Covalent Bonds versus van der Waals Forces: A Picture in Thermal Conduction of Organic Materials

We present a direct comparison of the heat transport properties between the state in which the constituent molecules are assembled by intermolecular forces and the one in which they are covalently bonded, in a molecular system with identical constituent elements and masses, as well as a nearly identical structure and density. This comparison leading to an essential understanding of thermal conduction in organic materials is made possible by the unique compound found by Wudl et al., which exhibits a single-crystal-to-single-crystal topochemical polymerization with a yield of >99%, in combination with microtemperature wave analysis (μTWA), which allows accurate measurements of the thermal diffusivity of small single crystals. At room temperature, the thermal conductivity of monomer and polymer single crystals is not significantly different. For both crystals, the thermal conductivity increases monotonically with decreasing temperature. However, below the Debye temperature, the thermal conductivity of the polymer single crystal increases exponentially, giving much larger values than those of the monomer single crystal. Based on physical quantities related to the behavior of phonons, derived from the specific heat analysis, we discuss the differences in heat transport properties in the two states and provide guidelines for achieving high thermal conductivity in organic materials.