Pantelidou Maria S, García Daza Fabián A, Avalos Josep Bonet, Mackie Allan D
Departament d'Enginyeria Química, ETSEQ, Universitat Rovira i Virgili, Tarragona 43007, Spain.
Department of Chemical Engineering, The University of Manchester, Manchester M13 9PL, United Kingdom.
Macromolecules. 2022 Feb 8;55(3):914-927. doi: 10.1021/acs.macromol.1c02387. Epub 2022 Jan 24.
The correlation function for the exit of poloxamer copolymers from equilibrated micelles is found to show up to four regimes depending on the chain flexibility: an initial fast reorganization, a logarithmic intermediate regime, followed by an exponential intermediate regime, and a final exponential decay. The logarithmic intermediate regime has been observed experimentally and attributed to the polydispersity of the polymer samples. However, we present dynamic single-chain mean-field theory simulations with chains of variable flexibility which show the same logarithmic relaxation but with strictly monodisperse systems. In agreement with our previous studies, we propose that this logarithmic response arises from a degeneracy of energy states of the hydrophobic block in the micelle core. For this to occur, a sufficiently large number of degenerate conformational states are required, which depend on the polymer flexibility and therefore should not be present for rigid polymers. Experimental results for monodisperse polymeric samples claiming the absence of such a logarithmic response may also lack a sufficient number of hydrophobic blocks for the required number of configurational states for this type of response to be seen. The insight gained from analyzing the simulation results allows us to propose a modified Eyring equation capable of reproducing the observed dynamic behavior. On scaling experimental results from different sources and systems according to this equation, we find a unique master curve showing a universal nature of the intermediate regimes: the logarithmic regime together with the secondary exponential decay. The terminal exponential regime at long times proposed by the standard Halperin and Alexander model is beyond the range of the data analyzed in this article. The universality observed suggests an entropic origin of the short-time dynamic response of this class of systems rather than the polydispersity.
泊洛沙姆共聚物从平衡胶束中析出的相关函数被发现，根据链的柔韧性可呈现多达四种状态：初始的快速重组、对数中间态、随后的指数中间态以及最后的指数衰减。对数中间态已通过实验观测到，并归因于聚合物样品的多分散性。然而，我们给出了具有可变柔韧性链的动态单链平均场理论模拟，结果显示了相同的对数弛豫，但体系是严格单分散的。与我们之前的研究一致，我们提出这种对数响应源于胶束核心中疏水嵌段能量状态的简并性。要出现这种情况，需要足够数量的简并构象状态，这取决于聚合物的柔韧性，因此刚性聚合物不应存在这种情况。声称不存在这种对数响应的单分散聚合物样品的实验结果，可能也缺乏足够数量的疏水嵌段来形成这种响应所需的构象状态数量。通过分析模拟结果获得的见解使我们能够提出一个修正的艾林方程，该方程能够重现观测到的动态行为。根据这个方程对来自不同来源和体系的实验结果进行标度时，我们发现了一条独特的主曲线，它显示了中间态的普遍性质：对数态以及二次指数衰减。标准的哈尔珀林和亚历山大模型提出的长时间的末端指数态超出了本文分析的数据范围。观测到的普遍性表明这类体系的短时间动态响应源于熵而不是多分散性。
