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Poly (p-phenylene terephthalamide) fibers prepared by wet or dry-jet wet spinning processes have a notable response to very brief heat treatment (seconds) under tension. The modulus of the as-spun fiber can be greatly affected by the heat treatment conditions (temperature, tension and duration). The crystallite orientation and the fiber modulus wil...
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... It is also well noted (in Figure 2(d) and (e)) that a considerable 48.95% reduction in crystallinity is observed at 130°C for 0.9% increase in dimension along a-length, while 0.7% increase in a-dimension brings about 46.89% reduction in crystallinity at 80°C. The similar thermally induced changes in crystallinity are also reported in other studies 16,25,[30][31][32] for Nomex and different grades of PPTA fibers. It indicates that changes in unit cell adimension, even at a small scale, can account for a significant reduction in crystallinity and thus behaves as an important crystallographic parameter upon prolonged thermal induction. ...
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 V 50 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.
... ref. [12]. Current experimental evidences are divided between these two suggestions. ...
In this article, we intend to characterize the dynamics of the molecular chains at different stages of stretching polymeric monofilament fibers. The stages characterized in this work are; undrawn, shear-bands, necking, drawn, crazes, cracks and fracture regions. We used the lookup table technique to calculate the dispersion curves of the stretched polymeric fibers at different deformed stages from one interferogram. This technique is suitable for dynamic stretching experiment of fibers. Digital photoelasticity and white-light two-beam interferometry are utilized to study each phenomenon by finding the dispersion curves and the 3D birefringence profiles using a single-shot pattern, which is performed for the first time, as far as we know. This study gives us clear vision on the behavior of the polymeric fibers at a molecular level during dynamic stretching until reaching its fracture point. Patterns of each deformation stage are given for illustration.
... [9][10][11][12] Traditionally, the development in understanding the relationship between processing conditions, structure, and mechanical properties, necessary for the improvement of aramid fibers, commonly involves the timeconsuming generation of samples and empirical testing of their properties. 13 Atomistic modeling has been increasingly used to assist in the development and understanding of structure-property relationships in high molecular weight polymers [14][15][16] and to shed light on important microscopic deformation mechanisms necessary to understand how these materials fail. 17,18 For example, atomistic modeling of polyethylene fibers with chain ends indicates that chain slip is the mechanism defining the tensile yield in these materials. ...
Aramid fibers composed of poly(p-phenylene terephthalamide) (PPTA) polymers are attractive materials due to their high strength, low weight, and high shock resilience. Even though they have widely been utilized as a basic ingredient in Kevlar, Twaron, and other fabrics and applications, their intrinsic behavior under intense shock loading is still to be understood. In this work, we characterize the anisotropic shock response of PPTA crystals by performing reactive molecular dynamics simulations. Results from shock loading along the two perpendicular directions to the polymer backbones, [100] and [010], indicate distinct shock release mechanisms that preserve and destroy the hydrogen bond network. Shocks along the [100] direction for particle velocity Up < 2.46 km/s indicate the formation of a plastic regime composed of shear bands, where the PPTA structure is planarized. Shocks along the [010] direction for particle velocity Up < 2.18 km/s indicate a complex response regime, where elastic compression shifts to amorphization as the shock is intensified. While hydrogen bonds are mostly preserved for shocks along the [100] direction, hydrogen bonds are continuously destroyed with the amorphization of the crystal for shocks along the [010] direction. Decomposition of the polymer chains by cross-linking is triggered at the threshold particle velocity Up = 2.18 km/s for the [010] direction and Up = 2.46 km/s for the [100] direction. These atomistic insights based on large-scale simulations highlight the intricate and anisotropic mechanisms underpinning the shock response of PPTA polymers and are expected to support the enhancement of their applications.
... 27 A crystalline form is necessary for the polyamide like poly(pphenylene terephthalamide) to acquire high strength through the dense π-π stacking between conjugated rings, and the crystallinity can be significantly affected by the experimental fabrication process. 28,29 Our ML models have a limitation in describing the morphological characteristic because the present GCN representation only contains the information limited in the monomer. Also, the GCN cannot account for the stereochemistry which requires information on the threedimensional configurations of the atoms and the techniques to process the molecular graph. ...
