Remarkable Toughening of Plastic with Monodispersed Nano-CaCO3: From Theoretical Predictions to Experimental Validation.

The structure-property relationship of poly(vinyl chloride) (PVC)/CaCO3 nanocomposites is investigated by all-atom molecular dynamics (MD) simulations. MD simulation results indicate that the dispersity of nanofillers, interfacial bonding, and chain mobility are imperative factors to improve the mechanical performance of nanocomposites, especially toughness. The tensile behavior and dissipated work of the PVC/CaCO3 model demonstrate that 12 wt % CaCO3 modified with oleate anion and dodecylbenzenesulfonate can impart high toughness to PVC due to its good dispersion, favorable interface interaction, and weak migration of PVC chains. Under the guidance of MD simulation, we experimentally prepared a transparent PVC/CaCO3 nanocomposite with good mechanical properties by in situ polymerization of monodispersed CaCO3 in vinyl chloride monomers. Interestingly, experimental tests indicate that the optimum toughness of a nanocomposite (a 368% increase in the elongation at break and 204% improvement of the impact strength) can be indeed realized by adding 12 wt % CaCO3 modified with oleic acid and dodecylbenzenesulfonic acid, which is remarkably consistent with the MD simulation prediction. In short, this work provides a proof-of-concept of using MD simulation to guide the experimental synthesis of PVC/CaCO3 nanocomposites, which can be considered as an example to develop other functional nanocomposites.

Structural and dynamical properties of thermoplastic polyurethane/fullerene nanocomposites: a molecular dynamics simulations study.

In this work, the structural and dynamical properties of thermoplastic polyurethane (TPU)/fullerene (C60) nanocomposites are investigated using atomistic molecular dynamics simulations, focusing on the glass transition, thermal expansion, polymer mobility, polymer-C60 interactions, and diffusion behavior of C60. The results show a slight increase in the glass transition temperature (Tg) with increasing C60 weight fraction (wt%), attributed to hindered polymer dynamics, and a remarkable reduction in the coefficient of thermal expansion above Tg. Results of the mean squared displacement and the time decay of bond-reorientation autocorrelation indicate that the mobility of TPU hard segments is more restricted than that of soft segments, owing to the electrostatic attractions and the π-π stacking between isocyanate groups and C60 molecules. Analysis of TPU-C60 interaction energy reveals that the electrostatic interactions are weakened with an increase in the C60 wt%, while the van der Waals contributions become more significant due to the TPU-C60 interfacial characteristics. Further analysis shows that the translational and rotational diffusion of C60 are both increasingly suppressed with the increase of C60 wt%, indicating a violation of Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations, presumably due to the polymer chain-mediated hydrodynamic interactions arising from chain bridges between neighboring C60 particles. This is highlighted by a stronger decoupling of translational-rotational diffusion and a lower ratio of translational-rotational diffusion coefficient (DT/DR) with increasing C60 wt%. This work elucidates an atomistic understanding of the structure and properties of polymer/C60 nanocomposites.

Exploring thermodynamic and structural properties of carbon nanotube/thermoplastic polyurethane nanocomposites from atomistic molecular dynamics simulations.

Carbon nanotubes (CNTs) and thermoplastic polyurethane (TPU) nanocomposites have emerged as promising materials for various applications in the field of nanotechnology. An understanding of the thermodynamic and structural properties is of fundamental significance in designing and fabricating CNT/TPU nanocomposites with desired properties. To this end, this work has employed atomistic molecular dynamics (MD) simulations to study the thermal properties and interfacial characteristics of TPU composites filled with pristine or functionalized single-walled carbon nanotubes (SWNTs). Simulations reveal that the introduction of SWNTs suppresses TPU chain dynamics and favors the hydrogen bond formation induced by the wrapping of TPU chains around SWNTs, leading to an increase of glass transition temperature (Tg) and a reduction of volumetric coefficient of thermal expansion (CTE) in the rubbery state. Compared to pristine and hydrogenated SWNTs, SWNTs featuring polar groups, such as carboxyl (-COOH), oxhydryl (-OH) and amine (-NH2) groups, show improved affinity for TPU molecules, suppressing polymer mobility. Analysis of SWNT/TPU binding energy and solubility parameter suggests that electrostatic interactions are responsible for such a functionalized SWNT/TPU interface enhancement. Furthermore, the amine groups exhibit the highest potential for forming hydrogen bonds with the urethane carbonyl (-C[double bond, length as m-dash]O) of TPU chains, resulting in lowest polymer mobility and highest Tg. In general, this research work could provide some guidance for material design of polymer nanocomposites and future simulations relevant to TPUs.

