Surface Structure of Nafion in Vapor and Liquid
Surface Structure of Nafion in Vapor and Liquid
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DOI：
10.1021/jp9113128
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发表时间：
2010-03-25
影响因子：
3.3
通讯作者：
Freger, Viatcheslav
中科院分区：
文献类型：
--
作者：
Bass, Maria;Berman, Amir;Freger, Viatcheslav
Nation的微观结构随水化作用的变化而变化。因此，它经历了从低水合下的具有水性核和融合的疏水壳的紧密堆积的反胶束束（“通心粉束”）到高水合下的正常类型（“意大利面条”）胶束的转变。最近有人假设，类似的“通心粉-意大利面条”过渡，即，当外部介质从蒸汽变为液态水时，在聚合物表面发生表面排列的通心粉破裂为随机取向的意大利面条，这可以解释Nation和类似的微相分离离聚物的一些令人困惑的特征。所得到的（非平衡）结构可能仍然局限于几纳米厚的表面区域。在这里，这张照片是证实使用掠入射小角X射线散射（GISAXS），接触角，和原子力显微镜（AFM）。增强对齐的束相邻的表面在蒸汽中，类似的双轴拉伸的效果，阐明了GISAXS的旋涂Nafion膜。这是从离聚物峰在X-Y（面内）和Z（面外）方向上的相对强度和位置随X射线穿透到膜中的不同深度的特征变化推断的。然而，接触角的测量表明，相对光滑和非常疏水的表面的Nafion在蒸汽转化为亲水表面，当蒸汽作为外部介质被替换为液态水。此外，AFM表明，在液体中的表面粗糙度显着增加。结果表明，Nafion和类似的微相分离材料的表面区域可能确实会受到剧烈的结构变化，即使固体基质的极慢松弛可能会阻止这种变化传播到体内。这些效应可能会对Nafion膜的宏观特性产生深远的影响，例如水合作用和电导率，以及它们在电化学和其他应用中作为离子选择性屏障的功能。
The microstructure of Nation varies in response to changes in hydration. Thus, it undergoes a transition from tightly packed bundles of inverted micelles with aqueous cores and fused hydrophobic shells ("macaroni bundles") at low hydrations to normal type ("spaghetti") micelles at high hydrations. It was postulated recently that a similar "macaroni-spaghetti" transition, i.e., breakup of surface-aligned macaroni to randomly oriented spaghetti, takes place at the polymer surface when the external medium is changed from vapor to liquid water, which can explain some puzzling features of Nation and similar microphase-separated ionomers. The resulting (nonequilibrium) structures may remain confined to a few nanometers thick surface region. Here, this picture is corroborated Using grazing-incidence small-angle X-ray scattering (GISAXS), contact angle, and atomic force microscopy (AFM). The enhanced alignment of bundles adjacent to the surface in vapor, similar to the effect of biaxial stretching, is elucidated by GISAXS of spin-cast Nafion films. It is inferred from the characteristic change in relative intensities and position of the ionomer peak in the X-Y (in-plane) and Z (out-of-plane) directions with varying X-ray penetration depths into (lie film. However, contact angle measurements show that the relatively smooth and very hydrophobic surface of Nafion in vapor transforms to a hydrophilic surface, when vapor as the external medium is replaced with liquid water. In addition, AFM indicates that the surface roughness significantly increases in liquid. The results demonstrate that the surface region of Nafion and similar microphase-separated materials may be indeed subject to drastic structural variations, even though the extremely slow relaxation of the solid matrix may preclude propagation of such changes into the bulk. These effects may have a profound effect on the macroscopic characteristics of Nafion membranes, such as hydration and conductivity, as well as their functioning as ion-selective barriers in electrochemical and other applications.