By Rafael A. Brito
Thermal switches are of great importance to thermal management in a wide variety of applications. A common characteristic associated with thermal switching is thermal conductivity. After noticing the change in thermal conductivity across LCST transitions, Zhiting Tian started researching polymers for this purpose. Poly(N-isopropylacrylamide) (PNIPAM) is the most studied thermoresponsive polymer and has a workable LCST of 32° C. The LCST transition of PNIPAM changes the chain-like formation of the polymer into an aggregation that shows a drastic decrease of thermal conductivity. This sharp change is due to LCST transitions being second-order, which are characterized by being almost instantaneous when the corresponding temperature is reached. Thermal conductivity was measured by applying a powerful approach: the transient thermal grating technique. It is used by heating a solution as a function of position creating a grating of temperature. This grating allows the use of the one-dimensional heat equation, which can be solved to give a relation between the thermal conductivity and temperature. The thermal conductivity can then be calculated using 𝑘 = 𝜌𝑐𝑝α, where 𝜌 is the density, is the specific heat capacity and α which is a function of temperature. After the setup was completed, solutions of varying concentrations were analyzed. For the solution with the highest concentration, the thermal switching ratio was measured to be 1.15 across the LCST transition. This shows a significant change between the two states of the polymer. The thermal conductivity of the PNIPAM aqueous solutions increases with temperature, the same as with water, until reaching the LCST. Then a drastic change is observed in the solution. The thermal switching ratio of PNIPAM aqueous solutions across the transition keeps increasing with increasing concentration, which is expected from the equation. To explain the thermal conductivity change due to the transition between the two modes of the polymer, the authors used the idea that the homogeneous phase of the solution separates into two phases that increases the thermal interface resistance resulting in a lower effective thermal conductivity.
As a summary, they reported the first direct measurement of thermal conductivity change in PNIPAM aqueous solutions across the LCST using a powerful approach, the laser-induced transient thermal grating technique. The results show an abrupt thermal conductivity drop across the transition temperature. The potential of using thermoresponsive polymer aqueous solutions of higher-order phase transitions for thermal switch applications has been demonstrated throughout this paper’s work.