The Microstructure and Mechanical Properties of Poplar Catkin Fibers Evaluated by Atomic Force Microscope (AFM) and Nanoindentation

Abstract

In this study, the microstructure and mechanical properties of poplar (Populus tomentosa) catkin fibers (PCFs) were investigated using field emission scanning electron microscope, atomic force microscopy (AFM), X-ray diffraction, and nanoindentation methods. Experimental results indicated that PCFs had a thin-wall cell structure with a large cell lumen and the hollow part of the cell wall took up 80 percent of the whole cell wall. The average diameters of the fiber and cell lumen, and the cell wall thickness were 5.2, 4.2, and 0.5 µm, respectively. The crystallinity of fibers was 32%. The AFM images showed that the orientation of microfibrils in cell walls was irregular and their average diameters were almost between 20.6-20.8 nm after being treated with 2 and 5 wt.% potassium hydroxide (KOH), respectively. According to the test of nanoindentation, the average longitudinal-reduced elastic modulus of the PCF S 2 layer was 5.28 GPa and the hardness was 0.25 GPa.Poplar (Populus tomentosa) catkin fiber (PCF) is a cotton-like catkin fiber covering the seeds of white poplar trees [7]. Poplar is a kind of fast-growing tree species with a short growth cycle and low price. It is mainly concentrated in the northeast, north China, and northwest areas. The total area of poplar forest in China exceeds 70,000 km 2 , and it increases year by year. Each poplar tree can harvest about 25 kg of catkins. The tassel fiber is mainly composed of cellulose and hemicellulose, as well as lignin, with a thin cell wall and large lumen [8]. During the period from April to June each year, PCFs float in the air, causing great trouble for the environment and residents' health. Zhang et al. [9] focused on poplar (Populus tomentosa) catkin fiber as a new resource for bioethanol production via enzymatic hydrolysis. Yin et al.[10] studied the water absorption and oil absorption of poplar catkins. They studied how to inhibit the production of PCFs, but not how to utilize them. There are few studies on the microstructure and mechanical properties of PCFs.In recent years, microscopic imaging and spectral characterization methods have been used in nanostructure and chemical composition analyses of cell walls of plant fibers like wood fibers [11][12][13][14][15][16], bamboo fibers [17][18][19][20][21][22], and cotton fibers [23,24]. Chen et al. [25] characterized the aggregation of microfibrils in the cross-section of thin-wall cells using the atomic force microscopy (AFM) technique. Xiao et al. [26] and Hao et al. [24] investigated the cell wall layer structure of kapok fibers using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) through formulating an efficient organic solvent system to isolate cellulose from lignin and concluded that both kapok and cotton fibers had similar multi-wall layer structures. Fernandes et al. [27] investigated the nanostructures of spruce fibers using a series of spectroscopic techniques, such as small angle neutron ray scanning (SANS) and wide-angle X-ray scattering (WAXS).Oliver and Pharr.[28]...

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