P-phenylene Terephthalamide Fibers Research Articles

Modeling failure mechanisms of poly(p-phenylene terephthalamide) fiber using reactive potentials

Failure mechanisms of poly(p-phenylene terephthalamide (PPTA) under extreme tensile deformation has been studied using reactive potentials with molecular dynamics simulations. Amorphous PPTA systems with different molecular weights generated using an in-house developed amorphous builder. Tensile modulus of amorphous PPTA has been calculated as up to 6.7GPa. Nitrogen and carbon vacancy defects were introduced to both crystalline and amorphous systems. The tensile modulus of defects-free crystalline PPTA calculated as 350GPa. Introduction of 5% nitrogen vacancy defects reduced the tensile modulus to 197GPa. To estimate fiber modulus, PPTA fiber considered to be composed of amorphous and crystalline phases. Rule of mixtures formula modified to incorporate influence of defects on tensile modulus. Histograms of various quantities such as bond lengths, bond angles and phenyl ring diameters were calculated at different strain levels. Tensile load was mostly accommodated through stretching of bonds between amide group and phenyl groups. Under extreme tensile deformation PPTA chains failed at these C–N bonds.

The effect of plain-weaving on the mechanical properties of warp and weft p-phenylene terephthalamide (PPTA) fibers/yarns

Coarse-grained molecular statics/dynamics methods are first used to investigate degradation in the PPTA fiber/yarn tensile strength, as a result of the prior compressive or tensile loading. PPTA fibers/yarns experience this type of loading in the course of a plain-weaving process, the process which is used in the fabrication of ballistic fabric and flexible armor. The more common all-atom molecular simulations were not used to assess strength degradation for two reasons: (a) the size of the associated computational domain rendering reasonable run-times would be too small and (b) modeling of the mechanical response of multi-fibril PPTA fibers could not be carried out (again due to the limited size of the computational domain). However, all-atom simulations were used to (a) define the coarse-grained particles (referred to as “beads”) and (b) parameterize various components of the bead/bead force-field functions. In the second portion of the work, a simplified finite-element analysis of the plain-weaving process is carried out in order to assess the extent of tensile-strength degradation in warp and weft yarns during the weaving process. In this analysis, a new material model is used for the PPTA fibers/yarns. Specifically, PPTA is considered to be a linearly elastic, transversely isotropic material with degradable longitudinal-tensile strength and the longitudinal Young’s modulus. Equations governing damage and strength/stiffness degradation in this material model are derived and parameterized using the coarse-grained simulation results. Lastly, the finite-element results are compared with their experimental counterparts, yielding a decent agreement.

Molecular-Level Study of the Effect of Prior Axial Compression/Torsion on the Axial-Tensile Strength of PPTA Fibers

A comprehensive all-atom molecular-level computational investigation is carried out in order to identify and quantify: (i) the effect of prior longitudinal-compressive or axial-torsional loading on the longitudinal-tensile behavior of p-phenylene terephthalamide (PPTA) fibrils/fibers; and (ii) the role various microstructural/topological defects play in affecting this behavior. Experimental and computational results available in the relevant open literature were utilized to construct various defects within the molecular-level model and to assign the concentration to these defects consistent with the values generally encountered under “prototypical” PPTA-polymer synthesis and fiber fabrication conditions. When quantifying the effect of the prior longitudinal-compressive/axial-torsional loading on the longitudinal-tensile behavior of PPTA fibrils, the stochastic nature of the size/potency of these defects was taken into account. The results obtained revealed that: (a) due to the stochastic nature of the defect type, concentration/number density and size/potency, the PPTA fibril/fiber longitudinal-tensile strength is a statistical quantity possessing a characteristic probability density function; (b) application of the prior axial compression or axial torsion to the PPTA imperfect single-crystalline fibrils degrades their longitudinal-tensile strength and only slightly modifies the associated probability density function; and (c) introduction of the fibril/fiber interfaces into the computational analyses showed that prior axial torsion can induce major changes in the material microstructure, causing significant reductions in the PPTA-fiber longitudinal-tensile strength and appreciable changes in the associated probability density function.

Filament-Level Modeling of Aramid-Based High-Performance Structural Materials

Molecular statics and molecular dynamics are employed to study the effects of various microstructural and topological defects (e.g., chain ends, axial chain misalignment, inorganic solvent impurities, and sheet stacking faults) on the strength, ductility, and stiffness of p-phenylene terephthalamide (PPTA) fibers/filaments. These fibers can be considered as prototypes for advanced high strength/high-stiffness fibers like Kevlar®, Twaron®, New Star®, etc. While modeling these fibers, it was taken into account that they are essentially crystalline materials consisting of stacks of sheets, with each sheet containing an array of nearly parallel hydrogen-bonded molecules/chains. The inter-sheet bonding, on the other hand, was considered as mainly being of van der Waals or p-electron character. The effects of various deviations of the PPTA fiber structure from that of the perfectly crystalline structure (i.e., microstructural/topological defects) on the material’s mechanical properties are then considered. The results obtained show that while the presence of these defects decreases all the mechanical properties of PPTA fibers, specific properties display an increased level of sensitivity to the presence of certain defects. For example, longitudinal tensile properties are found to be most sensitive to the presence of chain ends, in-sheet transverse properties to the presence of chain misalignments, while cross-sheet transverse properties are found to be most affected by the presence of sheet stacking faults.

