Molecular dynamics simulation of thermal transport in semicrystalline polyethylene: Roles of strain and the crystalline-amorphous interphase region

With potential thermal management applications, such as plastic heat exchangers and thermal interface materials, thermally conductive polymers have gained renewed interest in the past decade. Ultradrawn polyethylene fibers and films have been experimentally shown to have thermal conductivities at least two orders of magnitude of these in their amorphous counterparts. However, the theoretical molecular-level understanding of strain effects on the thermal transport in drawn semicrystalline polymers, such as polyethylene, especially the roles of different interlamellar chain topologies in the crystalline-amorphous interphase region, remains elusive. Using molecular dynamics simulations, we investigated the strain effects on the thermal conductivity and vibrational transport in a simplified sandwich semicrystalline structure. We found that the topology of the interlamellar chains determines the dependence of thermal conductivity on strains. Comparing thermal resistances at different regions in the interlamellar structure, thermal resistance at the amorphous region is not necessarily the highest; the interphase region with the transition from the crystalline to amorphous state can have a much higher resistance. We conducted the frequency domain analysis to obtain the heat flux spectrum in the crystalline-amorphous interphase region and found that the vibrational modes at intermediate and high frequencies can contribute more than these at relatively low frequencies to the total heat flux because of the complex interlamellar chain topologies (e.g., loop chains). Our work provides molecular-level understandings of the structural-property relationship in semicrystalline polymers with strains, which could assist the design and development of thermally conductive polymers for thermal management applications.