Dynamics and Deformation Response of Rod-Containing Nanocomposites

Molecular Origin of the Reinforcement Effect and Its Strain-Rate Dependence in Polymer Nanocomposite Glass

We investigate the molecular origin of mechanical reinforcement in a polymer nanocomposite (PNC) under a glass state via molecular dynamics simulations. The strength of the PNC system is found to be reinforced mainly via reduced plastic deformations of the nanoparticle neighborhood (NN). Such a reinforcement effect is found to decay with an increase in the strain rate. The Arrhenius-Eyring relation is used to analyze its origin. The amplitude of the reinforcement is found to be determined by the difference between the energy barrier (ΔE) for the activation of NN and the work (W) done by the applied stress to conquer that barrier. A larger strain rate is found to result in a larger W and, hence, a weaker reinforcement effect. Such a strain-rate dependence is verified in the experimental tensile tests of a poly(vinyl alcohol)/SiO2 composite system. These results not only provide a new understanding of the molecular origin of the reinforcement effect in the PNC system, but also pave the way for a better design of the PNC material properties.

Dispersion and orientation patterns in nanorod-infused polymer melts

Introducing nanorods into a polymeric matrix can enhance the physical and mechanical properties of the resulting material. In this paper, we focus on understanding the dispersion and orientation patterns of nanorods in an unentangled polymer melt, particularly as a function of nanorod concentration, using molecular dynamics simulations. The system is comprised of flexible polymer chains and multi-thread nanorods that are equilibrated in the NPT ensemble. All interactions are purely repulsive except for those between polymers and rods. Results with attractive vs repulsive polymer-rod interactions are compared and contrasted. The concentration of rods has a direct impact on the phase behavior of the system. At lower concentrations, rods phase separate into nematic clusters, whereas at higher concentrations more isotropic and less structured rod configurations are observed. A detailed examination of the conformation of the polymer chains near the rod surface shows extension of the chains along the director of the rods (especially within clusters). The dispersion and orientation of the nanorods are a result of the competition between depletion entropic forces responsible for the formation of rod clusters, the enthalpic effects that improve mixing of rods and polymer, and entropic losses of polymers interpenetrating rod clusters.

Ionic Polymer Nanocomposites Subjected to Uniaxial Extension: A Nonequilibrium Molecular Dynamics Study

We explore the behavior of coarse-grained ionic polymer nanocomposites (IPNCs) under uniaxial extension up to 800% strain by means of nonequilibrium molecular dynamics simulations. We observe a simultaneous increase of stiffness and toughness of the IPNCs upon increasing the engineering strain rate, in agreement with experimental observations. We reveal that the excellent toughness of the IPNCs originates from the electrostatic interaction between polymers and nanoparticles, and that it is not due to the mobility of the nanoparticles or the presence of polymer-polymer entanglements. During the extension, and depending on the nanoparticle volume fraction, polymer-nanoparticle ionic crosslinks are suppressed with the increase of strain rate and electrostatic strength, while the mean pore radius increases with strain rate and is altered by the nanoparticle volume fraction and electrostatic strength. At relatively low strain rates, IPNCs containing an entangled matrix exhibit self-strengthening behavior. We provide microscopic insight into the structural, conformational properties and crosslinks of IPNCs, also referred to as polymer nanocomposite electrolytes, accompanying their unusual mechanical behavior.

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.

Polymer Conformations, Entanglements and Dynamics in Ionic Nanocomposites: A Molecular Dynamics Study

We investigate nanoparticle (NP) dispersion, polymer conformations, entanglements and dynamics in ionic nanocomposites. To this end, we study nanocomposite systems with various spherical NP loadings, three different molecular weights, two different Bjerrum lengths, and two types of charge-sequenced polymers by means of molecular dynamics simulations. NP dispersion can be achieved in either oligomeric or entangled polymeric matrices due to the presence of electrostatic interactions. We show that the overall conformations of ionic oligomer chains, as characterized by their radii of gyration, are affected by the presence and the amount of charged NPs, while the dimensions of charged entangled polymers remain unperturbed. Both the dynamical behavior of polymers and NPs, and the lifetime and amount of temporary crosslinks, are found to depend on the ratio between the Bjerrum length and characteristic distance between charged monomers. Polymer-polymer entanglements start to decrease beyond a certain NP loading. The dynamics of ionic NPs and polymers is very different compared with their non-ionic counterparts. Specifically, ionic NP dynamics is getting enhanced in entangled matrices and also accelerates with the increase of NP loading.

