Stimpson Taylor C, Osorio Daniel A, Cranston Emily D, Moran-Mirabal Jose M
Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada.
Department of Chemical and Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada.
ACS Appl Mater Interfaces. 2021 Jun 23;13(24):29187-29198. doi: 10.1021/acsami.1c08056. Epub 2021 Jun 10.
To engineer tunable thin-film materials, the accurate measurement of their mechanical properties is crucial. However, characterizing the elastic modulus with current methods is particularly challenging for sub-micrometer thick films and hygroscopic materials because they are highly sensitive to environmental conditions and most methods require free-standing films which are difficult to prepare. In this work, we directly compared three buckling-based methods to determine the elastic moduli of supported thin films: (1) biaxial thermal shrinking, (2) uniaxial thermal shrinking, and (3) the mechanically compressed, strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) method. Nanobiocomposite model films composed of cellulose nanocrystals (CNCs) and polyethyleneimine (PEI) were assembled using layer-by-layer deposition to control composition and thickness. The three buckling-based methods yielded the same trends and comparable values for the elastic moduli of each CNC-PEI film composition (ranging from 15 to 44 GPa, depending on film composition). This suggests that the methods are similarly effective for the quantification of thin-film mechanical properties. Increasing the CNC content in the films statistically increased the modulus; however, increasing the PEI content did not lead to significant changes. For the CNC-PEI system, the standard deviation of elastic moduli determined from SIEBIMM was 2-4 times larger than that for thermal shrinking, likely due to extensive cracking due to the different stress applied to the film when subjected to compression of a relaxed substrate versus the shrinking of a pre-strained substrate. These results show that biaxial thermal shrinking is a reliable method for the determination of the mechanical properties of thin films with a simple implementation and analysis and low sensitivity to small deviations in the input parameter values, such as film thickness or substrate modulus.
为了设计出可调控的薄膜材料，准确测量其力学性能至关重要。然而，用现有方法表征亚微米厚薄膜和吸湿材料的弹性模量极具挑战性，因为它们对环境条件高度敏感，且大多数方法需要独立的薄膜，而这种薄膜很难制备。在这项工作中，我们直接比较了三种基于屈曲的方法来测定支撑薄膜的弹性模量：（1）双轴热收缩法，（2）单轴热收缩法，以及（3）用于力学测量的机械压缩应变诱导弹性屈曲不稳定性（SIEBIMM）方法。由纤维素纳米晶体（CNC）和聚乙烯亚胺（PEI）组成的纳米生物复合模型薄膜通过逐层沉积组装而成，以控制其组成和厚度。对于每种CNC-PEI薄膜组合物（弹性模量范围为15至44吉帕，取决于薄膜组成），这三种基于屈曲的方法得出了相同的趋势和可比的值。这表明这些方法在量化薄膜力学性能方面同样有效。薄膜中CNC含量的增加在统计学上提高了模量；然而，PEI含量的增加并未导致显著变化。对于CNC-PEI体系，由SIEBIMM测定的弹性模量的标准偏差比热收缩法的标准偏差大2至4倍，这可能是由于在对松弛基板进行压缩与对预应变基板进行收缩时，施加在薄膜上的应力不同，导致薄膜出现大量裂纹。这些结果表明，双轴热收缩法是一种可靠的测定薄膜力学性能的方法，其实施和分析简单，对输入参数值（如薄膜厚度或基板模量）的小偏差不敏感。