A high spatial resolution dataset of China's biomass resource potential.

Assessing biomass resource potential is essential for China's ambitious goals of carbon neutrality, rural revitalization, and poverty eradication. To fill the data gap of high spatial resolution biomass resources in China, this study estimates the biomass resource potential for all types of lignocellulosic biomass feedstock at 1 km resolution in 2018, including 9 types of agricultural residues, 11 types of forestry residues, and 5 types of energy crops. By combining the statistical accounting method and the GIS-based method, this study develops a transparent and comprehensive assessment framework, which is in accordance with the principle of food security, forest land and pasture protection, and biodiversity protection. In the end, we organize and store the data in different formats (GeoTIFF, NetCDF, and Excel) for GIS users, integrated modelers, and policymakers. The reliability of this high spatial resolution dataset has been proved by comparing the aggregated data at the subnational and national levels with the existing literature. This dataset has numerous potential uses and is a crucial input to many bioenergy-related studies.

Incorporating Health Cobenefits into Province-Driven Climate Policy: A Case of Banning New Internal Combustion Engine Vehicle Sales in China.

Incorporating health cobenefits from coabated air pollution into carbon mitigation policy making is particularly important for developing countries to boost policy efficiency. For sectors that highly depend on electrification for decarbonization, it remains unclear how the increased electricity demand and consequent health impacts from sectoral mitigation policy in one province would change the scale and the regional and sectoral distributions of the overall health impacts in the whole country. This study chooses the banning of new sales of internal combustion engine vehicles in the private vehicle sector in China as a case. The results show that, without carbon neutrality and air pollution control goals in electricity generation, 53% of CO2 reduction and 65% of health benefits from the private vehicle sector would be offset by increased electricity demand. The regional distributions of CO2 reduction and health benefits due to a province-driven ban policy are greatly uneven, as the top five provinces take up over one-third of the total impact in China. Health benefits per ton of carbon reduction (H/C) may vary by up to 8 times across provinces. Finally, the provinces in southeast China and the Sichuan Basin, with their stably high H/C values, are suggested to enact the province-driven ban policy first.

Preparation and properties of a novel poly(lactic-acid)-based thermoplastic vulcanizate from both experiments and simulations.

A novel bio-based thermoplastic vulcanizate (TPV) material consisting of poly(lactic acid) (PLA) and a novel polymeric slide ring material (SeRM) was fabricated via isocyanate-induced dynamic vulcanization. The microscopic morphology, thermal properties, biocompatibility, and mechanical properties of the SeRM/PLA TPV material were comprehensively investigated, in turn by transmission electron microscopy, differential scanning calorimetry, in vitro cytotoxicity test, electron tension machine, and molecular dynamics simulations. Phase inversion in TPV was observed during the dynamic vulcanization, and TEM images showed that SeRM particles that were dispersed in PLA continuous phase had an average diameter of 1-4 µm. Results also indicated that an optimum phase inversion morphology was obtained at the SeRM/PLA blending ratio of 70/30 w/w. Glass transition temperature of PLA was found to be slightly decreased, owing to the improvement in interface compatibility by chemically bonding the PCL side chains (of SeRM molecules) and PLA chains. The tensile strength and elongation at break of TPVs were approximately 14.7 MPa and 164%, respectively, at SeRM/PLA blending ratio of 70/30, owing to the unique sliding effect of SeRM molecules when subjected to deformations. Cytotoxicity test results proved that the bio-based TPVs were fully non-toxic to L929 cells. In such aspects we believe that the bio-based TPV can be a promising material in the biomedical applications as an alternative of traditional commodity plastics.

