Size and Dynamics of Ring Polymers under Different Topological Constraints

The impact of ring polymer length N and the influence of interchain and intrachain interactions on the size and dynamic behaviors of ring polymers,including the structural relaxation time τ_(R) and self-diffusion coefficient D,remain poorly understood at present due to a lack of systematic studies with relatively large N values.This work addressed this issue by applying dynamic Monte Carlo simulations with independently tuned interchain and intrachain interactions to investigate the size and dynamics of the ring melts with chain lengths over a wide range of 0.2N_(e)≤N≤80N_(e)(N_(e) is the entanglement length of corresponding linear chains)under different topological constraints,including all-crossing and intercrossing systems.We found that it was inappropriate to treat the unknotting constraint free energy of the ring chains in the melts as the free energy contributed by the excluded volume interactions of polymers in a good solvent.Scaling exponents of 2.5 and 1.5 reflecting the Ndependence of τ_(R) were obtained for long ring chains in non-crossing and intra-crossing systems,respectively,suggesting that the ring chains behaved as individual clusters and exhibited Zimm-like dynamics in intra-crossing systems.A single scaling exponent of-2 reflecting the Ndependence of D was obtained for ring chains in non-crossing and intra-crossing systems,indicating that the intrachain constraints affected only the value of D,and had little influence on the scaling relationship between D and N.Furthermore,the extended Stokes-Einstein relation broke down for the ring chains in the non-crossing and intra-crossing systems because the structural relaxation and translational diffusion were decoupled for the short ring systems,while both the translational diffusion and rotational relaxations,as well as diffusion at short and long time scales,were decoupled for long ring systems.

Polymer Translocation

The translocation of a polymer through a pore that is much smaller than its size is a fundamental and actively researched topic in polymer physics.An understanding of the principles governing polymer translocation provides important guidance for various practical applications,such as the separation and purification of polymers,nanopore-based single-molecule deoxyribonucleic acid/ribonucleic acid(DNA/RNA)sequencing,transmembrane transport of DNA or RNA,and infection of bacterial cells by bacteriophages.The past several decades have seen great progresses on the study of polymer translocation.Here we present an overview of theoretical,experimental,and simulational stduies on polymer translocation,focusing on the roles played by several important factors,including initial polymer conformations,external fields,polymer topology and architectures,and confinement degree.We highlight the physical mechanisms of different types of polymer translocations,and the main controversies about the basic rules of translocation dynamics.We compare and contrast the behaviors of force-induced versus flow-induced translocations and the effects of unknotted versus knotted polymers.Finally,we mention several opportunities and challenges in the study of polymer translocation.

书评

对<Nonlinear Polymer Rheology:Macroscopic Phenomenology and Molecular Foundation>(高分子非线性流变学:宏观唯象学及其分子基础)流变学专著进行了全面深入的分析与评价.著者通过深入浅出的语言、生动形象的图解、严密深邃的辩证逻辑和极为简洁的数学表述,对高分子非线性流变学中的基本概念和模型(例如:屈服、有限内聚力和解缠结等),以及高分子非线性流变学中的典型现象(例如:应力过冲、非静态松弛、剪切带和应变硬化等),进行了系统地分析与探索.在我国推介此书,对年轻学者快速了解该领域的前沿工作有重要的指导作用,即使对从事本领域研究的学者,在启发创新思维和提高品判能力方面仍有相当的帮助.

管子模型

高分子非线性流变学是发展高效、节能高分子材料及其加工技术的学科基础.针对缠结高分子流体,deGennes、Doi和Edwards基于单链平均场近似建立了“管子模型”.该模型奠定了现代高分子流体非线性流变学的基础,标志着人类描述缠结高分子流体步入了分子流变学时代.然而,“管子模型”过于简化的物理图像无法基于分子链本身和第一性原理,处理快速大形变条件下缠结高分子流体非线性流变学的一些问题;另一方面,“管子模型”无法统一描述近期的一些流变学实验现象.这就需要重新考虑现有的单链平均场假设和多链相互作用,建立分子流变学的新范式.本文介绍了“管子模型”及其修正理论的基本物理图像、概念及其发展历程,给出了该模型中核心方程的推导过程及其参数的物理意义,最后,对该领域的未来发展趋势以及面临的机遇和挑战进行了简明扼要的展望.希望通过本文能够加深读者对“管子模型”的认识和理解.

Effect of Interfacial Adsorption on the Stability of Thin Polymer Films in a Solvent-induced Process

The stability of ultrathin polymer films plays a crucial role in their technological applications.Here,we systematically investigated the influence of interfacial adsorption in physical aging and the stability of thin polymer films in the solvent-induced process.We further identify the stability mechanism from the theory of thin film stability.Our results show that the aging temperature and film thickness can strongly influence the stability of thin PS films in acetone vapor.Physical aging can greatly improve the stability of thin polymer films when the aging temperature T_(aging1)>T_(g).A thinner PS film more quickly reaches a stable state via physical aging.At short aging time,the formation of the adsorbed layer can reduce the polar interaction;however,it slightly influences the stability of thin polymer films in the solvent-induced process.At later aging stage,the conformational rearrangement of the polymer chains induced by the interfacial effect at the aging temperature Taging1 plays an important role in stabilizing the thin polymer films.However,at T_(aging2)<T_(g),the process of physical aging slightly influences the stability of the thin polymer films.The formation of the adsorbed layer at Taging2 can reduce the short-range polar interaction of the thin film system and cannot suppress the instability of thin polymer films in the solvent-induced process.These results provide further insight into the stable mechanism of thin polymer films in the solvent-induced process.

Diffusion of a Ring Threaded on a Linear Chain

A mesoscopic simulation is applied to investigate the effects of hydrodynamic interactions and axial chains on the dynamics of threaded rings.The hydrodynamic interactions significantly speed up the diffusion and relaxation of both free and threaded rings.The decoupled diffusion and relaxation dynamics indicate the broken of the Einstein-Stokes relationship.The diffusion of a ring threaded on a flexible chain exhibits a synergism effect compared to that on an axial rod,which originates from the self-diffusion of the ring and the reptation-like motion of the axial chain.Meanwhile,hydrodynamic interactions significantly improve the synergism effect,leading to an enhanced sliding motion of the threaded ring.The faster sliding of threaded rings suggests that the entropic barrier is negligible,which agrees well with the basic assumption of barrier-less confining tube at equilibrium in tube theory.Our results provide a new perspective on analysis of the effects of topology constraints on polymer dynamics.

Shape Accuracy and Residual Stress Distribution of Nano-molded Semicrystalline Polymer:A Simulation Study

The shape accuracy and residual stress distribution of a nano-molded semicrystalline polymer are studied by molecular dynamics simulations.Semicrystalline polyethylene flakes are obtained by continuous cooling inside templates with four shapes.We find that the curvature of contour curve near template corners decreases with corner angle.A simple 2D shape model of minimum surface energy is proposed to understand the shapes for repulsive and attractive templates.The confinement of template induces highly ordered chain packing in surface region.According to the spacial distribution of local stress,we find the contracting stress caused by volume shrinkage during cooling concentrates on the chains perpendicular to the direction of relative stress principal.The distribution of von Mises stress indicates the outer layer of semicrystalline polymer flake has lower distortion.Our results provide a theoretical insight for better nano-molding techniques.