Dynamics Of Polymer Chains Under Ultra-High Shear And Strong Confinement

The static and dynamic properties of polymers are strongly affected by a series of factors,such as topology,hydrodynamic interactions,flow and confinement.Clarifying the relationship between such effects and the static,dynamic,and rheological properties of polymer solutions or melts remains a challenging problem in polymers physics.From the experimental point of view,although the single molecule operation technology(such as microfluidics,optical/magnetic tweezers and single molecule fluorescence technology)has made great progress,however,the effective physical information obtained through the single molecule operation technology is insufficient due to technical limitations.Meanwhile,results of single molecule operation technology are vulnerable to the external disturbance.Therefore,by means of a hybrid simulation method that combines molecular dynamics simulation and multi-particle collision dynamics simulation,we hope to comprehensively and quantitatively investigate the effects of topology,hydrodynamic interactions,external flow and confinement on the static properties and dynamic response of single polymer chain on the molecular scale.The primary results are summarized as follows:1.Size and dynamic response of linear and ring polymer chains at high shear rates(1).When hydrodynamic interactions are neglected,the polymer size increases monotonically with the increase of shear rate,which is consistent with the predictions of the Rouse model.However,when hydrodynamic interactions are taken into account,both linear and ring polymers exhibit distinct nonmonotonic dependence of the relative extensions on the shear rate,and the counterintuitive nonmonotonic behavior can be divided into three main regions.(2).The average lifetime of the stretched conformation is positively correlated with the polymer size.For linear polymers at moderate shear rates,the probability distribution function of the rootmean-square radius of gyration indicates that the polymer tends to behave like a collapsed coil,corresponding to the initial decrease of polymer size.With the increase of shear rate,due to the force and moment balances,the linear polymer prefers to stay in a relatively stable stretched state with a cis-S-shaped conformation,resulting in the maximum size.(3).Due to the closed structure and the absence of free-end effects for ring polymers,there exist two different relatively stable stretched states:one with an oval-shaped conformation and the other with a cis-S-shaped conformation.For the former state,the tumbling motion almost disappears but the Tank-Treading motion persists;whereas for the latter state,both tumbling and Tank-Treading motions are greatly suppressed.2.Static and dynamics of polymer chains under confinement(1).The scaling relationships between axial diffusion coefficient D and the chain length N and channel size H follow:X?NH-2/3 and D?1/N,respectively.Such results are in agreement with predictions(the confined blobs engage in a self-avoiding walk along the centerline of the channel and the blob size is comparable to the confinement size)of de Gennes's blob theory.For weak confinement,the blob size and the number of monomers in the blob are relatively large.Each monomer experiences an altered velocity field due to coupling with the disturbances caused by all the other monomers.Therefore,hydrodynamic interactions are sufficiently strong to reasonably consider a confined blob as a non-draining sphere.The frictional force acting on the entire blob dominates,resulting in a Zimm-like behavior for a confined blob in weak confinement,where the blob can be considered as an equivalent non-draining sphere.As the channel size decreases,the blob size and the number of monomers in blob are also reduced.Therefore,the frictional force acting on individual monomers dominates,and the confined blobs become partially draining.The weak hydrodynamic interactions result in Rouse dynamics for highly confined blobs,which does not support the assumptions of complete screening and non-draining in blob theory.(2).For flow-induced translocations,the critical flux decreases at first and then approaches a stable value as the channel size increases,which is different from the predictions of de Gennes and Wu Chi.de Gennes treated the confined Blob as a non-draining sphere and the model can be reduced as a Zimm model on the Blob level.Wu chi treated the confined Blob as a full-draining sphere and the model can be reduced as a Rouse model on the Blob level.However,the confined Blob is probably a partially permeable sphere in practice.Extending the partially permeable sphere model to the Blob level can explain the scaling relationship between the critical flux and channel size.For flow-induced translocation,the Coil-to-Stretch transition of linear chains is not a well defined first-order phase transition.The transition width of ring chains is independent of channel size due to the folded-translocation type.While for the linear chains,the transition width of decreases as the channle size increases,because there exist two diffferent translocation ways:the folded-translocation and end-translocation 3.Diffusion of a ring threaded on a linear chain(1).Hydrodynamic interactions significantly speed up the diffusion and relaxation behaviors of both free and threaded rings.(2).The diffusion motion of ring itself couples with the reptation-like motion of the flexible axial chain,leading to an enhanced cooperative movement of the threaded ring.Both of the absolute diffusion and the reptation-like motion are accelerated by taking account of hydrodynamic interactions.The faster sliding dynamics of threaded ring on flexible axial chain in equilibrium implies that the entropy barrier for threaded ring exerted by the axial chain is negligible,which indirectly confirms the validity of the tube model in describing the entangled polymer melts in equilibrium.(3).The diffusion and relaxation behaviors for threaded rings are decoupled,indicating the breakdown of the Einstein-Stokes relationship.