Time-sectioning cryogenic scanning electron microscopy (cryo-SEM) is a unique method of visualizing how the microstructure of liquid coatings evolves during processing. Time-sectioning means rapidly freezing (nearly) identical specimens at successively later stages of the process; doing this requires that coating and drying be well controlled in the dry phase inversion process, and solvents exchange likewise in the wet phase inversion process. With control, frozen specimens are fractured, etched by limited sublimation, sputter-coated, and imaged at temperatures of ca −175°C.; The coatings examined were of cellulose acetate, of high and low molecular weights, and polysulfone in mixed solvents and nonsolvents: acetone and water with cellulose acetate undergoing dry phase inversion; and tetrahydrofuran, dimethylacetamide, ethanol with polysulfone undergoing dry-wet phase inversion. All coatings, cast on silicon substrates, were initially homogeneous. The initial compositions of the high and low molecular weight cellulose acetate ternary solutions were “off-critical” and “near-critical”, respectively, connoting their proximities to the critical or plait point of the phase diagram. The initial composition of the polysulfone quaternary solution was located near the binodal of the pseudo-ternary phase diagram. It appeared that as the higher molecular weight cellulose acetate coating dries, it nucleates and grows polymer-poor droplets that coalesce into a bicontinuous structure underlying a thin, dense skin. Bicontinuity of structure was verified by stereomicroscopy of the dry sample. The lower molecular weight cellulose acetate coating phase-separates, seemingly spinodally, directly into a bicontinuous structure whose polymer-rich network, stressed by frustrated in-plane shrinkage, ruptures far beneath the skin in some locales to form macrovoids. When, after partial drying, the polysulfone coating was immersed in a bath of water, a nonsolvent, it appeared to swell in thickness as it phase-separates. A dense skin, thinner than a micron, appeared to overlie a two-phase substructure that is punctuated with pear-shaped macrovoids. At early immersion times, this substructure is visibly bicontinuous or open-celled near the bath-side, and dispersion-like (droplets dispersed in a polymeric matrix) or closed-celled near the substrate-side. Moreover, in the bicontinuous regions, length-scales of the individual phases seem to increase across the coating thickness from the bath-side to the substrate-side. After prolonged immersion, the substructure, excluding the macrovoids, is entirely bicontinuous. The bicontinuity presumably results from a combination of spinodal decomposition and nucleation and growth plus coalescence. Quite strikingly, macrovoids are present exclusively in regions where phases are bicontinuous, and are absent where droplets are dispersed in the polymeric matrix. Evidence suggests that macrovoids result from an instability caused by a progressive rupture of polymer-rich links deeper and deeper beneath the skin, aggravated by stress localization in the rupturing network and a buildup of pressure in the polymer-poor phase (the pore space), as suspected by Gröbe and Meyer in 1959.
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机译：剖分式低温扫描电子显微镜（cryo-SEM）是一种独特的方法，可以可视化处理过程中液体涂层的微观结构如何演变。分时意味着在该过程的后续阶段快速冻结（几乎）相同的样本。为此，需要在干相转化过程中很好地控制涂布和干燥，并且在湿相转化过程中溶剂也要交换。在控制下，冷冻的样品断裂，通过有限的升华蚀刻，溅射涂层，并在 ca -175°C成像。所检查的涂料是高分子量和低分子量的醋酸纤维素，以及混合溶剂和非溶剂中的聚砜：丙酮和水，醋酸纤维素经过干法转化；四氢呋喃，二甲基乙酰胺，乙醇与聚砜经过干湿相转化。最初浇铸在硅基底上的所有涂层都是均匀的。高分子量和低分子量乙酸纤维素三元溶液的初始组成分别为“非临界”和“近临界”，这意味着它们接近相图的临界点或褶度点。聚砜四元溶液的初始组成位于拟三元相图的二倍体附近。看起来，随着较高分子量乙酸纤维素涂层的干燥，它会成核并生长出聚合物贫乏的液滴，这些液滴会聚结成薄而致密的皮肤下面的双连续结构。通过干样品的立体显微镜验证结构的双连续性。较低分子量的醋酸纤维素涂层似乎呈旋节状直接分离成双连续结构，其富含聚合物的网络由于受挫的平面收缩而受力，在某些位置远低于皮肤破裂，形成大孔。当部分干燥后，将聚砜涂层浸入非溶剂水浴中时，由于相分离，其厚度似乎膨胀。比微米薄的致密皮肤似乎覆盖了两相亚结构，并被梨形大孔打孔。在早期的浸入时间，该子结构在浴侧附近明显是双连续的或开孔的，而在基体侧附近则呈分散状（液滴分散在聚合物基质中）或闭孔。而且，在双连续区域中，各个相的长度尺度似乎从浴侧到基体侧在整个涂层厚度上增加。长时间浸泡后，不包括大孔的子结构完全是双连续的。双连续性大概是由于旋节线分解和成核以及生长和聚结的结果。令人惊讶的是，大孔隙仅存在于两相连续的区域，而液滴分散在聚合物基体中则不存在。有证据表明，大孔隙是由于皮肤深处越来越深的富含聚合物的连接逐渐破裂而引起的不稳定性所致，该破裂是由于破裂网络中的应力局部化以及贫聚合物相（孔隙空间）中压力的积累而加剧的， 1959年Gröbe和Meyer怀疑。
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