSe nanoplatelets within a styrene-butadiene-styrene block copolymer matrix we form homogeneous fluorescent hybrid films. Reversible orientation control of the nanoplatelets is simply achieved through stretching the film, leading to tuneable fluorescence anisotropy. Such adjustable polarization effects are useful for modulating the optical response in composite materials.

Effect of extrusion conditions and post-extrusion techniques on the morphology and thermal/mechanical properties of polycaprolactone/clay nanocomposites

The effect of extrusion conditions on the performance of polycaprolactone /organo-modified clay nanocomposites was studied. It was demonstrated that the extrusion parameters have negligible effect on the molecular weight of polycaprolactone, on the morphology of the nanocomposites and on the final thermal/mechanical properties of the materials. This result was a consequence of the previous optimization of both polymer/clay compatibility and clay processing stability. Finally, the molten–polycaprolactone/clay mixtures were post-processed by different techniques submitting the mixtures to extensional flow. Clay platelets alignment was observed as a function of the extensional flow intensity which further improved the mechanical properties of the nanocomposites.

In-situ SAXS Study of Aqueous Clay Suspensions Submitted to Alternating Current Electric Fields

Aqueous colloidal suspensions of clay platelets display a sol/gel transition that is not yet understood. Depending on the nature of the clay, liquid-crystalline behavior may also be observed. For example, the suspensions of beidellite display a nematic phase whereas those of montmorillonite do not. Both beidellite and montmorillonite have a "TOT" structure but the structural electric charge is located in the tetrahedral layer for the former and in the octahedral layer for the latter. We built a setup to perform SAXS experiments on complex fluids submitted to an electric field in situ. We found that the fluid nematic phase of beidellite suspensions readily aligns in the field. However, the field had no influence on the gels, showing that the orientational degrees of freedom of the platelets are effectively frozen. Moreover, strong platelet alignment was induced by the field in the isotropic phase of both clays, in a similar way, regardless of their ability to form a nematic phase. This surprising result would suggest that the orientational degrees of freedom are not directly involved in the sol/gel transition. The ability to induce orientational order in the isotropic phase of clay suspensions can be exploited to prepare materials of controlled anisotropy.

Crystal structure and orientation of uniaxially and biaxially oriented PLA and PP nanoclay composite films

AbstractCast films of poly(lactic acid) (PLA) and polypropylene (PP) with 2.5 and 5 wt % organo modified nanoclay were prepared and then uniaxially and biaxially hot drawn at T = 90 and 155°C, respectively, using a biaxial stretcher. The orientation of PLA and PP crystal unit cells, alignment of clay platelets, as well as the extent of intercalation and exfoliation were studied using wide angle X‐ray diffraction (WAXD). The measurement of d‐spacing of the 001 plane (normal to platelets plane) of the clay tactoids indicated the intercalation of the silicate layers for the PLA nanocomposite films, whereas the PP nanofilled films showed only dispersion of the nanoparticles (i.e., neither intercalation nor exfoliation were observed). The intercalation level of the clay platelets in PLA was almost identical for the uniaxially and biaxially drawn films. Our finding showed that the crystallite unit cell alignments are appreciably dependent on uniaxial and biaxial stretching. Moreover, the incorporation of clay to some extent influenced the orientation of the crystal unit cell axes (a, b, and c) of the oriented films. The silicate layers revealed a much higher orientation into the flow direction in the uniaxially stretched films compared to the biaxially drawn samples. In addition, the orientation of the 001 plane of nanoclays was significantly greater in the PLA compared to the PP nanoclay composite films probably due to a better intercalation and stress transfer in the former. Morphological pictograms illustrating the effects of uniaxial and biaxial stretching on the clay orientation are proposed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Orientation, mechanical, and optical properties of poly (lactic acid) nanoclay composite films

