Thermal Conductivity Switch due to Topochemical Polymerization of Organic Material

A material that exhibits significant change in thermal conductivity due to external stimuli can be used as a thermal switch and can allow the active control of heat flow. These materials are of high importance in various thermal management applications such as refrigeration and waste heat recovery. Due to their sensitivity to external stimuli, organic materials are considered as a promising class of materials in this regard. This study shows a potential thermal switching mechanism due to topochemical polymerization of [2,2′-bi-1H-indene]-1,1′-dione-3,3′-diheptylcarboxylate (BIT-Hep₂). The forward polymerization reaction of BIT-Hep₂monomer to polymer, P(BIT-Hep₂), occurs when exposed to light. Since polymerization alters the interchain interactions, breaking and reforming of bonds between repeat units can affect the thermal conductivity, . This study examines how changes in bond order affects the microscopic thermal conductivity. The thermal conductivities of these samples are measured using time domain thermoreflectance (TDTR). TDTR is a transient, non-contact, optical thermometry technique that utilizes a pump-probe experimental configuration. In addition, steady-state thermoreflectance (SSTR) technique was used to compose a map of thermal conductivity as a function of spatial coordinate for individual monomer and polymer crystallites. The complementary characterizations of these samples confirms that the topochemical polymerization of BIT-Hep₂to P(BIT-Hep₂) changes the bond order which affects the planarity of the repeat unit. Thus, the decrease in thermal conductivity due to this topochemical polymerization is likely due to a greater extent of disorder present in P(BIT-Hep₂). This study finds that the thermal conductivity within the crystallites differs significantly and reports the average values of 0.478±0.065 W/m/K and 0.110 ±0.060 W/m/K for the monomer and polymer, respectively.