Investigation on Tribological and Thermo-Mechanical Properties of Ti3C2 Nanosheets/Epoxy Nanocomposites.

In this study, two-dimensional Ti3C2 nanosheets were employed to improve the tribological and thermo-mechanical properties of epoxy resin. The Ti3C2 nanosheets were prepared by ultrasound-assisted delamination of multilayered Ti3C2 microparticles, and the Ti3C2 nanosheets/epoxy (Ti3C2/epoxy) nanocomposites were fabricated through physical blending and curing reaction. Scanning electron microscopy results showed that the Ti3C2 nanosheets were dispersed uniformly in the epoxy matrix. Tribological test results showed that the wear rate of Ti3C2/epoxy nanocomposites was only 6.61 × 10-14 m3/(N m) at a 1% mass fraction, which was reduced by 72.1% compared to that of neat epoxy. The morphologies of worn surfaces revealed that the wear form of Ti3C2/epoxy nanocomposites transformed gradually from fatigue wear to adhesive wear with the increase of mass fraction of Ti3C2 nanosheets. Moreover, the results of thermo-mechanical properties indicated that incorporation of Ti3C2 nanosheets effectively improved the storage modulus and glass-transition temperature (T g) of epoxy resin. This work provides guidance for improving the tribological and thermo-mechanical properties of epoxy resin.

Role of a nanoparticle network in polymer mechanical reinforcement: insights from molecular dynamics simulations.

Fully understanding the mechanism by which nanoparticles (NPs) strengthen polymer matrices is crucial for fabricating high-performance polymer nanocomposites (PNCs). Herein, coarse-grained molecular dynamics simulations were adopted to explicitly investigate the reinforcing effect of a NP network. Our results revealed that increasing the NP-NP interactions induced the self-assembly of NPs into a three-dimensional (3D) network that reinforced the polymer matrix. The reinforcing mechanism of NP-NP interactions was quite different from that of NP-polymer interactions. The latter promoted the orientation of polymer chains to transfer the external stress, while the former distributed the stress throughout the NP network. This work revealed the mechanism by which the NP network reinforced the polymer matrix at the molecular level and also provided guidelines for developing high performance PNCs via interfacial modification.

Tribological and Thermo-Mechanical Properties of TiO2 Nanodot-Decorated Ti3C2/Epoxy Nanocomposites.

The micromorphology of fillers plays an important role in tribological and mechanical properties of polymer matrices. In this work, a TiO2-decorated Ti2C3 (TiO2/Ti3C2) composite particle with unique micro-nano morphology was engineered to improve the tribological and thermo-mechanical properties of epoxy resin. The TiO2/Ti3C2 were synthesized by hydrothermal growth of TiO2 nanodots onto the surface of accordion-like Ti3C2 microparticles, and three different decoration degrees (low, medium, high density) of TiO2/Ti3C2 were prepared by regulating the concentration of TiO2 precursor solution. Tribological test results indicated that the incorporation of TiO2/Ti3C2 can effectively improve the wear rate of epoxy resin. Among them, the medium density TiO2/Ti3C2/epoxy nanocomposites gained a minimum wear rate. This may be ascribed by the moderate TiO2 nanodot protuberances on the Ti3C2 surface induced a strong mechanical interlock effect between medium-density TiO2/Ti3C2 and the epoxy matrix, which can bear a higher normal shear stress during sliding friction. The morphologies of worn surfaces and wear debris revealed that the wear form was gradually transformed from fatigue wear in neat epoxy to abrasive wear in TiO2/Ti3C2/epoxy nanocomposites. Moreover, the results of thermo-mechanical property indicated that incorporation of TiO2/Ti3C2 also effectively improved the storage modulus and glass transition temperature of epoxy resin.

Comparison of Four Amendments for Arsenic and Cadmium Combined Contaminated Soil.

