Bharadwaj Swaminath, Nayar Divya, Dalgicdir Cahit, van der Vegt Nico F A
Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, 64287 Darmstadt, Germany.
J Chem Phys. 2021 Apr 7;154(13):134903. doi: 10.1063/5.0046746.
Cosolvent effects on the coil-globule transitions in aqueous polymer solutions are not well understood, especially in the case of amphiphilic cosolvents that preferentially adsorb on the polymer and lead to both polymer swelling and collapse. Although a predominant focus in the literature has been placed on the role of polymer-cosolvent attractive interactions, our recent work has shown that excluded-volume interactions (repulsive interactions) can drive both preferential adsorption of the cosolvent and polymer collapse via a surfactant-like mechanism. Here, we further study the role of polymer-(co)solvent attractive interactions in two kinds of polymer solutions, namely, good solvent (water)-good cosolvent (alcohol) (GSGC) and poor solvent-good cosolvent (PSGC) solutions, both of which exhibit preferential adsorption of the cosolvent and a non-monotonic change in the polymer radius of gyration with the addition of the cosolvent. Interestingly, at low concentrations, the polymer-(co)solvent energetic interactions oppose polymer collapse in the GSGC solutions and contrarily support polymer collapse in the PSGC solutions, indicating the importance of the underlying polymer chemistry. Even though the alcohol molecules are preferentially adsorbed on the polymer, the trends of the energetic interactions at low cosolvent concentrations are dominated by the polymer-water energetic interactions in both the cases. Therefore, polymer-(co)solvent energetic interactions can either reinforce or compensate the surfactant-like mechanism, and it is this interplay that drives coil-to-globule transitions in polymer solutions. These results have implications for rationalizing the cononsolvency transitions in real systems such as polyacrylamides in aqueous alcohol solutions where the understanding of microscopic driving forces is still debatable.
助溶剂对聚合物水溶液中线圈-球状体转变的影响尚未得到很好的理解，特别是在两亲性助溶剂的情况下，它们优先吸附在聚合物上，导致聚合物膨胀和塌陷。尽管文献中的主要关注点一直是聚合物-助溶剂吸引相互作用的作用，但我们最近的研究表明，排除体积相互作用（排斥相互作用）可以通过类似表面活性剂的机制驱动助溶剂的优先吸附和聚合物塌陷。在这里，我们进一步研究聚合物-（共）溶剂吸引相互作用在两种聚合物溶液中的作用，即良溶剂（水）-良助溶剂（醇）（GSGC）和不良溶剂-良助溶剂（PSGC）溶液，这两种溶液都表现出助溶剂的优先吸附以及随着助溶剂的加入聚合物回转半径的非单调变化。有趣的是，在低浓度下，聚合物-（共）溶剂能量相互作用在GSGC溶液中阻碍聚合物塌陷，而在PSGC溶液中则相反地支持聚合物塌陷，这表明基础聚合物化学的重要性。尽管醇分子优先吸附在聚合物上，但在两种情况下，低助溶剂浓度下能量相互作用的趋势都由聚合物-水能量相互作用主导。因此，聚合物-（共）溶剂能量相互作用可以增强或补偿类似表面活性剂的机制，正是这种相互作用驱动了聚合物溶液中的线圈-球状体转变。这些结果对于合理化实际系统中的共溶剂化转变具有启示意义，例如在醇水溶液中的聚丙烯酰胺，其中对微观驱动力的理解仍存在争议。