AbstractIn this study, the orientation, mechanical as well as the optical properties of multilayer films of nanoclay filled poly (lactic acid) (PLA) as the core layer and polyethylene (PE) as the skin layer obtained from the film blowing process were investigated. The effect of nanoclay content in the range of 1–7.5 wt% was considered. The orientation of the polymers crystal unit cells, alignment of clay platelets as well as the extent of intercalation and exfoliation were studied using wide angle X‐ray diffraction (WAXD). The measurement of d‐spacing of the 001 plane (normal to platelets plane) of the clay tactoids indicated intercalation of the silicate layers in the PLA nanocomposite multilayer films. The Young modulus and tear resistance along the machine and transverse directions (MD and TD, respectively) improved significantly with the presence of the nanoparticles and percentage of nanofiller studied. The haze of the nanocomposite films decreased with clay concentration up to 5 wt% and increased by further addition of the nanoclay. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Analytical solutions of the displacement and stress fields of the nanocomposite structure of biological materials

Biological materials, such as bone and nacre, are nanocomposites of protein and mineral with superior mechanical properties. The basic building blocks of these materials feature a generic nanocomposite structure with staggered alignment of mineral platelets in protein matrix. Because of the structural complexity of the generic structure, its displacement and stress fields are currently still unknown. In this study, a perturbation method was applied for analytically solving the displacement and stress fields of the nanocomposite structure under uniaxial tension. The effect of the elastic modulus, aspect ratio and volume fraction of mineral and protein on the displacement and stress fields in the nanocomposite structure was studied. A non-dimensional parameter γ was then suggested for characterizing the stress and strain fields in this nanostructure. We showed that the assumption of uniform shear stress distribution at the mineral–protein interface in the TSC model is valid when γ is less than 4 which is broadly applicable to most biological materials. The analytical solutions of displacement and stress fields obtained in this study provide a solid basis for further analyses of mechanical properties, such as the buckling and the fracture behaviors of biological materials.

Tailoring the Microstructure of Cement‐Bonded Alumina–Magnesia Refractory Castables

Nature has various hierarchical structural materials, which show a high work of fracture despite the brittle characteristics of most of their constituents. This aspect is mainly a consequence of the appropriate alignment of mineral platelets and organic protein layers. As organic materials do not attain the high‐temperature working conditions of the refractory castables, an alternative should be considered in order to provide suitable thermo‐mechanical behavior (high creep and thermal shock resistance). Taking into account the usual microstructure of cement‐bonded alumina–magnesia refractory castables, comprising low‐temperature melting point phases in the CaO–MgO–Al2O3‐–SiO2 system and calcium hexaluminate (CA6) platelets, this material could be a good candidate to be designed based on the structure of biological materials, but fit to the high‐temperature working conditions. Therefore, this work addresses the evaluation of different liquid sources (transient or not) in order to master the CA6 grain morphology, their location, and stability in the castable's microstructure. As a consequence, outstanding creep resistance was attained, pointing out suitable alternatives to design technological bioinspired refractory castables.

Mathematical modelling of anisotropy of illite-rich shale

SUMMARY The estimation of illite-rich shale anisotropy to account for the alignment of clays and gas- or brine-filled cracks is presented via mathematical modelling. Such estimation requires analysis to interpret the dominance of one effect over another. This knowledge can help to evaluate the permeability in the unconventional reservoir, stress orientation, and the seal capacity for the conventional reservoir. Effective media modelling is used to predict the elastic properties of the illite-rich shale and to identify the dominant contributions to the shale anisotropy. We consider two principal reasons of the shale anisotropy: orientation of clay platelets and orientation of fluid-filled cracks. In reality, both of these two factors affect the shale anisotropy. The goal of this study is, first, to separately analyse the effect of these two factors to reveal the specific features in P- and S-wave velocity behaviour typical of each of the factors, and, then, consider a combined effect of the factors when the cracks are horizontally or vertically aligned. To do this, we construct four models of shale. The behaviour of P- and S-wave velocities is analysed when gas- and water-filled cracks embedded in a host matrix are randomly oriented, or horizontally or vertically aligned. The host matrix can be either isotropic or anisotropic (of VTI symmetry). In such a modelling, we use published data on mineralogy and clay platelet alignment along with other micromechanical measurements. In the model, where the host matrix is isotropic, the presence of a singularity point (when the difference changes its sign) in shear wave velocities is an indicator of brine-filled aligned cracks. In the model with the VTI host matrix and horizontally aligned cracks filled with gas, an increase in their volume concentration leads to that the azimuth at which the singularity is observed moves toward the symmetry axis. In this case, if the clay content is small (around 20 per cent), the singularity point may even vanish. The Thomsen parameters are helpful in fluid type indication in shale. An indicator of gas-filled aligned cracks is ɛ > γ. If aligned cracks in illite-rich shale are brine-filled, ɛ < γ. Negative value of δ indicates brine-filled cracks in illite-rich shale. A shale with brine-filled cracks exhibits higher Vp/Vs ratio in the vertical direction as compared to the gas-filled shale. A disorientation of clay platelets and brine-filled cracks may lead to that the singularity point is absent for brine-saturated shale as well. In this case one can also observe ɛ > γ and decreased values of Vp/Vs in the vertical direction as in the case of gas-filled cracks. In the presence of vertically aligned cracks, shales exhibit distinctly revealed features of orthorhombic symmetry. The results have important applications where seismic measurements are applied to predict the maturity state of the shale.