Arsenic (As) and cadmium (Cd) are common soil pollutants whose opposing geochemical behaviors must be taken into account in the development of cost-effective, environmentally friendly remediation strategies. In this study, a pot experiment with lettuce and a field experiment with wheat were performed to examine the impacts of zeolite, biochar, MnO2, zero-valent iron (ZVI) individually and in binary combinations thereof on As-Cd pollution. The results of the pot experiment showed that biochar, MnO2 and ZVI had good passivation effects on As and Cd when provided individually, but the effects of a combination of 0.2% ZVI/0.5% biochar or 0.2% MnO2/0.5% ZVI were even better. These amendments were further investigated in a field experiment, which confirmed the positive effect of 0.2% MnO2/0.5% ZVI. Therefore, ZVI/biochar and MnO2/ZVI mixtures may offer effective solutions to the remediation of farmland soil contaminated with both As and Cd.

Revisiting stress-strain behavior and mechanical reinforcement of polymer nanocomposites from molecular dynamics simulations.

Through coarse-grained molecular dynamics simulations, the effects of nanoparticle properties, polymer-nanoparticle interactions, chain crosslinks and temperature on the stress-strain behavior and mechanical reinforcement of polymer nanocomposites (PNCs) are comprehensively investigated. By regulating the filler-polymer interaction (miscibility) in a wide range, an optimal dispersion state of nanoparticles is found at moderate interaction strength, while the mechanical properties of PNCs are improved monotonically with the increase of the particle-polymer interaction due to the tele-bridge structures of nanoparticles via polymer chains. Although smaller-sized fillers more easily build interconnected structures, the elastic moduli of PNCs at the percolation threshold concentration where a three-dimensional filler network forms are almost independent of nanoparticle size. Compared with spherical nanoparticles, anisotropic rod-like ones, especially with larger aspect ratio and rod stiffness, contribute exceptional reinforcement towards polymer materials. In addition, the elastic modulus with the strain, derived from the stress-strain curve, shows an analogous nonlinear behavior to the amplitude-dependence of the storage modulus (Payne effect). Such a behavior originates essentially from the failure/breakup of the microstructures contributing to the mechanical reinforcement, such as bound polymer layers around nanoparticles or nanoparticle networking structures. The Young's modulus as a function of the nanoparticle volume fraction greatly exceeds that predicted by the Einstein-Smallwood model and Guth-Gold model, which arises primarily from the contribution of the local/global filler network. The temperature dependence of the Young's modulus is further examined by mode coupling theory (MCT) and the Vogel-Fulcher-Tammann (VFT) equation, and the results indicate that the time-temperature superposition principle holds modestly above the critical temperature on the short-time (small-length) scale of elastic response. This work is expected to provide some guidance on controlling and improving the mechanical properties and nonlinear behavior of PNCs.

Tailoring the mechanical properties of polymer nanocomposites via interfacial engineering.

The improvement of mechanical properties of polymer nanocomposites (PNCs) has been studied for many years, with the main focus on the structure of the nanofillers. Much less effort has been devoted to unraveling the factors controlling the structure of the grafted chains. Herein, through coarse-grained molecular-dynamics simulations, we have successfully fabricated an ideal, mechanically-interlocked composite structure composed of end-functionalized chains grafted to the nanoparticle surface forming rings and making the matrix chains thread through these rings. Depending on the details of the grafting, the reinforcement effect can be remarkable, improving the tensile stress of the system significantly up to 700%. Meanwhile, anisotropy of the system's mechanical response is also observed. Furthermore, the influence of the grafted chain distribution on the mechanical properties of the system has been investigated as well. We observe that the mechanical properties of the system are closely related to the total number of the beads in the grafted chains or the synergistic effect between the length and density of the grafted chains leads to no significant difference in the performance of systems. At constant grafting density, the mechanical properties of the systems correlate negatively to the grafted chain length. In general, our study should help to design and fabricate high-performance PNCs with excellent mechanical properties.

Strain rate and temperature dependence of the mechanical properties of polymers: A universal time-temperature superposition principle.

