Thermal Transport in Poly(p-phenylene): Anomalous Dimensionality Dependence and Role of π–π Stacking

Abstract

For heat conduction along polymer chains, a decrease in the axial thermal conductivity often occurs when the polymer structure changes from onedimensional (1D) to three-dimensional (3D). For example, a single extended aliphatic chain (e.g., polyethylene or poly(dimethylsiloxane)) usually has a higher axial thermal conductivity than its double-chain or crystal counterparts because coupling between chains induces strong interchain anharmonic scatterings. Intuitively, for chains with an aromatic backbone, the even stronger π−π stacking, once formed between chains, should enhance thermal transport across chains and suppress the thermal conductivity along the chains. However, we show that this trend is the opposite in poly(p-phenylene) (PPP), a typical chain with an aromatic backbone. Using molecular dynamics simulations, we found that the axial thermal conductivity of PPP chains shows an anomalous dimensionality dependence where the thermal conductivity of double-chain and 3D crystal structures is higher than that of a 1D single chain. We analyzed the probability distribution of dihedral angles and found that π−π stacking between phenyl rings restricts the free rotation of phenyl rings and forms a long-range order along the chain, thus enhancing thermal transport along the chain direction. Though possessing a stronger bonding strength and stabilizing the multiple-chain structure, π−π stacking does not lead to a higher interchain thermal conductance between phenyl rings compared with that between aliphatic chains. Our simulation results on the effects of π−π stacking provide insights to engineer thermal transport in polymers at the molecular level.

Anomalous Correlation between Thermal Conductivity and Elastic Modulus in Two-Dimensional Hybrid Metal Halide Perovskites

“…Fundamentally, this finding also challenges the current understanding on thermal transport in 2D HOIPs that appear to coherently transport heat across the organic–organic interface. , The unusual correlation between thermal conductivity and elastic modulus highlights the importance of optical phonons in regulating room temperature heat flow in 2D HOIPs depending on the interface type. , Optical phonons may not necessarily contribute to thermal conductivity, but rather provide strong scattering channels for acoustic phonons, as claimed for all-inorganic 2D HOIPs . Another possibility is the large mismatch of phonon spectrum at the aryl–aryl and alkyl–aryl interfaces induces a strong phonon filtering effect . Indeed, studies on thermal transport in 2D HOIPs have only recently begun to emerge and would require further investigations.…”

“…This comparison seems to indicate that the rings are more closely packed (smaller spacing) in full-chiral 2D HOIPs than the aryl–alkyl and aryl–aryl spacings in half-chiral 2D HOIPs. This is also indicative of the importance of orientation of the rings in determining thermal transport across aryl–aryl type interface . A notable difference between the structures of [SMePEA] 2 PbI 4 and [PEA] 2 PbI 4 is the presence of several diverse orientations of rings in the former.…”

“…The universality of this correlation in HOIPs cannot be assumed without understanding how both properties depend on different types of organic–organic interfaces, beyond a typical alkyl–alkyl type of interface with only van der Waals interactions . If aryl ligands are introduced into the structure, there will be π–π and CH−π interactions at the organic–organic interface. , Our recent simulation work suggests that π–π stacking enhances the interaction strength of this interface but decreases the thermal transport capability across the interface, which could potentially induce an anomalous correlation between elastic modulus and thermal conductivity . Synthetically, this has been made possible by replacing an existing alkyl cation with an aryl one.…”

Molecular perspective and engineering of thermal transport and thermoelectricity in polymers

“…12(b)). Conversely, the relation between thermal conductivity and dimensionality can be inverted if the chains are bonded and aligned in the perpendicular plane to the chain length, as demonstrated by Yang et al 233 in their study of aromatic poly( p -phenylene). They showed that the 3-D crystalline form exhibits greater thermal conductivity compared to both double-chain (2-D) and single-chain (1-D) configurations, as depicted in Fig.…”

“…(c) Thermal conductivity for single-chain, double-chain, and crystal as a function of a number of segments of poly(pphenylene). Reproduced with permission 233. Copyright, 2023, American Chemical Society -JPCB.…”