Microstructure evolution in dry cast cellulose acetate membranes by cryo-SEM
Microstructure evolution in dry cast cellulose acetate membranes by cryo-SEM
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通过冷冻扫描电镜观察干铸醋酸纤维素膜的微观结构演变
DOI：
10.1016/j.memsci.2006.07.001
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发表时间：
2006
影响因子：
9.5
通讯作者：
L. Scriven
中科院分区：
文献类型：
--
作者：
S. Prakash;L. Francis;L. Scriven
用“时间切片”低温扫描电子显微镜(Cryo-SEM)观察了水/丙酮/醋酸酯纤维素膜在干燥诱导相转化或干法浇注过程中微观结构的变化，发现涂层在干燥过程中形成了非对称分离膜。涂层样品的制备通过以下步骤进行：将聚合物溶液均匀涂覆或浇注到基材上，在环境控制下干燥特定时间，在液体冷冻剂中快速冷冻样品，破裂以显示涂层横截面，短暂升华以进行形貌对比，溅射涂层以防止荷电和低温扫描电子显微镜成像。标本是用不同的干燥时间制作的，因此每个标本被称为“时间段”。最早的时间切片，来自沉积后不久的涂层，显示的是一个没有特征的样品，正如对于均匀的聚合物溶液所预期的那样。随着干燥的进行，时间切片首先揭示了分散在富含聚合物的基质中的贫聚液滴的成核过程，整个区域由上面的自由表面和下面的相分离前沿限制。在进一步干燥时，这一前沿向下移动到衬底；稀疏的聚合物液滴生长并聚合，形成平稳的互联相，随着干燥过程的进行，最终成为蜂窝状结构的孔隙空间。与此同时，在自由表面，干燥时形成了看似致密的皮肤，而在较薄的皮肤和较厚的蜂窝状亚结构之间出现了结节状的中间层。用理论模拟得到的组成路径对图像进行分析，以阐明不对称膜中微结构发展的基本原理。
The development of microstructure during drying-induced phase inversion or dry casting of homogeneous water/acetone/cellulose acetate coatings, which evolve into asymmetric separation membranes, was witnessed using ‘time-sectioning’ cryogenic scanning electron microscopy (cryo-SEM). Coating specimens were prepared via the following sequential steps: uniformly coating or casting the polymer solution onto a substrate, drying with environmental control for a specific time, rapidly freezing the specimen in liquid cryogen, fracturing to reveal the coating cross-section, subliming briefly for topographical contrast, sputter-coating to prevent charging and cryo-SEM imaging. Specimens were created with different drying times and hence each specimen is called a ‘time-section’. The earliest time-section, one from a coating soon after deposition, shows a featureless specimen, as expected for a homogeneous polymer solution. As drying proceeds, time-sectioning reveals first the nucleation of polymer-lean droplets dispersed within a polymer-rich matrix across a region bound by the free surface above and a phase separation front below. On further drying, this front travels down to the substrate; the polymer-lean droplets grow and coalesce, forming a smoothly interconnected phase, which eventually becomes the pore space of a honeycomb-like structure as drying progresses. Meanwhile at the free surface, a seemingly dense skin develops on drying, while a nodular intermediate layer appears between the thinner skin and the thicker honeycomb-like substructure. The images are analyzed with composition paths derived from theoretical modeling to elucidate the fundamentals of microstructure development in asymmetric membranes.