Department of Chemistry and Chemical Biology, McMaster University, 1280 Main St. W., Hamilton, ON, CanadaL8S 4M1.
Nanoscale. 2017 Jun 14;9(23):7781-7790. doi: 10.1039/c7nr00251c.
Precise knowledge of the mechanical properties of emerging nanomaterials and nanocomposites is crucial to match their performance with suitable applications. While methods to characterize mechanical properties exist, they are limited by instrument sensitivity and sample requirements. For bio-based nanomaterials this challenge is exacerbated by the extreme dependence of mechanical properties on humidity. This work presents an alternative approach, based on polymer shrinking-induced wrinkling mechanics, to determine the elastic modulus of nanobiocomposite films in a humidity-independent manner. Layer-by-layer (LbL) films containing cellulose nanocrystals (CNCs) and water-soluble polymers were deposited onto pre-stressed polystyrene substrates followed by thermal shrinking, which wrinkled the films to give them characteristic topographies. Three deposition parameters were varied during LbL assembly: (1) polymer type (xyloglucan - XG, or polyethyleneimine - PEI); (2) polymer concentration (0.1 or 1 wt%); and (3) number of deposition cycles, resulting in 10-600 nm thick nanobiocomposite films with tuneable compositions. Fast Fourier transform analysis on electron microscopy images of the wrinkled films was used to calculate humidity-independent moduli of 70 ± 2 GPa for CNC-XG, 72 ± 2 GPa for CNC-PEI, and 32.2 ± 0.8 GPa for CNC-PEI films. This structuring method is straightforward and amenable to a wide range of supported thin films.
准确了解新兴纳米材料和纳米复合材料的力学性能对于将其性能与合适的应用相匹配至关重要。虽然存在用于表征力学性能的方法，但这些方法受到仪器灵敏度和样品要求的限制。对于基于生物的纳米材料，这种挑战因机械性能对湿度的极端依赖性而加剧。这项工作提出了一种替代方法，基于聚合物收缩诱导的皱缩力学，以非依赖性湿度的方式确定纳米生物复合材料薄膜的弹性模量。含有纤维素纳米晶体 (CNC) 和水溶性聚合物的层层 (LbL) 薄膜被沉积到预加应力的聚苯乙烯基底上，然后进行热收缩，使薄膜起皱以赋予其特征形貌。在 LbL 组装过程中，三个沉积参数发生了变化：(1)聚合物类型（木葡聚糖 - XG 或聚乙烯亚胺 - PEI）；(2)聚合物浓度（0.1 或 1wt%）；和 (3)沉积循环次数，从而制成了厚度可调的 10-600nm 厚的纳米生物复合材料薄膜。对皱缩薄膜的电子显微镜图像进行快速傅里叶变换分析，计算出 CNC-XG 的湿度独立模量为 70±2GPa，CNC-PEI 的为 72±2GPa，CNC-PEI 薄膜的为 32.2±0.8GPa。这种结构化方法简单直接，适用于广泛的支持薄膜。
