Molecular Dynamics Simulation Of Tensile Properties Of Amorphous Polymers With Different Chain Length Distributions

Many properties of polymer materials are related to their molecular weight and molecular weight distribution(MWD).Molecular weight and its distribution are internal factors that affect polymer structure-performance.Therefore,Understanding the influence of molecular weight and MWD on the properties of the polymer has a very profound guiding significance for people to choose the raw materials meeting the requirements.However,due to the limitations of the experiments,some micro-scale information is difficult to obtain through traditional experiments.The relationship between molecular weight,MWD and polymer mechanical behavior has not been well revealed.In this paper,a large-scale molecular dynamics simulations were used to study deformation mechanisms during uniaxial tensile deformation of an amorphous polyethylene polymer with different chain lengths.The effects of mixing ratio,molecular weight distribution and the molecular weight of the polydisperse polymer system on the mechanical properties of the polymer were explored.The main research content includes :(1)By simulating the uniaxial stretching of an amorphous polyethylene system containing two chain lengths in different proportions,the influence of themixing ratio on the deformation behavior of the amorphous polymer was explored.The stress-strain curves,energy evolution,orientation and entanglement behavior of polymer chains in the tensile stress field of amorphous polymers were analyzed.The results show that the mixing ratio has almost no effect on the elastic deformation,yield,and strain softening stages during the uniaxial stretching process.But,in the strain-hardening region,as the mixing ratio decreases(ie,the longer chains in the system increase),the Strain hardening stage of the stretching process becomes more pronounced and longer.And the smaller the mixing ratio,the greater the maximum stress that can be achieved.At the same time,the orientation and entanglements of the chains also mainly occurred in the Strain hardening stage,And the smaller the mixing ratio,the greater the degree of chain orientation and disentanglement.(2)We have used molecular dynamics simulations to explore the effect of the MWD of amorphous polyethylene on uniaxial tensile deformation.The stress-strain curves,energy evolution,orientation and entanglement behavior of polymer chains in the tensile stress field of amorphous polymers were analyzed.The results show that the MWD has almost no effect on the the elastic,yield peak and softening regions of the stress-strain curve,but there is a significant difference in the strain hardening region.and this effect is not simply relationship that the wider the molecular weight distribution is,the greater the strain hardening modulus is,but it is positively correlated within a certain distribution range.And in the tensile deformation,the ultimate degree oforientation and disentanglement of the systems is proportional to the duration of strain hardening region.(3)Through the uniaxial tensile simulation of polydisperse amorphous polyethylene systems with different molecular weights,the effect of molecular weight on the stress-strain behavior of polydisperse amorphous polymers was investigated.The stress-strain curves,energy evolution,orientation and entanglement behavior of polymer chains in the tensile stress field of amorphous polymers were analyzed.The analysis results show that the elastic modulus of polydisperse polymer systems with different molecular weights is the same in the tensile deformation,but the higher the molecular weight,the more pronounced the yield and strain softening,and the longer the strain hardening stage,the maximum stress reached during the stretching does not increase with the increase of the molecular weight.However,after each system is stretched,the ultimate degree of orientation of the systems increases with the increase of the molecular weight of the system,and the ultimate degree of orientation of the systems is proportional to the duration of strain hardening region.At different stretching temperatures,the elastic modulus and strain-hardening modulus of each system are the same,but the lower the stretching temperature,the greater the yield stress and the greater the maximum stress reached.However,after stretching,the ultimate degree of orientation of the systems is basically the same at different stretching temperatures,but the ultimate degree of entanglement of the systems decreases as the stretching temperature increases.In this paper,molecular dynamics simulations of uniaxial stretching of amorphous polyethylene systems with different chain lengths are performed.The effects of mixing ratio,molecular weight distribution and the molecular weight of polydisperse polymer system on the mechanical properties of the polymer were investigated.It was found that the mixing ratio and molecular weight distribution had little effect on the elastic,yield,and strain-softening stages of the stress-strain behavior of amorphous polymers.However,systems with different chain length distributions have a significant impact on the strain-hardening stage,and the chain orientation and disentanglement also mainly occur at this stage.