Volume Fraction Of Nanorods Research Articles

Charge-driven liquid-crystalline behavior of ligand-functionalized nanorods in apolar solvent.

Concentrated colloidal suspensions of nanorods often exhibit liquid-crystalline (LC) behavior. The transition to a nematic LC phase, with long-range orientational order of the particles, is usually well-captured by Onsager's theory for hard rods, at least qualitatively. The theory shows how the volume fraction at the transition decreases with increasing aspect ratio of the rods. It also explains that the long-range electrostatic repulsive interaction occurring between rods stabilized by their surface charge can significantly increase their effective diameter, resulting in a decrease in the volume fraction at the transition, as compared to sterically stabilized rods. Here, we report on a system of ligand-stabilized LaPO4 nanorods, of aspect ratio ≈11, dispersed in apolar medium exhibiting the counter-intuitive observation that the onset of nematic self-assembly occurs at an extremely low volume fraction of ≈0.25%, which is lower than observed (≈3%) with the same particles when charged-stabilized in polar solvent. Furthermore, the nanorod volume fraction at the transition increases with increasing concentration of ligands, in a similar way as in polar media where increasing the ionic strength leads to surface charge screening. This peculiar system was investigated by dynamic light scattering, Fourier-transform infrared spectroscopy, zetametry, electron microscopy, polarized light microscopy, photoluminescence measurements, and X-ray scattering. Based on these experimental data, we formulate several tentative scenarios that might explain this unexpected phase behavior. However, at this stage, its full understanding remains a pending theoretical challenge. Nevertheless, this study shows that dispersing anisotropic nanoparticles in an apolar solvent may sometimes lead to spontaneous ordering events that defy our intuitive ideas about colloidal systems.

Effect of Nanorod Physical Roughness on the Aggregation and Percolation of Nanorods in Polymer Nanocomposites.

Using molecular dynamics simulations, we elucidate the effect of nanorod roughness on nanorod aggregation, dispersion, and percolation in polymer nanocomposites (PNCs). By choosing coarse-grained models that enable systematic variation of the nanorod roughness and by selecting purely repulsive pairwise interactions for nanorods and polymer chains, we show how nanorod roughness affects the entropic driving forces for various PNC morphologies. At this entropically driven limit, we find that increasing nanorod roughness hinders nanorod aggregation and promotes nanorod percolation in the polymer melt. As nanorod roughness increases, the nanorod volume fraction needed to induce nanorod aggregation also increases. Increasing nanorod roughness increases the configurational entropy of the polymer chains and lowers the entropically induced depletion attraction between nanorods.

Phase behavior of polymer-nanorod composites: A comparative study using PRISM theory and molecular dynamics simulations.

Well-dispersed composites of polymer and nanorods have many emerging applications and, therefore, are an important area of research. Polymer reference interaction site model (PRISM) theory and molecular dynamics simulations have become powerful tools in the study of the structure and phase behavior of polymer nanocomposites. In this work, we employ both PRISM theory and molecular dynamics simulations to determine the structure and spinodal phase diagram of 1% volume fraction of nanorods in a polymer melt. We make quantitative comparisons between the phase diagrams, which are reported as a function of nanorod aspect ratio and polymer-nanorod interactions. We find that both PRISM theory and molecular dynamics simulations predict the formation of contact aggregates at low polymer-nanorod attraction strength (γ) and bridged aggregates at high polymer-nanorod attraction strength. They predict an entropic depletion-driven phase separation at low γ and a bridging-driven spinodal phase separation at high γ. The polymer and nanorods are found to form stable composites at intermediate values of the polymer-nanorod attraction strength. The fall of the bridging boundary and the gradual rise of the depletion boundary with the nanorod aspect ratio are predicted by both PRISM theory and molecular dynamics simulations. Hence, the miscible region narrows with increasing aspect ratio. The depletion boundaries predicted by theory and simulation are quite close. However, the respective bridging boundaries present a significant quantitative difference. Therefore, we find that theory and simulations qualitatively complement each other and display quantitative differences.

Molecular Modeling and Simulation of Polymer Nanocomposites with Nanorod Fillers.

