Crystalline polymers with exceptionally low thermal conductivity studied using molecular dynamics
چکیده
Semi-crystalline polymers have been shown to have greatly increased thermal conductivity compared to amorphous bulk polymers due to effective heat conduction along the covalent bonds of the backbone. However, the mechanisms governing the intrinsic thermal conductivity of polymers remain largely unexplored as thermal transport has been studied in relatively few polymers. Here, we use molecular dynamics simulations to study heat transport in polynorbornene, a polymer that can be synthesized in semi-crystalline form using solution processing. We find that even perfectly crystalline polynorbornene has an exceptionally low thermal conductivity near the amorphous limit due to extremely strong anharmonic scattering. Our calculations show that this scattering is sufficiently strong to prevent the formation of propagating phonons, with heat being instead carried by non-propagating, delocalized vibrational modes known as diffusons. Our results demonstrate a mechanism for achieving intrinsically low thermal conductivity even in crystalline polymers that may be useful for organic thermoelectrics. ∗ aminnich@caltech.edu 1 ar X iv :1 50 9. 04 36 5v 1 [ co nd -m at .m es -h al l] 1 5 Se p 20 15 Bulk polymers are generally considered heat insulators due to ineffective heat transport across the weak van der Waals bonds linking polymer chains. However, both computational[1– 5] and experimental[2, 6–10] studies have demonstrated that some semi-crystalline polymers can have large thermal conductivity exceeding that of many metals. This enhancement occurs when the polymer chains are highly aligned, allowing heat to preferentially transport along the strong covalent backbone bonds. Thermally conductive polymers could find great use in a variety of heat dissipation applications including electronics packaging and LEDs. Computational studies of heat transport in polymers have predicted the high thermal conductivity of crystalline polymers and also identified key molecular features that contribute to this high thermal conductivity. While most studies have focused on polyethylene (PE), comparisons of thermal conductivity among polymers have helped to identify factors of particular importance in setting a polymer’s intrinsic upper limit to thermal conductivity. First, backbone bond strength[1] has been identified as heavily influencing group velocity, leading to higher thermal conductivity. Additionally, chain segment disorder[1, 11, 12], or the random rotations of segments in a chain, has been shown to lead to lower thermal conductivity. An important goal in exploiting the high intrinsic thermal conductivity of certain polymers is to fabricate semi-crystalline polymers at a large scale. A morphology potentially suited to this purpose is the polymer brush, or an array of polymer chains attached at one or both ends to a substrate[13]. A promising synthesis technique, known as surface-initiated ring-opening metathesis polymerization (SI-ROMP), is able to uniformly grow tethered polymer chains from a substrate in the desired aligned structure. Polynorbornene (PNb) and its derivatives are well studied in the ROMP synthesis technique and are thus of interest as thermal interface materials [14]. However, the intrinsic thermal transport properties of PNb remain unknown. In this Letter, we use molecular dynamics (MD) to study heat conduction in PNb. While PNb meets the standard criterion for high thermal conductivity, our simulations indicate that even perfectly crystalline PNb has an exceptionally low thermal conductivity nearly at the amorphous limit. We show that this low thermal conductivity arises from significant anharmonicity in PNb, causing high scattering rates that prevent the formation of phonons, resulting in heat being carried by non-propagating vibrations known as diffusons [15, 16]. Our work shows how intrinsically low thermal conductivity can be realized in fully crystalline