Nanotechnology has an important part to play in the inception of cutting-edge fabrics for novel applications. In order to increase the incorporation of nanomaterials into fabrics already on the market, it is vital to appreciate the additional properties that they have to offer.
To illustrate this point, nanofibers that are known to be considerably difficult to tear could be weaved into existing fabrics that are resistant to tearing, enhancing this fabric property without adding large amounts of additional weight to the material. Fiberglass is a prime example of a nanoscale analog being integrated into an existing light fiber plastic in order to enhance the wear and tear properties.
Researchers wanting to investigate the interactions between the fibers and the material matrix of the fabric to be combined with nanomaterials will use Atomic Force Microscopy (AFM). AFM is a helpful tool that allows information that will aid the fine-tuning of ratios of textile components to create materials with the optimal performance gain for the smallest increase in production cost.
Professor Yan Vivian Li, whilst at the Colorado State University, was given a silicon matrix material that had fibers of polyester yarn woven into it . The investigation had access to the commercial AFM operating system Park NX10 from Park Systems , and carried out experiments in ambient air conditions, collecting data in non-contact mode for both topography and phase imaging. Data for both types of imaging, topography and phase, were obtained simultaneously.
Topography imaging is used to obtain the three-dimensional features of the sample material surface, whilst phase provides information which can be used to work out the elasticity of the material. In addition, force-volume and force-distance spectroscopy were used to collect data on the nanomechanical characteristics of the material and correlated to the phase imaging data.
Plotting distance and force against each other on a graph produces a curve that can be used to calculate the force that the AFM probe exerts at each point at a right angle to the material surface. These graphical representations are also referred to as f-d (force-distance) curves and are plots of the cantilever’s deflection, recorded by a position-sensitive photodetector, against the extension of a piezoelectric scanner .
Force-volume mapping involves measuring a selection of single data points and changes the accumulation of f-d curves that have taken over the whole sample surface and turns it into a 2D map showing the property of hardness, building on force-distance spectroscopy . The gradient of the f-d curve tells us the hardness in units of N/m.
Finally, phase imaging is an AFM method which utilizes the change in the cantilever’s oscillation as the probe end goes from one feature to another on the sample. A shift phase is produced, which is the input signal for the probe end’s oscillation minus the arising oscillations output signal . This information is then put in relation to multiple material properties for example elasticity.
Results and Discussions
Figure 1 depicts a cross-sectional view of the silicon matrix with a strand of polyester yarn (the shaded round feature in the center, 100-150 µm in diameter), which was acquired using methodology that included an optical microscopy that had a Park NX10 AFM system built-in.
The exposed strand or yarn, the interface between the yarn and the silicon matrix, and a region made of only the matrix were labeled Region 1, 2 and 3 respectively. These areas were of particular interest to researchers because of the focus of the experiments.
Figure 1. Cross-section of Silicon gel matrix with embedded strand of polyester yarn. Three regions were selected for further investigation: Region 1 on the yarn itself, Region 2 at the interfacial region between the yarn and matrix, and Region 3 on the matrix surrounding the yarn.
Figure 2. Contains non-contact AFM topography images taken from each of the regions selected in Figure 1 as well as f-d curves that were measured at selected sites within each of those regions. The shape of the f-d curves correlate to a measure of the physical interaction between the tip and sample; in this case, the tip-sample distance vs. the force load on the tip cantilever. The slope of an f-d curve is steeper when the tip presses on a harder sample.
An area of the material’s yarn, in the bottom-left corner of Region 1 in Figure 2 was chosen for analysis with force-distance spectroscopy. To the right of the image is the f-d curve produced at this site which shows the force given by the probe end. The material is raised by about 4 µN for a length of 0.5 µm as the probe end is forced down onto the surface.
Looking at Region 2, two different areas were recorded at the material’s interfacial region: the area located on the left side of the image is made up of yarn and the second, on the right-hand side of the image, is made up of the matrix.
The corresponding f-d curves to the right of Figure 2 are top for area one and below is for area two. Area one of Region 2, like in Region 1, shows a force of approximately 4 µN being given by the end of the probe to the material for a length of 0.5 µm. Area two’s f-d curve below and of the matrix part of Region 2 have dissimilar results.
It starts with a raise just shy of 40 nN in the force given by the probe end to the material and continues for a length of around 1.25 µm. Then there is a sharp decrease of around 40 nN in the probe end force being applied and recorded. It is feasible that the decrease in force being applied is a result of the probe end being taken onto the surface of the silicon matrix. The resulting measurement taken is approximately 50 nM for a length of 0.5 µm.
Finally, bringing our attention to Region 3 of Figure 2, this area is of the material’s silicon matrix. Here it can be seen that the force exerted onto the surface occurs in a two-step manner increasing in an almost identical way to the one that was produced by the matrix half of Region 2. At the beginning, the force exerted by the probe end reaches to approximately 40 nN for a length of 1.5 µm, but then suddenly reduces to almost 0 nN.
It is believed that the sharp reduction in exerted pressure by the probe end depicts the moment that the probe end has moved onto the surface of the silicon matrix. Following this, another raise in the probe end exerted force is seen as it moves down onto the matrix causing an increase in the force of around 40 nN for a length of 0.5 µm.
