Water Is a Poor Solvent for Densely Grafted Poly(ethylene oxide) Chains: A Conclusion Drawn from a Self-Consistent Field Theory-Based Analysis of Neutron Reflectivity and Surface Pressure鈥揂rea Isother

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• 作者：Hoyoung Lee ; Dae Hwan Kim ; Kevin N. Witte ; Kimberly Ohn ; Je Choi ; Bulent Akgun ; Sushil Satija ; You-Yeon Won
• 刊名：The Journal of Physical Chemistry B
• 出版年：2012
• 出版时间：June 21, 2012
• 年：2012
• 卷：116
• 期：24
• 页码：7367-7378
• 全文大小：495K
• 年卷期：v.116,no.24(June 21, 2012)
• ISSN：1520-5207

文摘

By use of a combined experimental and theoretical approach, a model poly(ethylene oxide) (PEO) brush system, prepared by spreading a poly(ethylene oxide)鈥損oly(n-butyl acrylate) (PEO鈥揚nBA) amphiphilic diblock copolymer onto an air鈥搘ater interface, was investigated. The polymer segment density profiles of the PEO brush in the direction normal to the air鈥搘ater interface under various grafting density conditions were determined by using the neutron reflectivity (NR) measurement technique. To achieve a theoretically sound analysis of the reflectivity data, we used a data analysis method that utilizes the self-consistent field (SCF) theoretical modeling as a tool for predicting expected reflectivity results for comparison with the experimental data. Using this data analysis technique, we discovered that the effective Flory鈥揌uggins interaction parameter of the PEO brush chains is significantly greater than that corresponding to the 胃 condition in Flory鈥揌uggins solutions (i.e., 蠂_{PEO鈥搘ater}(brush chains)/蠂_{PEO鈥搘ater}(胃 condition) 鈮?1.2), suggesting that contrary to what is more commonly observed for PEO in normal situations (蠂_{PEO鈥搘ater}(free chains)/蠂_{PEO鈥搘ater}(胃 condition) 鈮?0.92), the PEO chains are actually not 鈥渉ydrophilic鈥?when they exist as polymer brush chains, because of the many body interactions that are forced to be effective in the brush situation. This result is further supported by the fact that the surface pressures of the PEO brush calculated on the basis of the measured 蠂_{PEO鈥搘ater} value are in close agreement with the experimental surface pressure鈥揳rea isotherm data. The SCF theoretical analysis of the surface pressure behavior of the PEO brush also suggests that even though the grafted PEO chains experience a poor solvent environment, the PEO brush layer exhibits positive surface pressures, because the hydrophobicity of the PEO brush chains (which favors compression) is insufficient to overcome the opposing effect of the chain conformational entropy (which resists compression).