Heterogeneous drying of colloidal polymer films: Dependence on added salt
Heterogeneous drying of colloidal polymer films: Dependence on added salt
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DOI：
10.1021/la800525n
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发表时间：
2008-07-15
期刊：
影响因子：
3.9
通讯作者：
Johannsmann, Diethelm
中科院分区：
文献类型：
--
作者：
Koenig, Alexander M.;Weerakkody, Tecla G.;Johannsmann, Diethelm
利用磁共振剖面和动态光散射，我们研究了水性聚合物分散体在成膜过程中导致部分聚结表面层或“开放皮肤”形成的机制。我们首次使用自由水分布的偏度作为干燥空间不均匀性的无模型指标。在光散射数据中出现强烈的静态分量时，偏度达到最大值，该分量可能来自膜/空气界面处的皮肤。在分散液中加入盐会增加自由水分布的偏度和形成皮肤的倾向。令人惊讶的是，干燥不仅受到盐中离子的浓度和价的影响，而且受到特定离子的影响。在中等颗粒密度下，盐的加入显著降低了协同扩散系数D-coop。当颗粒紧密堆积时，D-coop急剧增大。如果颗粒很容易结合，则增加的扩散率的影响将被抵消，从而诱导皮肤的形成。利用D-coop提出了一个改进的Peclet数Pe，从而明确地考虑了盐的存在。这种改良的聚乙烯能够预测干燥过程中导致皮肤形成的不均匀性。
Using magnetic resonance profiling coupled with dynamic light scattering, we have investigated the mechanisms leading to the formation of a partly coalesced surface layer, or "open skin", during film formation from waterborne polymer dispersions. We present the first use of the skewness of the distribution of free water as a model-free indicator of the spatial nonuniformity of drying. The skewness reaches a maximum at the same time at which a strong, static component, presumably originating from a skin at the film/air interface, appears in the light scattering data. Addition of salt to the dispersion increases both the skewness of the distribution of free water and the propensity for skin formation. Surprisingly, the drying is influenced not only by the concentration and valency of the ions in the salt but also by the particular ion. At intermediate particle densities, added salt strongly lowers the cooperative diffusion coefficient, D-coop. When the particles reach close packing, D-coop sharply increases. If the particles readily coalesce, the effects of the increased diffusivity will be counteracted, thereby inducing the formation of a skin. A modified Peclet number, Pe, using D-coop, is proposed, so that the presence of salt is explicitly considered. This modified Pe is able to predict the nonuniformity in drying that leads to skin formation.