Filament-based flexible material extrusion additive manufacturing is highly desirable yet faces critical challenges such as limited print resolution and repeatability. To enhance printability of a thermoplastic elastomer (TPE), a series of core-shell filaments comprising a TPE shell (Shore hardness 75 A) and a rigid ABS core are fabricated, with the ABS volume fraction varying from 11% to 78%. The presence of an ABS core imparts rigidity to the filament to inhibit buckling and allow for successful high-fidelity 3D printing. Rheological characterizations of TPE and ABS using capillary and parallel-plate viscometry point to the optimized extrusion parameters suitable for filament coextrusion, printability, and wettability between the print interfaces. Complex benchmark geometries are printed to verify the print resolution. Printed specimens with less than 20% ABS preserve the shore hardness of 75 A, providing flexibility and a soft touch to the printed structures. Izod impact, 3-point bending, and tensile tests reveal tunable mechanical properties of parts printed in z-direction with ABS+TPE filaments. Lower ABS content exhibits higher flexibility and impact resistance, while higher ABS imparts higher stiffness and tensile strength. Notably, the parts printed with 30-40% ABS exhibit higher toughness and impact strength than specimens composed of ABS or TPE alone. Lastly, a coil spring printed with a ABS+TPE filament exhibits reversible extension up to 120 mm, at least twice the reversible extension of an ABS-printed coil spring.

A detailed understanding of all phenomena that determine high-rate impact response of woven polyaramid systems is needed to design more effective materials and save lives. This study investigates the detailed nanostructures of Kevlar® KM2® Plus fibers from woven systems subjected to controlled rheometric deformations and high-rate impact. The rheometric tension, axial compression following tensile failure, and transverse compression are basic deformations not previously studied by our methods and hypothesized in high-rate impact. The impact of a high-rate object causes significant, layer-dependent nanostructural changes to woven systems. Multichannel AM-FM atomic force microscopy characterizes interior fiber nanostructure with spatially-resolved topological and viscoelastic property measurements of pleat lengths, crystal misorientation angles, fibril widths, and void widths. Statistical analyses were developed to assess convergence of nanostructure feature measurement distributions and compare the correlated nanostructure feature distributions between samples. The data and analyses indicate that tension is not the only mechanism of energy transfer from the impact of a high-rate object and that compressions, both in the axial and transverse directions, are also significant. The varying localized responses to high-rate impact and the presence of both tension and compression imply better systems can be created by improving tensile and compressive properties, tailoring fabric layers to perform to specific types of mechanical deformations.

While significant research has investigated the processing and properties of high-performance terpolymer fibers, much remains to be understood about the internal nano- and microstructures of these fibers, and how these morphologies relate to fiber properties. Here we use a focused ion beam notch technique and multifrequency atomic force microscope mapping to characterize the internal structure and local mechanical properties within Technora® fibers. We find a highly fibrillated structure that appears to connect with both the fiber's molecular chemistry and full-fiber mechanical properties. In addition, through detailed comparisons with Kevlar® K29 fibers, we find remarkable differences between the internal structures of the two fibers, and posit connections between our measurements and multifunctional performance studies from the literature.

High performance ultra high molecular weight polyethylene (UHMWPE) fibers are primarily composed of extended chain (shish) and lamellae (kebab) crystalline domains. Characterization of these architectures commonly utilizes x-ray scattering techniques, which rely upon models to transform from inverse scattering space to real space. Scattering models for lamellae and straight chain crystalline domains were developed and intended for use in ideal scattering scenarios; ie. purely extended chain or purely lamellae. Simultaneous implementation of both models with real (non-ideal) fibers takes for granted the validity of either model in the presence of the other crystalline feature. In this work, we utilize atomic force microscopy (AFM) to directly couple real space stiffness measurements to the analysis of scattering data. The validity of scattering models is tested against real space image analysis and conclusions are drawn regarding the analysis of scattering data to quantify crystalline domain sizes. We find that the lamellae long spacing estimated from scattering data is consistent with real space image analysis. Whereas significant differences are observed for the analysis of lamellae diameter. The characteristic size of extended chain crystalline domains is measured via the streak analysis method on both SAXS 2D scattering data and the Fast Fourier Transform (FFT) of AFM stiffness maps. The streak analysis results in comparable length scales when applied to SAXS and FFT AFM images. A comparison of the streak analysis with novel filtered AFM images suggest that the streak analysis is capturing continuity of straight chain crystalline domains.

