Eskilsson K, Tiberg F
Physical Chemistry 1, Chemical Centre, Lund University, Box 124, S-221 00 Lund, Sweden, and Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden.
Macromolecules. 1998 Jul 28;31(15):5075-83. doi: 10.1021/ma9715637.
We report on the adsorption of a series of poly(ethylene oxide)-polytetrahydrofuran-poly(ethylene oxide) copolymers, EOn/2THFmEOn/2, at hydrophilic silica surfaces and relate our findings to the corresponding behavior at hydrophobic surfaces. The adsorption of these copolymers is similar to that of poly(ethylene oxide) homopolymers at low bulk concentrations. However, the copolymer adsorption increases strongly above a certain threshold concentration. This increase, which begins more than 1 order of magnitude below the critical micellar concentration (cmc), is related to the concomitant formation of micellar-like structures at the hydrophilic surfaces. We show in this work that a commercial (ethylene oxide-propylene oxide-ethylene oxide) triblock copolymer, Pluronic F127, exhibits a similar behavior at silica. Due to surface aggregation, much thicker layers are measured on silica than at the hydrophobic surface, where the adsorption results in the formation of a monolayer structure. The adsorbed amount and layer thickness measured on bare silica tend to decrease when the bulk concentration is raised above the cmc. We infer that this is due to changes of the molecular weight distribution and relative block sizes of the copolymers in the surface aggregates, i.e., a polydispersity effect. This study also covers some aspects of the adsorption and desorption kinetics exhibited by the copolymers at silica. As is common for adsorbing polymers, the concentration dependent adsorption process is generally observed to be much faster than the desorption process. The adsorption process is in parts diffusion controlled but overall to a complex to be fully analyzed. During adsorption from solutions with bulk concentrations exceeding the cmc, a clear overshoot of the surface excess is observed after intermediate adsorption times. Again, this is interpreted as being due to polydispersity. Finally, after an initial rapid desorption regime, the surface excess exhibits a logarithmic decay with time during desorption.
我们报道了一系列聚（环氧乙烷）-聚四氢呋喃-聚（环氧乙烷）共聚物EOn/2THFmEOn/2在亲水性二氧化硅表面的吸附情况，并将我们的研究结果与它们在疏水性表面的相应行为进行关联。在低本体浓度下，这些共聚物的吸附情况与聚（环氧乙烷）均聚物相似。然而，在超过某个阈值浓度后，共聚物的吸附量会大幅增加。这种增加在比临界胶束浓度（cmc）低一个多数量级时就开始了，它与在亲水性表面同时形成的类似胶束的结构有关。我们在这项工作中表明，一种商业用的（环氧乙烷-环氧丙烷-环氧乙烷）三嵌段共聚物Pluronic F127在二氧化硅上表现出类似的行为。由于表面聚集，在二氧化硅上测得的吸附层比在疏水性表面上厚得多，在疏水性表面吸附会形成单层结构。当本体浓度升高到cmc以上时，在裸二氧化硅上测得的吸附量和层厚度往往会降低。我们推断这是由于表面聚集体中共聚物的分子量分布和相对嵌段尺寸发生了变化，即多分散效应。这项研究还涵盖了共聚物在二氧化硅上的吸附和解吸动力学的一些方面。正如吸附聚合物常见的情况那样，通常观察到浓度依赖性吸附过程比解吸过程快得多。吸附过程部分受扩散控制，但总体过于复杂难以完全分析。在从本体浓度超过cmc的溶液中吸附时，在中间吸附时间后会观察到表面过量明显超调。同样，这被解释为是由于多分散性。最后，在初始的快速解吸阶段之后，表面过量在解吸过程中随时间呈对数衰减。
