Simulation Study On The Sliding Dynamics Of Mechanically Interlocked Molecules And Interaction Between Nanorods Immersed In Active Particles

Mechanically interlocked molecules such as catenane and rotaxane have attracted much attention due to their unique molecular topology and their wide applications in nanotechnology,biology,and materials.Based on the advantages of the dynamic properties of mechanical bondsthe lower sliding or rotational barriers of the rings along the axial chain,mechanically interlocked molecules have higher degrees of freedom and flexibility than traditional covalent bonding molecules,which makes this type of polymer material has unique mobility,stimuli responsiveness and superior mechanical properties.The sliding dynamics of ring in mechanically interlocked molecular can cause changes in molecular physical or chemical properties,bringing them application prospects in artificial molecular machines and new functional materials.Therefore,it is of great significance to study the structure-property relationship of novel materials such as catenane/rotaxane for understanding and exploiting their superior properties.In Chapter 1,we present the research background including the development,structural characterization and applications based on internal kinetic properties of two typical mechanically interlocked molecules(catenane and rotaxane),as well as interesting collective behaviors in active particle baths.And an overview of the research model,methodology and research purpose is provided.In Chapter 2,we use molecular dynamics simulations to study the sliding dynamics of multirings on semiflexible polymer in radial poly[n]catenanes.The structure of poly[n]catenanes is that n-1 rings are threaded on a central large ring.Two conformations of the central cyclic polymer chain are considered:fixed and fluctuating(non-fixed).As the rigidity of the central cyclic polymer chain increases,for the fixed case,the diffusion coefficient increases monotonically because the tortuosity of the sliding path decreases;for the fluctuating case,the diffusion coefficient decreases.This indicates that the contribution of the polymer fluctuation is weakened with the further increase of rigidity.Compared with the one ring case,the mean-square displacement of multiple rings exhibits unique sub-diffusion behavior on intermediate time scales due to the repulsion between two adjacent rings.In addition,for multi-rings systems,the overall diffusion is relatively slow,but the local dynamics of individual rings and the rotational diffusion of the central cyclic polymer are faster.These results help us to understand the diffusion motion of rings in radial poly[n]catenanes from a fundamental perspective,and provide a theoretical basis for molecular design ideas.In Chapter 3,we use the molecular dynamics simulation method and the Lifson-Jackson formula to study the sliding dynamics of the ring along the block polymer chain(ANABNB)in rotaxane,respectively.We found that the sliding dynamics of the ring is strongly dependent on the block length(NA+NB),the A-block content fa and the block-ring interaction strength ε.From the free energy perspective,the width of the free energy well and the height of the free energy barrier together determine the sliding diffusion behavior of the ring.Based on the Potential of Mean Force(PMF)of the ring sliding along the block copolymer,we find that our results are in good agreement with the theoretical analysis using the Lifson-Jackson formula.Our study of the sliding dynamics of rings on block copolymers can provide a very simple and practical model for studying the diffusion of particles in periodic media,which is one of the interesting problems in many different scientific fields such as physics,chemistry and biology.In Chapter 4,we investigate the effective force between two asymmetric wedges immersed in a two-dimensional active bath through the Brownian dynamics simulations.First,by exchanging the positions of the two asymmetric wedge-shaped nanorods,an attraction-repulsion transition between the two asymmetric nanorods is induced by active particles,which is completely different from passive Brownian particles.Second,the transition of the effective force between the two asymmetric wedge rods is symmetric for the long-range distance,but not for the short-range case.Our study opens up new possibilities for controlling the motion and assembly of microscopic objects using the self-propelled behavior of active particles.