Microstructure Effect On Thermal Conductivity Of Polymers: A Molecular Dynamics Simulation

In the wake of developments in industrial production, science and technology, polymershave been widely used in many fields as functional or structural materials because of itsexcellent allround properties and low-cost. However, polymers also have very low thermalconductivity on the order of0.1W·m^-1·K^-1at room temperature. They can’t adapt to high heatemission generated by high integration density and power which can lead to thermal fatiguedamage. Now improving the thermal conductivity of polymers has already become one of theimportant goal for the world to strive for. In this thesis, nonequlibrium molecular dynamics（NEMD） simulations of thermal conductivity in polymers were performed, the effect ofmicrostructure, such as chain length, branched chain, or double bond, on thermal conductivityof polymers was analyzed, and the thermal mechanism in the heat transport process isexplored. This study can be used to guide the future development of advanced polymerproducts with excellent thermal conductivity.The effects of conjugate double bond on single polymer chains have been investigated.Based on the inhomogeneous nonequlibrium molecular dynamics （Inhomogeneous NEMD）,the heat transport in single polymer chains of polyacetylene （PAc）, polyethylene （PE）, andcis-1,4-polybutadiene （cis-1,4-PB） was simulated. The results show that the inverse thermalconductivity （1/κ） of polymer chains are linear with the inverse chain length （1/L）. And thepolymer chain with non-conjugated double bond has the most obvious finite-size effects dueto its longer ballistic phonon transport time. Also, it is found that the polymer chain withconjugate double bond has the highest thermal conductivity, such as the thermal conductivityof PAc is higer than PE. Instead, non-conjugated double bond has inhibitory effects on the heat transport of polymer chains, for example, the thermal conductivity of cis-1,4-PB arelower than PE. The mean square radius of gyration （<R_g²>） and mean squrare distancement（MSD） are able to reflect the the sizes and situation changes of molecular conformation,respectively. The conjugated double bond has inhibitory effects on <R_g²> and MSD ofpolymer chains, while the non-conjugated double bond has promotion effects. Radialdistribution function （RDF） can reflect the the distribution of atoms in the polymer chains,double bond makes the peak of RDF higher. In addition, the analysis of vibration density ofstates （VDOS） has been applied and its high frequency peaks which represents differentvibrations （stretching, bending, etc.） in polymer chains showed that the polymer chain withnon-conjugate double bond have higher peaks and conjugate double bond system is lower.The low frequency peaks of spectrum of VDOS which associated with low frequency modesof vibrations was strengthened because of double bond, known to be significant for thermalconduction due to their thermal diffusivity.The effects of branching on the thermal conductivity of single polymer chains have beeninvestigated. Inhomogeneous nonequilibrium molecular dynamics simulation method wasapplied to simulate the thermal conduction process of alternating isotacticpropylene-ethylene-propylene copolymer （alternating isotactic PEP） chain and polyisoprene（PI） chain. By comparing with the thermal conductivity of polyethylene （PE） and cis-1,4-polybutadiene （cis-1,4-PB）, it is found that polymer chains with branched chain have mostobvious finite-size effects and lower thermal conductivity. The <R_g²> and MSD of polymerwith branched chain are larger. For double bond system, branched chain makes the RDF ofpolymer chains larger, while for system without double bond, the RDF of polymer chainswith branched chain is smaller. In addtion, The high frequency peaks of spectrum of VDOSshowed that polymer chains with branched chain have higher peaks, this is represents moreintense atom motions in polymer chains. And the low frequency peaks showed that polymerchains with branched chain have lower peaks.The effects of tensile strain （ε） on thermal conductivity of single polymer chains and bulk polymers are investigated. Inhomogeneous nonequilibrium molecular dynamics has beenused to simulate the heat transport in single polyethylene （PE） chains that stretched along themain chain direction, while homogeneous nonequlibrium molecular dynamics has been usedto simulate the heat transport in stretched bulk polyethylene （PE）. The simulation results showthat the thermal conductivity of single PE chains and bulk PE increase with the increasingchain length （L） and ε, and the trend get gently. For single PE chains, this trend about L isunderstood to be a result of competition between ballistic phonon transport and diffusivephonon transport in a single polymer chain and limitation from the finite-size effects. Thetrend about ε is determined by the growing phonon mean free path and decreasing numberdensity of atoms during drawing. The <R_g²>, MSD, RDF and high frequency peaks ofspectrum of VDOS decreased with the increasing ε. For bulk PE, this trend about L isunderstood to be a result of heat transport competition between along chain backbone andbetween chains and limitation from the finite-size effects. The trend about ε is determined bythe variation of interchain spacing between chains during drawing. The <R_g²>, MSD, RDFand high frequency peaks of spectrum of VDOS which showed an increase firstly butdecreased with the increasing ε, showed an increase firstly but getting gently with theincreasing ε, as the L of polymer chains in bulk PE is longer. In addition, the low frequencypeaks of spectrum of VDOS for single polymer chains and bulk polymers were strengthenedat first and then weakened with the increasing ε, which represents that the heat transport inpolymer chains was strengthened at first and then weakened, too, as a result of the increasingtrend of thermal conductivity get gently.