Effect of Multiaxial Tensile Deformation on the Mechanical Properties of Semiflexible Polymeric Samples

Molecular dynamics simulation is used to investigate the mechanical properties of the semiflexible polymer during multiaxial tensile deformations. The multiaxial tensile deformations can be imposed in totally or partially constrained modes. These types of deformations may be observed during the sudden deformation of polymeric material in the areas of aerospace, automobile, defense applications, etc. It is found that the constrained multiaxial deformation leads to the formation of nanovoids into the polymer sample. The high Young's modulus and yield strength for the totally constrained modes of tensile deformation are due to the energy required to create voids. The variation in von Misses stress, void volume, and bond order parameter with strain indicates the occurrence of brittle fracture during totally constrained tensile deformations. The partially constrained tensile deformations lead to the improvement in bond order parameter and lesser creation of nanovoids within the system. The system shows the characteristic strain hardening before failures.

Molecular dynamics simulation of the electrical conductive network formation of polymer nanocomposites by utilizing diblock copolymer-mediated nanoparticles

It is very important to improve the electrical conductivities of polymer nanocomposites (PNCs) as this can widen their application. In this work, by employing a coarse-grained molecular dynamics simulation, we investigated the effect of the amphiphilic diblock copolymer (BCP)-mediated nanoparticle (NP) on the conductive probability of polymer nanocomposites (PNCs) in the quiescent state and under a shear field. The conductive probability of PNCs first increases and then decreases with increasing content of BCPs while, interestingly, it exhibits an N-type dependence on the A-Block-NP interaction. Furthermore, the conductive probability shows a non-monotonic dependence on the fraction of A block (f_A) in the BCPs, which reaches the maximum value at moderate f_A. Under the shear field, NPs self-assemble to form the sandwich-like structures in the matrix above a critical concentration of BCPs, which leads to the anisotropic conductive probability of PNCs. In addition, the sandwich-like structures of NPs will be broken down at a high shear rate, which reduces the difference of the directional conductive probabilities. Last, the mechanism of the formation of the sandwich-like structures of NPs is discussed. In summary, this work presents a simple method to control the conductive network formation, which can help to design PNCs with high electrical conductivity, and especially anisotropy.

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

This topical review discusses the theoretical progress made in the field of polymer nanocomposites, i.e., hybrid materials created by mixing (typically inorganic) nanoparticles (NPs) with organic polymers. It primarily focuses on the outstanding issues in this field and is structured around five separate topics: (i) the synthesis of functionalized nanoparticles; (ii) their phase behavior when mixed with a homopolymer matrix and their assembly into well-defined superstructures; (iii) the role of processing on the structures realized by these hybrid materials and the role of the mobilities of the different constituents; (iv) the role of external fields (electric, magnetic) in the active assembly of the NPs; and (v) the engineering properties that result and the factors that control them. While the most is known about topic (ii), we believe that significant progress needs to be made in the other four topics before the practical promise offered by these materials can be realized. This review delineates the most pressing issues on these topics and poses specific questions that we believe need to be addressed in the immediate future.

Controlling the electrical conductive network formation in nanorod filled polymer nanocomposites by tuning nanorod stiffness

In this work, by employing a coarse-grained molecular dynamics simulation, we have investigated the effect of the nanorod (NR) stiffness on the relationship between the NR microstructure and the conductive probability of NR filled polymer nanocomposites (PNCs) under the quiescent state and under the shear field. The conductive probability of PNCs is gradually enhanced with the increase of NR stiffness in the quiescent state; however, it first increases and then decreases under the shear field. As a result, the largest conductive probability appears at moderate NR stiffness, which results from the competition between the improved effective aspect ratio of the NR and the breakage of the conductive network. Meanwhile, compared with in the quiescent state, under the shear field the decrease or the increase of the conductive probability depends on the NR stiffness. At low NR stiffness, the increase of the effective aspect ratio of NR enhances the conductive probability, while at high NR stiffness, the breakage of the conductive network reduces the conductive probability. For flexible NRs, the conductive probability first increases and then decreases with increasing the shear rate. The maximum effective aspect ratio of NRs appears at the moderate shear rate, which is consistent with the conductive probability. In summary, this work presents some further understanding about how NR stiffness affects the electric conductive properties of PNCs under the shear field.

In this work, by employing a coarse-grained molecular simulation, we investigated the effect of the nanorod stiffness on the relationship between the microstructure and the conductive probability under the quiescent state and under the shear field.