Polymer nanocomposites with layered silicates

The topics of current research in the field of polymer nanocomposites with layered silicates are addressed. The factors influencing the efficiency of the clay exfoliation in a polymer during melt processing are summarized, namely type of a machine used, mixing conditions and molecular characteristics of polymers utilized, including that of polymeric compatibilizers. New methods leading to the clay exfoliation in a polymer matrix are described. Application of exfoliation promoter allows to eliminate the necessity to compatibilize organo-modified clay with hydrophobic polymer matrix. Shear induced crystallization of polyolefin based nanocomposites is also addressed; even weak shearing induces alignment of clay platelets, results in polymer chains orientation and enhances the nucleation of crystallization.

Orientation of montmorillonite clay in dicyclopentadiene/organically modified clay dispersions and nanocomposites

AbstractHighly delaminated dispersions of the organically modified clay I‐28 (Nanocor, Inc.) in liquid dicyclopentadiene (DCPD) were prepared. In situ ring‐opening metathesis polymerization of I‐28/DCPD nanodispersions generated I‐28/poly(DCPD) nanocomposites. When clay/DCPD dispersions were cured under shear, alignment of clay platelets, tactoids, and small particles was captured. This orientation was confirmed by X‐ray diffraction and transmission electron microscopy. The Herman's orientation parameters were calculated for the oriented nanocomposites. Viscosities of these liquid nanodispersions exhibited thixotropic flow behavior, prior to curing. The time‐dependent viscosity effects became more pronounced with an increase in delamination. Initial viscosities increased with progressive clay platelet generation during delamination and nanodispersion within the liquid monomer. Viscosity can be used to follow clay exfoliation/delamination. Etching the surface of a 2 wt % I‐28 clay/poly(DCPD) nanocomposite with oxygen plasma eroded the matrix, exposing clay tactoids protruding from the surface. These surfaces were examined by SEM and energy dispersive X‐ray spectroscopy. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2743–2751, 2006

Mechanical properties of nanostructure of biological materials

Natural biological materials such as bone, teeth and nacre are nanocomposites of protein and mineral with superior strength. It is quite a marvel that nature produces hard and tough materials out of protein as soft as human skin and mineral as brittle as classroom chalk. What are the secrets of nature? Can we learn from this to produce bio-inspired materials in the laboratory? These questions have motivated us to investigate the mechanics of protein–mineral nanocomposite structure. Large aspect ratios and a staggered alignment of mineral platelets are found to be the key factors contributing to the large stiffness of biomaterials. A tension–shear chain (TSC) model of biological nanostructure reveals that the strength of biomaterials hinges upon optimizing the tensile strength of the mineral crystals. As the size of the mineral crystals is reduced to nanoscale, they become insensitive to flaws with strength approaching the theoretical strength of atomic bonds. The optimized tensile strength of mineral crystals thus allows a large amount of fracture energy to be dissipated in protein via shear deformation and consequently enhances the fracture toughness of biocomposites. We derive viscoelastic properties of the protein–mineral nanostructure and show that the toughness of biocomposite can be further enhanced by the viscoelastic properties of protein.