We present a coarse-grained (CG) molecular dynamics (MD) simulation study of polymer nanocomposites (PNCs) containing nanorods with homogeneous and patchy surface chemistry/functionalization, modeled with isotropic and directional nanorod-nanorod attraction, respectively. We show how the PNC morphology is impacted by the nanorod design (i.e., aspect ratio, homogeneous or patchy surface chemistry/functionalization) for nanorods with a diameter equal to the Kuhn length of the polymer in the matrix. For PNCs with 10 vol % nanorods that have an aspect ratio ≤5, we observe percolated morphology with directional nanorod-nanorod attraction and phase-separated (i.e., nanorod aggregation) morphology with isotropic nanorod-nanorod attraction. In contrast, for nanorods with higher aspect ratios, both types of attractions result in aggregated nanorods morphology due to the dominance of entropic driving forces that cause long nanorods to form orientationally ordered aggregates. For most PNCs with isotropic or directional nanorod-nanorod attractions, the average matrix polymer conformation is not perturbed by the inclusion of up to 20 vol % nanorods. The polymer chains in contact with nanorods (i.e., interfacial chains) are on average extended and statistically different from the conformations the matrix chains adopt in the pure melt state (with no nanorods); in contrast, the polymer chains far from nanorods (i.e., bulk chains) adopt the same conformations as the matrix chains adopt in the pure melt state. We also study the effect of other parameters, such as attraction strength, nanorod volume fraction, and matrix chain length, for PNCs with isotropic or directional nanorod-nanorod attractions. Collectively, our results provide valuable design rules to achieve specific PNC morphologies (i.e., dispersed, aggregated, percolated, and orientationally aligned nanorods) for various potential applications.

Numerical study of the effects of nanorod aspect ratio on Poiseuille flow and convective heat transfer in a circular minichannel

The effects of nanorod aspect ratio on Poiseuille flow and convective heat transfer of a nanofluid were studied numerically. A coupled model was proposed that considered the non-uniformity of the nanoparticle volume fraction and the orientation distribution. The thermal properties of the base fluid varied with temperature, and the nanofluid viscosity and thermal conductivity were correlated with the particle volume fraction based on experimental data. The model was verified useful to predict nanofluid flow that contains nanorods by comparing the numerical results with experimental data. The non-uniformity of nanorod volume fraction distribution increases near the boundary when the aspect ratio is larger. The nanorods orient nearly randomly-in-space along the flow center and align around the flow direction when particles migrate towards the wall. The flow resistance increases with the augment of the nanorod aspect ratio. The radial velocity profile of the fully developed Poiseuille flow is flattened due to the non-uniformity of the apparent stress, and the effect is enhanced with the increase of the nanorod aspect ratios. The convective heat transfer and overall thermal performance of the nanofluids is enhanced with the augment of the nanorod aspect ratio, especially in the low-flow-rate region. Hence, non-spherical nanofluids can be applied to cooling applications in minichannels with low-power consumption.

Synergistic effect in improving the electrical conductivity in polymer nanocomposites by mixing spherical and rod-shaped fillers.

In this work, coarse-grained molecular dynamics simulation is adopted to investigate the effect of hybrid fillers [nanospheres (NSs) and nanorods (NRs)] on the conductive probability of polymer nanocomposites (PNCs) in the quiescent state and under the shear field. The percolation threshold gradually rises as the volume fraction ratio (α) of NSs to all the fillers increases in the quiescent state. Compared to the NSs, the greater number of beads in the NRs help them connect to other NRs to form the conductive network. Meanwhile, compared to NSs, more NRs participate in building the conductive network. A transition from the synergistic effect to the antagonistic effect occurs as the NS-NR tunneling distance is reduced. Furthermore, the shear field induces a more direct aggregation structure of NSs, which act as linkers between fillers to protect the conductive network. This result is confirmed by the fact that more NSs occupy the conductive network under the shear field. As a result, the percolation threshold declines with increasing shear rate. Finally, compared to in the quiescent state, the percolation threshold increases at α = 0.0 and remains nearly unchanged for α = 0.25 under the shear field, while it gradually decreases for α≥ 0.5. In total, the results further our understanding of how to realize the synergistic effect between NSs and NRs when forming a conductive network of PNCs.

Investigation of thermal conductivity property of plasmonic nanofluids based on gold nanorods prepared by seed-mediated growth method

In this paper, nanofluids were prepared based on gold nanorods in basic fluid, water, by single-stage chemical reduction and in different volume fractions and the used gold nanorods were synthesized by seed-mediated growth method in different dimensional ratios. The properties of the prepared nanoparticles, including crystalline size, aspect ratio, surface properties, nanoparticle purity, shape and morphology of nanostructures were investigated using x-ray diffraction, UV-vis spectroscopy, FT-IR, and transmitted electron microscopy. The effect of changing parameters of Nano rod dimensions, changes in Nano rod volume fraction in water and also the effect of temperature on the nanofluid thermal conductivity coefficient were investigated using transient hot wire method. The results showed that reducing the aspect ratio, increasing the volume fraction and increasing the temperature increase the thermal conductivity. In fact, results show that an increase in the nanorods aspect ratio with a constant volume fraction of 1:50 of gold in water nanorod and at room temperature leads to a decrease in the thermal conductivity of the nanofluid. Also, increasing the two parameters of volume fraction and temperature significantly increases the thermal conductivity coefficient.

