The inclusion of nanotechnology in textile development can create futuristic fabrics for various new exciting applications. In order to predict how targeted enhanced properties will manifest in novel composites, it is essential to understand the characteristics of the nanomaterials that are being integrated with existing fabric matrices.
For instance, the strength of tear-resistant fabric can be further increased without significantly increasing the fabric’s weight by weaving in nanofibers. This nanoscale method is similar to the methods that have been used in the development of strong materials, such as fiber-reinforced plastics.
Next-generation fabrics aim to emulate the weight advantages and durability of fiber-reinforced plastics by finding solutions at a smaller scale.
Atomic Force Microscopy (AFM) can be used to study the interactions between the fibers and the fabric matrix, their interfacial regions, and the composite's topography—all points of knowledge helpful in adjusting the composite's makeup for maximizing performance gain and minimizing production cost. To this end, AFM was used to characterize the nanomechanical properties of a polyester fiber interaction with a silicone matrix.
Prof. Yan Vivian Li and Dr. Dean Hendrickson from Colorado State University provided the silicone matrix sample embedded with strands of polyester yarn[1-2]. This sample was used to conduct the AFM experiment in ambient air conditions with a Park NX10 SPM system in non-contact AFM mode for phase imaging and topography imaging.
Both sets of data are obtained at the same time. While phase imaging provides data that can be correlated to the sample's elastic properties, topography imaging is useful mainly to observe 3D features on the sample surface. Force-distance spectroscopy and force volume mapping were used to characterize the sample’s nanomechanical properties. These results were correlated to phase imaging data.
Force-distance (f-d) curves in force-distance spectroscopy are used to measure the force that is vertically applied by an AFM probe to a single point on a sample surface. F-d curves are plots of the cantilever's deflection that are measured using a position-sensitive photodetector vs. the extension of a piezoelectric scanner.
Force-volume mapping builds on force-distance spectroscopy - it uses an array of single measurement points and transforms the collected f-d curves across the surface of the sample into a 2D characterization map of hardness . Hardness is characterized as the slope of the f-d curve given in the units of N/m.
Finally, phase imaging is an AFM technique that uses the shift in a cantilever's oscillation when the tip moves across various features on the sample. The difference in the input signal for tip's oscillation and the resulting oscillation's output signal is known as a shift in phase. This signal can be correlated to many material properties, including elasticity.
Results and Discussions
Figure 1 shows the cross-section view of the silicone matrix (pink in color) with a strand of polyester yarn which is indicated by a dark circular feature in center, with a diameter of 100 - 150 µm. Three regions were selected for investigation here.
Region 1 is located directly on the exposed strand of embedded polyester yarn. Region 2 is the interface between the silicone matrix and the embedded polyester yarn surrounding it. Region 3 is an area composed entirely of the silicone matrix.
Non-contact AFM topography images that were taken from the regions selected in Figure 1 and the f-d curves that were measured at the selected sites within all of those regions are shown in Figure 2. The shape of the f-d curves correlate to a measure of the physical interaction between the sample and the tip - which is the sample-tip distance vs. the force load on the tip cantilever in this case. As a result, the f-d curve is at its steepest when the tip presses on a harder sample.
A site in the lower-left corner of Region 1, a location on the sample's polyester yarn, was chosen for force-distance spectroscopy. At this site, the f-d curve shows that the force applied by the tip to the sample increased by approximately 4 µN across a distance of 0.5 µm when the tip presses down on the surface.
In Region 2, a pair of sites was measured at the sample's interfacial region: the first site is in the left half of the image (the area composed of the polyester yarn) and a second site in the right half of the image (an area composed of the silicone matrix). The f-d curve for the polyester yarn side of the interfacial region again exhibits a force of about 4 μN being applied by the tip to the sample over a distance of about 0.5 μm. The f-d curve at the site corresponding to the silicone matrix portion of the region yields different data.
There is an initial increase of just below 40 nN in the force applied by the tip to the sample across a distance of about 1.25 µm. At this point, there is a steep decrease of approximately 40 nN in the tip-applied force. It is possible that the drop-off in applied force due to the tip being pulled onto the surface of the silicone matrix.
As the probe continues pressing onto the matrix, tip-applied force increases for the second time. The increase is measured to be approximately 50 nN across a distance of 0.5 µm.
In a location on the sample's silicone matrix of Region 3, there was a two-stage force load increase that was similar to the one observed in the silicone matrix side of the previously analyzed interfacial region. The initial force applied by the tip increases to approximately 40 nN across a distance of 1.5 µm, and then drops to approximately 0 nN.
This sudden drop in the applied force is hypothesized to be the moment in which the tip snaps onto the silicone matrix surface. After a short period, there is a second increase in the tip-applied force when the probe continues to be pushed onto the matrix. This results in a load increase of approximately 40 nN across a distance of approximately 0.5 µm.