The processing conditions used in the production of advanced polymer fibers facilitate the formation of an oriented fibrillar network that consists of structures spanning multiple length scales. The irregular nature of fiber tensile fracture surfaces suggests that their structural integrity is defined by the degree of lateral (interfacial) interactions that exist within the fiber microstructure. To date, experimental studies have quantified interfacial adhesion between nanoscale fibrils measuring 10-50 nm in width, and the global fracture energy through applying peel loads to fiber halves. However, a more in-depth evaluation of tensile fracture indicates that fiber failure typically occurs at an intermediate length scale, involving fibrillation along interfaces between fibril bundles of a few 100s of nanometers in width. Interaction mechanisms at this length scale have not yet been studied, due in part to a lack of established experimental techniques. Here, a new focused ion beam-based sample preparation protocol is combined with nanoindentation to probe interfaces at the intermediate length scale in two high-performance fibers, a rigid-rod poly(p-phenylene terephthalamide) and a flexible chain ultrahigh molecular weight polyethylene fiber. Higher interfacial separation energy recorded in the rigid-rod fiber correlated with less intensive fibrillation during failure and is discussed in the context of fiber chemistry and processing. Power law scaling of the total absorbed interfacial separation energy at three different scales in the polyethylene fiber is observed and analyzed, and distinct energy absorption mechanisms, featuring a degree of self-similarity, are identified. The contribution of these mechanisms to the overall integrity of the fiber is discussed, and the importance of the intermediate scale is elucidated. Results from this study provide new insights into the mechanical implications of hierarchical lateral interactions and will aid in the development of novel fibers with further improved mechanical performance.

Real-space methods of characterizing high-performance fibers’ inherent morphologies will greatly enhance our understanding of the key structural features within fibers and their impacts on mechanical performance. Here, we report on structure–property correlations of two new classes of commercial DuPont Kevlar fibers, termed “K29 sample test” and “K49 sample test,” as well as conventional K29 and K49 fibers. * Through multifrequency atomic force microscope scans of internal fiber surfaces prepared by a focused ion beam notch technique, we directly capture nano- and microstructural features that define the inherent structures of these fibers. Integrating these findings with X-ray scattering experiments, we relate crystallographic and real-space measurements to each other, highlighting how multiscale structural motifs manifest within fibers. By carrying out tensile tests on single fibers drawn from the same tows, we also glean new insights into the structure–property relationships that dictate the mechanical behavior of these fibers.

ABSTRACTThe goal of this research is to quantify the fibrillar adhesive energy in ultra‐high molecular weight polyethylene fibers, characteristic of nanoscale fibril interactions. Quantification of these energies is vital to the understanding of fibrillar deformation mechanisms that have been shown to play an important role in fiber performance. This is achieved through the development and implementation of a nanosplitting technique developed through the use of AFM‐enabled nanoindentation. This technique allows the quantification of nanoscale adhesive energies through careful monitoring of load and unload curves as well as examination of the residual split through high‐resolution AFM images. Results indicate that the average nanoscale fibril adhesive energy is over 3 times larger than the energy expected from van der Waals interactions alone. This indicates that a significant degree of physical interactions exist between fibrils, beyond van der Waals interactions, in the form of tie‐molecules, fibrillar network junctions, and bridging lamellar crystals. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 391–401

High-performance materials like ballistic fibers have remarkable mechanical properties owing to specific patterns of organization ranging from the molecular scale, to the micro scale and macro scale. Understanding these strategies for material organization is critical to improving the mechanical properties of these high-performance materials. In this work, atomic force microscopy (AFM) was used to detect changes in material composition at an extremely high resolution with transverse-stiffness scanning. New methods for direct quantification of material morphology were developed, and applied as an example to these AFM scans, although these methods can be applied to any spatially-resolved scans. These techniques were used to delineate between subtle morphological differences in commercial ultra-high-molecular-weight polyethylene (UHMWPE) fibers that have different processing conditions and mechanical properties as well as quantify morphology in commercial Kevlar®, a high-performance material with an entirely different organization strategy. Both frequency analysis and visual processing methods were used to systematically quantify the microstructure of the fiber samples in this study. These techniques are the first step in establishing structure-property relationships that can be used to inform synthesis and processing techniques to achieve desired morphologies, and thus superior mechanical performance.

