Stimuli-Responsive Behavior of PNiPAm Microgels under Interfacial Confinement
Stimuli-Responsive Behavior of PNiPAm Microgels under Interfacial Confinement
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DOI：
10.1021/acs.langmuir.9b01208
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发表时间：
2019-08-13
期刊：
影响因子：
3.9
通讯作者：
Vogel, Nicolas
中科院分区：
文献类型：
--
作者：
Harrer, Johannes;Rey, Marcel;Vogel, Nicolas
微凝胶的体积相变是刺激响应性的最典型的例子之一，能够在外部刺激下从高度膨胀的微凝胶状态崩溃到密集卷曲的状态。虽然在整体上很好地描述了相变的特征，但仍然不清楚限制界面的存在是如何影响相变的。在这里，我们证明了温度诱导的聚N-异丙基丙烯酰胺微凝胶的体积相变，通常被认为是聚合物的固有分子性质，当微凝胶被吸附到气/液界面时，实际上在很大程度上被抑制了。我们进一步观察到在加热和冷却循环之间的核心形态和界面压力的滞后。我们的结果得到了分子动力学模拟的支持，表明微凝胶粒子悬挂的高分子链在表面张力的影响下散布在界面上，不会发生任何体积相变。界面约束从根本上改变了导致体积相变的自由能平衡。这些结果表明，这类体系的重要技术性质，包括温度引起的乳液不稳定，并不是通过微凝胶界面覆盖率的减少而发生的。
The volume phase transition of microgels is one of the most paradigmatic examples of stimuli-responsiveness, enabling a collapse from a highly swollen microgel state into a densely coiled state by an external stimulus. Although well characterized in bulk, it remains unclear how the phase transition is affected by the presence of a confining interface. Here, we demonstrate that the temperature-induced volume phase transition of poly(N-isopropylacrylamide) microgels, conventionally considered an intrinsic molecular property of the polymer, is in fact largely suppressed when the microgel is adsorbed to an air/liquid interface. We further observe a hysteresis in the core morphology and interfacial pressure between heating and cooling cycles. Our results, supported by molecular dynamics simulations, reveal that the dangling polymer chains of microgel particles, spread at the interface under the influence of surface tension, do not undergo any volume phase transition. The balance in free energy responsible for the volume phase transition is fundamentally altered by interfacial confinement. These results imply that important technological properties of such systems, including the temperature-induced destabilization of emulsions, do not occur via a decrease in the interfacial coverage of the microgels.