Nanofiller Aspect Ratio Research Articles

Numerical simulation and mathematical modeling of 2D multi-scale diffusion in lamellar nanocomposite

In this work, FE analysis was used to study steady-state diffusion into 2D polymer nanocomposites. The developed FE model is made of randomly distributed and randomly oriented permeable lamellar stacks made of a certain number of platelets, separated by galleries. The model is able to account for diffusion occurring between lamellar stacks, as well as within stacks, inside lamellar galleries. This allows to account for different morphologic features of the nanofiller, including the number of lamellae in each stack, which defines the degree of dispersion, and the lamellar gallery thickness, indicative of the degree of intercalation. Simulation results showed that the normalized coefficient of diffusion only depends on the normalized path length, which is, in turn, dependent on the morphology of the nanocomposite. Besides the aspect ratio and volume fraction of the nanofiller, also the degree of intercalation and the degree of dispersion play a significant role in determining the barrier properties of nanocomposites.The behavior of nanocomposites made of permeable lamellar stacks was represented by introducing a geometrical model, which is based on the probability of collision of diffusing particles on the lamellar surface. For a random orientation of lamellar stacks, the developed model showed an excellent agreement with the simulation results. The developed model also allowed to estimate the error arising from the assumption of impermeable stacks when using permeability data in order to calculate the aspect ratio of nanofillers.

Engineering the coefficient of thermal expansion and thermal conductivity of polymers filled with high aspect ratio silica nanofibers

The thermomechanical properties of epoxy filled with two different types of silica nanofillers: spherical nanoparticles and nanofibers were investigated as a function of silica nanofiller aspect ratio and concentration. Results indicated that at room temperature and at 8.74% silica nanofiber concentration (by volume) the thermal conductivity of epoxy increased twofold and coefficient of thermal expansion (CET) decreased by ∼40%. Silica nanofiber filled epoxy showed 1.4 times greater CET and 1.5 times greater thermal conductivity compared to spherical nanoparticle filled epoxy. The significant changes observed in thermomechanical properties of silica nanofiber filled epoxy were attributed to its high aspect ratio by constraining the polymer matrix as well as reducing the phonon scattering due to the formation of a continuous fiber network within the matrix. In addition to being electrically insulating, the improved properties of silica nanofiber filled epoxy make it an extremely attractive material as underfill and encapsulant in advanced electronic packaging industry.

Synthesis and characterization of fibrous nanosilica/polydimethylsiloxane composites

This article reports the synthesis of silica nanofiber/polydimethylsiloxane composites. Hollow silica nanofibers were synthesized through soft‐template mediated synthesis using nitric acid catalyst and extensively characterized. Amorphous nanofibers were obtained by this method with mean diameter of 80 nm and length extending over several micrometers. Nanofibers were also prepared by varying the surfactant concentration and its effect on aspect ratio was investigated. Incorporation of these nanofibers into PDMS matrix was accomplished through in situ and ex situ preparative techniques. The composites prepared exhibited a significant enhancement of various properties. In situ prepared composites yielded maximum improvement in properties with 120% improvement in tensile strength, 175% improvement in room temperature storage modulus and 190°C increase in temperature of maximum degradation (Tmax) even at low filler loading. Extent of nanofiller dispersion solely determined the property improvement of the composites. Nanofiller aspect ratio was also found to play an important role on the property improvement of the composites. Composite formation and hence property improvement were explained in depth in the light of free energy change and interaction parameter of the components of the hybrid material. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1005‐1019, 2013

Is it possible to strengthen rigid polymers by adding low aspect ratio nanofillers?

twenty years ago, represented by the application of clay/nylon-6 nanocomposites at Toyota Central R&D Labs, they were considered as revolutionary alternative to microcomposites. As the interface between nanofillers and matrix would constitute large area within the bulk material, properties of polymer nano composites were expected to be greatly improved. With growing experience, however, it seems that enhancement of mechanical properties of rigid polymers (which are so named to distinguish them from rubbers and gels) resulting from addition of low aspect ratio nanofillers (e.g., clay, nanoparticles, etc.) is not so easy as people imagine. In general, stiffening characterized by increase of modulus can be acquired due to the inherent rigidity of inorganic nanofillers. Toughening is also possible when nanofillers are capable of highly mobile under applied stress, which leads to greater ability of energy dissipation and hence higher ductility of nano composites. The strategy has been evidenced by nanoclay/poly(vinylidene fluoride) or polystyrene (Adv. Mater., DOI: 10.1002/adma.200400984), nano-SiO2/polypropylene or polystyrene (Adv. Mater., DOI: 10.1002/adma.200602611), and carbon nanotubes/polypropylene (J. Mater. Chem., DOI: 10.1039/C1JM14474J). Nevertheless, the reported strengthening effect is far from satisfactory no matter how the composites are prepared. The poor loadbearing capability of the low aspect ratio nanofillers should take the main responsibility. It is worth noting that the results of our recent exploration indicate that the problem might be hopefully solved (J. Mater. Chem., DOI: 10.1039/ C2JM16097H). We found that when nano-SiO2 is present in drawn polypropylene, the aligned chains in amorphous regions can be tied by the well distributed nanofillers to share the stress together. Above the critical content of nanoparticles or drawing ratio, the nanoparticles form percolated network throughout the matrix, facilitating stress transfer in the amorphous phases during tensile test. Additionally, the nanoparticles favor microfibrillation of matrix polymer in the late stage of tensile testing (note: the microfibrils are mainly constituted by the crystalline phases). As a result, the high strength covalent bonds of macromolecules in both amorphous and crystalline phases are brought into full play. The matrix acts as the key load-bearer, while nano-SiO2 gives indispensable assistance. This is different from the mechanism involved in conventional fiber/ polymer composites. Our approach is not a total solution for the aforesaid problem and needs optimization for specific polymers, but it demonstrates the possibility of strengthening rigid linear polymers with low aspect ratio nanofillers. Continuous efforts are needed in this aspect to find out new strengthening mechanism.