Advancements in processing techniques have facilitated the development of a series of unique high-performance DuPont™ Kevlar® fibers. Directly characterizing interior structures of these fibers is vital for connecting processing variations with resulting mechanical properties. Here we report detailed investigations of processing-internal structure-property relationships among four distinct classes of Kevlar® fibers (K119, K29, KM2+, and K49). We employ a novel focused ion beam (FIB) notch technique to obtain pristine interior fiber planes and measure morphology and transverse stiffness across these surfaces via multifrequency atomic force microscopy (AFM) maps. Combining these observations with tensile tests of single fibers drawn from the same tows, we acquire remarkable insights into structural motifs that strongly correlate with measured mechanical properties. Most notably, we uncover alternating stiffness bands that are especially apparent in low-modulus, non-heat-treated fibers, revealing a unique manifestation of internal structural variations that helps explain the observed trends in material response.

The potential advantages of cell-based biohybrid devices over conventional nonliving systems drive the interest to control the behavior of the underlying biological cells in microdevices. Here, the authors studied how shear influenced the geometry and elongation of fimbriated filaments on affinity substrates. The cells were engineered to express FimH, which binds to mannose with a high affinity. A microfluidic channel was functionalized with RNAse B, which is rich in mannose residues, and the device was used to control the hydrodynamic force on live Escherichia coli under filamentous growth. It was discovered that filamentous E. coli cells adopt buckled geometry when the shear rate is low, but assume an extended geometry at high shear and align with the flow direction. The extension moves from bidirectional to preferentially downstream as the shear rate increases. Furthermore, living filaments slide easily on the substrate, and detach from the substrates at a rate nearly ten times greater than unfilamented live E. coli at high shear conditions (1000-4000 s-1). The hydrodynamic force and binding force experienced by the cells are further analyzed by COMSOL simulation and atomic force microscopy measurements, respectively, to explore the mechanism behind the living cell dynamics. Knowledge from this work helps guide design of interfacial properties and shear environments to control the geometry of living filamentous bacteria.

The unique thermo-electro-mechanical properties of polycrystalline silicon carbide (poly-SiC) make it a desirable candidate for structural and electronic materials for operation in extreme environments. Necessitated by the need to understand how processing additives influence poly-SiC structure and electrical properties, the distribution of lattice defects and impurities across a specimen of hot-pressed 6H poly-SiC processed with p-type additives was visualized with high spatial resolution using a conductive atomic force microscopy approach in which a contact forming a nano-Schottky interface is scanned across the sample. The results reveal very intricate structures within poly-SiC, with each grain having a complex core-rim structure. This complexity results from the influence the additives have on the evolution of the microstructure during processing. It was found that the highest conductivities localized at rims as well as at the interface between the rim and the core. The conductivity of the cores is less than the conductivity of the rims due to a lower concentration of dopant. Analysis of the observed conductivities and current-voltage curves is presented in the context of nano-Schottky contact regimes where the conventional understanding of charge transport to diode operation is no longer valid.

Wind tunnel experiments examined the coupled effects of relative humidity (RH) and surface and particle properties on aerodynamically induced resuspension. Hydrophilic glass spheres and hydrophobic polyethylene spheres similar to 20 mu m in diameter, with nanoscale surface features, were resuspended from hydrophilic glass, hydrophobic chemical agent resistant coating (CARC), and gold surfaces. Roughness of the glass and gold surfaces was on the nanoscale, whereas CARC surfaces had microscale roughness. Different particle-surface combinations yielded van der Waals interactions that varied by a factor of 4, but these differences had a relatively minor effect on resuspension. Wind tunnel RH was varied between 7% and 78%. Overall, RH affected the resuspension of hydrophilic particles on hydrophilic surfaces most strongly and that of hydrophobic particles on hydrophobic surfaces the least. For each particle-surface combination there was a threshold RH value below which resuspension rates were essentially constant and in good agreement with a dimensionless model of particle resuspension.Copyright (c) 2016 American Association for Aerosol Research

