Molecular Simulation Study On Crystallization And Thermal Conductivity Of Polymers

Polymer materials are widely used in the fields of electronics,engineering and machinery.However,the poor thermal conductivity of polymer materials limits its application in the field of thermal management materials.The typical characteristic of polymers is that the thermal conductivity is only 0.1-1W/mK,which is much lower than that of ceramic and metal materials.The thermal conductivity of the composite system can be effectively improved by adding thermal conductive fillers.However,the interface between the organic and inorganic of the filled polymer composites seriously affects the mechanical properties of the composites.With the development of electronic information and new material industry,how to comprehensively improve the thermal conductivity and mechanical properties of polymer materials has become the focus of polymer research.The physical properties of polymers are closely related to their microstructure.The size and orientation of the crystal,the composite state of the heat conducting filler and the matrix,the physical/chemical bonding between the filler particles and other factors have important influence on the properties of the polymer system.In this paper,the influence factors of crystallization and thermal conductivity of polymers and polymer crosslinked nanoparticles and the evolution rules of microstructure were studied by computer simulation.Firstly,the thermal conductivity of polyethylene and polyethylene/boron nitride(BN)composites under mechanical strain was simulated by molecular dynamics method.The relationship between the thermal conductivity and tensile deformation,tensile rate,temperature and molar mass of the polymer was measured by means of non-equilibrium molecular dynamics method.The results show that the thermal conductivity of the pure polyethylene system can be improved through changing the orientation structure of the molecule.With the mechanical straining,the states of the polyethylene molecules were changed from the disorder state to the state of crystal structure and high order along the stretching direction,in this way the heat transfer process along the chain transfer is greater than the intermolecular transfer,which will greatly improve the thermal efficiency.The strain rate,temperature and the molecular weight of the polyethylene molecules also affect the orientation structure,with the smaller stretching rate when the system reached the same deformation conditions,the molecular orientation is higher;the temperature also affects the orientation,stretching rate corresponding to different temperature,the effect is not the same;the molecular size also affects the heat conduction of the polymer,in the same orientation,the polymer with larger molecular mass has a higher thermal conductivity.For polyethylene and boron nitride composite system,the boron nitride sheet can induce the formation of ordered structure of polyethylene molecules due to their unique properties.Our results show that the effect of nano additive is not only the conductive path formed in the polymer system to improve the thermal conductivity of materials with high thermal conductivity properties,and the interaction between polymer materials changing the structure of the polymer is also an important factor affecting the thermal conductivity of materials.Secondly,the effects of two different configurations of boron nitride nano materials on polymer crystallization were studied.In the melt conditions,by studying the distribution of molecular evolution,molecular conformation of crystal in the process of crystallization in the space and the study of molecular diffusioncharacteristics,we compared the characteristics of Boron nitride sheet and Boron nitride nanotubes inducing the crystallization of polyethylene.the results show that the rate of boron nitride nanotubes inducing PE alkanes crystallization was more quickly than that of boron nitride layers.Because of the different dimensions of nano materials,the ability of inducing polymers crystallization is not the same,The radius of curvature of the boron nitride nanostructures affects the rate of crystallization of the polymer on its surface.Analysis of the conformational change and the evolution of bond orientation parameters account for the difference,causing the boron nitride layer inducing crystallization ability is lower than that of the boron nitride nanotubes is that PE molecules on the surface of boron nitride multiple orientation,and the orientation of PE molecules in the surface of boron nitride nanotubes are along the nanotube axial direction,indicating the nanometer material dimension is an important factor affecting the polymer crystallization.Finally,we discuss the influence factors of the synthesis of nano filler nano chain by DPD method,and study the influence factors of the reaction of the surface grafted nanoparticles in solution.We consider the length of the grafted chain(from 30 to 90)and the grafting density from low to high(from 0.04 to 0.16)and the rigidity of the grafted chain.The relationship between the bonding rate of the nanoparticles and the length of grafted chain,the grafting density,the grafting chain stiffness and the concentration was determined.The results prove that controlling the length of the grafted chain of nanoparticles is the most effective method,The length of the grafting chain has an optimal value,in different grafting density,when the graft chain length is less than the best quality increasing the length of grafting chain is conducive to the link between the nano particles,when the grafting chain length is higher than the optimal value increasing the grafting chain length have the opposite effects.In addition,changing the length of the grafting chain,the stiffness of the grafted chain is also considered as an important factor.The grafting density also has an important effect on bonding reaction,the bonding rate of the particles increases with the increase of grafting density.The effect of concentration on bond formation is similar to that of grafting density.