The Design Of Elastomer Physical Network And Rheological Properties:Molecular Dynamics Simulation

The flow behavior of polymer melt is still an extremely important but obscure subject in polymer physics,especially shear thinning,which is a common property of most polymer fluids and is a basic feature to distinguish polymer fluids from Newtonian fluids.The end-functionalized polymer will change the structure and dynamics of the whole molecular chain greatly compared with its linear counterpart,but there are few studies on it.Therefore,we need to understand the interrelation between the peculiar rheology of polymer melts and the underlying microstructural changes.Through experiments,the current instrument characterization methods are still unable to understand the dynamics of polymers from the molecular and atomic point of view.Therefore,computer simulation technology is playing an increasingly important role in understanding and predicting the rheological properties of polymer materials.Based on the background of the above topics,this thesis mainly uses coarse-grained molecular dynamics simulations to explore the microscopic mechanism of the influence of terminal functionalization and bimodal molecular weight distribution on the structural properties and rheological properties of polymer materials.Our research work is mainly carried out from the following two aspects:1.Study on the microscopic mechanism of the influence of the interaction strength between end functional groups on the structural properties and rheological properties of single-chain polymer systemsBy using non-equilibrium molecular dynamics(NEMD)simulation,we constructed a series of end-functionalized polymer systems with different terminal interaction strengths.Firstly,we reproduced the shear thinning behavior of the pure linear polymer chains,attributed to the deformation and extension of the polymer chains,as well as the arrangement and orientation along the flow direction.It is also found that the degree of shear thinning is significantly influenced by the chain length,i.e.,the shear thinning behavior weakens and the zero-shear viscosity derived corresponding from the Carreau Model decreases for short polymer chains,which is also evidenced by calculating the gyration tensor and the bond orientation.Then we tailored the physical polymer network by varying the interaction strength between the end functional groups,a prominent shear thinning behavior also occurs.And this shear thinning is attributed to the breakage of the physical network resulting from the chain alignment and orientation induced by the shear,which is further supported by the observation that the chain size becomes much larger under the shear flow compared to the case without any shear,and increases with the interaction strength between ending beads.This is caused by the contraction of the polymer network induced by the attraction between the ending beads,while the polymer network becomes expanded and elongated contributed by the shear flow.2.Research on the influence mechanism of long chain ratio on the structural properties and rheological properties of dual-distributed terminal functionalized polymersLastly,we constructed physical polymer networks with bimodal molecular weight distribution by changing the long chain content,which show that the shear viscosity and shear stress decrease with the long chain content.In addition,large shearing force will destroy part of the physical network,but small shear rate promotes the formation of the physical network.For the evolution process of the shear stress with strain corresponding to different flow field strengths of the LC20 system,only when the shear rate is greater than 0.01τ-1,a weak stress overshoot phenomenon occurs,and the greater the shear rate is,the more obvious the stress yields phenomenon is.When the different long-chain content systems are at the same shear rate(0.2τ-1),there is an obvious stress overshoot phenomenon in each system.