The evolution of structure and properties in poly(p-phenylene terephthalamide) fibers

Probabilistic Approach to Low Strain Rate Atomistic Simulations of Ultimate Tensile Strength of Polymer Crystals

Molecular dynamics simulations of the tensile ultimate properties of polymer crystals require the use of empirical potentials that model bond dissociation. However, fully reactive potentials are computationally expensive such that reactive simulations cannot reach the low strain rates of typical experiments. Here, we present a hybrid approach that uses the simplicity of a classical, nonreactive potential, information from bond dissociation energy calculations, and a probabilistic expression that mimics bond breaking. The approach is demonstrated for poly(p-phenylene terephthalamide) and, with one tunable parameter, the calculated tensile ultimate stress matches that obtained using a fully reactive simulation at high strain rates. Then, the hybrid simulations are run at much lower strain rates where the ultimate tensile stress is strain rate-independent and consistent with the expected experimental range.

Effect of thermally induced microstructural changes on the mechanical properties and ballistic performance of poly (p-phenylene terephthalamide) fibers

Microstructural variations have a strong influence on the load transfer capacity of the high-performance polymeric fibers, which is also reflected in their ballistic property changes. The focus of the present study is to investigate thermally induced microstructural changes and their reflection on the mechanical properties and theoretical ballistic limit of poly ( p-phenylene terephthalamide) fibers by a correlation. From the quantitative analysis of XRD, thermally induced changes in unit cell a-dimension show profound sensitivity in affecting the tenacity and modulus of the fibers. Based on the physicochemical changes in FTIR and FESEM analysis, significant surface deterioration and changes in the chemical network are observed. However, dimensional variations of the crystal structure along a-direction show a stronger influence than the chemical and morphological changes, reflecting sigmoidal responses with tenacity, modulus and theoretical V50 by correlations. As an effect of unit cell dimensional variation, changes in crystallinity are resulted and lead to the loss in theoretical ballistic limit of the fibers by following first-order kinetics. Lastly, angular separation and (200) orientation angle are determined to build a global correlation with modulus and theoretical ballistic limit for quickly decoding macro-changes in terms of micro-properties. The given correlations can help to identify crystallographic transformations upon other induction techniques and view their effect on mechanical and ballistic parameters. In addition, the given approach can be extended for different ballistic materials under any environmental conditions.

Ferrous-Oxalate-Modified Aramid Nanofibers Heterogeneous Fenton Catalyst for Methylene Blue Degradation

The heterogeneous Fenton system has drawn great attention in recent years due to its effective degradation of polluted water capability without limitation of the pH range and avoiding excess ferric hydroxide sludge. Therefore, simple chemical precipitation and vacuum filtration method for manufacturing the heterogeneous Fenton aramid nanofibers (ANFs)/ferrous oxalate (FeC₂O₄) composite membrane catalysts with excellent degradation of methylene blue (MB) is reported in the study. The morphology and structure of materials synthesized were characterized by scanning electron microscope (SEM), X-ray energy spectrum analysis (EDS), infrared spectrometer (FTIR), and X-ray diffraction (XRD) equipment. The 10 ppm MB degradation efficiency of composite catalyst and ferrous oxalate (FeC₂O₄) within 15 min were 94.5% and 91.6%, respectively. The content of methylene blue was measured by a UV-Vis spectrophotometer. Moreover, the dye degradation efficiency still could achieve 92% after five cycles, indicating the composite catalyst with excellent chemical stability and reusability. Simultaneously, the composite catalyst membrane can degrade not only MB but also rhodamine B (RB), orange II (O II), and methyl orange (MO). This study represents a new avenue for the fabrication of heterogeneous Fenton catalysts and will contribute to dye wastewater purification, especially in the degradation of methylene blue.

Thermal insulating, light-weight and conductive cellulose/aramid nanofibers composite aerogel for pressure sensing

Conductive nanocellulose aerogels have attracted significant attention in pressure sensing for wearable devices owing to lightweight, sustainability and good chemical stability. Limited by its flammability and weak mechanical properties, aramid nanofiber (ANF) was designed as reinforcement to overcome the shortcoming mentioned above. Herein, the unidirectional freeze casting method was proposed to fabricate nanocellulose/aramid nanofiber (CA) aerogel. Then, the CA/PPy (CAP) aerogel was obtained by using the oriented structure of CA aerogel as a template for inducing conductive polypyrrole (PPy) in-situ formation inside the composite aerogel. The conductive aerogel with the ordered microstructure exhibited the anisotropic mechanical properties and thermal conductivity. And it could withstand high temperature without any destruction phenomenon. Moreover, the aerogel sensor revealed high strain sensitivity and satisfactory electrochemical performance. Lightweight CAP aerogel with controllable alignment, sensitive sensing property and thermal stability is very promising in pressure sensor under some extreme conditions.

