Adsorbed and spread layers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers at the air-water interface studied by sum-frequency vibrational spectroscopy and tensiometry
Adsorbed and spread layers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers at the air-water interface studied by sum-frequency vibrational spectroscopy and tensiometry
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DOI：
10.1021/ma061161b
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发表时间：
2006-12-26
期刊：
影响因子：
5.5
通讯作者：
Hakkel, Orsolya
中科院分区：
文献类型：
--
作者：
Kiss, Eva;Keszthelyi, Tamas;Hakkel, Orsolya
采用张力测定法和和频振动谱法研究了三种不同组成的水溶性两亲性PEO-PPO-PEO三嵌段共聚物（Pluronic 6100、6400和6800）的表面性质。我们比较了三种不同方法制备的表面层结构的浓度依赖性：(i)从水溶液中吸附，（ii）从有机溶液中滴状扩散到恒定面积的表面，（iii）在Langmuir槽中压缩扩散膜。聚合物的表面密度和构象由中心疏水性PPO嵌段甲基的振动带强度和表面张力等温线推断。通过张力测量法观察到，具有较短PEO块的Pluronics吸附和压缩的Langmuir膜具有与构象变化相关的转变范围，而具有最长PEO块的Pluronic膜表面压力呈现渐变变化，没有相变迹象。这种差异揭示了亲水和疏水嵌段的相对长度对嵌段共聚物表层结构变化的影响。在和频实验中还观察到亲水性块对界面处整个分子结构的影响。在一定浓度以上，具有最长PEO链的Pluronic化合物表现出较低的和频强度和甲基对称/不对称振幅比。液滴扩散层和压缩朗缪尔层表面性质的差异揭示了聚合物单层形成动力学方面的重要性。在展开膜的情况下，可能存在的较大程度的非平衡链缠结可能会阻碍聚合物链的优选取向和构象的实现。
The surface properties of three water-soluble and amphiphilic PEO-PPO-PEO triblock copolymers of different composition (Pluronic 6100, 6400, and 6800) are investigated by tensiometry and sum-frequency vibrational spectroscopy. We compared the concentration dependence of the structure of surface layers prepared by three different methods: (i) adsorption from aqueous solution, (ii) dropwise spreading from an organic solution onto a surface of constant area, and (iii) compression of the spread film in a Langmuir trough. The surface density and conformation of the polymers were deduced from the vibrational band intensities of the methyl groups of the central hydrophobic PPO block and from the surface tension isotherms. A transition range related to a conformational change was observed by tensiometry for the adsorbed and the compressed Langmuir films of Pluronics with short PEO blocks, whereas the Pluronic with longest PEO blocks displays a gradual change of surface pressure without the sign of a phase transition. This difference reveals the effect of the relative lengths of the hydrophilic and hydrophobic blocks on the structural changes in the surface layers of block copolymers. A clear indication of the influence of the hydrophilic blocks on the structure of the whole molecule at the interface was also observed in the sum-frequency experiments. Above a given concentration the Pluronic with longest PEO chains exhibited lower sum-frequency intensities and methyl symmetric/asymmetric amplitude ratios than the other Pluronic compounds for all of the layers formed by the various methods. The differences in the surface properties of the dropwise spread layers and of the compressed Langmuir layers exposed the importance of the kinetic aspects of polymer monolayer formation. In the case of the spread films the possible large degree of nonequilibrium chain entanglements might hinder the accomplishment of preferred orientation and conformation of the polymer chains.