Correlations between mechanical, structural, and dynamical properties of polymer nanocomposites

Polymer solution structure and dynamics within pores of hexagonally close-packed nanoparticles

Using coarse-grained molecular dynamics simulations, we examine structure and dynamics of polymer solutions under confinement within the pores of a hexagonally close-packed (HCP) nanoparticle system with nanoparticle diameter fifty times that of the polymer Kuhn segment size. We model a condition where the polymer chain is in a good solvent (i.e., polymer-polymer interaction is purely repulsive and polymer-solvent and solvent-solvent interactions are attractive) and the polymer-nanoparticle and solvent-nanoparticle interactions are purely repulsive. We probe three polymer lengths (N = 10, 114, and 228 Kuhn segments) and three solution concentrations (1, 10, and 25%v) to understand how the polymer chain conformations and chain center-of-mass diffusion change under confinement within the pores of the HCP nanoparticle structure from those seen in bulk. The known trend of bulk polymer R_g² decreasing with increasing concentration no longer holds when confined in the pores of HCP nanoparticle structure; for example, for the 114-mer, the HCP 〈R_g²〉 at 1%v concentration is lower than HCP 〈R_g²〉 at 10%v concentration. The 〈R_g²〉 of the 114-mer and 228-mer exhibit the largest percent decline going from bulk to HCP at the 1%v concentration and the smallest percent decline at the 25%v concentration. We also provide insight into how the confinement ratio (CR) of polymer chain size to pore size within tetrahedral and octahedral pores in the HCP arrangement of nanoparticles affects the chain conformation and diffusion at various concentrations. At the same concentration, the N = 114 has significantly more movement between pores than the N = 228 chains. For the N = 114 polymer, the diffusion between pores (i.e., inter-pore diffusion) accelerates the overall diffusion rate for the confined HCP system while for the N = 228 polymer, the polymer diffusion in the entire HCP is dominated by the diffusion within the tetrahedral or octahedral pores with minor contributions from inter-pore diffusion. These findings augment the fundamental understanding of macromolecular diffusion through large, densely packed nanoparticle assemblies and are relevant to research focused on fabrication of polymer composite materials for chemical separations, storage, optics, and photonics. We perform coarse-grained molecular dynamics simulations to understand structure and dynamics of polymer solutions under confinement within hexagonal close packed nanoparticles with radii much larger than the polymer chain's bulk radius of gyration.

Atomistic-scale analysis of the deformation and failure of polypropylene composites reinforced by functionalized silica nanoparticles

Interfacial adhesion between polymer matrix and reinforcing silica nanoparticles plays an important role in strengthening polypropylene (PP) composite. To improve the adhesion strength, the surface of silica nanoparticles can be modified by grafted functional molecules. Using atomistic simulations, we examined the effect of functionalization of silica nanoparticles by hexamethyldisilazane (HMDS) and octyltriethoxysilane (OTES) molecules on the deformation and failure of silica-reinforced PP composite. We found that the ultimate tensile strength (UTS) of PP composite functionalized by OTES (28 MPa) is higher than that of HMDS (25 MPa), which is in turn higher than that passivated only by hydrogen (22 MPa). To understand the underlying mechanistic origin, we calculated the adhesive energy and interfacial strength of the interphase region, and found that both the adhesive energy and interfacial strength are the highest for the silica nanoparticles functionalized by OTES molecules, while both are the lowest by hydrogen. The ultimate failure of the polymer composite is initiated by the cavitation in the interphase region with the lowest mass density, and this cavitation failure mode is common for all the examined PP composites, but the cavitation position is dependent on the tail length of the functional molecules. The present work provides interesting insights into the deformation and cavitation failure mechanisms of the silica-reinforced PP composites, and the findings can be used as useful guidelines in selecting chemical agents for surface treatment of silica nanoparticles.

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.