The introduction of benzimidazole and ether moieties into poly(p-phenylene terephthalamide): effects on its microstructure, interactions and properties

Co-poly(p-phenylene terephthalamide) (co-PPTA) fibers containing 4,4′-oxidianiline (4,4′-ODA) and 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) in terephthaloyl chloride (TPC) and p-phenylene diamine (p-PDA) were prepared via a wet spinning method, followed by water washing and drawing at a high temperature. With the addition of a new acid-binding agent, imidazole, the solution prepared by low-temperature polycondensation had suitable viscosity for spinning. Herein, the properties of six co-PPTA fibers with different contents of BIA and 4,4′-ODA segments were studied. The mechanical properties of the co-PPTA fibers were improved with the addition of BIA and ODA; they reached the optimum tensile strength of 2.45 GPa at a p-PDA/ODA/BIA molar ratio of 2/4/4, which showed a higher degree of orientation and the highest crystallinity, and the strength further increased on increasing the thermal drawing ratio. X-ray diffraction indicated that the fibers exhibited highly ordered structures, while two-dimensional wide angle X-ray diffraction showed that molecular packing regions with highly oriented structures were formed. In addition, the co-PPTA fibers exhibited excellent thermal stability when the 5% weight loss temperature was above 492 °C under nitrogen, and glass transition occurred at about 290 °C.

Co-poly(p-phenylene terephthalamide) fibers containing 4,4′-oxidianiline and 2-(4-aminophenyl)-5-aminobenzimidazole in terephthaloyl chloride and p-phenylene diamine were prepared via a wet spinning method, followed by water washing and drawing at high temperature.

From Monomers to a Lasagna-like Aerogel Monolith: An Assembling Strategy for Aramid Nanofibers

The manipulation of nanobuilding blocks into a 3D macroscopic monolith with ordered hierarchical structures has been much desired for broad and large-scale practical applications of nanoarchitectures. In this paper, we demonstrate a fully bottom-up strategy for the preparation of aramid aerogel monoliths. The process starts from the synthesis of poly(p-phenylene terephthalamide) (PPTA) through the polycondensation of p-phenylenediamine and terephthaloyl chloride, with the assistance of a nonreactive dispersing agent (polyethylene glycol dimethyl ether), which helps the dispersal of the as-synthesized PPTA in an aqueous medium for the formation of p-aramid nanofibers (ANF). Then the vacuum-assisted self-assembly (Vas) technique is skillfully connected with the ice-templated directional solidification (I) technique, and the combined VasI method successfully tailors the self-assembly of ANF to transform the 1D nanofibers into a 3D aerogel monolith with a specific long-range aligned, lasagna-like, multilaminated internal structure. The study of the aerogel microstructure revealed the dependence of the lamina orientation on the direction of the freezing front of ice crystals. This direction should be parallel to the deposition plane of the Vas process if a long-range aligned lamellar structure is desired. The anisotropy of the multilaminated aerogel was proven by the different results in the radial and axial directions in the compression and thermal conductivity tests. As a kind of organic aerogel, the ANF monolith has typical low density, high porosity, and low thermal conductivity. Additionally, the ANF monolith exhibits high compressive stress and excellent thermal stability. Considering its high performance and facile preparation process, potential applications of the ANF aerogel monolith can be expected.

Study on Crystallization Behaviors and Properties of F-III Fibers during Hot Drawing in Supercritical Carbon Dioxide

In order to obtain F-III fibers with high mechanical properties, pristine F-III fibers were hot drawn at the temperature of 250 °C, pressure of 14 MPa, tension of 6 g·d⁻¹, and different times, which were 15 min, 30 min, 45 min, 60 min, 75 min, 90 min, and 105 min, respectively, in supercritical carbon dioxide (Sc-CO₂) in this article. All the samples, including the pristine and treated F-III fibers, were characterized by a mechanical performance tester, wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and thermogravimetric analysis (TGA). The results showed that the thermal stability of F-III fibers was enhanced to some extent, and the tensile strength and modulus of F-III fibers had great changes as the extension of treatment time during hot drawing in Sc-CO₂, although the treatment temperature was lower than the glass transition temperature (Tg) of F-III fibers. Accordingly, the phase fraction, orientation factor f_c of the (110) crystal plane, fibril length l_f, and misorientation angle B_φ of all the samples were also investigated. Fortunately, the hot drawing in Sc-CO₂ was successfully applied to the preparation of F-III fibers with high mechanical properties.