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.

Polyethylene nanocomposite dielectrics: Implications of nanofiller orientation on high field properties and energy storage

Nanocomposite formation, through the incorporation of high aspect ratio nanoparticles, has been proven to enhance the dielectric properties of thermoplastic polymers, when the mitigation of internal charges and the nature of the interfacial regions are properly adjusted. Here, we explore polyethylene/montmorillonite nanocomposites, and we specifically investigate how to impart desirable dielectric behavior through controlled nanoscale texturing, i.e., through control of the spatial arrangement of the high aspect ratio nanofiller platelets. In particular, it is shown that filler alignment can be used to improve the high electric-field breakdown strength and the recoverable energy density. The origins of the improved high field performance were traced to improved charge-trapping by a synergy of nanofillers and polar maleic anhydride (MAH) groups—introduced via polyethylene-MAH copolymers—as templated by the inorganic nanofillers. Further, it is conclusively demonstrated that the alignment of the two-dimensional nanoparticles has a measurable positive effect on the breakdown strength of the materials and, consequently, on the maximum recoverable energy density.

The use of functionalized nanoparticles as non‐specific compatibilizers for polymer blends

AbstractThe ability to form blends of polymers offers the opportunity of creating a new class of materials with enhanced properties. In addition to the polymer components, recent advances in nanoengineering have resulted in the development of nanosized inorganic particles that can be used to improve the properties of the blend, such as the flammability and the mechanical properties. While traditional methods using copolymer compatibilizers have been used to strengthen polymer blends, here, we show that the inorganic nanosized filler additive can also serve as a compatibilizer as it can localize to the interface between the polymers. We use experimental and theoretical studies to show the fundamental mechanisms by which inorganic fillers with large aspect ratio and at least one‐dimension in the nanometer range, can act as non‐specific compatibilizers for polymer blends. We examine a series of nanosized fillers, ranging from nanotubes to nanoclays (with varying aspect ratios) in a model polystyrene (PS)/poly(methylmethacyralate) (PMMA) blend. Using a number of experimental techniques such as transmission electron microscopy (TEM), scanning tunneling X‐ray microscopy (STXM), and atomic force microscopy (AFM) we postulate that the mechanism of compatibilization occurs as a result of the fillers forming in situ grafts with the immiscible polymers. We also use theoretical studies to show that the aspect ratio and the bending energy of the fillers play a key role in the compatibilization process. Our results indicate that the compatibilization is a general phenomenon, which should occur with all large aspect ratio nanofiller additives to polymer blends. Copyright © 2010 John Wiley & Sons, Ltd.

On the overall elastic moduli of polymer–clay nanocomposite materials using a self-consistent approach. Part I: Theory

Although few investigations recently proposed to describe the overall elastic response of polymer–clay nanocomposite materials using micromechanical-based models, the applicability of such models for nanocomposites is far from being fully established. The main point of criticism to mention is the shelving of crucial physical phenomena, such as interactions and length scale effects, generally associated by material scientists, in addition to the nanofiller aspect ratio, to the remarkable mechanical property enhancement of polymer–clay nanocomposites. In this Part I of two-part paper, we present a micromechanical approach for the prediction of the overall moduli of polymer–clay nanocomposites using a self-consistent scheme based on the double-inclusion model. This approach is used to account for the inter-inclusion and inclusion–matrix interactions. Although neglected in the models presented in the literature, the active interaction between the nanofillers should play a key role in the reinforcing effect of nano-objects dispersed in a polymer matrix. The present micromechanical model incorporates the nanostructure of clay stacks, modeled as transversely isotropic spheroids, and the so-called constrained region, modeled as an interphase around reinforcements. This latter is linked to the interfacial interaction between matrix and reinforcements that forms a region where the polymer chain mobility is reduced. To account for length scale effects, interphase thickness and particle dimensions are taken as explicit model parameters. Instead of solving iteratively the basic homogenization equation of the self-consistent scheme, our formulation yields to a pair of equations that can be solved simultaneously for the overall elastic moduli of composite materials. When the interphase is disregarded for spheroids with zero aspect ratio, our formulation coincides with the Walpole solution (J Mech Phys Solids 1969;17:235–251). Using the proposed general form, a parametric study is presented to analyze the respective influence of aspect ratio, number of silicate layers, interlayer spacing and nanoscopic size of the transversely isotropic spheroids on the overall elastic moduli of nanocomposite materials.

