Structure-property analysis for gel-spun, ultrahigh molecular mass polyethylene fibers

Hierarchical Mechanisms of Lateral Interactions in High-Performance Fibers

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.

A Novel Ziegler⁻Natta-Type Catalytic System-TiCl₄/2,2'-Dimethoxy-1,1'-Binaphthalene/Et₃Al₂Cl₃/Bu₂Mg for Production of Ultrahigh Molecular Weight Polyethylene Nascent Reactor Powders, Suitable for Solvent-Free Processing

A series of ultrahigh molecular weight polyethylenes with viscosity-average molecular weights in the range of 1.6⁻5.6 × 10⁶ have been prepared by using a novel Ziegler⁻Natta-type catalytic system-TiCl₄/2,2'-dimethoxy-1,1'-binaphthalene/Et₃Al₂Cl₃/Bu₂Mg at different temperatures (Tpoly) in the range between 10 and 70 °C in toluene. The morphology of the nascent reactor powders has been studied by scanning electron microscopy, wide-angle X-ray diffraction, and the DSC melting behavior. Polymers are suitable for the modern processing methods-the solvent-free solid-state formation of super high-strength (tensile strength over 1.8⁻2.5 GPa) and high-modulus (elastic modulus up to 136 GPa) oriented film tapes. With decrease of Tpoly, the drawability of the reactor powders increased significantly.

Oxidation Reactions in Kink Banded Regions of UHMMPE Fiber-Based Laminates Used in Body Armor: A Mechanistic Study

This work demonstrates the synergy between the thermo-mechanical and humidity induced degradation as well as the oxidation reactions in the kink-banded areas of ultra-high molar mass polyethylene (UHMMPE) fiber-based laminates used in body armor. For aged materials, the energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) results reveal high concentrations of oxygen containing products, and the EPR results demonstrate the presence of the peroxyl radicals (RO₂ ^• ) in the kink-banded areas. After one year of dark ambient storage, very long-lived RO₂ ^• radicals were observed primarily in the samples exposed to ageing conditions of elevated temperatures, humidity, and mechanical stress. The total percentage of crystallinity, as measured by differential scanning calorimetry, of the kinkbanded fibers was unchanged, indicating that the degradation occurs primarily in the amorphous region, and may also involve recrystallization processes of the degraded chains. However, the most abundant orthorhombic crystalline phase decreases from 77 % to 70 %. This decrease in the orthorhombic structure leads to more diffusion of oxygen into the kink-banded region, enhancing the oxidation processes. No changes are observed in the monoclinic phase of the kinked fibers, which remained constant and constituted ~2 % of the total crystallinity.

Significant Improvement in Thermal and UV Resistances of UHMWPE Fabric through in Situ Formation of Polysiloxane-TiO2 Hybrid Layers

Anatase nanocrystalline titanium dioxide coatings were produced on ultrahigh molecular weight polyethylene (UHMWPE) fabric by radiation-induced graft polymerization of γ-methacryloxypropyl trimethoxysilane (MAPS) and subsequent cohydrolysis of the graft chains (PMAPS) with tetrabutyl titanate, followed by boiling water treatment for 180 min. The resulting material was coded as UHMWPE-g-PMAPS/TiO2 and characterized by attenuated total reflection infrared spectrometry, differential scanning calorimetry, X-ray diffraction, thermal gravimetry, and ultraviolet absorption spectroscopy, among others. The predominant form of TiO2 in the thin film was anatase. The coating layer was composed of two sublayers: an inner part consisting of an organic-inorganic hybrid layer to prevent photocatalytic degradation of the matrix by TiO2 film, and an outer part consisting of anatase nanocrystalline TiO2 capable of UV absorption. This UHMWPE-g-PMAPS/TiO2 composite exhibited much better thermal resistance than conventional UHMWPE fabric, as reflected by the higher melting point, decreased maximum degradation rate, and higher char yield at 700 °C. Compared with UHMWPE fabric, UHMWPE-g-PMAPS/TiO2 exhibited significantly enhanced UV absorption and excellent duration of UV illumination. Specifically, the UV absorption intensity was 2.4-fold higher than that of UHMWPE fabric; the retention of the break strength of UHMWPE-g-PMAPS/TiO2 reached 92.3% after UV irradiation. This work provides an approach for addressing the issue of self-degradation of TiO2-coated polymeric materials due to the inherent photoactivity of TiO2.