Effect of oxygen plasma treatment of PPTA and PBO fibers on the interfacial properties of single fiber/epoxy composites studied by Raman spectroscopy

The interfacial micromechanics of single poly( p-phenylene terephthalamide (PPTA) and poly( p-phenylene benzobisoxazole (PBO) fibers embedded in an epoxy resin has been investigated by determining the interfacial shear stress distributions along the fiber length. The effects of an oxygen plasma treatment on the interfacial shear stress of the fiber–epoxy systems are analyzed. Raman spectroscopy was used to map the stress distributions along the fiber when the composite is subjected to a small axial tensile strain (3.5% for PPTA and 2.5% for PBO). The quality of the interface or adhesion was improved after the surface treatment, supporting the ability of plasma oxidation to enhance the adhesion of high-performance fibers to epoxy resins. The tensile behavior of fiber-reinforced systems was different in each case. PPTA reinforcements underwent fragmentation, likely by fiber microfailure, whereas debonding or bridging is the most probable fragmentation mechanism in the case of PBO.

ON THE FORMATION OF POLY (P-PHENYLENE TEREPHTHALAMIDE) PULPS FROM STIRRED SOLUTIONS

PPTA (poly (p-phenylene terephthalamide)) may precipitate directly from stirred solution by the use of glycerol or glycerol/water coagulant in fibrous pulp form—short fibers with attached fine fibrils that are called PPTA pulp. Scanning electron micrographs show that the precipitated PPTA pulps have a ribbon-like structure similar to Twaron pulps. The wide-angle X-ray diffraction patters of the resultant pulps exhibit three diffraction peaks at 2θ–20.5, 23.5, and 28.5 corresponding to the {110}, {200}, and {004} crystal faces of PPTA fibers, and the peaks are not as sharp as those of fibers. The process yields PPTA pulps with properties such as specific surface areas and mean length depending on the stirring speed, and the ratio of the viscosity of the PPTA/H2SO4 solution to that of the coagulant. The inherent viscosity of the pulps determined by that of the PPTA bulk, which can be easily polycondensated to the desired specification.

Structure–property relation in poly(p-phenylene terephthalamide) (PPTA) fibers

Structure parameters of various poly(p-phenylene terephthalamide fibers have been investigated using WAXD and correlated with mechanical properties. The mechanical properties examined were modulus E and strength σ; the pertinent structural parameters include orientation angle φ200, lattice constants a, b, c, paracrystalline parameter gII,apparent crystal sizes ACS110, ACS200, ACS00l, intensity ratio I110/I200 and transverse crystallinity X. The parameters, c, gII and I110/I200 are found to be interrelated and to provide indications of nonreversible chain conformational changes due to post-treatment. It is concluded that the fiber modulus is determined by the combination of the orientation of the crystallites and the paracrystalline parameter through the following equation:(10)1/Ef=(1/E0+D1gII2)+A〈sin2φ〉in which Ef is the fiber modulus; gII the paracrystalline parameter; φ the orientation angle; and E0, D1 and A the material constants. This relationship is derived from our proposed morphological model in which crystallites are: (a) formed from chains have nonlinear conformations; and (b) packed with an orientation distribution. The correlation of structure with strength has also been studied. In addition, different types of Kevlar® fibers, Kevlar® 119, Kevlar® 29, Kevlar® 49 and Kevlar® 149 show slight, systematic, variations in structure. In particular, all Kevlar® fibers except Kevlar® 149 show the forbidden 001 diffraction reflection, which has been related to conformational differences.

Complications of cylindrical anisotropy on the properties of fibres

Cylindrically orthotropic materials having linear elastic stress-strain relations can possess a unique type of coupling between the radial and tangential directions. This type of anisotropy occurs when fibres have onion skin or radial morphologies which are characteristic of some graphite fibres and poly- p-phenylene terephthalamide fibres. Pulling on a long slender fibre having this type of anisotropy does not produce a uniaxial state of stress because the unequal Poisson ratios also cause both radial and tangential stresses. The magnitude of these predicted transverse stresses in relation to the axial stress is similar to the relationship of strengths in these principal directions. Hence, it is conceivable that fibre failure could be caused by fibre splitting from high transverse stresses rather than the longitudinal tensile stresses. Temperature changes and other factors causing dimensional changes such as swelling and pressure can similarly cause large stresses if the radial and tangential expansion coefficients are different. This type of axial-transverse coupling is known to limit certain designs in composite structures having this type of anisotropy, yet little has been done to understand fibres possessing these characteristics. This paper examines the complex state of stress that can result in fibres having cylindrically orthotropic linear elastic properties for several simple modes of deformation.

The influence of epoxide binder components on the supermolecular structure and properties of reinforcing fibres based on rigid chain polymers

The change in supermolecular structure of fibres based on poly- p-phenylene terephthalamide (PPTA) and polyamidobenzimidazole (PABI) treated with components of epoxide binders ED-20, TEAT and DEG-1 has been studied by X-ray diffusion and by NMR methods. It was shown that the interaction of the oligomer components of binder with PABI cause structural changes, decreasing the molecular mobility, whilst the structure of PPTA is practically unchanged after the same treatment. The differing character of interaction of PABI and PPTA fibres with epoxy compounds and tertiary amines is related to the different supermolecular structures of these fibres and also to the lower chemical activity of the amide groups in PPTA, compared with those in PABI. The physico-mechanical characteristics of fibres treated with epoxide binder components are given.