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Structural and thermodynamic aspects of the cylinder-to-sphere transition in amphiphilic diblock copolymer micelles

机译:两亲性二嵌段共聚物胶束中圆柱到球形转变的结构和热力学方面

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摘要

The structure of diblock copolymers micelles depends on a delicate balance of thermodynamic forces driving the system towards equilibrium and kinetic factors which limit the systems' exploration of the phase space. The factors governing the morphological transition between cylindrical and spherical micelles are related to a fine balance between entropic forces from chains within the micellar core and corona. In order to understand and control these structures, it is important to gain insight into the fundamental thermodynamic driving forces governing the structure and answer fundamental questions concerning its equilibrium nature. In this work we aim to understand the relationship between thermodynamics and morphological transitions by investigating the detailed structure of a system undergoing a cylinder-to-sphere transition. We focus on the structural properties of micelles constituted of poly(ethylene-alt-propylene)–poly(ethylene oxide) (PEP1–PEO1, the numbers indicate the molar mass in kg/mole) diblock copolymers in dimethylformamide (DMF)/water solvent mixtures. This system is ideal for fundamental studies as it represents a classical well-segregated block copolymer micelle system where the interfacial tension can be controlled in detail without significantly changing other thermodynamic properties. Using small-angle neutron scattering (SANS) it is shown that the system undergoes a cylinder-to-sphere transition upon addition of DMF which lowers the interfacial tension. By applying a detailed thermodynamic model we show that both the dependence of the structural parameters with the interfacial tension as well as the morphological transition can be quantitatively understood. The transition itself is governed by the interfacial tension which dictates the stretching of chains within both corona and core. At high interfacial tensions (in water-rich solutions) discrepancies between structural data and predictions from the thermodynamic model are observed. A qualitative comparison with some preliminary results on the chain exchange kinetics in the system show that these deviations coincide with the region where this equilibration mechanism is not active, i.e. when the kinetics are frozen at high interfacial tensions.
机译:二嵌段共聚物胶束的结构取决于驱动系统趋向平衡和动力学因素的热力学力的微妙平衡,这限制了系统对相空间的探索。控制圆柱和球形胶束之间形态转变的因素与胶束核心和日冕中来自链的熵力之间的良好平衡有关。为了理解和控制这些结构,重要的是深入了解控制该结构的基本热力学驱动力,并回答有关其平衡性质的基本问题。在这项工作中,我们旨在通过研究经历圆柱到球面转变的系统的详细结构来理解热力学和形态转变之间的关系。我们专注于由聚(乙烯-丙烯-丙烯-丙烯)-聚(环氧乙烷)(PEP1-PEO1)组成的胶束的结构性质,数字表示在二甲基甲酰胺(DMF)/水溶剂中的二嵌段共聚物的摩尔质量(千克/摩尔)混合物。该系统非常适合基础研究,因为它代表了经典的,良好隔离的嵌段共聚物胶束系统,在此系统中界面张力可以得到详细控制,而不会显着改变其他热力学性质。使用小角度中子散射(SANS)表明,该系统在添加DMF时会经历圆柱到球形的过渡,从而降低了界面张力。通过应用详细的热力学模型,我们表明结构参数与界面张力的依赖关系以及形态转变都可以被定量地理解。过渡本身受界面张力控制,界面张力决定了电晕和核内链的拉伸。在高界面张力下(在富水溶液中),观察到结构数据与热力学模型的预测之间存在差异。与系统中链交换动力学的一些初步结果的定性比较表明,这些偏差与该平衡机制不起作用的区域(即动力学在高界面张力下冻结)一致。

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