Insights from modeling into structure, entanglements, and dynamics in attractive polymer nanocomposites

Conformations, entanglements and dynamics in attractive polymer nanocomposites are investigated in this work by means of coarse-grained molecular dynamics simulation, for both weak and strong confinements, in the presence of nanoparticles (NPs) at NP volume fractions φ up to 60%. We show that the behavior of the apparent tube diameter dapp in such nanocomposites can be greatly different from nanocomposites with nonattractive interactions. We find that this effect originates, based on a mean field argument, from the geometric confinement length dgeo at strong confinement (large φ) and not from the bound polymer layer on NPs (interparticle distance ID <2Rg) as proposed recently based on experimental measurements. Close to the NP surface, the entangled polymer mobility is reduced in attractive nanocomposites but still faster than the NP mobility for volume fractions beyond 20%. Furthermore, entangled polymer dynamics is hindered dramatically by the strong confinement created by NPs. For the first time using simulations, we show that the entangled polymer conformation, characterized by the polymer radius of gyration Rg and form factor, remains basically unperturbed by the presence of NPs up to the highest volume fractions studied, in agreement with various experiments on attractive nanocomposites. As a side-result we demonstrate that the loose concept of ID can be made a microscopically well defined quantity using the mean pore size of the NP arrangement.

Application and Suitability of Polymeric Materials as Insulators in Electrical Equipment

In this paper, the applications of thermoplastic, thermoset polymers, and a brief description of the functions of each subsystem are reviewed. The synthetic route and characteristics of polymeric materials are presented. The mechanical properties of polymers such as impact behavior, tensile test, bending test, and thermal properties like mold stress-relief distortion, generic thermal indices, relative thermal capability, and relative thermal index are mentioned. Furthermore, this paper covers the electrical behavior of polymers, mainly their dielectric strength. Different techniques for evaluating polymers’ suitability applied for electrical insulation are covered, such as partial discharge and high current arc resistance to ignition. The polymeric materials and processes used for manufacturing cables at different voltage ranges are described, and their applications to high voltage DC systems (HVDC) are discussed. The evolution and limitations of polymeric materials for electrical application and their advantages and future trends are mentioned. However, to reduce the high cost of filler networks and improve their technical properties, new techniques need to be developed. To overcome limitations associated with the accuracy of the techniques used for quantifying residual stresses in polymers, new techniques such as indentation are used with higher force at the stressed location.

Effect of graphene and carbon-nitride nanofillers on the thermal transport properties of polymer nanocomposites: A combined molecular dynamics and finite element study

Low thermal conductivity of polymers, which is one of the considerable drawbacks of commonly used composite structures, has been the focus of many researchers aiming to achieve high-performance polymer-based nanocomposites through the inclusion of highly thermally conductive fillers inside the polymer matrices. Thus, in the present study, a multiscale scheme using nonequilibrium molecular dynamics and the finite element method is developed to explore the impact of different nanosized fillers (carbon-nitride and graphene) on the effective thermal conductivity of polyethylene-based nanocomposites. We show that the thermal conductivity of amorphous polyethylene at room temperature using the reactive bond order interatomic potential is nearly 0.36±0.05W/mK. Also, the atomistic results predict that, compared to the C_{3}N and graphene nanosheets, the C_{2}N nanofilm presents a much stronger interfacial thermal conductance with polyethylene. Furthermore, the results indicate that the effective thermal conductivity values of C_{2}N-polyethylene, C_{3}N-polyethylene, and graphene-polyethylene nanocomposite, at constant volume fractions of 1%, are about 0.47, 0.56, and 0.74W/mK, respectively. In other words, the results of our models reveal that the thermal conductivity of fillers is the dominant factor that defines the effective thermal conductivity of nanocomposites.

Cavitation, crazing and bond scission in chemically cross-linked polymer nanocomposites

It is very important to understand the molecular mechanism of the fracture behavior of chemically cross-linked polymer nanocomposites (PNCs). Thus, in this work, by employing a coarse-grained molecular dynamics simulation we investigated the effect of the cross-link density and the cross-link distribution on it by calculating the void formation and the chemical bond scission. Considering the fracture energy, the optimal fracture properties of PNCs are realized at the moderate cross-link density which results from the competition between the chain slippage induced voids and the bond scission induced voids. Meanwhile, more bond scission occurs on the chain backbone while a high broken percentage of the cross-link bonds appears between chains because of the higher average stress borne by one cross-linked bead than by one other bead. In addition, the number of voids is quantified which first increases and then decreases with the strain at low cross-link density. However, the number of newly formed voids increases again at high cross-link density. Finally, it decreases because of the low rate of bond scission. Furthermore, the chemical bonds are broken at a similar strain for the uniform cross-link distribution while they are broken at any strain for the nonuniform cross-link distribution. The low number of broken bonds induces the disappearance of the second peak of the number of voids with the strain for the nonuniform cross-link distribution. In summary, this work could provide a clear understanding of the fracture mechanism of the chemically cross-linked PNCs on the molecular level.