Understanding the structural evolution under the oscillatory shear field to determine the viscoelastic behavior of nanorod filled polymer nanocomposites

By taking advantage of the classic coarse-grained molecular dynamics simulation, we have examined the effect of oscillatory shear strain amplitude on the viscoelastic behavior of nanorod (NR) filled polymer nanocomposites (PNCs) by tuning the interfacial interaction between polymer and NRs. We have observed the Payne effect (referred to the nonlinear viscoelastic behavior) at strong interfacial interaction. Payne-effect magnitude gradually increases with the interfacial interaction and the NR volume fraction. To understand the origin of Payne-effect, we examine the NR microstructures and their evolution during the shear field. We find that at low interfacial interaction, the probability of forming NR network is nearly unchanged with the shear strain amplitude; while at strong interfacial interaction, on the contrary the probability significantly decreases with the shear strain amplitude. We infer that various interfacial interactions change the NR network, influencing their evolution behavior under the shear flow. Meanwhile, we calculate the main cluster size and the total number of clusters for the NR network at different shear strain amplitudes, which is consistent with the probability of forming NR network. Thus, the original NR microstructure is significantly broken down under the external shear field, which can also be reflected by the number for polymer-mediated NR network bridged by glassy chains and the connected polymer beads between NRs. In addition, the polymer chains can slip on the filler surface under the shear field. As a result, the observed Payne effect comes from the slippage of the interfacial chains and breakage of the polymer-mediated NR network bridged by glassy chains at strong interfacial interaction. While at low interfacial interaction, it exhibits a weak non-linear behavior. Additionally, both reinforcement and Payne-effect magnitude exhibit a linear dependence on the inverse of the aspect ratio of the NRs. At last, different contributions to reinforcement due to hydrodynamic effects, “occluded rubber”, and “filler network” are quantified where the “filler network” is the main contribution. In summary, this work provides some interesting results to help further understand the relationship between the viscoelastic behavior and the NR network under the shear flow.

Controlling the conductive network formation of polymer nanocomposites filled with nanorods through the electric field

Abstract Facilitating the electric field to fabricate high performance polymer nanocomposites (PNCs) is always a great and promising strategy. In this work, the effect of the electric field on the conductive property of the PNCs is investigated by adopting coarse-grained molecular dynamics simulation. The translational and rotational diffusion of the nanorod gradually decreases with the increase of the nanorod aspect ratio; however, it is nearly independent of the nanorod volume fraction. Under the electric field, it exhibits a limited decrease, but more anisotropy of the translational diffusion of the nanorod. Meanwhile, the nanorods can not experience the random rotation, which is attributed to the electrostatic force. In addition, on one hand, the electrostatic force exerting on the nanorods induces the nanorod orientation, leading to the decrease of the homogeneous conductive probability; however, the electrostatic interaction induces the connection of the nanorod, resulting in the increase of the homogeneous conductive probability. Additionally, the directional conductive probability parallel to the electric field direction increases; however, the directional conductive probability perpendicular to the electric field direction shows a continuous decrease. Considering these two effects, the decrease or the increase of the homogeneous conductivity probability depends on the competition between the electrostatic force and the electrostatic interaction. The relationship among the anisotropy of the conductive probability, the nanorod volume fraction, the electrostatic force and the electrostatic interaction can be described quantitatively by an empirical formula. Another empirical formula is adopted to quantitatively describe the relationship among the percolation threshold, the electrostatic force and the electrostatic interaction. Meanwhile, the evolution process of the conductivity network structure is studied with or without the electric field. In summary, this work quantitatively describes the effect of the electric field on the conductive property of the PNCs, which can provide some useful guidances to design and fabricate PNCs with excellent conductivity properties by taking advantage of the external electric field.

Destruction and recovery of a nanorod conductive network in polymer nanocomposites via molecular dynamics simulation.