The f-d curves taken from a region in a strand of polyester yarn, exhibit a force load on the tip's cantilever that is nearly 100 times higher than the load exhibited on an f-d curve measured from the silicone matrix at the comparable tip-sample distances.
Force-volume mapping was applied to increase the area of observation from single points to whole sections of the sample's surface (Figure 3). A 2D map of the nanomechanical properties of the sample – hardness in this case – can be created using this technique.
A small area in Region 2 that is shown in Figures 1 and 2 was chosen for hardness mapping, as both the silicone matrix and polyester yarn can be measured simultaneously in the interfacial region. The inset red square seen in the non- contact AFM topography image for Region 2 (Figure 2) indicates this location.
The location chosen for force-volume mapping measures 5 x 5 µm, and it is shown in all the images making up Figure 3. A 16 x 16 grid was then overlaid on the location to be mapped forming a group of 256 total sites. F-d curves were measured at the midpoint of all these sites, and the data was converted into a 256 pixel hardness map, with each pixel representing the sample hardness measured at the mid-point of each site.
There is a distinct difference in the colors of the hardness map’s pixels that follows the border of the interfacial region between the silicone matrix and the polyester yarn as seen in the topography image of Figure 3.
A similar phenomenon is observed with the phase image as well—the upper-left portion of the phase image corresponds to the embedded yarn and has a profoundly different phase signal than the remaining portion of the image depicting the silicone 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.
The f-d curve comparison shown in Figure 2 was repeated in the final leg of the experiment. However, the sites chosen for analysis were exclusive to the interfacial region shown in Figure 3.
Out of the 256 superimposed on the topography image shown in Figure 3, four sites were selected. These four sites are referred to as sites 31, 88, 191, and 227, from now on. While the first three sites are in the silicone matrix side of the interfacial region, site 227 is in the polyester yarn side.
Figure 4 shows the f-d curves for each of these four sites. The shape of the f-d curve of site 227 is as expected based on the results of measurements in Region 1 (polyester yarn) shown in Figure 2. There was a force load of about 4 µN across a distance of about 0.5 µm.
The f-d curves of sites 31, 88, and 191 were also anticipated, as Region 3 has already been measured on Figure 2. The force load increases of these three sites are the same, with a steep drop in the tip-applied force as the probe snaps onto the matrix. The second force load increase is measured to be around 40 nN across a distance of 0.5 µm.
This 2D mapping data correlates with the single-point force- distance spectroscopy data that was obtained earlier in the investigation.
Since the nanofibers of polyester yarn investigated here are approximately 100 times harder than the gel matrix they are embedded in, it can be concluded that the resistance of the gel to specific types of damage may have been increased by the embedded yarn.
Hardness is the nanochemical property that was focused on in this investigation. A material’s hardness is typically related to the plasticity, elasticity, and/or its inclination/resistance to fracture. The embedded fibers in fiber-reinforced plastics enable the novel composite to remain in a single piece even when a large, shattering force is applied to it.
When compared to the material it is being designed to substitute, the novel composite may also offer considerable weight savings. The nanofibers such as the one investigated here can be embedded in more materials than just gel matrices when they are used in the textiles and apparel industry.
Nanofibers have been woven into fabrics such as cotton to grant traits such as increased aerosol filtration [4,5]. A nanofiber chosen particularly for its hardness can improve the damage resistance of a fabric, leading to utilization in ballistics and other applications, where increased durability is required in clothing.
The phase image, topography, and a nanomechanical property map (based on f-d curve data) of a polyester yarn and silicone gel sample were obtained using a Park NX10 SPM system. The investigation shows that the strands of polyester yarn are about 100 times harder compared to the silicone gel matrix they are embedded in.
All data acquisition was carried out at the nanoscale, with the forces on the order of nanonewtons being measured across distances of a few micrometers. Performing measurements at this scale indicates AFM’s ability to characterize the main properties of nanomaterials used for developing novel composites such as next-generation fabrics.
Gaining an insight into the nanoscale behavior of such materials can enable informed speculation on the macroscopic effects that can be induced by their inclusion in a composite.
 Li Research Group. (n.d.). Retrieved April 22, 2016, from https://sites.google.com/site/smartextilesnanotech/
 SurgiReal. (.d.). Retrieved April 22, 2016, from https://www.surgireal.com/about/
 Park AFM Nanomechanical Mode. (n.d.). Retrieved April 22, 2016, from http://www.parkafm.com/index.php/park-afm-modes/nanomechanical-modes
 17.3.3 Phase Imaging. (n.d.). In W. N. Sharpe (Ed.), Spring Handbook of Experimental Solid Mechanics (p. 430). Springer Science & Business Media.
 10.3 Nanofibers as Filters. (n.d.). In Kiekens, P., & Jayaraman, S. (Ed.), Intelligent Textiles and Clothing for Ballistic and NBC Protection: Technology at the Cutting Edge. (p.210). Springer Science & Business Media.
This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.
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