... While the rigidity would frustrate the dense packing during cooling, 23 the aging of the glass will result in volume relaxations compensating the nonequilibrium effect. 54 Although the incorporation of conjugated rings into the backbone chain to enhance the rigidity is a generic strategy to increase the elastic moduli of polymer materials, 28,29 the diminished correlations in the E dataset are consistent with the fact that interchain interactions through the crystallization are also critical. As the correlation between the backbone rigidity and the target property decreases, the prediction performance of our GCN models is also reduced. ...
We present machine learning models for the prediction of thermal and mechanical properties of polymers based on the graph convolutional network (GCN). GCN-based models provide reliable prediction performances for the glass transition temperature (T g), melting temperature (T m), density (ρ), and elastic modulus (E) with substantial dependence on the dataset, which is the best for T g (R 2 ∼ 0.9) and worst for E (R 2 ∼ 0.5). It is found that the GCN representations for polymers provide prediction performances of their properties comparable to the popular extended-connectivity circular fingerprint (ECFP) representation. Notably, the GCN combined with the neural network regression (GCN-NN) slightly outperforms the ECFP. It is investigated how the GCN captures important structural features of polymers to learn their properties. Using the dimensionality reduction, we demonstrate that the polymers are organized in the principal subspace of the GCN representation spaces with respect to the backbone rigidity. The organization in the representation space adaptively changes with the training and through the NN layers, which might facilitate a subsequent prediction of target properties based on the relationships between the structure and the property. The GCN models are found to provide an advantage to automatically extract a backbone rigidity, strongly correlated with T g, as well as a potential transferability to predict other properties associated with a backbone rigidity. Our results indicate both the capability and limitations of the GCN in learning to describe polymer systems depending on the property.
... These issues will be detailed in this section. Figure 5. Stress x strain diagram of fibers used in HPC and UHPC [191,192]. Table 5. Properties of fibers used in HPC and UHPC. Source: [193][194][195][196][197][198][199]. ...
This review article proposes the identification and basic concepts of materials that might be used for the production of high-performance concrete (HPC) and ultra-high-performance concrete (UHPC). Although other reviews have addressed this topic, the present work differs by presenting relevant aspects on possible materials applied in the production of HPC and UHPC. The main innovation of this review article is to identify the perspectives for new materials that can be considered in the production of novel special concretes. After consulting different bibliographic databases, some information related to ordinary Portland cement (OPC), mineral additions, aggregates, and chemical additives used for the production of HPC and UHPC were highlighted. Relevant information on the application of synthetic and natural fibers is also highlighted in association with a cement matrix of HPC and UHPC, forming composites with properties superior to conventional concrete used in civil construction. The article also presents some relevant characteristics for the application of HPC and UHPC produced with alkali-activated cement, an alternative binder to OPC produced through the reaction between two essential components: precursors and activators. Some information about the main types of precursors, subdivided into materials rich in aluminosilicates and rich in calcium, were also highlighted. Finally, suggestions for future work related to the application of HPC and UHPC are highlighted, guiding future research on this topic.
... During the coagulation step of the wet spinning process, the filament skin has formed the instant they come into contact with the coagulation bath, while the nucleus has time to relax. This phenomenon results in a microstructure where the surface polymeric chains are oriented axially and in a compacted way, while the nucleus chains are packaged inefficiently, resulting in internal pores observed in the SEM micrographs of the CS threads [26,27]. CS multifilament threads preserved the structure of single filament thread, compacted external surface, and porous nucleus. ...