Molecular dynamics simulation study of the fracture properties of polymer nanocomposites filled with grafted nanoparticles

By employing coarse-grained molecular dynamics simulations, we investigated the fracture behavior of polymer nanocomposites (PNCs) filled with polymer-grafted nanoparticles (NPs) in detail by particularly regulating the grafting density and the length of the grafted chain.

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.

Entanglements in polymer nanocomposites containing spherical nanoparticles

We investigate the polymer packing around nanoparticles and polymer/nanoparticle topological constraints (entanglements) in nanocomposites containing spherical nanoparticles in comparison to pure polymer melts using molecular dynamics (MD) simulations. The polymer-nanoparticle attraction leads to good dispersion of nanoparticles. We observe an increase in the number of topological constraints (decrease of total entanglement length Ne with nanoparticle loading in the polymer matrix) in nanocomposites due to nanoparticles, as evidenced by larger contour lengths of the primitive paths. An increase of the nanoparticle radius reduces the polymer-particle entanglements. These studies demonstrate that the interaction between polymers and nanoparticles does not affect the total entanglement length because in nanocomposites with small nanoparticles, the polymer-nanoparticles topological constraints dominate.

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.

Partial discharge characteristics of polymer nanocomposite materials in electrical insulation: a review of sample preparation techniques, analysis methods, potential applications, and future trends

Polymer nanocomposites have recently been attracting attention among researchers in electrical insulating applications from energy storage to power delivery. However, partial discharge has always been a predecessor to major faults and problems in this field. In addition, there is a lot more to explore, as neither the partial discharge characteristic in nanocomposites nor their electrical properties are clearly understood. By adding a small amount of weight percentage (wt%) of nanofillers, the physical, mechanical, and electrical properties of polymers can be greatly enhanced. For instance, nanofillers in nanocomposites such as silica (SiO₂), alumina (Al₂O₃) and titania (TiO₂) play a big role in providing a good approach to increasing the dielectric breakdown strength and partial discharge resistance of nanocomposites. Such polymer nanocomposites will be reviewed thoroughly in this paper, with the different experimental and analytical techniques used in previous studies. This paper also provides an academic review about partial discharge in polymer nanocomposites used as electrical insulating material from previous research, covering aspects of preparation, characteristics of the nanocomposite based on experimental works, application in power systems, methods and techniques of experiment and analysis, and future trends.

Time dependence of the segmental relaxation time of poly(vinyl acetate)-silica nanocomposites

The aging-time dependence of the segmental relaxation time of poly(vinyl acetate) (PVAc) in the glassy state is investigated in the bulk polymer and its nanocomposites with silica (SiO(2)). These systems present identical segmental dynamics, when this is probed in the equilibrium supercooled liquid by broadband dielectric spectroscopy. An acceleration of the physical aging process of PVAc with SiO(2) was detected by monitoring the enthalpy recovery through differential scanning calorimetry. The segmental relaxation time during physical aging, followed by means of BDS, has been shown to increase more rapidly the higher the SiO(2) concentration in PVAc is. Thermally stimulated depolarization current experiments show that this is the case over the whole probed glassy state. This means that nanocomposites displaying a relatively slow segmental mobility evolve toward equilibrium more rapidly than the bulk. Furthermore, despite the faster increase in the relaxation time with aging time, so-called self-retardation, the nanocomposites and their bulk counterpart reach the same values of equilibrium relaxation time. These findings not only confirm the assumption of identical equilibrium dynamics even in the aging regime for all nanocomposites and bulk polymers, proposed in previous works, but also highlight the fact that the physical aging rate is not determined solely by the polymer segmental dynamics, the amount of interface being an additional relevant parameter.