Establishing the Time-Temperature and Frequency-Temperature Superposition Principles (TTSP and FTSP) to describe the mechanical behavior of polymeric materials is always of paramount significance. In this work, by adopting the classic coarse-grained model, we investigate the validity of these superposition principles for a series of networks, such as the pure polymer network, interpenetrating polymer networks composed of stiff and flexible networks (IPNs-SF), interpenetrating polymer networks composed of different cross-linking networks (IPNs-DC), polymer nanocomposites (PNCs), and surface grafted modified PNCs. The study focuses on the three critical mechanical properties such as the stress relaxation, the storage modulus versus the frequency obtained from the dynamic periodic shear deformation, and the uniaxial tensile stress-strain. The glass transition temperature (Tg) is about 0.47 for the simulated polymer network (CL400), and a smooth master curve is obtained for the stress relaxation process by setting the reference temperature Tref = 0.6 via the horizontal shifting process, indicating the validity of TTSP. Furthermore, similar smooth master curves are also achieved for both dynamic periodic shear and uniaxial tensile deformation, which exhibit similar trends and share the identical linear viscoelastic regime in the temperature interval above Tg: 0.55

Tailoring the mechanical properties by molecular integration of flexible and stiff polymer networks.

Designing a multiple-network structure at the molecular level to tailor the mechanical properties of polymeric materials is of great scientific and technological importance. Through the coarse-grained molecular dynamics simulation, we successfully construct an interpenetrating polymer network (IPN) composed of a flexible polymer network and a stiff polymer network. First, we find that there is an optimal chain stiffness for a single network (SN) to achieve the best stress-strain behavior. Then we turn to study the mechanical behaviors of IPNs. The result shows that the stress-strain behaviors of the IPNs appreciably exceed the sum of that of the corresponding single flexible and stiff network, which highlights the advantage of the IPN structure. By systematically varying the stiffness of the stiff polymer network of the IPNs, optimal stiffness also exists to achieve the best performance. We attribute this to a much larger contribution of the non-bonded interaction energy. Last, the effect of the component concentration ratio is probed. With the increase of the concentration of the flexible network, the stress-strain behavior of the IPNs is gradually enhanced, while an optimized concentration (around 60% molar ration) of the stiff network occurs, which could result from the dominant role of the enthalpy rather than the entropy. In general, our work is expected to provide some guidelines to better tailor the mechanical properties of the IPNs made of a flexible network and a stiff network, by manipulating the stiffness of the stiff polymer network and the component concentration ratio.

Designing the Slide-Ring Polymer Network with both Good Mechanical and Damping Properties via Molecular Dynamics Simulation.

Through coarse-grained molecular dynamics simulation, we have successfully designed the chemically cross-linked (fixed junction) and the slide-ring (SR) systems. Firstly, we examine the dynamic properties such as the mean-square displacement, the bond, and the end-to-end autocorrelation functions as a function of the cross-linking density, consistently pointing out that the SR system exhibits much lower mobility compared with the fixed junction one at the same cross-linking density. This is further validated by a relatively higher glass transition temperature for the SR system compared with that of the fixed junction one. Then, we calculated the effect of the cross-linking density on the stretch-recovery behavior for the SR and fixed junction systems. Although the chain orientation of the SR system is higher than that of the fixed-junction system, the tensile stress is smaller than the latter. We infer that much greater chain sliding can occur during the stretch, because the movable ring structure homogeneously sustains the external force of the SR system, which, therefore, leads to much larger permanent set and higher hysteresis during the recovery process compared with the fixed-junction one. Based on the stretch-recovery behavior for various cross-linking densities, we obtain the change of the hysteresis loss, which is larger for the SR system than that of the fixed junction system. Lastly, we note that the relatively bigger compressive stress for the SR system results from the aggregation of the rigid rings compared with the fixed junction system. In general, compared with the traditionally cross-linked system, a deep molecular-level insight into the slide-ring polymer network is offered and thus is believed to provide some guidance to the design and preparation of the slide-ring polymer network with both good mechanical and damping properties.

