Smart polymer solution and thermal conductivity: How important is an exact polymer conformation?

TL;DR

This study investigates how the thermal conductivity switching in smart polymers depends more on solvent interactions than on precise polymer conformations, using generic simulations of collapse mechanisms.

Contribution

It demonstrates that solvent-monomer interactions are more critical than exact polymer conformation in thermal switching behavior, challenging the assumption that conformation alone governs thermal conductivity changes.

Findings

Collapse via temperature correlates with thermal switching.

Collapse via cosolvent mole fraction shows no correlation with thermal switching.

Solvent-monomer interactions are more influential than conformation in thermal conductivity changes.

Abstract

Heat management in devices is a key to their efficiency and longevity. Here, thermal switches (TS) are of great importance because of their ability to transition between different thermal conductivity $\kappa$ states. While traditional TS are bulky and slow, recent experiments have suggested "smart" responsive (bio--inspired) polymers as their fast alternatives. One example is poly(N--isopropylacrylamide) (PNIPAM) in water, where $\kappa$ drops suddenly around a temperature $T_{\ell} \simeq 305$ K when a PNIPAM undergoes a coil--to--globule transition. At a first glance, this may suggest that the change in polymer conformation has a direct influence on TS. However, it may be presumptuous to trivially "only" link conformations with TS, especially because many complex microscopic details control macroscopic conformational transition. Motivated by this, we study TS in "smart" polymers using generic simulations. As the test cases, we investigate two different modes of polymer collapse using external stimuli, i.e., changing $T$ and cosolvent mole fraction $x_{\rm c}$. Collapse upon increasing $T$ shows a direct correlation between the conformation and $\kappa$ switching, while no correlation is observed in the latter case. These results suggest that the (co--)solvent--monomer interactions play a greater important role than the exact conformation in dictating TS. While some results are compared with the available experiments, possible future directions are also highlighted.