This work elucidates the undisturbed interior morphology of commercial ultra-high-molecular-weight polyethylene (UHMWPE) high-performance fibers through atomic force microscopy (AFM) modulus mapping of interior surfaces exposed by a novel focused ion beam (FIB)-notched sample preparation technique. The imaging shows unequivocally the presence of epitaxial crystals in the interior of highly drawn UHMWPE fibers. Overall, AFM observations and measurements were made for three different commercial fiber types, illustrating five basic UHMWPE morphologies: (i) extended chain fibrils (shish), (ii) interlocking and (iii) standalone kebab or epitaxial crystals, (iv) voids or interface between fibrils, and (v) tie chains across voids. Furthermore, microfibrils were bundled into groups separated by voids or amorphous material. Bundled groups of microfibrils group together to form macrofibrils, typically separated by larger voids or amorphous material. Additionally, stretched tie-chain bridges provide connectivity between some fibrils. Each of these five features (extended chain fibrils, interlocking and standalone epitaxial crystals, voids and tie-chains) varies across the three types of commercial fibers, and sometimes across the individual fiber interiors. AFM contact modulus values measured transverse to the fiber draw axis were found to vary considerably within different morphological domains. The distribution of morphology sizing was quantified for each of the examined fibers. The measurement of internal facets have major implications for fiber modeling and processing, such as optimization of draw ratios to affect the extent and arrangement of epitaxial crystalline morphologies and thus improve mechanical performance.

This work introduces an innovative technique to characterize the internal morphology of high-performance fibers by using a focused ion beam (FIB) sample preparation method and subsequent atomic force microscopy (AFM). A FIB is used to mill opposing notches that facilitate direct failure along a longitudinal shear plane, and expose the internal surface of the fiber. By exposing this surface via longitudinal shear, distortion of the cleaved surface is minimal, which is an advantage over surface preparation methods such as microtoming. After cleaving the notched fibers, an AFM technique is used to generate modulus maps of the fiber fracture surfaces. These modulus maps provide qualitative and quantitative microstructural information. Initial results obtained from Kevlar KM2 and Dyneema SK76 fibers are presented and a brief analysis of the observed internal features is provided. Extending the technique to image internal features in other materials is also discussed.

The data included here provides a basis for understanding "Interior morphology of high-performance polyethylene fibers revealed by modulus mapping" (K.E. Strawhecker, E.J. Sandoz-Rosado, T.A. Stockdale, E.D. Laird, 2016) [1], in specific: the multi-frequency (AMFM) atomic force microscopy technique and its application to ultra-high-molecular-weight Polyethylene (UHMWPE) fibers. Furthermore, the data suggests why the Hertzian contact mechanics model can be used within the framework of AMFM theory, simple harmonic oscillator theory, and contact mechanics. The framework is first laid out followed by data showing cantilever dynamics, force-distance spectra in AC mode, and force-distance in contact mode using Polystyrene reference and UHMWPE. Finally topography and frequency shift (stiffness) maps are presented to show the cases where elastic versus plastic deformation may have occurred.

ABSTRACTWe describe an experimental approach for characterizing the local mechanical behavior of acrylonitrile butadiene styrene (ABS) structures processed through fused deposition modeling. ABS test specimens processed in various build orientations were subject to multiscale mechanical tests as well as local morphology and chemical analyses. Instrumented indentation, local dynamic mechanical analysis, and atomic force microscopy tests were used to explore the mechanical behavior and morphology of build surfaces and weld interfaces. An interfacial stiffening effect was found for the majority of the specimens tested, with up to a 40% increase in the indentation elastic modulus measured with respect to the build surfaces. Raman spectroscopy mapping of the interfacial areas revealed ∼30% less butadiene/styrene and butadiene/acrylonitrile ratios with respect to analysis of the build surfaces. The results provide insight into the multiscale behavior of additive manufactured structures and offer the potential to guide processing–structure–property understanding of these materials. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43671.