Self-Assembly of Block Copolymer Chains To Promote the Dispersion of Nanoparticles in Polymer Nanocomposites.

In this paper we adopt molecular dynamics simulations to study the amphiphilic AB block copolymer (BCP) mediated nanoparticle (NP) dispersion in polymer nanocomposites (PNCs), with the A-block being compatible with the NPs and the B-block being miscible with the polymer matrix. The effects of the number and components of BCP, as well as the interaction strength between A-block and NPs on the spatial organization of NPs, are explored. We find that the increase of the fraction of the A-block brings different dispersion effect to NPs than that of B-block. We also find that the best dispersion state of the NPs occurs in the case of a moderate interaction strength between the A-block and the NPs. Meanwhile, the stress-strain behavior is probed. Our simulation results verify that adopting BCP is an effective way to adjust the dispersion of NPs in the polymer matrix, further to manipulate the mechanical properties.

Stress-strain behavior of block-copolymers and their nanocomposites filled with uniform or Janus nanoparticles under shear: a molecular dynamics simulation.

Although numerous research studies have been focused on studying the self-assembled morphologies of block-copolymers (BCPs) and their nanocomposites, little attention has been directed to explore the relation between their ordered structures and the resulting mechanical properties. We adopt coarse-grained molecular dynamics simulation to study the influence of the morphologies on the stress-strain behavior of pure block copolymers and block copolymers filled with uniform or Janus nanoparticles (NPs). At first, we examine the effect of the arrangement (di-block, tri-block, alternating-block) and the components of the pure block copolymers, and by varying the component ratio between A and B blocks, spherical, cylindrical and lamellar phases are all formed, showing that spherical domains bring the largest reinforcing effect. Then by studying BCPs filled with NPs, the Janus NPs induce stronger bond orientation of polymer chains and greater mechanical properties than the uniform NPs, when these two kinds of NPs are both located in the interface region. Meanwhile, some other anisotropic Janus NPs, such as Janus rods and Janus sheets, are incorporated to examine the effect on the morphology and the stress-strain behavior. These findings deepen our understanding of the morphology-mechanics relation of BCPs and their nanocomposites, opening up a vast number of approaches such as designing the arrangement and components of BCPs, positioning uniform or Janus NPs with different shapes and shear flow to tailor their stress-strain performance.

Tuning the structure and mechanical property of polymer nanocomposites by employing anisotropic nanoparticles as netpoints.

Introducing carbon nanotubes or graphene sheets into polymer matrices has received lots of scientific and technological attention. For the first time, we report a new kind of polymer nanocomposite (PNC) by means of employing anisotropic nanoparticles (NPs) as netpoints (referred to as an end-linked system), namely with NPs acting as netpoints to chemically connect the dual end-groups of each polymer chain to form a network. By taking advantage of this strategy, the anisotropic NPs can be uniformly distributed in the polymer matrix, with the NPs being separated via the connected polymer chains. And the separation distance between NPs, the stress-strain behavior and the dynamic hysteresis loss (HL) can be manipulated by varying the temperature and the polymer chain flexibility. Meanwhile, the physically mixed system is investigated by changing the interaction strength between polymer and NPs, and the temperature. It is emphasized that compared to the physically mixed system, the end-linked system which employs carbon nanotubes or graphene as netpoints possesses good thermal stability because of its thermodynamically stable morphology, exhibiting both excellent static and dynamic mechanical properties. These results help us to design and fabricate high performance and multi-functional PNCs filled with carbon nanotubes or graphene, facilitating the potentially large industrial application of these nanomaterials.

The Young's moduli of three types of carbon allotropes: a molecular mechanics model and a finite-element method.

The close-form expressions of the Young's moduli and the fracture stresses of cyclicgraphene, graphyne and supergraphene along their armchair and zigzag directions are derived based on a molecular mechanics model. Checking against present finite-element calculations of their Young's moduli shows that the explicit solutions are reasonable. The obtained analytical solutions should be of great help for understanding the mechanical properties of the graphene-like materials.