Uncovering the rupture mechanism of carbon nanotube filled cis-1,4-polybutadiene via molecular dynamics simulation

In this work, by employing molecular dynamics simulations in a united atomistic resolution, we explored the rupture mechanism of carbon nanotube (CNT) filled cis-1,4-polybutadiene (PB) nanocomposites. We observed that the rupture resistance capability increases with the interfacial interaction between PB and CNTs, as well as the loading of CNTs, attributed to the enhanced chain orientation along the deformed direction to sustain the external force, particularly those near voids. The number of voids is quantified as a function of the strain, exhibiting a non-monotonic behavior because of the coalescence of small voids into larger ones at high strain. However, the number of voids is greatly reduced by stronger PB–CNT interaction and higher loading of CNTs. During the rupture process, the maximum van der Waals energy change reflects the maximum conformational transition rate and the largest number of voids. Meanwhile, the strain at the maximum orientation degree of bonds is roughly consistent with that at the maximum square radius of gyration of chains. After the failure, the stress gradually decreases with the strain, accompanied by the contraction of the highly orientated polymer bundles. In particular, with weak interfacial interaction, the nucleation of voids occurs in the interface, and in the polymer matrix in the strong case. In general, this work could provide some fundamental understanding of the voids occurring in polymer nanocomposites (PNCs), with the aim to design and fabricate high performance PNCs.

In this work, by employing molecular dynamics simulations in a united atomistic resolution, we explored the rupture mechanism of carbon nanotube (CNT) filled cis-1,4-polybutadiene (PB) nanocomposites.

Molecular dynamics simulation of the electrical conductive network formation of polymer nanocomposites with polymer-grafted nanorods

Grafting chains on the surface of a filler is an effective strategy to tune and control the filler conductive network, which can be utilized to fabricate polymer nanocomposites (PNCs) with high electrical conductivity.

Strain induced insulator-to-conductor transition in conducting polymer composites from the auxetic behaviour of hierarchical microstructures

The auxetic behaviour of the hierarchichal microstructure present in polyaniline composites is shown to result in an insulator-to-conductor transition and a reduction in the percolation threshold upon the application of strain.

Bound Layers "Cloak" Nanoparticles in Strongly Interacting Polymer Nanocomposites

Polymer-nanoparticle (NP) interfacial interactions are expected to strongly influence the properties of nanocomposites, but surprisingly, experiments often report small or no changes in the glass transition temperature, T_g. To understand this paradoxical situation, we simulate nanocomposites over a broad range of polymer-NP interaction strengths, ε. When ε is stronger than the polymer-polymer interaction, a distinct relaxation that is slower than the main α-relaxation emerges, arising from an adsorbed "bound" polymer layer near the NP surface. This bound layer "cloaks" the NPs, so that the dynamics of the matrix polymer are largely unaffected. Consequently, T_g defined from the temperature dependence of the routinely measured thermodynamics or the polymer matrix relaxation is nearly independent of ε, in accord with many experiments. Apparently, quasi-thermodynamic measurements do not reliably reflect dynamical changes in the bound layer, which alter the overall composite dynamics. These findings clarify the relation between quasi-thermodynamic T_g measurements and nanocomposite dynamics, and should also apply to thin polymer films.

The dynamics of unentangled polymers during capillary rise infiltration into a nanoparticle packing

Although highly packed polymer nanocomposites (PNCs) are important for a wide array of applications, preparing them remains difficult because of the poor dispersion of NPs at high loading fractions. One method to successfully prepare PNCs with high loadings is through capillary rise infiltration, as previously shown by Huang et al., although the mechanism of polymer infiltration remains largely unknown. We use molecular dynamics simulations to directly simulate the process of capillary rise infiltration, and we show that the polymers follow Lucas-Washburn dynamics. We observe a wetting front that precedes bulk infiltration, and chains belonging to this front are highly adsorbed to NPs. We also investigate the viscosity of the model polymers both globally and locally in supported and free-standing films, and we find reduced viscosity near the surface of the films and increased viscosity near the supporting substrate, similar to the results of local relaxation times. The reduction in the viscosity at the free surface for short, oligomeric polymers is smaller than for higher molecular weight polymers, and the ratio of the surface viscosities is most consistent with the predictions of the Lucas-Washburn equation. Our results introduce the mechanism by which polymers infiltrate a highly packed NP film, which may shed light on better ways to prepare these materials for energy storage applications and protective coatings.

Molecular dynamics simulation of the conductivity mechanism of nanorod filled polymer nanocomposites

We adopted molecular dynamics simulation to study the conductive property of nanorod-filled polymer nanocomposites by focusing on the effects of the interfacial interaction, aspect ratio of the fillers, external shear field, filler-filler interaction and temperature. The variation of the percolation threshold is anti N-type with increasing interfacial interaction. It decreases with an increase in the aspect ratio. At an intermediate filler-filler interaction, a minimum percolation threshold appears. The percolation threshold decreases to a plateau with temperature. At low interfacial interaction, the effect of an external shear field on the homogeneous probability is negligible; however, the directional probability increases with shear rate. Moreover, the difference in conductivity probabilities is reduced for different interfacial interactions under shear. Under shear, the decrease or increase of conductivity probability depends on the initial dispersion state. However, the steady-state conductivity is independent of the initial state for different interfacial interactions. In particular, the evolution of the conductivity network structure under shear is investigated. In short, this study may provide rational tuning methods to obtain nanorod-filled polymer nanocomposites with high 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.