# Topochemical Reaction Induces Anisotropy, Decreasing Solid-State Thermal Conductivity

## TL;DR

This paper shows that a light-induced chemical reaction in a material significantly lowers its ability to conduct heat, especially in a specific direction.

## Contribution

The study introduces a new method to dynamically reduce thermal conductivity in organic materials using topochemical reactions.

## Key findings

- Photoinduced polymerization of BIT into polyBIT decreases thermal conductivity by over 4-fold in the through-plane direction.
- Theoretical calculations link the reduced thermal conductivity to induced anisotropy in the polymer structure.
- Changes in morphology and phase transitions limit thermal depolymerization in the material.

## Abstract

Realizing organic
materials that exhibit a dynamic thermal conductivity
requires a fundamental understanding of how molecular structure and
processing affect thermal transport. Herein, we demonstrate that the
photoinduced polymerization of [2,2′-bi-1H-indene]-1,1′-dione-3,3′-diheptylcarboxylate (BIT)
into polyBIT results in over a 4-fold decrease in thermal conductivity
as measured on polycrystalline thin-films in the through-plane direction,
mostly perpendicular to the chain growth direction. Experimental determination
of the material’s decreased heat capacity supports this view.
Through theoretical calculations, we attribute this decrease in thermal
conductivity in part to induced anisotropy in the polymer. We also
discuss the non-negligible changes in morphology, phase transitions,
and thermal degradation that serve to limit the thermal depolymerization
reaction. This work highlights the different contributions one must
consider when designing an organic thermal switch that operates in
the solid-state.