Effect of Different Pressures on Microstructure and Mechanical Performance of F-III Fibers in Supercritical Carbon Dioxide Fluid

F-III fibers were treated at different pressures in supercritical carbon dioxide fluid and all samples including untreated and treated F-III fibers were characterized by a mechanical performance tester, wide-angle X-ray scattering and small-angle X-ray scattering. By studying the relationship between mechanical performance and microstructural changes of the samples, it was found that microstructural change was the main cause of variation in mechanical performance. Results revealed that the maximum tensile strength and modulus of F-III fibers were acquired at 14 MPa within the pressure range of 8 MPa to 16 MPa when the temperature, tension and time were 250 °C, 6 g·d⁻¹ and 40 min, respectively. Correspondingly, the microstructures of the samples, including the phase fraction, crystal size, orientation factor, fibril radius, fibril length and misorientation angle, have been investigated. It was fortunate that the supercritical carbon dioxide fluid could be used as a medium during the hot-stretch process to improve the mechanical performance of F-III fibers, although the treatment temperature was lower than the glass transition temperature of the F-III fibers.

Quantifying High-Performance Material Microstructure Using Nanomechanical Tools with Visual and Frequency Analysis

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.

High strength films from oriented, hydrogen-bonded "graphamid" 2D polymer molecular ensembles

The linear polymer poly(p-phenylene terephthalamide), better known by its tradename Kevlar, is an icon of modern materials science due to its remarkable strength, stiffness, and environmental resistance. Here, we propose a new two-dimensional (2D) polymer, “graphamid”, that closely resembles Kevlar in chemical structure, but is mechanically advantaged by virtue of its 2D structure. Using atomistic calculations, we show that graphamid comprises covalently-bonded sheets bridged by a high population of strong intermolecular hydrogen bonds. Molecular and micromechanical calculations predict that these strong intermolecular interactions allow stiff, high strength (6–8 GPa), and tough films from ensembles of finite graphamid molecules. In contrast, traditional 2D materials like graphene have weak intermolecular interactions, leading to ensembles of low strength (0.1–0.5 GPa) and brittle fracture behavior. These results suggest that hydrogen-bonded 2D polymers like graphamid would be transformative in enabling scalable, lightweight, high performance polymer films of unprecedented mechanical performance.

Ultrahigh strength and modulus copolyamide films with uniaxially cold-drawing induced molecular orientation

The tensile strength of polymer films is generally lower than 200 MPa. Copolyamide (CPA) films with ultrahigh strength (approximately 317–865 MPa) and high modulus (approximately 3.96–11.76 GPa) were obtained by uniaxial cold-drawing and heat treatment. The relationship between structures and properties in the orientation state was also investigated. The results of Fourier transform infrared (FTIR) spectroscopy indicate that hydrogen bonding interactions change a little with drawing. Wide-angle X-ray diffraction patterns show orientational stretch induces π–π ordered packing after drawing 100%. The tensile strength and modulus of the drawn CPA films are improved approximately 41%–173% and 9%–197% compared to undrawn films. Dichroic ratios obtained under polarized FTIR spectroscopy represent overall orientation factors. The decrease of dichroic ratios at about 3300 cm−1 and 1650 cm−1 and the increase of dichroic ratios at 1515 cm−1 prove that the orientation of macromolecular chains is improved along stretch direction. What’s more, the degree of orientation is further increased after heat treatment, which is the phenomenon of spontaneous orientation. The degree of spontaneous orientation increases with the increase of cold-drawing ratio. Thus, the initial orientation in cold-drawing and spontaneous orientation during heat treatment leads to ultrahigh modulus and high strength.

In situ synchrotron small- and wide-angle X-ray study on the structural evolution of Kevlar fiber under uniaxial stretching

In situ SAXS and WAXS study on the structural evolution and mechanism of two different Kevlar fibers during stretching.

Pre-drawing induced evolution of phase, microstructure and property in para-aramid fibres containing benzimidazole moiety

Copoly(p-phenylene-benzimidazole-terephthalamide) (PBIA) fibre was spun by wet-spinning and drawn in a coagulating bath with different pre-drawing ratios (R).

Preparation of poly(p-phenylene terephthalamide) in a microstructured chemical system

PPTA was prepared in a microstructured chemical system, and the apparent polycondensation kinetics was studied.

Analysing the nanoporous structure of aramid fibres

After consideration of the applicability of classical methods, a novel analysis method for the characterization of fibre void structures is presented, capable of fitting the entire anisotropic two-dimensional scattering pattern to a model of perfectly aligned, polydisperse ellipsoids. It is tested for validity against the computed scattering pattern for a simulated nanostructure, after which it is used to fit the scattering from the void structure of commercially available heat-treated poly(p-phenylene terephtalamide) fibre and its as-spun precursor fibre. The application shows a reasonable fit and results in size distributions for both the lengths and the widths of the ellipsoidal voids. Improvements to the analysis methods are compared, consisting of the introduction of an orientation distribution for the nano-ellipsoids, and the addition of large scatterers to account for the effect of fibrillar scattering on the scattering pattern. The fit to the scattering pattern of as-spun aramid fibre is improved by the introduction of the large scatterers, while the fit to the scattering pattern obtained from the heat-treated fibre improves when an orientation distribution is taken into account. It is concluded that, as a result of the heat treatment, the average width and length of the scatterers increase.