Kantardjiev Alexander, Ivanov Petko M
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, ul. Acad. G. Bonchev, bloc 9, 1113 Sofia, Bulgaria.
Entropy (Basel). 2020 Oct 21;22(10):1187. doi: 10.3390/e22101187.
We attempted to attain atomic-scale insights into the mechanism of the heat-induced phase transition of two thermoresponsive polymers containing amide groups, poly(-isopropylacrylamide) (PNIPAM) and poly(2-isopropyl-2-oxazoline) (PIPOZ), and we succeeded in reproducing the existence of lower critical solution temperature (LCST). The simulation data are in accord with experimental findings. We found out that the entropy has an important contribution to the thermodynamics of the phase separation transition. Moreover, after decomposing further the entropy change to contributions from the solutes and from the solvent, it appeared out that the entropy of the solvent has the decisive share for the lowering of the free energy of the system when increasing the temperature above the LCST. Our conclusion is that the thermoresponsive behavior is driven by the entropy of the solvent. The water molecules structured around the functional groups of the polymer that are exposed to contact with the solvent in the extended conformation lower the enthalpy of the system, but at certain temperature the extended conformation of the polymer collapses as a result of dominating entropy gain from "released" water molecules. We stress also on the importance of using more than one reference molecule in the simulation box at the setup of the simulation.
我们试图深入了解含酰胺基团的两种热响应性聚合物聚（N-异丙基丙烯酰胺）（PNIPAM）和聚（2-异丙基-2-恶唑啉）（PIPOZ）热诱导相变的机制，并成功再现了下临界溶液温度（LCST）的存在。模拟数据与实验结果一致。我们发现熵对相分离转变的热力学有重要贡献。此外，在将熵变进一步分解为溶质和溶剂的贡献后，结果表明，当温度升高到LCST以上时，溶剂的熵对系统自由能的降低起决定性作用。我们的结论是，热响应行为是由溶剂的熵驱动的。聚合物功能基团周围以伸展构象与溶剂接触的水分子降低了系统的焓，但在一定温度下，聚合物的伸展构象由于“释放”水分子的熵增占主导而坍塌。我们还强调了在模拟设置时在模拟盒中使用不止一种参考分子的重要性。
