Study On The Thermal Conductivity And Mechanical Properties Of Crystalline Polyethylene By Molecular Dynamics Simulation

Polymer materials have the advantages of low cost,low density,corrosion resistance,easy processing,etc.,so they are widely used in various fields.However,traditional polymer materials have poor thermal conductivity and mechanical properties due to the chaotic chain segment structure and internal defects,which limits its scope of use.Research has found that ordered polymer materials can not only improve thermal conductivity and mechanical properties,but also maintain the original advantages of the material.At present,the research on the performance of ordered polymers has been discussed in experiments.However,it only stays at the macro level and lacks theoretical research on the molecular morphology and structure of materials.Therefore,this thesis uses molecular dynamics(MD)methods to precisely control the microstructure of ordered polymer materials,and further explored the relationship between material structure and performance.The coarse-grained potential field of polyethylene(PE)was established to study the thermal conductivity and mechanical properties of complex crystalline systems.At the same time,a rigid-soft polyethylene block model was designed,which opened up new ideas for the study of high thermal conductivity intrinsic polymers.Firstly,the Coarse-Grained(CG)force field of polyethylene was established through the Boltzmann inversion method combined with experimental data to optimize the iterative method based on the polyethylene all-atom model.It was fully verified by thermodynamic data such as glass transition temperature(Tg),thermal expansion coefficient(),mean square radius of gyration(Rg),diffusion coefficient(D),and characterization of the thermal conductivity and mechanical properties of the random bulk system and crystalline system.The accuracy and rationality of the potential energy parameters had laid a foundation for the study of the structural properties of the complex crystalline system of polyethylene.Secondly,a coarse-grained model of the polyethylene spherulite system was established,that was,models with different crystallinity and different crystal sizes.The developed coarse-grained force field was used to explore the thermal conductivity and mechanical properties of the spherulite model.The research results showed that the crystallinity will affect the thermal conductivity of the system.The higher the crystallinity,the greater the thermal conductivity.And the higher the crystallinity and the larger the size of crystal region,the higher the thermal conductivity and the better the mechanical properties under the condition of stretching to the same deformation.In addition,the dynamic process of stretching was further explored,and it was found that the crystalline and amorphous regions in the spherulites formed regions with different mechanical strength.And the amorphous region was easier to orient than the crystalline region.The influence of the complex spherulitic structure on the thermal conductivity and mechanical properties of the system was analyzed from a microscopic point of view.Finally,two kinds of polyethylene rigid-soft block models of rigid-soft diblock and rigid-soft alternating block were designed to be inspired by the stretching dynamics of the spherulite system.Through mechanical stretching and the formation of hydrogen bonds,the thermal resistance of the interface was reduced and the thermal conductivity of the material was improved.The results showed that the improvement of the molecular chain structure and the enhancement of interface interaction had significantly improved the thermal conductivity of the rigid-soft block model.The research results provided a theoretical basis for the design of new high thermal conductivity materials.The thermal conductivity and mechanical properties of crystalline polyethylene was systematically studied in this thesis.And the influence of the complex crystal structure and rigid-soft block model on the performance of the system was analyzed.It has important guiding significance for the design and preparation of high-performance polymer materials in experiments.