Dispersion, Dynamics, and Mechanics of Polymer Nanocomposites Filled with Rigid-Soft Janus Nanoparticles via Molecular Dynamics Simulation.

The introduction of nanoparticles (NPs) presents boundless possibilities for enhancing the performance of polymer nanocomposites (PNCs). Consequently, the design of novel NPs becomes of paramount significance for PNCs. In our study, we employ the dumbbell two-component model of Janus nanoparticles (JNPs) and design rigid-soft JNPs as fillers. Using coarse-grained molecular dynamics simulations, we systematically investigate the dispersion, dynamics, and mechanical properties of these novel PNCs. First, we determine the optimal dispersion conditions by studying rcutoff and εnp. The simulation indicates that when the interaction between polymer chains and JNPs is a repulsive potential, the JNPs tend to aggregate together, forming a cluster with soft NPs inside and rigid NPs outside. Conversely, under attractive interactions, JNPs show superior dispersion uniformity compared to the repulsive system, and as εnp increases, the dispersion improves. Then, the mean square displacement (MSD) indicates that JNPs effectively impede the mobility of polymer chains, with the degree of hindrance increasing as εnp grows; this effect is more pronounced in attractive systems. Comparing JNPs of different particle sizes, we find that smaller JNP systems exhibit higher temperature sensitivity. Furthermore, there exists a critical particle size (Dnp ≈ 5σ) under a constant filling fraction at which the NPs exert the most pronounced restriction effect on the polymer. Next, upon examining the mechanical behavior, we find that the rigid-soft JNPs demonstrate notable elasticity and variability compared to traditional NPs. This observation is confirmed through measurements of the bond orientation and mean square radius of gyration of the soft segments of JNPs. In summary, this research provides a comprehensive understanding of the intricate interplay among various factors, offering valuable insights for optimizing JNP dispersion and enhancing the mechanical properties of PNCs.

Advancing elastomer performance with dynamic bond networks in polymer-grafted single-chain nanoparticles: a molecular dynamics exploration.

This research introduces a method to enhance the mechanical properties of elastomers by grafting polymer chains onto single-chain flexible nanoparticles (SCNPs) and incorporating dynamic functional groups. Drawing on developments in grafting polymers onto hard nanoparticle fillers, this method employs the distinct flexibility of SCNPs to diminish heterogeneity and enhance core size control. We use molecular dynamics (MD) simulations for a mesoscale analysis of structural properties, particularly the effects of dynamic functional group quantities and their distribution. The findings demonstrate that increased quantities of functional groups are correlated with enhanced mechanical strength and toughness, showing improved stress-strain responses and energy dissipation capabilities. Moreover, the uniformity in the distribution of these functional groups is crucial, promoting a more cohesive and stable dynamic bonding network. The insights gained from MD simulations not only advance our understanding of the microstructural control necessary for optimizing macroscopic properties, but also provide valuable guidance for the design and engineering of advanced polymer nanocomposites, thereby enhancing the material performance through strategic molecular design.

Manipulating the Properties of Polymer Vitrimer Nanocomposites by Designing Dual Dynamic Covalent Bonds.

Polymer vitrimer is a novel material that contains dynamic covalent bonds (DCBs) allowing it to combine the desirable characteristics of both thermoplastics and thermosets. Similar to the traditional polymer nanocomposites, introducing nanoparticles into polymer vitrimer is also an effective strategy to further enhance its properties. However, a comprehensive understanding of matrix and interfacial bond exchange reactions (BERs) to tailor the properties of polymer vitrimer nanocomposites (PVNs) is still lacking. Herein, we utilized coarse-grained molecular dynamics simulations to investigate model PVNs in which there are two different kinds of DCBs in the vitrimer matrix and at the interface. Our results show that the normalized bond autocorrelation function (Csw) confirms the independence of BERs in the vitrimer matrix and in the interface. By varying the bond swap energy barrier (ΔEsw) in the matrix ΔEswmat or in the interface ΔEswint, or in both ΔEswall, a maximum mechanical property is observed at the moderate value of ΔEswmat, ΔEswint, orΔEswall. Meanwhile, the effect of ΔEsw on the stress relaxation and the bond orientation as a function of the time under a fixed strain is well probed, which both decay more slowly at greater ΔEsw. We simulated the tension-recovery curve to examine the effect of ΔEsw on the hysteresis loss and permanent deformation of PVNs, finding an optimal value to achieve its minimum energy dissipation and maximum recovery ratio. Lastly, we investigated the efficiency of self-healing by building and removing walls from the system. Interestingly, a maximum self-healing efficiency of the stress-strain behavior is observed at moderate ΔEsw. Overall, this study provides valuable insights into the relationship between the structure and properties of PVNs, offering implications for the manipulation of their mechanical properties and enhancement of their self-healing capabilities.

Columnar optical-radiative properties and components of aerosols in the Arctic summer from long-term AERONET measurements.