Solid-state (13)C NMR and synchrotron SAXS/WAXS studies of uniaxially-oriented polyethylene

We report solid-state (13)C NMR and synchrotron wide-and small-angle X-ray scattering experiments (WAXS, SAXS) on metallocene linear low density polyethylene films (e.g., Exceed™ 1018 mLLDPE; nominally 1MI, 0.918 density ethylene-hexene metallocene copolymer) as a function of uniaxial draw ratio, λ. Combined, these experiments provide an unambiguous, quantitative molecular view of the orientation of both the crystalline and amorphous phases in the samples as a function of draw. Together with previously reported differential scanning calorimetry (DSC), gas transport measurements, transmission electron microscopy (TEM), optical birefringence, small angle X-ray scattering (SAXS) as well as other characterization techniques, this study of the state of orientation in both phases provides insight concerning the development of unusually high barrier properties of the most oriented samples (λ=10). In this work, static (non-spinning) solid-state NMR measurements indicate that in the drawn Exceed(TM) films both the crystalline and amorphous regions are highly oriented. In particular, chemical shift data show the amorphous phase is comprised increasingly of so-called "taut tie chains" (or tie chains under any state of tautness) in the mLLDPE with increasing draw ratio - the resonance lines associated with the amorphous phase shift to where the crystalline peaks are observed. In the sample with highest total draw (λ=10), virtually all of the chains in the non-crystalline region have responded and aligned in the machine (draw) direction. Both monoclinic and orthorhombic crystalline peaks are observed in high-resolution, solid-state magic-angle spinning (MAS) NMR measurements of the oriented PE films. The orientation is comparable to that obtained for ultra-high molecular weight HDPE fibers described as "ultra-oriented" in the literature. Furthermore, the presence of a monoclinic peak in cold-drawn samples suggests that there is an appreciable internal stress associated with the LLDPE. The results are confirmed and independently quantified by Herman's Orientation Function values derived from the WAXS measurements. The degree of orientation approaches theoretically perfect alignment of chains along the draw direction. We deduce from this observation that a high fraction of the non-crystalline chains are either tie chains that directly connect adjacent lamellae or are interlocking loops from adjacent lamellae. In either case, the chains are load-bearing and are consistent with the idea of "taut tie chains". We note that transmission electron micrographs recorded for the ultra-oriented Exceed showed the lamellae are often appreciably thinner and shorter than they are for cast or blown Exceed 1018. Combined with higher crystallinity, the thinner lamellae statistically favor more tie chains. Finally, the remarkably large decrease in permeability of the λ=10 film is primarily attributed to the high degree of orientation (and loss of entropy) of the amorphous phase.

Fracture behavior of UHMWPE in non-implanted, shelf-aged knee prostheses after gamma irradiation in air

We studied non-implanted ultra-high molecular weight polyethylene (UHMWPE) knee components, that were gamma-sterilized in air and shelf-aged during 6 and 7 years, to explore fracture mechanisms in the degraded material. Prior to the fractography study, we assessed degradation through FTIR and optical density measurements in microtome sections. Then, we morphologically compared the surface fractography of degraded, oxidized UHMWPE with that of non-degraded material in specimens from the same implants, fractured after uniaxial tensile tests in standardized samples. Fractography surface analysis clearly differentiated the behavior of non-degraded and degraded UHMWPE. Failure in non-degraded samples occurred through craze formation, followed by fast crack propagation. Quantitative fracture analysis suggested a ductile behavior. This was supported by the identified local plasticity in the region close to the initial flaw. In the fractography surface analysis of degraded material, a fibrillar behavior was observed. A heterogeneous domain structure appeared, showing different coarseness with multiple void sizes. Our results, based on mechanical measurements and fracture morphology in the subsurface region, confirmed the embrittlement of oxidized UHMWPE.