Molecular Dynamics Simulations of Polymer Nanocomposites Welding: Interfacial Structure, Dynamics and Strength.

Polymer welding has received numerous scientific attention, however, the welding of polymer nanocomposites (PNCs) has not been studied yet. In this work, via coarse-grained molecular dynamics simulation, the attention on investigating the welding interfacial structure, dynamics, and strength by constructing the upper and lower layers of PNCs, by varying the polymer-nanoparticle (NP) interaction strength εNP-p is focused. Remarkably, at low εNP-p , the NPs gradually migrate into the top and bottom surface layer perpendicular to the z direction during the adhesion process, while they are distributed in the middle region at high εNP-p . Meanwhile, the dimension of polymer chains is found to exhibit a remarkable anisotropy evidenced by the root-mean-square radius of gyration in the xy- (Rg,xy ) and z- (Rg,z ) component. The welding interdiffusion depth increases the fastest at low εNP-p , attributed to the high mobility of polymer chains and NPs. Lastly, although the mechanical properties of PNCs at high εNP-p is the strongest because of the presence of the NPs in the bulk region, the welding efficiency is the greatest at low εNP-p . Generally, this work provides a fundamental understanding of the interfacial welding of PNCs, in hopes of guiding to design and fabricate excellent self-healable PNCs.

Effects of Forest Type on Nutrient Fluxes in Throughfall, Stemflow, and Litter Leachate within Acid-Polluted Locations in Southwest China.

Although new inputs of acidic anions are decreasing, soil acidification still deserves more academic attention because of the effects of historical stores of SO42- already absorbed into soils. Forest canopy has large, species-specific effects on rainwater chemistry, for which the hydrological mechanism remains unclear. We investigated precipitation, throughfall, stemflow, and litter leachate across three forest types in a severely acid-polluted site located in Southwest China. Precipitation monitored over 4 months, representing summer, fall, winter, and spring, indicated neutral precipitation in Tieshanping with pH ranging from 6.58-7.33. Throughfall and litter leachate in Pinus massoniana Lamb. stands were enriched with greater cation and anion fluxes, as well as more dissolved organic carbon (DOC) flux. Rainwater from pure stands of Cinnamomum camphora (Linn) Presl yielded lower N and DOC inputs to soils with higher base saturation, which would reduce soil acidification and, therefore, improve the sustainability of forest ecosystems.

Achieving Efficient p-Type Organic Thermoelectrics by Modulation of Acceptor Unit in Photovoltaic π-Conjugated Copolymers.

π-Conjugated donor (D)-acceptor (A) copolymers have been extensively studied as organic photovoltaic (OPV) donors yet remain largely unexplored in organic thermoelectrics (OTEs) despite their outstanding mechanical bendability, solution processability and flexible molecular design. Importantly, they feature high Seebeck coefficient (S) that are desirable in room-temperature wearable application scenarios under small temperature gradients. In this work, the authors have systematically investigated a series of D-A semiconducting copolymers possessing various electron-deficient A-units (e.g., BDD, TT, DPP) towards efficient OTEs. Upon p-type ferric chloride (FeCl3 ) doping, the relationship between the thermoelectric characteristics and the electron-withdrawing ability of A-unit is largely elucidated. It is revealed that a strong D-A nature tends to induce an energetic disorder along the π-backbone, leading to an enlarged separation of the transport and Fermi levels, and consequently an increase of S. Meanwhile, the highly electron-deficient A-unit would impair electron transfer from D-unit to p-type dopants, thus decreasing the doping efficiency and electrical conductivity (σ). Ultimately, the peak power factor (PF) at room-temperature is obtained as high as 105.5 µW m-1 K-2 with an outstanding S of 247 µV K-1 in a paradigm OPV donor PBDB-T, which holds great potential in wearable electronics driven by a small temperature gradient.

Bimodal Polymer End-Linked Nanoparticle Network Design Strategy to Manipulate the Structure-Mechanics Relation.

A kind of bimodal polymer end-linked network employing nanoparticles (NPs) as net points has been designed and constructed through coarse-grained molecular dynamics simulation. We systematically explore the effects of the molecular weight (length of the long polymer chains), chain flexibility, and temperature on the accurate distribution of the spherical NPs and the resulting mechanical properties of the bimodal network. It is found that the NPs can be dispersed well, and a larger average distance between the NPs is realized with the increase of the length of the long polymer chains, the rigidity of short and long chains, and the temperature. There is a linear relationship between the average interparticle distance of NPs and the arithmetical average of the root-mean-square end-to-end distance of long and short chains. By adopting the uniaxial deformation, the stress-strain behavior and the bond orientation are examined. The results illustrate that introducing the short chains into the uniform long chains network can notably improve the tensile stress-strain performance. The bond orientation behaviors present that short chains are more prone to be oriented and stretched, contributing to more stress during the stretching process. Furthermore, enhanced stress-strain behaviors can be observed by manipulating the chain stiffness and temperature. Interestingly, the bimodal end-linked network reveals a distinctively enhanced stress-strain behavior versus the temperature, which is opposite to that of traditional physically mixed polymer nanocomposites (PNCs), attributed to a higher entropic elasticity and the uniform dispersion of NPs of the end-linked system at high temperatures. The network exhibits a linear relationship for the stress at a fixed strain versus the temperature. Notably, it is indicated that the contribution of entropy accounts for most of the total stress, while the change of internal energy only accounts for a small part, which is consistent with the experimental observation of the classic rubber elastic theory. In general, our study demonstrates a rational route to precisely control the spatial dispersion of the NPs and effectively tailor the mechanical properties of PNCs.