修正电子显微镜对膜孔径的低估，发现真实孔隙率是此前认知的3倍。
From Coated to Uncoated: Scanning Electron Microscopy Corrections to Estimate True Surface Pore Size in Nanoporous Membranes
- 通过低电压成像和数字膨胀法，校正金属涂层导致的孔径缩小
- 未涂层膜孔隙率高达23%（原测5.8%），孔径扩大2倍以上
- 结果与实验渗透数据吻合，适用于膜材料结构研究者
扫描电子显微镜（SEM）是表征超滤（UF）膜及反渗透（RO）支撑层纳米级表面孔结构的主流方法。传统认知认为膜表面孔隙率普遍低于10%。我们推测，高加速电压和溅射金属涂层会系统性低估孔隙率与孔径。实验显示，将商用UF膜成像电压从1 kV升至10 kV，孔隙率由10.3%降至6.3%；铂涂层厚度从1.5 nm增至5 nm，使UF膜孔隙率下降54%（12.9%→5.8%），RO支撑层下降46%（13.1%→7.0%）。为此，我们提出一种数字膨胀法，模拟涂层引起的孔径扩张，估算无涂层状态下的真实结构。结果显示，未涂层孔隙率为23%（UF膜）和20%（RO支撑），约为4 nm涂层下测量值的3倍；平均孔径分别扩大2倍（UF）和1.5倍（RO）。关键的是，膨胀后孔径分布与低通量葡聚糖截留数据（用Bungay-Brenner模型拟合）一致。结果表明，纳米多孔膜的真实孔隙率与孔径远高于以往认知，对结构-传输关系理解具有重要意义。建议未来膜材料纳米孔分析采用1 kV低电压、1-2 nm最小涂层，并结合数字膨胀校正涂层伪影。
原文摘要 · Abstract (English)
Scanning electron microscopy (SEM) is the premier method for characterizing the nanoscale surface pores in ultrafiltration (UF) membranes and the support layers of reverse osmosis (RO) membranes. Based on SEM, the conventional understanding is that membranes typically have low surface porosities of <10%. We hypothesized that high acceleration voltage during SEM imaging and sputter metal coatings required for SEM have led to systematic underestimations of porosity and pore size. We showed that imaging a commercial UF membrane at 1, 5, and 10 kV reduced measured porosity from 10.3% (1 kV) to 6.3% (10 kV), while increasing Pt coating thickness from 1.5 to 5 nm lowered porosity by 54% for the UF membrane (12.9% to 5.8%) and 46% for an RO support (13.1% to 7.0%). To account for coating thickness, we developed a digital correction method that simulates pore dilation, enabling the pore structure to be estimated for uncoated membranes. Dilation yielded uncoated porosity values of 23% for the UF membrane and 20% for the RO support, about 3-fold greater than values observed with a 4 nm coating. Mean pore diameters were 2-fold greater for the UF membrane and 1.5-fold greater for the RO support. Critically, dilation-derived pore-size distributions agreed with low-flux dextran-retention data fitted with the Bungay-Brenner model. Our results suggest that surface porosities and pore sizes of nanoporous membranes are much larger than previously understood, with major implications for structure/transport relationships. For future nanoscale pore analysis of membranes (and other nanoporous materials), we recommend low acceleration voltage (1 kV), minimal coatings (1-2 nm), and digital dilation to account for coating artifacts
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