Disentangling High Strength Copolymer Aramid Fibers to Enable the Determination of Their Mechanical Properties

The copolymer aramid fibers studied here are difficult to separate from yarn bundles into individual fibers for testing. The fibers are entangled and coated with processing chemicals that make them very difficult to separate without damaging the fibers. Figure 3 shows the structural morphology of fibers within a yarn. Even as part of a larger bundle, the fiber surfaces show extensive roughness and tears that are likely caused by strong adhesion to adjacent fibers. In previous work by McDonough⁴, et al., water was used to separate the fibers prior to tensile testing, however, the chemical analysis of fibers prepared using this method raised important questions regarding the sample preparation and its effects on mechanical properties. In the first part of this work, the effectiveness of three different solvents (chosen by the elimination from the entire polarity range of solvents), including water, are compared using SEM to examine the effect of different washing protocols on the physical appearance of the separated fibers. The water and acetone immersed fibers were rinsed in methanol after washing to remove any solvent residue and aid in drying the water immersed fibers more quickly. Figure 2 shows an overview of the fiber bundle disentanglement procedure. The washed fibers are also compared against as received fibers that were separated from the yarn bundle without any further sample preparation. Resulting micrographs are presented in Figure 4, Figure 5, Figure 6.

In Figure 4a, note that the physical damage to the PBIA-co-PPTA1 fiber in the form of fibrillation when the "dry" fiber was separated without the use of any solvents. Also note the presence of flaking and longitudinal grooves on the fiber surfaces due to immersion in water (Figure 4b), which could be indicative of degradation mechanisms such as hydrolysis, or caused by incomplete removal of the chemical coating from the fiber. These features are moderately observed in the methanol (Figure 4c)) and acetone (Figure 4d) immersed fibers, but the acetone immersed fiber appears to have the least solvent-induced damage and predominantly exhibits a clean and smooth surface. As the primary goal of the study was to develop a methodology to separate individual fibers for mechanical testing while ensuring minimal fiber damage (physical or chemical) during the separation process, traces of residual chemical coating can be observed in SEM images of the washed fibers (Figure 5a). The goal was not to completely dissolve the coating, just enough to be able to separate the yarns with minimal damage.

In Figure 5a, the physical damage to the PBIA-co-PPTA3 fiber in the form of longitudinal grooves and fibrillation are observed especially at the fiber edges of the "dry" fiber separated without any immersion. The fiber immersed in water (Figure 5b) also shows some damage to the edges where it appears to have been adhered to an adjacent fiber prior to the separation. The methanol (Figure 5c) and acetone (Figure 5d) immersed fibers both show much less fibrillation, but as observed previously, the fibers immersed in acetone qualitatively appear to have lesser surface artifacts than the other fibers.

In Figure 5, the physical damage to the dry PBIA is observed to be less severe than the other two fibers, but there is some evidence of longitudinal grooves along the fiber in the lower part of the image (Figure 6a). The fiber immersed in water (Figure 6b) shows minor damage at the edges caused by strong attachment to an adjacent fiber. The methanol and acetone immersed fibers (Figure 6c-d) show similar physical characteristics as the water immersed fiber.

To further examine the effect of the acetone rinsing on the fibers, FTIR spectroscopy was performed. The result of this analysis is presented in Figure 7. Some intensity changes are observed after washing, but no major changes in spectra indicative of chemical degradation (e.g., changes in the OH/NH region around 3300 cm^-1 or the formation of a carbonyl peak around 1700 cm^-1) are observed. Therefore, the acetone rinsing procedure was selected as the best fiber preparation method for the rest of the study.

The next step in this study was to determine the best method of tensile testing single fibers with the existing equipment setup. An effort was made to directly test fibers by mounting fibers in the grips and performing the test. As this method requires minimal sample preparation, and the samples exhibited no slippage from the grip, this was regarded as the most rapid way to perform the test. However, most of the fibers tested in this manner broke right at the grip face, a phenomenon known as a "jaw break". As described in ASTM D3822¹⁴, this result indicates that the test is invalid. Therefore, based on the recommendations of the ASTM D3822 standard, the single fibers were then mounted on a cardstock template before testing.

The fibers were adhered to the cardstock templates using either epoxy or cyanoacrylate and allowed to cure for at least 24 h before testing. Two types of epoxy adhesive were tested, one requiring a 24 h cure and the other requiring a 1 h cure. Nearly all samples adhered to the paper templates with the epoxy adhesive (both the slow and the fast cure) exhibited an uncharacteristic slipping behavior and jagged stress strain curves, as shown by the representative example given in Figure 8a. However, Figure 8b depicts a representative stress-strain curve obtained with the cyanoacrylate adhesive, which is predominantly devoid of sample slippage. A similar behavior was observed in all the fiber systems used in the current study, thereby making cyanoacrylate the most suitable test adhesive to glue the fibers on the templates. Following the success of cyanoacrylate adhesive, all samples were tested according to the recommendations provided in a previous study on single fiber testing of polyethylene¹⁴. Overall, the fibers adhered with the cyanoacrylate generally had smooth and continuous stress-strain curves and didn't exhibit significant slippage. While a few fibers failed near the top of the gauge area of the fiber, the use of the template helped us exclude these fibers effectively.