This study aimed to develop meshes from the weaving of mono- and multifilament wet-spun chitosan (CS), for possible biomedical applications. In the wet-spinning process, CS solution (4% w/v) was extruded in a coagulation bath containing 70% sodium hydroxide solution (0.5 M), and 30% methanol was used. The multifilament thread was prepared by twisted of two and three monofilaments. CS threads obtained were characterized by tensile tests and scanning electron microscopy (SEM). Moreover, it was verified from the morphological tests that threads preserve the characteristics of the individual filaments and present typical “skin-core” microstructure obtained by wet spinning. CS woven meshes obtained were evaluated by optical microscopy (OM), tensile test, swelling degree, and in vitro enzymatic biodegradation. Mechanical properties, biodegradation rate, and amount of fluid absorbed of CS woven meshes were influenced by thread configuration. Hydrated CS meshes showed a larger elastic zone than the dry state. Therefore, CS woven meshes were obtained with modular properties from thread configuration used in weaving, suggesting potential applications in the biomedical field, like dressings, controlled drug delivery systems, or mechanical support.
... The chemical formula of Kevlar® monomer is [− CO− C 6 H 4 − CO− NH− C 6 H 4 − NH− ] n while its molecular structure is depicted below in Fig. 1. The fibers are fully crystalline with a small fraction of randomly oriented material [36,37]. ...
Carbon-based nanostructures are considered promising materials for gas storage thanks to robust structure, tunable porosity, lightweight, high thermal/chemical stability and easy production. This study reports the development of activated carbon fibers prepared from commercial Kevlar® through an innovative pyrolysis method, consisting of carbonization in inert ambient and subsequent physical activation in oxidizing atmosphere, using a unique apparatus. Varying three key parameters, time (range 60− 240 min), temperature (range 1023− 1123 K) and gas-flow (0.3/0.9/1.2 Nl/min of CO 2), the correlation between the activation procedure and the resulting samples structure was evaluated. The best results in terms of microporosity and gas adsorption properties have been obtained by reducing the activation times of the material. Furthermore, the purpose has been to optimize the characteristics in terms of Specific Surface Area, Total Pore Volume and optimal Pore Size Distribution. The method made it possible to develop an adsorbent material with a high fraction of micropores up to 94 % of the total pore volume, straddling the supermicroporosity (0.7− 2 nm) and ultramicroporosity (<0.7 nm) region. Such textural properties have resulted in high gases storage capacities, tested at different temperatures (280, 298, 314 K), with maximum uptake of 46 wt% for CO 2 and almost 10 wt% for CH 4. All produced samples were characterized by helium picnometry to estimate skeletal density, Scanning Electron Microscopy to obtain morphological information, porosimetry to know structural properties. The adsorption behaviour was tested using a Sievert-type apparatus in the pressure range of 0− 15 bar for CO 2 and 0− 40 bar for CH 4 .
... Table 5 lists the crystallographic parameters, the interplanar distances (d 110 and d 200 ), the unit cell parameters ("a" and "b") and the percentage of aramid crystallinity for all investigated groups. In these groups, it was observed that the crystallographic parameters obtained from the aramid unit cell were of the same order of magnitude as those found in the literature: a =7.87 Å, b =5.18 Å e c =12.9 Å [38][39][40][41]. It is then proposed that the weathering exposure did not promote significant changes in the aramid crystalline lattice. ...
Moisten by washing a b s t r a c t The influence of weathering on the ballistic performance of an armor made with aramid fabric was for the first time investigated, by standard exposure related to degradation conditions of: (i) ultraviolet radiation (UV); (ii) moisten by washing (MW) and (iii) UV+MW. Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterized the modifications that occurred on the aramid fabric armor (AFA). Samples of both non-exposed (NE) and UV, MW and UV+MW exposed AFA, were subjected to ballistic tests against 0.380-caliber ammunition. The results of UV and UV+MW exposed samples revealed sensible macromolecular alterations, due to the scission of the polymeric chain by the processes of hydrolysis and photolysis, causing oxidation on the surface and breaking of the crystalline domains inside the aramid fiber. There was also a decrease in the crystallinity of the polyaramid macromolecular chain, being more severe after exposure to simple MW. However, no changes were observed in the thermal behavior of the AFA. The ballistic behavior was significantly affected by the weathering simulation conditions. The damage caused in the AFA by the ballistic impact was comparable within all groups exposed to degradation conditions. Practically, the same trauma, 28.2-32.1 mm and the same number, two (2), of perforated layers were observed for all exposed groups. These common ballistic results were less favorable than those of 25.7 mm and one layer, respectively, for the non-exposed AFA. The penetration traumas remained below the standard 44 mm lethal limit.
