CALCULATION OF CONFORMATIONAL ENTROPY AND FREE ENERGY OF POLYSILANE CHAIN

The conformational entropy S and free energy F were calculated by exact enumeration of polysilane chain up to 23 segments with excluded volume (EV) and long-range van der Waals (VW) interaction. A nonlinear relation between SEV+VW and chain length n was found though S-EV was found to vary linearly with n. We found that the second-order transition temperature of polysilane chain with VW interaction increases with the increase of chain length, while that of polysilane chain without VW interaction is chain length independent. Moreover, the free energies FEV+VW and F-EV are both linearly related with n, and FEV+VW < F-EV for all temperatures.

Entropy dominated behaviors of confined polymer-nanoparticle composites

Stretched polymers will lose their possible configurations if they are mixed with nanoparticles or touch a hard wall, which leads to a strong depletion attraction responsible for the enrichment of nanoparticles near substrates. Moreover, it is found that there exists a sacrifice mechanism in confined pure polymer samples or polymer-nanoparticle mixtures, that part of the polymers, in order to reach a minimum free energy for the total system, are adsorbed on hard walls even though they lose their conformation. The current study provides a simple yet effective approach for the design of thin polymer composites.

COMPUTER SIMULATION OF POLYMER SOLUTION THERMODYNAMICS

The statistical counting method for the computer simulation of the thermodynamic quantities of polymer solution has been reviewed. The calculating results for a single athermal chain confirm the theory of the renormalization group. The results for the athermal solution are consistent with the scaling law of the osmotic pressure with the exponent 2.25. The results for a single chain with the segmental interaction are in a good agreement with the exact results obtained by the direct counting method. The results for the polymer solution show us that the Flory-Huggins parameter is strongly dependent on both the polymer concentration and the interaction energy between segments. (Author abstract) 15 Refs.

Single long-polymer translocation through a long pore

This paper theoretically studies the free energy and conformational entropy of a long polymer threading a long nanopore （no/N ≥0.1） on external electric field. The polymer expanded model is built in this paper, that is, a single long polymer chain with N monomers （each of size a） threading a pore with no monomers can be regarded as polymer with N ＋ no monomers translocating a 2-dimension hole embedded in membrane. A theoretical approach is presented which explicitly takes into account the nucleation theory. Our calculations imply that, the structure of polymer changes more acutely than other situation, while its leading monomer reaches the second vacuum and its end monomer escapes the first vacuum. And it is also shown that the length scale of polymer and pore play a very important role for polymer translocation dynamics. The present model predicts that the translocation time depends on the chemical potential gradient and the property of the solvent on sides of pore to some extent.

Theoretical model of biomacromolecule through nanopore including effects of electrolyte and excluded volume

A theoretical model for the translocation process of biomacromolecule is developed based on the self-consistent field theory （SCFT）, where the biomacromolecule is regarded as a self-avoiding polymer chain actuated by the external potential. In this theoretical model, the external potential, the Coulomb electrostatic potential of the charged ions （the electrolyte effect）, and the attractive interaction between the polymer and the nanopore （the excluded volume effect） are all considered, which have effects on the free energy landscape and conformation entropy during the translocation stage. The result shows that the entropy barrier of the polymer in the solution with high valence electrolyte is much larger than that with low valence electrolyte under the same condition, leading to that the translocation time of the DNA molecules in the solution increases when the valence electrolyte increases. In addition, the attractive interaction between the polymer and the nanopore increases the free energy of the polymer, which means that the probability of the translocation through the nanopore increases. The average translocation time decreases when the excluded volume effect parameter increases. The electrolyte effect can prolong the average translocation time. The simulation results agree well with the available experimental results.

New class of high-entropy pseudobrookite titanate with excellent thermal stability,low thermal expansion coefficient,and low thermal conductivity

As a type of titanate,the pseudobrookite(MTi_(2)O_(5)/M_(2)TiO_(5))exhibits a low thermal expansion coefficient and thermal conductivity,as well as excellent dielectric and solar spectrum absorption properties.However,the pseudobrookite is unstable and prone to decomposing below 1200℃,which limits the practical application of the pseudobrookite.In this paper,the high-entropy pseudobrookite ceramic is synthesized for the first time.The pure high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) with the pseudobrookite structure and the biphasic high-entropy ceramic composed of the highentropy pseudobrookite(Cr,Mn,Fe,Al,Ga)_(2)TiO_(5) and the high-entropy spinel(Cr,Mn,Fe,Al,Ga,Ti)_(3)O_(4) are successfully prepared by the in-situ solid-phase reaction method.The comparison between the theoretical crystal structure of the pseudobrookite and the aberration-corrected scanning transmission electron microscopy(AC-STEM)images of high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) shows that the metal ions(M and Ti ions)are disorderly distributed at the A site and the B site in high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5),leading to an unprecedentedly high configurational entropy of high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5).The bulk high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) ceramics exhibit a low thermal expansion coefficient of 6.35×10^(−6) K^(−1) in the temperature range of 25-1400℃ and thermal conductivity of 1.840 W·m^(−1)·K^(−1) at room temperature,as well as the excellent thermal stability at 200,600,and 1400℃.Owing to these outstanding properties,high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) is expected to be the promising candidate for high-temperature thermal insulation.This work has further extended the family of different crystal structures of high-entropy ceramics reported to date.