By adopting coarse-grained molecular dynamics simulation, we investigate the effects of end-functionalization and shear flow on the destruction and recovery of a nanorod conductive network in a functionalized polymer matrix. We find that the end-functionalization of polymeric chains can enhance the electrical conductivity of nanorod filled polymer nanocomposites, indicated by the decrease of the percolation threshold. However, there exists an optimal end-functionalization extent to reach the maximum electrical conductivity. In the case of steady shear flow, both homogeneous conductive probability and directional conductive probability perpendicular to the shear direction decrease with the shear rate, while the directional conductive probability parallel to the shear direction increases. Importantly, we develop a semi-empirical equation to describe the change of the homogeneous conductive probability as a function of the shear rate. Meanwhile, we obtain an empirical formula describing the relationship between the anisotropy of the conductive probability and the orientation of the nanorods. In addition, the conductivity stability increases with increasing nanorod volume fraction. During the recovery process of the nanorod conductive network, it can be fitted well by the model combining classical percolation theory and a time-dependent nanorod aggregation kinetic equation. The fitted recovery rate is similar for different nanorod volume fractions. In summary, this work provides some rational rules for fabricating polymer nanocomposites with excellent performance of electrical conductivity.

Molecular dynamics simulation of the rupture mechanism in nanorod filled polymer nanocomposites.

Through coarse-grained molecular dynamics simulation, we aim to uncover the rupture mechanism of polymer-nanorod nanocomposites by characterizing the structural and dynamic changes during the tension process. We find that the strain at failure is corresponding to the coalescence of a single void into larger voids, namely the change of the free volume. And the minimum of the Van der Walls (VDWL) energy reflects the maximum mobility of polymer chains and the largest number of voids of polymer nanocomposites. After the failure, the stress gradually decreases with the strain, accompanied by the contract of the highly orientated polymer bundles. In particular, we observe that the nucleation of voids prefers to occur from where the ends of polymer chains are located. We systematically study the effects of the interfacial interaction, temperature, the length and volume fraction of nanorods, chain length, bulk cross-linking density and interfacial chemical bonds on the rupture behavior, such as the stress at failure, the tensile modulus and the rupture energy. The rupture resistance ability increases with the increase of the interfacial interaction, rod length, and bulk cross-linking density. With an increase in the interfacial interaction, it induces the rupture transition from mode A (no bundles) to B (bundles). The transition point of the stress at failure as a function of the temperature roughly corresponds to the glass transition temperature. At longer chain length, a non-zero stress plateau occurs. And excessive chemical bonds between polymers and nanorods are harmful to the rupture property. We find that an optimal volume fraction of nanorods exists for the stress-strain behavior, which can be rationalized by the formation of the strongest polymer-nanorod network, leading to the slowest mobility of nanorods.

Uniaxial deformation of nanorod filled polymer nanocomposites: a coarse-grained molecular dynamics simulation.

A coarse-grained molecular dynamics simulation was used to investigate the stress-strain behavior of nanorod-filled polymer composites. The effects of the interfacial interaction, aspect ratio of fillers, filler functionalization, chemical couplings between the polymer and the filler and the filler loading on the mechanical reinforcement were explored. The results indicate that there exists an optimal nanorod volume fraction for elastomer reinforcement. The strong polymer-nanorod interaction enhances the reinforcement of polymer nanocomposites. Meanwhile, it is found that nanorods with longer length and smaller diameter, and the chemical functionalization of nanorods can help realize the efficient interfacial stress transfer. And excessive chemical couplings between polymers and nanorods are harmful to mechanical properties. An upturn in the modulus at large deformation is observed in the Mooney-Rivlin plot, attributed to the limited chain extensibility. Particularly, the medium polymer-nanorod interfacial strength and low nanorod volume loading will lead to better dispersion of nanorods. It is suggested that the reinforcement mechanism comes from the nanorod alignment and bond orientation, as well as from the limited extensibility of chain bridges at large deformation. In addition, an optimal nanorod volume fraction can also be explained by the strong polymer-nanorod network. Compared to glassy systems, the mechanism for the significantly enhanced reinforcement of rubbery systems is also demonstrated. In short, our simulation study of nanorod-induced mechanical reinforcement will provide a basic understanding of polymer reinforcement.

Collective alignment of nanorods in thin Newtonian films

In this paper, we provide a complete analytical description of the alignment kinetics of magnetic nanorods in magnetic field. Nickel nanorods were formed by template electrochemical deposition in alumina membranes from a dispersion in a water–glycerol mixture. To ensure uniformity of the dispersion, the surface of the nickel nanorods was covered with polyvinylpyrrolidone (PVP). A 40–70 nm coating prevented aggregation of the nanorods. These modifications allowed us to control alignment of the nanorods in a magnetic field and test the proposed theory. An orientational distribution function of nanorods was introduced. We demonstrated that the 0.04% volume fraction of nanorods in the glycerol–water mixture behaves as a system of non-interacting particles. However, the kinetics of alignment of a nanorod assembly does not follow the predictions of the single-nanorod theory. The distribution function theory explains the kinetics of alignment of a nanorod assembly and shows the significance of the initial distribution of nanorods in the film. It can be used to develop an experimental protocol for controlled ordering of magnetic nanorods in thin films.