Effect of nanofiller aspect ratio on the stress relaxation and creep response of toughened pom composites

Ternary composites composed of polyoxymethylene (POM), polyurethane (PU), and sodium fluorohectorite (FH) or sodium bentonite (BN) were produced by the melt compounding masterbatch (MB) technique. The related MB was produced by mixing the PU latex with water-swellable FH or BN. The dispersion of the nanofillers in the composites was studied by X-ray diffraction techniques. The crystallization of the POM-based systems was inspected by polarized optical microscopy (PLM). The stress relaxation and creep properties of the composites were determined in short-time stress relaxation and creep tests (creep at various temperatures), respectively. The POM/PU/FH composites produced by the MB technique outperformed the POM/PU blend and the POM/PU/BN system in respect to most of the stress relaxation and creep characteristics. This fact was attributed to the higher aspect ratio of FH compared with that of BN. The master curves (creep compliance vs. time) constructed by employing the time-temperature superposition principle showed that the Findley power law was fully applicable to the experimental results obtained.

Quantifying the degree of nanofiller dispersion by advanced thermal analysis: application to polyester nanocomposites prepared by various elaboration methods

An innovative thermal analysis methodology is applied for the characterization of poly(e-caprolactone) (PCL) nanocomposites containing layered silicates, needle-like sepiolite or polyhedral oligomeric silsesquioxane (POSS) nano-cages, aiming at assessing the key factors affecting nanofiller dispersion and nanocomposite properties. This methodology takes benefit of the fact that—for a given nanofiller aspect ratio—the magnitude of the excess heat capacity recorded during quasi-isothermal crystallization is directly related to the occurrence of pronounced changes to the PCL crystalline morphology. The extent of these changes, in turn, directly depends on the amount of matrix/filler interface and can therefore be considered a reliable measure for the degree of nanofiller dispersion, as supported by complementary morphological characterization. The importance of processing parameters is demonstrated in a comparative study using various melt processing conditions, evidencing the need for high shear to effectively exfoliate and disperse individual nanoparticles throughout the polymer matrix. Furthermore, the choice of the nanocomposite elaboration method is shown to profoundly affect the final morphology, as illustrated in a comparison between nanocomposites prepared by melt mixing, by in situ polymerization and by a masterbatch approach. Grafting PCL onto the filler strongly enhances its dispersion quality as compared to conventional melt mixing; subsequently further dispersing such grafted nanohybrids into the polymer matrix through a masterbatch approach provides a highly efficient method for the elaboration of well-dispersed nanocomposites. Finally, the crucial issue of interfacial compatibility is addressed in a comparison between various surface-treated layered silicates, showing that high degrees of filler dispersion in a PCL matrix can only be achieved upon polar modification of the silicate.

Nanofillers for papermaking wet end applications

The papermaking industry can benefit a lot from nanotechnology. This versatile technology can also be used in the area of fillers for papermaking wet end applications. In such applications the main technological examples currently available include wet end addition of commercially available nanofillers, formation of nanofiller/fiber or nanofiller/fibril hybrids, development of novel categories of nanofillers such as high aspect ratio nanofillers, and combination of microfillers with nanostructures by specially controlled routes to obtain composite nanofillers. It is worth noting that there are certain challenges associated with nanofillers, such as high cost, difficulty in structure and performance control, poor dispersability and retention, possible severe negative effects on paper strength, possible detrimental interactions between nanofillers with some wet end additives, and the industry-related limitations. However, in the long run, the research and development in the area of nanofillers will surely create many fruitful results.

Toughening of polymerized cyclic butylene terephthalate with carbon nanotubes for use in composites

Low viscosity cyclic butyleneterephthalate (CBT) is a promising material for thermoplastic composites, however after isothermal polymerization (to PBT) and crystallization the material is rather brittle. In the present work, high aspect ratio nanofiller, carbon nanotubes (CNTs), are added to improve ductility. The dispersion of CNTs in CBT is achieved by rotational mixing and appears optimal. Concentrations up to 0.1 wt.%CNT are added to investigate their influence on the mechanical behavior. The addition leads to an increase in stiffness, strength and energy to failure, however, the failure strain slightly decreases. Thermal measurements indicate that the CNTs have no influence on the crystallinity. However, when the mixture of CBT and CNTs is processed in fiber reinforced composites processed by vacuum infusion, the CNTs show a tendency of filtration through the fabric.

Fracture behaviour of nanoplatelet reinforced polymer nanocomposites

The authors present a review of the recent research effort towards developing a fundamental understanding of fracture behaviour of polymer nanocomposites that contain nanoplatelet fillers. Various toughening mechanisms reported in the literature are summarized and discussed. Within this review, an emphasis is placed on the up to date understanding of how the nanofiller aspect ratio affects the fracture behaviour of polymer nanocomposites. Besides, the effect of nanoplatelet dispersion and exfoliation on the fracture behaviour of polymer nanocomposites will be addressed.