低醇浓度下聚N-异丙基丙烯酰胺在水/乙醇混合溶液中的线团-小球转变的分子描述
Molecular Description of the Coil-to-Globule Transition of Poly(N-isopropylacrylamide) in Water/Ethanol Mixture at Low Alcohol Concentration
摘要：
聚（N-异丙基丙烯酰胺），PNIPAM，是一种广泛研究的聚合物，它是具有响应特性的纳米结构软材料的关键组成部分。在升高温度时，PNIPAM聚合物链在~ 305 K下经历可逆的螺旋到球转变，这通过交联结构（例如微凝胶）中的体积相变来反映，这对于许多实际应用是有价值的。添加共溶剂是根据特定目的调节转变温度的简单方法。在这项研究中，我们使用原子分子动力学模拟探索的溶液行为的PNIPAM链在水和乙醇的混合物，作为共溶剂，在低醇浓度。我们的模拟再现了发生的线圈到球的聚合物链在289 K，温度低于在水中测得的，与实验结果完全一致。通过监测PNIPAM-水-乙醇三元体系的结构和动力学性质的温度演化，我们检测到乙醇分子在聚合物界面处的定位，主要是由于异丙基和乙基之间的相互作用。我们观察到，发生的过渡没有释放吸附的乙醇分子，但从周围的PNIPAM疏水部分，有利于乙醇分子的聚集接近聚合物的水分子的损失。我们的研究结果支持了这样的想法，即在混合物的体积和水和乙醇分子之间的竞争在与聚合物的相互作用中的水的化学势降低发挥了驱动作用的过渡。
英文摘要：
Poly(N-isopropylacrylamide), PNIPAM, is a widely studied polymer, which serves as a key constituent of nanostructured soft materials with responsive properties. Upon increasing temperature the PNIPAM polymer chain undergoes a reversible coil-to-globule transition at ~305K, which is reflected by a volume phase transition in cross-linked architectures, such as microgels, valuable for many practical applications. The addition of a cosolvent is a simple method to tune the transition temperature according to the specific purpose. In this study, we use atomistic molecular dynamics simulations to explore the solution behavior of a PNIPAM chain in a mixture of water and ethanol, acting as cosolvent, at low alcohol concentration. Our simulations reproduce the occurrence of the coil-to-globule transition of the polymer chain at 289 K, a temperature lower than that measured in water, in full agreement with experimental findings. By monitoring the temperature evolution of structural and dynamical properties of the PNIPAM-water-ethanol ternary system, we detect a localization of ethanol molecules at the polymer interface, mainly due to interactions between isopropyl and ethyl groups. We observe that the transition occurs without a release of adsorbed ethanol molecules, but with a loss of water molecules from the surrounding of PNIPAM hydrophobic moieties that favours the aggregation of ethanol molecules close to the polymer. Our results support the idea that both the decreased chemical potential of water in the bulk of the mixture and the competition between water and ethanol molecules in the interactions with the polymer play a driving role in the transition.
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