Thermal Conductivity of Pentiptycene-Based Poly(o-hydroxyimide) Copolymers: A Study via Integrated Experiments and Simulations

Abstract

Polymers play important roles in thermal transport applications, and conjugated polymers have been identified as promising candidates to achieve high thermal conductivity. In this paper, pentiptycene-based poly­(o-hydroxyimide) (PPHI)-conjugated copolymers, made of a bulky pentiptycene segment and a −CF3 substituent segment, with different segment ratios are studied. Both experimental and molecular dynamics (MD) simulation results show that polymer thermal conductivity increases as the ratio of the bulky segment increases. The experimental in-plane thermal conductivity is significantly higher than the out-of-plane value (up to 6.38 times), which is due to the chain alignment effect in the in-plane direction. The highest measured in-plane thermal conductivity reaches 0.982 W/m·K, which is among the highest in polymer thermal conductivity. Experimental results show that the volumetric heat capacity increases with the bulky segment ratio which should have a positive impact on the thermal conductivity. MD results further show that the bulky segment helps increase the spatial extension of polymer chains and the amount of strong interatomic forces related to the aromatic carbon atoms, both leading to increasing thermal conductivity. In addition, we also find that the fractional free volume in the PPHI copolymers decreases as the bulky segment ratio increases, which should also have contributed to the increase in thermal conductivity.

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

“…128 The experimental in-plane thermal conductivity is ∼6 times higher than the out-of-plane value, which is due to the chain alignments in the in-plane direction. 128 The in-plane thermal conductivity at 0.982 W m −1 K −1 has been measured. 128 Experimental results show that the volumetric heat capacity increases with the bulky segment ratios which should have a positive impact on the thermal conductivity.…”

“…9). 102,128 Both experimental and molecular dynamics simulation results show that the thermal conductivity of pentiptycene-based poly(o-hydroxyimide) increases as the ratio of the bulky pentiptycene segment increases (Fig. 9).…”

“…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][182][183][184][185][186] to decrease interface scattering, [187][188][189][190] to increase interchain bonding (e.g., hydrogen bonds and π-π interactions), 13,44,[109][110][111]180,191 and to reduce anharmonicity/bond strength inhomogeneity. 14,146,152,192 Thus, thermally conductive polymers synthesized at monomer levels by various bottom-up approaches have been developed.…”

“…This leads to an increase in thermal conductivity. 128 As discussed above, high thermal conductivities have been measured in polymers, which are made by the bottom-up methods including chemical vapor deposition, 44,193,194 template-assisted electrochemical polymerization, 75 copolymerization, 74,185 cross-linking reaction, 13,79,95,96,195 and surface-initiated polymerization. 121 However, thermal transport mechanisms in these polymers are not fully understood.…”

“…9 (a) Relationships between the bulky segment ratio and thermal conductivity for pentiptycene-based poly(o-hydroxyimide) copolymers. Thermal conductivities are calculated by molecular dynamic simulation (MD simulation) 128 and experimental results for the in-plane direction and out-of-plane direction. 128 (b) Chemical structures of pentiptycene-based poly(o-hydroxyimide) copolymers and schematic of bulky and flexible segments.…”