Matsumoto Mitsuhiro, Asoh Taka-Aki, Shoji Tatsuya, Tsuboi Yasuyuki
Division of Molecular Materials Science, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi, Osaka 558-8585, Japan.
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
Langmuir. 2021 Mar 9;37(9):2874-2883. doi: 10.1021/acs.langmuir.0c03009. Epub 2021 Feb 22.
We demonstrate liquid-liquid phase separation involving both coacervation and coil-to-globule phase transition of a thermoresponsive polymer. By focusing a near-infrared laser beam into an aqueous solution of poly(-isopropylacrylamide) (PNIPAM), a single phase-separated polymer microdroplet can be formed and stably trapped at the focal point. Such droplet formation is induced by a local elevation in temperature (induced by a photothermal effect) and an optical force. The technique allows us to selectively analyze a single polymer droplet trapped at the focal point. In this study, we applied this technique to poly(-diethylacrylamide) (PDEA) in water and generated a double-layered PDEA droplet. Such an inhomogeneous and complex microstructure has not been previously observed both in steady-state heating of a PDEA solution and in the PNIPAM system. Moreover, we used micro-Raman spectroscopy to clarify that PDEA underwent dehydration due to a coil-to-globule phase transition. Despite this, the polymer concentration (C) of the trapped PDEA droplet was very low and was around 30 wt %. C depended on the molecular weight of PDEA and the laser power that regulates the temperature elevation. These results strongly indicate that PDEA undergoes coacervation in addition to a coil-to-globule phase transition. This study will help provide us with a fundamental understanding of the phase separation mechanisms of thermoresponsive polymers.
我们展示了涉及热响应聚合物的凝聚和线圈到球状体相变的液-液相分离。通过将近红外激光束聚焦到聚（N-异丙基丙烯酰胺）（PNIPAM）的水溶液中，可以形成单个相分离的聚合物微滴并稳定地捕获在焦点处。这种微滴的形成是由局部温度升高（由光热效应引起）和光力诱导的。该技术使我们能够选择性地分析捕获在焦点处的单个聚合物微滴。在本研究中，我们将该技术应用于水中的聚（N，N-二乙基丙烯酰胺）（PDEA），并生成了双层PDEA微滴。这种不均匀且复杂的微观结构在PDEA溶液的稳态加热和PNIPAM系统中均未被观察到。此外，我们使用显微拉曼光谱来阐明PDEA由于线圈到球状体的相变而发生脱水。尽管如此，捕获的PDEA微滴的聚合物浓度（C）非常低，约为30 wt%。C取决于PDEA的分子量和调节温度升高的激光功率。这些结果有力地表明，PDEA除了发生线圈到球状体的相变外，还会发生凝聚。这项研究将有助于我们对热响应聚合物的相分离机制有基本的了解。
