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.
Rivera Lab at UPR 
Rating (synopsis): 4/5
Rating (figure): 5/5
I’m not sure if PNIPAM is the most studied system but we sure have focused in it this whole semester. I may not be from the conductivity area, so I do have to ask, what makes 32 °C workable? As his narrative goes, I got a little bit lost on the explanation of the solutions of experiments, and the concentrations (concentrations of PNIPAM?). Of course, by reading the paper all that is cleared out. But when writing summaries, we have to make sure to pay attention in the detail. One of “the curse of knowledge” is thinking that everyone already knows what you are talking about. Also “The thermal conductivity of the PNIPAM aqueous solutions increases with temperature, the same as with water, until reaching the LCST.”; isn’t it already in water? Or is he comparing PNIPAM with water? Maybe some different wording might have helped there. I have to say it is a very good summary telling the main rational without loosing the reader. I just identify certain things because I do have the same issues when writing. As his image goes, it did convey the general idea.
Rating (synopsis): 5/5
Rating (figure): 4/5
Rafael summarized the paper very well. The paper and supporting information show a couple mathematical expressions and the experimental setup of the laser which might seem complicated. Even though, he was able to focus on the important parts and findings of the paper which makes it easier for the reader to understand the synopsis. I enjoyed a lot the transient thermal grating technique used to measure the thermal conductivity of the polymer. The experimental technique reminded me a lot of the research I did in Rutgers, because they also worked with optical tables to set up the laser to study the grating patters of the polymer sample.
The figure presents the idea of the paper. The two pump laser beams which generate the gratings to study the thermal conductivity is shown in the figure. Also, the relation with the LCST temperatures of the polymer in solution presented in the TOC makes it easy to understand the paper.
Rating (Synopsis): 4/5
Rating (Figure): 4/5
In his synopsis, Rafael was able to explain the article’s authors’ research and summarize the article’s main results in a way that is very easy to comprehend. However, I wish he had included more of his opinion on the article (in a few sentences) since I’m curious to see how Rafael correlates this article’s findings to his research in the lab. Nevertheless, I think he did a great job with his synopsis. I like the figure constructed by Rafael, it is attractive and gets the point across yet is a little crowded. Also, shouldn’t the arrow for delta(Thermal Conductivity) with LCST transition be a reversible-process arrow instead of the resonance-structure arrow that was used?