Dynamics of polymer chains confined to a periodic cylinder: molecular dynamics simulation vs. Lifson-Jackson formula.

The dynamics of polymer chains confined to a periodic cylinder is explored using molecular dynamics simulation and theoretical analysis. The cylinder is divided into two cavities in one periodicity: one cavity consists of a channel of length L1 and diameter D1 and another cavity is a channel of length L2 and diameter D2. For L1 = L2 = L/2, the diffusion coefficient D of a single confined polymer chain decreases rapidly with increase in periodicities L. For a fixed periodicity with L = L1 + L2 = constant, the diffusion coefficient D of a single confined polymer chain shows strong dependence on L1 (or L2). Moreover, for a multi-chain system with L1 = L2, the diffusion coefficient D shows strong non-monotonic dependence on the chain monomer density ρ, and the confined polymer chains diffuse fastest for ρ = 0.068, in which there are three polymer chains in two periodicities as well as two opposing effects: one is that the excluded volume effect between polymer chains can reduce the free energy barrier, and another is that when the chain monomer density ρ increases further, the entanglement effect increases, which leads to that the diffusion coefficient D decreases as ρ increases. Finally, we found that the diffusion coefficient D has a similar oscillation relationship with the ratio of R/L for different chain lengths N and different periodicity L, and the oscillation amplitude decreases gradually as R/L increases; here R is the mean end-to-end distance of a single confined polymer chain, i.e., . From the view of free energy potential, both the width of the free energy potential well and the height of the free energy potential barrier govern simultaneously the diffusion behavior of confined polymer chains. According to the mean force potential (PMF) based on the weighted histogram analysis method (WHAM), we found that our results agree very well with the theoretical analysis using the Lifson-Jackson formula. Our investigation may help us understand the dynamics of particles in a periodic medium, which is one of the interesting problems in many different fields of science, such as physics, chemistry and biology.

Study on the grain refinement mechanism of Mg-Al alloy based on carbon addition.

In this study, a comprehensive treatment process based on the rotary injection of Ar+CO2 Mg-Al alloy melt is proposed. The effect of carbon on the grain refinement of Mg-Al alloy is studied according to the proposed integrated treatment process. The regularity of carbon refinement in the Mg-Al alloy is examined by microstructural observation and theoretical calculation. The results show that carbon has no effect on the grain refinement of Mg-Al alloy when the Al content is less than 1wt.%. However, when the Al content reaches 2 wt.%, the refining effect is obvious, and the grain refinement efficiency is 62%. The refining effect increases with the increase in the Al content, and the refinement efficiency becomes 79% when the Al content reaches 9 wt.%. The size of Al-C-O in the matrix is approximately 5µm, which confirms the existence of Al4C3 phase exists as a heterogeneous nucleating agent. The theoretical calculations suggest that the Al4C3 heterogeneous nucleating agent cannot be formed when the Al content in the Mg alloy is less than 1.34%, so there is no thinning effect under such Al content. The crystallographic calculations reveal that the mismatch between the Al4C3 phase and Mg alloy matrix is only 4.05%, and Al4C3 can exist as a heterogeneous nucleating agent for α-Mg phase. Combining the measured solidification curves with the classical nucleation theory, the wetting angle of Mg-Al alloy on Al4C3 is calculated to be 24.3°.

Strengthened Optical Nonlinearity of V2C Hybrids Inlaid with Silver Nanoparticles.

The investigation of nonlinear optical characteristics resulting from the light-matter interactions of two-dimensional (2D) nano materials has contributed to the extensive use of photonics. In this study, we synthesize a 2D MXene (V2C) monolayer nanosheet by the selective etching of Al from V2AlC at room temperature and use the nanosecond Z-scan technique with 532 nm to determine the nonlinear optical characters of the Ag@V2C hybrid. The z-scan experiment reveals that Ag@V2C hybrids usually exhibits saturable absorption owing to the bleaching of the ground state plasma, and the switch from saturable absorption to reverse saturable absorption takes place. The findings demonstrate that Ag@V2C has optical nonlinear characters. The quantitative data of the nonlinear absorption of Ag@V2C varies with the wavelength and the reverse saturable absorption results from the two-photon absorption, which proves that Ag@V2C hybrids have great potential for future ultrathin optoelectronic devices.

Two Highly Stable Luminescent Lead Phosphonates Based on Mixed Ligands: Highly Selective and Sensitive Sensing for Thymine Molecule and VO3 - Anion.

Two luminescent lead phosphonates with two-dimensional (2D) layer and three-dimensional (3D) framework structure, namely, Pb3[(L1)2(Hssc)(H2O)2] (1) and [Pb2(L2)0.5(bts)(H2O)2]·H2O (2) (H2L1 = O(CH2CH2)2NCH2PO3H2, H4L2 = H2PO3CH2NH(C2H4)2NHCH2PO3H2, H3ssc = 5-sulfosalicylic acid, NaH2bts = 5-sulfoisophthalic acid sodium) have been prepared via hydrothermal techniques. The two compounds not only show excellent thermal stability but also remain intact in aqueous solution within an extensive pH range. Moreover, the atomic absorption spectroscopy analysis experiment indicates that there does not exist the leaching of Pb2+ ions from the lead phosphonates, which show they are nontoxic in aqueous solution. In compound 1, the Pb(1)O4, Pb(2)O7, Pb(3)O4, and CPO3 polyhedra are interlinked into a one-dimensional chain, which is further connected to adjacent chain by sharing the Hssc2- to form a 2D layer. Interestingly, compound 1 as a highly selective and sensitive luminescent material can be used to detect the thymine molecule with a very low detection limit of 8.26 × 10-7 M. In compound 2, the Pb(1)O6 and Pb(2)O5 polyhedra are interlinked into a dimer via edge sharing, which is further connected to adjacent dimer to form a tetramer via corner sharing, and such a tetramer is then interlinked into a 2D layer through bts3- ligands; the adjacent 2D layers are finally constructed to a 3D structure by sharing the L2 4- ligand. Compound 2 can be applied as an excellent luminescent sensor for sensing of VO3 - anion. Furthermore, the probable fluorescent quenching mechanisms of the two compounds have also been studied.