Aerosols as an external factor have an important role in the amplification of Arctic warming, yet the geography of this harsh region has led to a paucity of observations, which has limited our understanding of the Arctic climate. We synthesized the latest decade (2010-2021) of data on the microphysical-optical-radiative properties of aerosols and their multi-component evolution during the Arctic summer, taking into consideration the important role of wildfire burning. Our results are based on continuous observations from eight AERONET sites across the Arctic region, together with a meteorological reanalysis dataset and satellite observations of fires, and utilize a back-trajectory model to track the source of the aerosols. The summer climatological characteristics within the Arctic Circle showed that the aerosols are mainly fine-mode aerosols (fraction >0.95) with a radius of 0.15-0.20 µm, a slight extinction ability (aerosol optical depth âˆ¼ 0.11) with strong scattering (single scattering albedo ∼0.95) and dominant forward scattering (asymmetry factor âˆ¼ 0.68). These optical properties result in significant cooling at the Earth's surface (∼-13 W m-2) and a weak cooling effect at the top of the atmosphere (∼-5 W m-2). Further, we found that Arctic region is severely impacted by wildfire burning events in July and August, which primarily occur in central and eastern Siberia and followed in subpolar North America. The plumes from wildfire transport aerosols to the Arctic atmosphere with the westerly circulation, leading to an increase in fine-mode aerosols containing large amounts of organic carbon, with fraction as high as 97-98 %. Absorptive carbonaceous aerosols also increase synergistically, which could convert the instantaneous direct aerosol radiative effect into a heating effect on the Earth-atmosphere system. This study provides insights into the complex sources of aerosol loading in the Arctic atmosphere in summer and emphasizes the important impacts of the increasingly frequent occurrence of wildfire burning events in recent years.

Conformation and Dynamics along the Chain Contours of Polymer-Grafted Nanoparticles.

Densely grafted polymer chains onto spherical nanoparticles produce a diverse range of conformations. At high grafting densities, the corona region near the nanoparticle surface undergoes intense confinement due to a high concentration of chains in the concentrated polymer brush (CPB) region, which results in strong stretching for portions of the chains located within. In contrast, a semi-dilute polymer brush (SDPB) forms farther away from the core and offers reduced confinement for the polymer and more ideal conformations. However, conventional experimental methods are limited in their ability to provide detailed information on individual segments of grafted polymers in these regions; hence, molecular dynamics (MD) simulations are essential for gaining comprehensive insights into the behavior of the grafted chains. This study aims to explore the variations in polymer structure and dynamics that occur along the contour of the grafted chains as influenced by spatial confinement. We focus on the motions and relative positions of each bead along grafted polymers. Our results show that only the initial few grafted beads near the nanoparticle surface exhibit the strong stretching attributed segments in the CPB region of the brush. Increased grafting density or decreased chain flexibility leads to more stretched grafted chains and more aligned bond vectors. As a result, the relaxation dynamics of local regions of the polymer are also strongly influenced by these parameters. Although the grafted beads in the interior of the CPB region are highly sensitive to these parameters, those farther from the nanoparticle core experience significantly diminished effects. In comparison to the Daoud-Cotton (DC) model's predictions of CPB size, beads near the nanoparticle surface show slower dynamic decay, especially in high grafting densities, aligning with the DC model's estimates. Finally, we compare our simulations to previous works for additional insight into polymer-grafted nanoparticles.

Heterogeneous Dynamics of Polymer Melts Exerted by Chain Loops Anchored on the Substrate: Insights from Molecular Dynamics Simulation.

Understanding polymer-substrate interfacial dynamics at the molecular level is crucial for tailoring the properties of polymer ultrathin films (PUFs). Herein, through coarse-grained molecular dynamics simulation, the effect of length (Nloop) and rigidity (Kloop) of loop chains on the dynamics of linear chains is systematically explored, in which the loop chains are adsorbed on a solid substrate and the linear chains are covered on the loop chains. It is found that there is an optimal Kloop, which strongly confines the motion of the linear chains. Meanwhile, compared to increasing the rigidity of the loop chains, increasing the length of the loop chains can more effectively confine the motion of the linear chains. More interestingly, we observe that the mismatch of the length (ΔN) and rigidity (ΔK) between the loop and linear chains leads to dynamic asymmetry (ΔDc). The relationship between the ΔN, ΔK, and ΔDc are found to follow the mathematical expression of ΔDc ∼ (ΔN)α(ΔK)ß, in which the values of α and ß are around 4.58 and 0.83, separately. Remarkably, using the Gaussian process regression model, we construct a master curve of diffusion coefficient on the segmental and chain length scales of the linear chains as a function of Nloop and Kloop, which is further validated by our simulated prediction. In general, this work provides a fundamental understanding of polymer interfacial dynamics at the molecular level, enlightening some rational principles for manipulating the physical properties of PUFs.