Cellulose nanowhiskers (CNWs) are biopolymer nanomaterials that appear and act in many ways as carbon nanotubes. Unlike carbon nanotubes that are difficult to synthesize in volume, CNWs are manufactured from trees and other plants, so there is an endless supply and the mass production of it is cost effective.
Motivated by such economic, technical, and environmental advantages, people have looked at CNWs to develop new nanocomposite materials that are stronger and lighter, rather than using carbon nanotubes.
The mechanical property of nanocomposite materials can largely depend on exact shape, morphology, and size of CNW elements that go in to reinforce it. So, novel metrology technique suitable for accurately characterizing CNW's shape, size and morphology are needed[3, 4].
Three samples of cellulose nanowhiskers suspension were prepared for scanning. The CNW suspension samples were labeled as Sample 1, Sample 2, and Sample 3.
A droplet of each sample additionally diluted in deionized water was placed onto a freshly cleaved mica substrate. Liquid droplet residue was blown off using puffs of air and the sample was left in ambient air to dry. The sample was then mounted onto a commercial Atomic Force Microscope’s (AFM) stage for imaging. The sample was imaged in air in a fast nanomechanical mode, also called PinPoint mode AFM, using silicon-based cantilever AFM probes.
All three CNW samples displayed similar sizes and shapes in AFM topographies. In Figure 1a, the topography of Sample 1 shows that the CNW rods are comparatively straight and long. The CNWs are about 1 - 3 nm wide and 100 - 1000 nm long, as measured from the AFM height profile in Figure 1b.
It is thought that the CNW rods have circular cross-sections and the width is established by measuring the height of the CNWs. An interesting observation of the CNWs is that they are well distributed and seem to be oriented along a diagonal direction.
The puff pf air that was used to blow off the residual droplet from the mica surface was positioned so that the droplet was driven to flow from one side to the other. This quick droplet flow may have induced the force gradient suitable for the CNWs to re-orient in the direction of the liquid flow.
Although the CNWs are not as preferentially oriented, as illustrated in Figures 2a and 3a, similar shapes of CNWs can be viewed from the AFM topographies of the other two samples, Sample 2 and Sample 3, respectively. They also appear relatively straight and well dispersed but not as densely distributed–bending or entanglement of CNWs were minimal.
The width of the CNWs from Sample 2 and 3 ranging from 1 to 3 nm can be observed from the AFM height profiles of Figure 2b and 3b, respectively. More nanofibrils are observed from the Sample 1 topography than those of Sample 2 and Sample 3 because sample 1 was not diluted with DI water as much as the other two samples.
Topography of CNWs spread on mica is illustrated in Figure 4a. It is overlaid with a color scale from Young’s modulus values determined in PinPoint mode AFM. In this mode, the force distance (f-d) curve is obtained from each pixel in the areas where topography is imaged, and from each f-d curve elastic modulus is measured and mapped out in real-time together with the corresponding topography image.
Figure 4a shows a darker color scale, indicating a lower modulus value. The CNWs are darker compared to the surrounding mica, which signifies that the CNWs are not as stiff as the substrate. The modulus line profile (Figure 4b) shows ~180 GPa as the modulus value for CNWs and ~ 210 GPa for mica.
The calculated value of the CNW’s Young’s modulus is not too far off the value of 150 GPa that is theoretically estimated for crystalline forms of the material .
The CNWs were successfully imaged in high quality and high resolution using commercially available AFM. The CNW samples that were well spread on mica substrates without much entanglement; this enabled easy measurements of the length, width, as well as Young’s modulus of individual CNWs.
The results indicate that AFM can be applied for dimensional nanometrology and quantitative nanomechanical property measurements for CNW characterization.
References and Further Reading
- Yanxia Zhang, Tiina Nypelo, Carlos Salas, Julio Arboleda, Ingrid C. Hoeger, Orlando J. Rojas, J. Renew. Mater., Vol. 1, No. 3, July 2013.
- Xuezhu Xu,Haoran Wang,Long Jiang, Xinnan Wang, Scott A. Payne, J. Y. Zhu, and Ruipeng Li⊥, Macromolecules 2014, 47, 3409−3416
- Michael T Postek, Andr´as Vlad´ar1, John Dagata1, Natalia Farkas, Bin Ming1, Ryan Wagner, Arvind Raman, Robert J Moon, Ronald Sabo, Theodore H Wegner and James Beecher, Meas. Sci. Technol. 22 (2011) 024005.
- Ingrid Hoeger,a Orlando J. Rojas, Kirill Efimenko,c Orlin D. Velevc and Steve S. Kelleya, Soft Matter, 2011, 7, 1957.
- Park NX20 - Overview | Park Atomic Force Microscope - http://www.parkafm.com/index.php/products/research-afm/park-nx20/overview
- Park AFM Modes - http://www.parkafm.com/index.php/park-afmmodes/nanomechanical-modes
- Xiaodong Cao, Youssef Habibi1, Washington Luiz Esteves Magalhães, Orlando J. Rojas, and Lucian A. Lucia, CURRENT SCIENCE, VOL. 100, NO. 8, 25 APRIL 2011 (and references within).
This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.
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