In the experiments carried out, it was found that f-d curves produced from a region found in a strand of yarn have 100 times more force exerted by the probe end’s cantilever than when being taken from a silicon matrix maintaining similar distances between the probe end and the sample.
To increase the scope of the study from single points to whole sections of the sample’s surface, force-volume mapping was used as depicted in Figure 3. This technique allows for the creation of a 2D map of the sample’s nanomechanical properties — in this case, hardness. To begin, a small area within Region 2 as shown in Figures 1 and 2 was chosen for hardness mapping as the interfacial region of the sample would allow the investigation of both the yarn and matrix simultaneously.
This location is specified by the inset red square in the non-contact AFM topography image for Region 2 shown in Figure 2. The location for force-volume mapping measures 5 x 5 µm and is shown in all of the images making up Figure 3. The next stage was to overlay a 16 x 16 grid over the location to be mapped, therefore creating an array of 256 total sites.
F-d curves were measured at the midpoint of each of these sites and the collected data were translated into a 256 pixel hardness map where each pixel represents the sample hardness detected at each site’s midpoint. Note the sharp difference in the colors of the hardness map’s pixels which closely follows the interfacial region’s border between the yarn and the matrix as shown in the topography image in Figure 3.
A similar phenomenon is observed with the phase image as well — the upper-left portion of the phase image corresponds to the yarn and has a profoundly different phase signal than the remaining portion of the image depicting the matrix. This indicates that the phase shifts in the cantilever’s oscillation are markedly different when the probe moves across the yarn versus across the matrix—suggesting a difference in material elasticity.
Figure 3. Force-volume mapping array of the sample’s interfacial region and the hardness map generated from it. The hardness data correlates to the phase image of the same interfacial region.
The final stage of the investigation repeated the f-d curve comparison that was depicted in Figure 2; however, this time the chosen sites for analysis were exclusive to the interfacial region of the sample focused on in Figure 3. A total of 4 sites were selected from the array of 256 superimposed on the topography image in Figure 3 and are referred to hereafter as sites 31, 88, 191, and 227.
The first three sites are located within the matrix area of the interfacial zone whereas site 227 is located on the yarn. The f-d curves for each of the four sites are seen in Figure 4. The f-d curve of site 227 is shaped as expected given the experience of measuring Region 1 (yarn) on Figure 2.
Again, a force load of about 4 µN over a distance of about 0.5 µm is seen. Sites 31, 88, and 191 also have f-d curves that were also anticipated by having previously measured Region 3 (matrix) on Figure 2.
These three sites all have the same force load increase marked by a steep drop-off in tip-applied force as the probe snaps onto the matrix. The ensuing second force load increase is again measured to be around 40 nN over a distance of 0.5 µm. This 2D mapping data is consistent with the single-point force-distance spectroscopy data gained earlier in the investigation.
Figure 4. F-d curves of sites 31, 88, 191, and 227 from the sample’s interfacial region hardness map as shown on Figure 3.
Given that the polyester yarn has been observed to be around 100 times harder in comparison to the gel matrix it is embedded in, it is reasonable to propose that the gel’s original resistance to certain types of damage may have been positively augmented by the embedded yarn.
The nanomechanical property which was focused on during this investigation was hardness. The hardness of a material is usually correlated to its elasticity, plasticity, and/or its resistance to fracture. In fiber reinforced plastics, embedded fibers allow a novel composite to remain in one piece despite having a large, shattering force applied to it.
The composite also has potentially significant weight savings when compared to the material it is replacing. When applied to textiles and apparel, nanofibers such as the yarn investigated here can be embedded in more than just gel matrices and have been woven into existing fabrics such as cotton to confer traits such as increased aerosol filtration  or the ability to self-clean .
A nanofiber specifically chosen for its hardness may conceivably increase the damage resistance of a composite fabric, leading to immediate uses in ballistics as well as other applications that require clothing with heightened durability with acceptable or even negligible increases in weight.
The topography image, phase image, and a nanomechanical property map (based on f-d curve data) of a silicon gel matrix and polyester yarn sample were all created using the Park NX10 from Park Systems, a commercially available AFM system .
The data collected here suggests the yarn is around 100 times harder than the matrix it is embedded in. All data acquisition was performed with forces being measured on the order of nanonewtons across distances as small as micrometers. Carrying out such high-precision measurements demonstrates AFM’s ability to characterize key material properties at nanoscale. This is particularly important to understand the interactions of components in novel composites such as next-generation textiles which are now being designed with nanomaterials in mind.
Investigations such as the one conducted here can help researchers understand the source of macroscopic effects blending materials may manifest starting at the smallest of scales. This knowledge can in turn inform subsequent strategies to design future iterations with increasing performance and decreasing cost.
Gerald Pascual, Byong Kim, Mina Hong, John Paul Pineda, and Keibock Lee, Technical Marketing, Park Systems.
References and Further Reading
 Li Research Group. (n.d.). Retrieved April 22, 2016, from https://sites.google.com/site/smartextilesnanotech/
 Park NX10 – Overview. (n.d.). Retrieved April 30, 2016 from http://www.parkafm.com/index.php/products/research-afm/park-nx10/overview
This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.
For more information on this source, please visit Park Systems Inc.