The Thermal Transport And Regulation Mechanism In Polymers By Molecular Dynamics Simulation

Faced with the harsh energy and environmental situation,the development of energy-saving and emission-reduction technologies is of crucial importance to our country's strategy of achieving sustainable development.Researching and developing new type of thermally conductive materials,which are low energy consumption,low emission and low cost,brings a lot of benefits.It can not only replace the traditional materials of high energy consumption,high emission,high cost,and reduce the market scale of high cost traditional materials,but also promote industrial upgrading and improve the heat transfer efficiency.Polymer materials are environment-friendly and excellent in physical and chemical properties.The research of high-thermal-conductivity polymers will enable them to be applied to the wide field of heat transfer in the future.It is of great importance for our country to achieve energy-saving and emission reduction targets.In this dessertation,the importance of phonon transport theory in polymer thermal conductivity is introduced.The advantages of the molecular dynamics simulation method for polymer thermal conductivity and its mechanism are elucidated.Then the important concepts associated with the phonon transport theory in polymer,and the important concept and calculating methods in the research of polymer using molecular dynamics method for heat conduction problem are introduced briefly.The combination of phonon transport theory and molecular dynamics simulation provides an effective way for studying the thermal conduction and mechanism of polymers and their composites.For highly thermal conductive polymer-based composites,this dessertation presents a single-walled carbon nanotube?SWCNT?/polyethylene?PE?oriented composite.It researches the effect of PE chain number on their thermal conductivities,evaluates the thermal conductivity contribution of SWCNT and PE chains,and analyzes the thermal conduction of PE chains in composites.The results show that the thermal conductivity of the oriented composite structure has been significantly improved.The non-bond interaction between SWCNT and PE chain improved the thermal conductivity of PE chain on average by about 23%.The average contribution rate of PE chain to the overall thermal conductivity has reached³0%.The improvement of the thermal conductivity of PE chains is crucial to the overall performance enhancement.Secondly,we find the mass transport phenomenon of PE molecules in SWCNT,and analyze the formation mechanism of this phenomenon.The simplified SWCNT is used to study the relationship between the self-migration of PE molecules and several parameters.A SWCNT-PE nano-heat pipe is proposed based on this mass transport phenomenon.The results show that the temperature,the length of PE molecular and the depth of potential trap can significantly affect the self-migration of PE molecules.The SWCNT-PE nano-heat pipe array with a packing density of 7×10¹³ cm^-2 can reach a heat transfer coefficient of 450WK^-1cm^-2 at 500K,which has the potential of application in nanoscale thermal management and interface heat transfer.The research enriches the mass transport theory of self-migrating phenomenon and proposes the design concept of nano-heat pipe,which is beneficial to the design of nanoscale heat and mass transport devices.Thirdly,we propose a bifurcated branch to construct thermally conductive channels in polymer composites.The bifurcated branch structure is constructed using PE and PA chains,respectively.The thermal conductivities of the PE/PA-CNT interfaces and their composite materials are studied.The results show that the tree-structured PE/PA-CNT expands the range of original CNT by 25 times.In their effective range,the interface thermal conductivity has also been significantly improved.The high thermal conductivity channels can improve and perfect the integrity and efficiency of the thermal conductive network.Then,the properties of PE/PA-CNT in PE matrix are studied.The thermal conductivity of PE/PA-CNT-PE composites is found to be significantly improved.The high thermal conductivity of bifurcated branches can regulate the heat transport in the composites,redistribute the temperature of the system,and improve the heat transport among the dopant particles.This research helps fabricate polymer composites with high thermal conductivity.Additionally,for intrinsically high thermal conductive polymer,in this dessertation,the relationship between the thermal conductivity of single PE chain and temperature and strain is studied.Then the atomic mass modification model is used to simulate the influence of functional groups on the thermal conductivity of single-strand carbon chain polymer.The phonon spectrum is used to reveal the inner link between the above factors and thermal conductivity.The results show that the thermal conductivity of single PE chains decreases with the increase of temperature,and the tensile strain significantly enhances the thermal conductivity.The studies on the thermal conductivity of single-strand carbon chain polymers show that atomic mass modification can change the phonon spectrum of carbon atoms in the main chain and thus significantly affect the heat transport on the carbon chain.Fianally,the effects of tensile and hydrogen bonding on the structure and thermal conductivity of polymers are investigated by using PVA and PP.The results show that the thermal conductivity of the hydrogen bonding polymer PVA is much higher than that of the non-hydrogen bonding polymer PP under unstretched conditions.Hydrogen bonds make PVA have better orientation effects under stretching effect.Under the tensile condition,the thermal conductivity of PVA nanofibers is obviously higher than that of PP nanofibers.Under the combined effect of hydrogen bonding and stretching,the thermal conductivity of PVA nanofibers reaches 3 to 4 times that of PP nanofibers,which shows the synergistic effect of stretching and hydrogen bonding on the thermal conductivity of polymers.For polymer composites,this dissertation focuses on the coupling effects of dopants and matrix at orientating area,and the interfaces of dopants in composites.For intrinsic polymers,this dissertation focuses on the effects of functional groups upon thermal conductivity of polymer chains,and the synergistic effect of stretching and hydrogen bonding on the thermal conductivity of polymers.The results reflect the general researching ideas.This dissertation is of great benefits to polymer materials with high thermal conductivity.