Structure and defects in high-performance aramid fibers
High performance aramid fibers, such as Kevlar, AuTx, and Twaron have been designed for ultra-high specific strength and stiffness. These fibers derive their superior properties from the high degree of molecular orientation that allows for efficient load transfer to the backbone covalent bond axis. While the experimental Young’s modulus of aramid fibers approaches the theoretical limits, the measured mechanical strength is quite lower than the predicted values. The objective of this dissertation research was to obtain an understanding of the subtle differences in the structure of commercial grade aramid fibers with different tensile strength and modulus values. Towards this goal, single fiber tension experiments were conducted with various fiber gauge lengths to evaluate the existence of fiber length size effects and, therefore, a statistical distribution of defects limiting the fiber strength. The test results were supported by several studies of the fiber structure via various modalities of high resolution microscopy. Uniaxial tension tests were carried out with AuTx and KM2 fibers with gauge lengths between 100 µm and 10 mm. The average strength of KM2 fibers was 4.4 ± 0.4 GPa, with a high value of 5.2 GPa and a low value of 3.5 GPa. The average tensile strength decreased by 20% as the gauge length increased from 100 µm to 10 mm. The tensile modulus of KM2 fibers was calculated to be 85.6 ± 3.7 GPa. Similarly, the average strength of AuTx fibers was 5.5 ± 0.7 GPa, with a high value of 6.8 GPa and a low value of 4.4 GPa. A reduction of approximately 20% in the average tensile strength value was also established when the gauge length increased from 100 µm to 10 mm. The relative insensitivity of the fiber tensile strength to the gauge length suggests that failure in both kinds of aramid fibers could be due to processes or defects at length scales well below the micron-scale. High resolution SEM images were obtained to understand the surface morphology and the internal structure of KM2 and AuTx fibers. A process of crushing and splitting of KM2 fibers revealed the presence of circular microfibril building blocks that were as small as 5 nm in diameter, with lengths exceeding 500 nm. AuTx fibers formed ribbon-like fibrils when deformed, with finer and more ordered microfibrils compared to those found in the fracture sections of KM2 fibers. Furthermore, the fiber failure sections from tensile tests revealed several distinguishing features. Both types of fibers had distinct skin but with different thickness and morphology. Specifically, the skin of KM2 fibers was relatively featureless in comparison to AuTx. The core of both types of aramid fibers was comprised of microfibrils that were qualitatively more ordered and better oriented in AuTx than in KM2 fibers. Webs of individual microfibrils formed in the interior of failed KM2 and AuTx fibers, whose structure provided evidence for stronger cohesive forces present in AuTx compared to KM2 fibers. AFM imaging was conducted both on the fiber surface and on fiber cross-sections obtained by a microtome blade. Ordinary AFM tips (tip radius of ~10 nm) resolved surface microfibrils that were aligned with the axis of KM2 and Twaron fibers, while more complex surface features were evidenced in AuTx fibers. Ultra-sharp AFM tips (tip radius of ~2 nm) revealed a crystallite-like structure of the microfibrils on the surface of KM2 fibers. Longitudinal microtome fiber cuts allowed for excellent resolution images, especially with the use of 2-nm radius AFM tips. Unfortunately, transverse microtome fiber cuts were subjected to severe knife damage which prevented detailed AFM imaging. Finally, TEM imaging was carried out on microtome cut fiber sections. The aramid fibers were resistant to all applied staining methods, and, therefore, the additional information from TEM imaging was limited. KM2 fiber sections demonstrated periodic defect banding along the fiber axis, with larger microtome knife damage near the fiber skin. Meanwhile, AuTx fiber cuts demonstrated a mix of low damage areas, and areas covered with ripples. Finally, transverse microtome sections of AuTx showed both extensive tearing and folding, which prohibited their use for high magnification TEM imaging.
kevlar, aramid, high-performance fiber, AuTx, polymer, liquid crystal
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign