Temperature-Induced Reversible Morphological Changes of Polystyrene-block-Poly(ethylene Oxide) Micelles in Solution

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• 作者：Prachur Bhargava ; Yingfeng Tu ; Joseph X. Zheng ; Huiming Xiong ; Roderic P. Quirk ; Stephen Z. D. Cheng
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：February 7, 2007
• 年：2007
• 卷：129
• 期：5
• 页码：1113 - 1121
• 全文大小：867K
• 年卷期：v.129,no.5(February 7, 2007)
• ISSN：1520-5126

文摘

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) inN,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, themicelle morphology observed in transmission electron microscopy changed from vesicles at roomtemperature to worm-like cylinders and then to spheres with increasing temperature. Mixed morphologieswere also formed in the intermediate temperature regions. Cooling the system back to room temperatureregenerated the vesicle morphology, indicating that the morphological changes were reversible. Nohysteresis was observed in the morphological changes during heating and cooling. Dynamic light scatteringrevealed that the hydrodynamic radius of the micelles decreased with increasing temperature. Combinedstatic and dynamic light scattering results supported the change in morphology with temperature. The criticalmicellization temperatures and critical morphological transition temperatures were determined by turbiditymeasurements 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/watersystem was low, or else the mobility of the PS blocks would be severely restricted. The driving force forthese morphological changes was understood to be mainly a reduction in the free energy of the coronaand a minor reduction in the free energy of the interface. Morphological observations at different time periodsof isothermal experiments indicated that in the pathway from one equilibrium morphology to another, largecompound micelles formed as an intermediate or metastable stage.