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

Abstract

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.

Thermal Conduction and Phonon Transport in Folded Polyethylene Chains

“…In Figure , we consider the system of multiple lamella with two hairpin folds along the polymer chain (fixed N stem = 3 and L stem = 30 nm, while L changes). This is a single chain representation of semicrystalline polymer with multiple crystalline domains connected via bridge chains, specifically under high mechanical strain . From the temperature profile for a chain with two lamella in Figure a, we observe the same temperature jumps at all fold interfaces.…”

“…Analogous to these analytical predictions, a few simulation studies have considered explicit semicrystalline sandwich structures containing various chain topologies within the interlamellar region. 14,15 These studies show that an increase in thermal conductivity of semicrystalline polymer fibers or films under mechanical stretching can be attributed to changes in the conformations of chains in their interlamellar region. The crystalline lamellae can break into several smaller crystalline regions, which are sandwiched by amorphous regions containing intrafibrillar bridge chains, with relatively small strains.…”

“…The crystalline lamellae can break into several smaller crystalline regions, which are sandwiched by amorphous regions containing intrafibrillar bridge chains, with relatively small strains. These bridges extend, align themselves with increasing strain, and become the dominant medium of thermal energy transport …”

“…In a tail configuration, one end is connected to a lamellar stem and the other end terminates within the interlamellar region. These interlamellar chain topologies play a substantial role in determining the overall thermal conductivity and specifically bridges are the most important in efficient thermal transport. , …”

State-of-the-art, opportunities, and challenges in bottom-up synthesis of polymers with high thermal conductivity

“…Typical strategies for enhancing efficient thermal transport in polymers are to improve chain orders ( e.g. , chain alignments and inter-polymer chain packing), 29,56,75,106,128,163,172,181–186 to decrease interface scattering, 187–190 to increase interchain bonding ( e.g. , hydrogen bonds and π–π interactions), 13,44,109–111,180,191 and to reduce anharmonicity/bond strength inhomogeneity.…”