Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles in solution
Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles in solution
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DOI：
10.1021/ja0653019
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发表时间：
2007-02-07
影响因子：
15
通讯作者：
Cheng, Stephen Z. D.
中科院分区：
文献类型：
--
作者：
Bhargava, Prachur;Tu, Yingfeng;Cheng, Stephen Z. D.
温度诱导的可逆形态变化的聚苯乙烯-嵌段-聚（环氧乙烷）胶束的聚合度为962的PS和227的PEO块（PS 962-b-PEO 227）在N，N-二甲基甲酰胺（DMF）/水，其中水是一个选择性溶剂的PEO块，观察。对于在DMF/水中具有0.2wt%共聚物浓度和4.5wt%水浓度的系统，在透射电子显微镜中观察到的胶束形态随着温度的升高从室温下的囊泡变为蠕虫状圆柱体，然后变为球体。在中温区也形成了混合形貌。将系统冷却回室温再生囊泡形态，表明形态变化是可逆的。在加热和冷却过程中的形态变化没有观察到滞后。动态光散射结果表明，胶束的流体动力学半径随温度的升高而减小。结合静态和动态光散射结果支持形态随温度的变化。临界胶束化温度和临界形态转变温度由浊度测量确定，并被发现是依赖于共聚物和水的浓度在DMF/水系统。只有当DMF/水体系中的水浓度较低时，才可能发生形态变化，否则PS嵌段的流动性将受到严重限制。这些形态变化的驱动力被理解为主要是在电晕的自由能的减少和界面的自由能的轻微减少。等温实验不同时间段的形态观察表明，在从一种平衡形态到另一种平衡形态的途径中，大的化合物胶束形成为中间或亚稳态阶段。
Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles with degrees of polymerization of 962 for the PS and 227 for the PEO blocks (PS962-b-PEO227) in N,N-dimethylformamide (DMF)/water, in which water is a selective solvent for the PEO block, were observed. For a system with 0.2 wt % copolymer concentration and 4.5 wt % water concentration in DMF/water, the micelle morphology observed in transmission electron microscopy changed from vesicles at room temperature to worm-like cylinders and then to spheres with increasing temperature. Mixed morphologies were also formed in the intermediate temperature regions. Cooling the system back to room temperature regenerated the vesicle morphology, indicating that the morphological changes were reversible. No hysteresis was observed in the morphological changes during heating and cooling. Dynamic light scattering revealed that the hydrodynamic radius of the micelles decreased with increasing temperature. Combined static and dynamic light scattering results supported the change in morphology with temperature. The critical micellization temperatures and critical morphological transition temperatures were determined by turbidity measurements and were found to be dependent on the copolymer and water concentrations in the DMF/water system. The morphological changes were only possible if the water concentration in the DMF/water system was low, or else the mobility of the PS blocks would be severely restricted. The driving force for these morphological changes was understood to be mainly a reduction in the free energy of the corona and a minor reduction in the free energy of the interface. Morphological observations at different time periods of isothermal experiments indicated that in the pathway from one equilibrium morphology to another, large compound micelles formed as an intermediate or metastable stage.