After settling on the cardstock template and cyanoacrylate glue method, the tensile strength and strain to failure of all three fibers could be measured. The results of these tests are presented in Table 1. For each fiber type, 35 samples were tested, and the fourth column of the table reports the successful number of tests in each data set (between 15 and 26 tests). A nominal diameter of 14 µm was used to compute the tensile strength for all fibers, based on previous work and measurements from micrographs of more than 30 fibers. SEM imaging of the failed fibers (Figure 9) indicates that all the fibers undergo brittle fracture resulting in fibrillation. As the fibers used in the current study are mostly non-crystalline (as shown by the wide angle X-ray scattering (WAXS) measurements in Figure 10), minimal plastic deformation is observed in SEM images of these fiber cross-sections, with no evidence of necking.

The mounting configuration for the WAXS analysis is shown in Figure 1, and the results of this analysis are presented in Figure 10. The WAXS analysis indicated that the equatorial diffraction scattering of the PBIA, PBIA-co-PPTA1, and PBIA-co-PPTA3 fibers was very similar, consisting of a broad asymmetrical peak at a 2θ of about 22°. This is indicative of a non-crystalline structure with an absence of orientation in the plane perpendicular to the polymer chain axis. However, the diffraction pattern and the diffractograms of the meridional scattering revealed the presence of two major Bragg peaks at 2θ angles of about 26° and 28° (Figure 10). The strongest peak of the two is at a 2θ of 28° with a d-spacing of about 0.31 nm, and is also present on the meridional diffraction scans of typical PPTA fibers¹⁵. The fact that these Bragg peaks on the PBIA, PBIA-co-PPTA1, and PBIA-co-PPTA3 fibers are very weak is indicative of the very low amount of the PPTA linkages in the copolymer structure of these fibers. Additionally, the PBIA-co-PPTA1 and PBIA-co-PPTA3 fibers diffractograms of the meridional scattering revealed the presence of two weak peaks at 2θ angles of about 18° and 21°. Ultimately, these fibers show a very low degree of crystallinity along the chain axis.

Figure 1: An illustrative process to show the methodology for mounting fibers on washers for analysis by WAXS. The silver behenate control sample is not pictured in this photograph. Please click here to view a larger version of this figure.

Figure 2: An illustrative process to disentangle a single fiber from the yarn bundle for tensile testing. Please click here to view a larger version of this figure.

Figure 3: Representative scanning electron micrographs of the fibers within a fiber yarn. (a) PBIA-co-PPTA1, (b) PBIA-co-PPTA3, and (c) PBIA. Please click here to view a larger version of this figure.

Figure 4: Representative scanning electron micrographs of separated single fibers of PBIA-co-PPTA1 after treatment. (a) Separated dry fiber (no immersion), (b) fiber after immersion in water, (c) fiber after immersion in methanol, and (d) fiber after immersion in acetone. Please click here to view a larger version of this figure.

Figure 5: Representative scanning electron micrographs of separated single fibers of PBIA-co-PPTA3 after treatment. (a) Separated dry fiber (no immersion), (b) fiber after immersion in water, (c) fiber after immersion in methanol, and (d) fiber after immersion in acetone. Please click here to view a larger version of this figure.

Figure 6: Representative scanning electron micrographs of separated single fibers of PBIA after treatment. (a) Separated dry fiber (no immersion), (b) fiber after immersion in water, (c) fiber after immersion in methanol, and (d) fiber after immersion in acetone. Please click here to view a larger version of this figure.

Figure 7: Representative ATR-FTIR spectra of as received dry (black) and acetone washed (red) fibers. Other than slight intensity changes, no major differences indicating chemical changes were observed in the fibers before and after washing. All spectra presented are the average of at least 3 measurements and were collected at a resolution of 4 cm^-1. The standard uncertainty in absorbance for this technique is approximately 5%. Please click here to view a larger version of this figure.

Figure 8: Representative stress-strain curves of PBIA-co-PPTA1 fiber prepared with epoxy adhesive (left) and cyanoacrylate adhesive (right). Note the jagged character of the epoxy curve and the higher strain to failure, which may be representative of slippage in the adhesive. Please click here to view a larger version of this figure.

Figure 9: Scanning electron micrographs of failed single fiber cross-sections after acetone treatment: (a) PBIA-co-PPTA1, (b) PBIA-co-PPTA3, and (c) PBIA. All fiber specimens exhibit fibrillation and brittle fracture. Please click here to view a larger version of this figure.

Figure 10: Wide-angle X-ray diffraction patterns of PBIA-co-PPTA3 (a), PBIA-co-PPTA1 (c), and PBIA (e) fibers. Meridional wide-angle X-ray diffractograms of PBIA-co-PPTA3 (b), PBIA-co-PPTA1 (d), and PBIA (f) fibers. Please click here to view a larger version of this figure.

Table 1: Mean single fiber tensile properties of acetone washed PBIA-co-PPTA1, PBIA-co-PPTA3, and PBIA. The standard deviation is reported in parentheses next to the value.