Topological entanglement length in polymer melts and nanocomposites by a DPD polymer model

We investigate the topological constraints (entanglements) in polymer–nanorod nanocomposites in comparison to polymer melts using dissipative particle dynamics (DPD) polymer model simulations. The nanorods have a radius smaller than the polymer radius of gyration and an aspect ratio of 7.5. We observe an increase in the number of entanglements (50% decrease of Ne with 11% volume fraction of nanorods dispersed in the polymer matrix) in the nanocomposites as evidenced by larger contour lengths of the primitive paths. The end-to-end distance is essentially unchanged with the nanorod volume fraction (0–11%). Interaction between polymers and nanorods affects the dispersion of nanorods in the nanocomposites.

Light‐Induced Charge Transfer in Pyrene/CdSe‐SWNT Hybrids

On account of their nano-scale size, large aspect ratio and high conductivity, single-walled carbon nanotubes (SWNTs) have emerged as an attractive choice for conducting composite materials. Composites incorporating SWNTs show percolation dominated conductivity with a much lower volume threshold (volume fraction ≈ 10), compared to those with nanoparticles. Two-thirds of SWNTs are p-type semiconductors with holes as the charge carriers. Using the holeblocking nature of SWNTs, conducting polymers having SWNTs as dopants are effectively used as the hole buffering and electron transport layers in organic light-emitted diodes (OLEDs). For the active layer of OLEDs and organic photovoltaic (OPV) devices, incorporation of SWNTs enhances the charge separation and facilitates charge transport, hence improving the performance, i.e., the short circuit current, the filling factor and power conversion efficiency. However, since SWNTs are a mixture of metallic and semiconducting nanotubes with a small bandgap (≈ 0.6 eV), both electrons and holes in the composite matrix prefer to transfer onto and then be quenched on the SWNTs. Directly incorporating SWNTs into active layers of OLEDs and OPV devices does not facilitate the electron/hole separation, nor does it improve the performances of devices. On the other hand, semiconductor nanoparticles, possessing large and tunable bandgaps (1–2 eV), can be incorporated into conducting polymers to form hybrid solar cells. Holes are transported along the polymer chains and electrons hop along the nanoparticle network. However, nanoparticles with large volume fractions are needed because of their high percolation threshold (>≈30%). By contrast, the volume fraction for the conducting polymer is small, leading to low hole mobility and hence limiting the solar cell efficiency. Semiconductor nanorod-polymer hybrid solar cells benefit from the quasi-one-dimensional (1-D) electron transport along the rods, a mechanism which allows the use of a smaller volume fraction of nanorods and a larger volume fraction of polymer to conduct holes with higher mobilities. Although Alivisatos et al. have demonstrated that longer nanorods lead to higher energy conversion efficiency, nanorods of larger lengths and uniformly distributed diameters are difficult to fabricate. Semiconductor nanoparticle-SWNT hybrids have been the subject of recent interest as a consequence of the development of methods for the chemical modification of SWNTs. Such hybrids are well suited for use in optoelectronic devices, given the tunable bandgap of nanoparticles, quasione-dimensional (1-D) transport of SWNTs, and the ease of chemical fabrication. As part of a drive towards finding applications, an examination of the charge transfer (CT) between the various components of the hybrids is needed. By using such hybrid materials as photo-electrodes, efficient electron transfer from semiconductor nanoparticles, such as CdS, CdSe, and CdTe (donor) to SWNTs (acceptor) has been demonstrated by several groups to lead to increased photon generated current (Fig. 1). The CT also results in photolumiC O M M U N IC A IO N

A comparative study of microstructure of RuO 2 nanorods via Raman scattering and field emission scanning electron microscopy

Raman scattering (RS) and field emission scanning electron microscopy (FESEM) have been used to extract microstructural information of RuO 2 nanorods (NRs) and a two-phase system comprising NRs embedded in polycrystalline matrix deposited on different substrates by the metal-organic chemical vapor deposition method. The red shifts and asymmetric broadening of the Raman line shape for the NRs are analyzed by the spatial correlation model. The deduced spatial correlation length l is found to be much smaller than that of the average size L 0 estimated from the FESEM images. The Raman features for the two-phase system can be resolved into two parts: a Lorentzian line shape feature corresponding to the polycrystallite at higher frequency side and an asymmetrically broadened NRs' signature located at lower frequency end. The volume fraction of NRs in the two-phase system can be determined from the analysis. These results demonstrate the significance of RS as a structural characterization method when used in conjunction with FESEM.