... They mainly include the spinning temperature, air gap, coagulation condition, and draw ratio. Among these parameters, the increase in the draw ratio can significantly improve the orientation and crystallization degree of fibers, especially for high-viscosity spinning solution system, including cellulose [17], polyacrylonitrile (PAN) [15], poly(p-phenylene terephthalamide) PPTA [18], and poly(p-phenylene benzobisoxazole) PBO [19] fibers. Therefore, the draw ratio is the key to improving the mechanical properties of fibers during the dry jet-wet spinning process. ...
Aromatic copolysulfonamide (co-PSA) fiber produced via wet spinning exhibits poor mechanical properties. Hence, dry jet-wet spinning was introduced to prepare the co-PSA fiber. Dry jet-wet spinning exhibits the advantage of positive stretching to the extruded fiber before coagulation. The fibers were spun at different jet draw ratios and second draw ratios and were examined for scanning electron microscopy, mechanical properties, sound velocity, small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS). The results revealed that all the fibers exhibited a uniform and dense structure and nearly round cross section at different drawing conditions. The strength and orientation of the fiber increased slightly as the jet draw ratio was increased to the second draw ratio of 2.0. Furthermore, the fibers formed a periodic ordered structure along the fiber axis. However, the strength of the fibers decreased significantly when the jet draw ratio continued to increase and the second draw ratio decreased year-on-year to maintain the total draw ratio at 11.0. Additionally, the fibers exhibited lower orientation because the molecular chain segments could not continue to extend and orient even at larger jet draw ratios, and thus they could not be further well-stretched at subsequent lower stretching ratios. Meanwhile, the periodic ordered structure disappeared and more content of entanglement structure existed in the fibers. During the heat drawing process, the results of SAXS and WAXS indicated that the better ordered molecular chain segments within the fiber were easier to arrange and reconstruct to significantly improve the content of crystalline and mesophase structures in the fibers. Hence, after the heat drawing process, the strengths of co-PSA-H2 and co-PSA-H3 fibers increased to more than 620 MPa, which is significantly higher than those of commercialized fibers and fibers reported in extant studies.
... However, commercial p-aramid is insoluble in most organic solvents. This behavior can primarily be attributed to the strong intermolecular hydrogen bonds that are formed between different amide groups in two neighboring polymer molecules [31,32]. KOtBu, methanol, and DMSO were used in our study to disrupt the hydrogen bonds existing between the p-aramid fibers in order to obtain welldispersed ANFs. ...
p-Aramid is an ideal building block for forward osmosis (FO) membranes due to its extraordinary thermal resistance, chemical stability, and mechanical properties. However, existing aramid membranes have certain limitations such as large pore diameters and low salt rejection rates. In this work, we describe a facile solvent exchange-delay phase inversion strategy to prepare p-aramid nanofibrous membranes that would be suitable for FO applications. In this strategy, p-aramid nanofibers with an average diameter of 16 ± 4 nm and an average length of 382 ± 89 nm were employed as membrane matrices. Prior to the immersion of the cast film into a coagulation bath, a pre-evaporation protocol was carefully designed and introduced to provide a slower exchange rate between the good solvent and the non-solvent, which delayed the demixing process between p-aramid nanofibers and thus yielded an asymmetric membrane with a denser active layer as well as a loose substrate layer. The resultant membrane showed excellent FO water flux, NaCl rejection ratios, tensile strength, thermal properties, and solvent resistance. The membrane reported in this work may provide a promising candidate for separation applications and the results reported herein will facilitate the development of high-performance nanofibrous membranes.
