Gel-spun Ultra-high Molecular Weight Polyethylene Research Articles

Fast, Light-Responsive, Metal-Like Polymer Actuators Generating High Stresses at Low Strain

Summary Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultra-high molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzene-doped UHMWPE films have high Young's moduli (∼100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast ( 6 × 104 Pa (kg m−3)−1) to UV or visible light at a low strain (∼0.06%). The actuator responds to rotating linearly polarized light, causing a photo-induced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics.

Structural difference of gel-spun ultra-high molecular weight polyethylene fibers affected by cold drawing process

The UHMWPE fibers with different cold drawing ratio (DR0) were obtained from the industrial UHMWPE fibers production line. The effect of cold drawing before the extraction of paraffin oil process on final fibers was investigated by tensile testing, small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD). The tensile strength and modulus with 5.0 DR0 were 2.99 and 151.5 GPa, respectively, which were 13.3 % and 41.9 % higher than those with 1.5 DR0. With the increase of DR0, the values of average shish length decreased obviously, while the shish orientation increased and the apparent crystal size along two lattice directions ((110)o and (200)o) in UHMWPE fibers decreased. The increase of degree of orientation and crystallization were verified that better folded chains and amorphous chains were involved in forming shorter and better oriented shish.

Structure and properties of gel-spun ultra-high molecular weight polyethylene fibers with high gel solution concentration

The gel-spun ultra-high molecular weight polyethylene (UHMWPE) fibers were prepared at the industrial production line with different gel solution concentrations of 15 wt%, 18 wt% and 24 wt%. The difference in ultimate structure and mechanical properties of UHMWPE fibers for different gel solution concentrations were analyzed by tensile testing, differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). With the increase of gel solution concentration, the ultimate mechanical properties of UHMWPE fibers were decreased and the crystallization and orientation of UHMWPE fibers became inferior. Besides, both the average shish length (<L shish>) and shish misorientation (B ϕ ) of UHMWPE fibers were decreased with the increase of gel solution concentration. In addition, the appropriate increase of spinning temperature led to the further optimization of the ultimate structure and mechanical properties of UHMWPE fibers.

Effect of Gel Solution Concentration on the Structure and Properties of Gel-Spun Ultrahigh Molecular Weight Polyethylene Fibers

The gel-spun ultrahigh molecular weight polyethylene (UHMWPE) fibers were prepared at the industrial production line with different gel solution concentration of 10–18 wt %. The effect of gel solution concentration on the structure and performance of UHMWPE fibers was studied by tensile testing, differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). With the increase of gel solution concentration, the ultimate mechanical properties of UHMWPE fibers decreased while the melting temperature increased. And the increase of gel solution concentration made the crystallization and orientation of UHMWPE fibers become worse. Furthermore, the average length (⟨Lshish⟩) and misorientation (Bϕ) of shish structure in UHMWPE fibers decreased with the increase of gel solution concentration.

Ultra-strong gel-spun ultra-high molecular weight polyethylene fibers filled with chitin nanocrystals

Ultra-high molecular weight polyethylene (UHMWPE)/chitin nanocrystals (CNC) and UHMWPE/acetylated chitin nanocrystals (ACNC) fibers were prepared.

Evolution of Intercrystallite “Bridges” During Multi-Step Drawing of Gel-Spun Polyethylene Fibers

In 1978, Gibson et al. [1] observed a paradoxical x-ray scattering pattern in oriented ultra-high-molecularweight polyethylene (UHMWPE) that indicated that the crystallite thickness in the fiber exceeded significantly the long period. The researchers proposed a bridging model that assumed the presence of a large number of straightened connecting molecules (SCM) joining like bridges successive crystallites along the fiber axis in order to explain this obvious dimensional disparity. Subsequently, this model was confirmed in other x-ray investigations [2-5] and independently from results of low-frequency Raman spectroscopy [6-8]. The bridging model was used successfully to interpret several mechanical properties of highly oriented UHMWPE fibers, in particular, to explain the behavior of Young’s modulus during various sample-preparation procedures and various test conditions [9 and references therein]. The model turned out to be useful also for understanding the solid-state extrusion process [9]. Nevertheless, little reliable information about the formation of intercrystallite bridges in drawn fibers is available in terms of a general concept of the oriented state. The goal of the present work was to study the formation of SCM-bridges in gel-spun UHMWPE fibers. Hot drawing of fibers from a spinning solution is usually carried out in several stages in industry. Each stage has a different special structural organization of drawn fibers that can be characterized using instrumental methods. Herein we present results from a study of the formation of intercrystallite bridges in fibers drawn at the pilot plant (SRISF, Tver´). Fibers that were obtained at various stages of oriented drawing were studied using low-frequency Raman spectroscopy and wide-angle x-ray scattering. Samples were prepared from UHMWPE reactor powder with crystallinity degree 64% (according to differential scanning calorimetry) and molecular weight 2.7"10 6 g/mol (according to viscosity measurements). The reactor powder and drawing temperature—time regime were chosen based on previous results [10, 11]. The drawing process included the following successive stages. Reactor powder was dissolved with stirring in mineral oil at 172°C. The resulting homogeneous solution of concentration 3 mass% was forced at the same temperature by a dispensing six-channel pump through six spinnerettes with 48 openings of diameter 0.5 mm each. The ratio of capillary length to diameter was 5. The solution from the spinnerette head was fed into a cooling bath filled with mineral oil at 43.0 ± 1.5°C. The fluid lost its flowability and converted into a gel as a result of phase separation during such rapid cooling. The spun gel-threads were continuously transferred from the cooling bath to a mineral-oil bath at 70°C. Then, the threads were freely placed in a container where they were shrunk to the equilibrium state.

Morphology, Chain Dynamics, and Domain Sizes in Highly Drawn Gel-Spun Ultrahigh Molecular Weight Polyethylene Fibers at the Final Stages of Drawing by SAXS, WAXS, and 1H Solid-State NMR

Morphology, phase composition, and molecular mobility for a series of semicommercial gel-spun UHMWPE fibers were studied using a combination of WAXS, SAXS, and 1H solid-state NMR methods. The fibers show uncommon for this type of fibers decrease in the break load with increasing draw ratio, whereas their modulus and the tenacity reach very high ultimate values. The X-ray and NMR methods have provided complementary information about the fiber morphology and structural reorganizations occurring at the final stage of the fiber drawing. The results suggest that the fiber morphology can be described by a mixture of crystalline fibrils with long period of ∼35–45 nm, as shown by SAXS, and large, so-called, chain-extended crystals. The presence of large crystals with embedded defects is shown by NMR. The drawing causes increase in the crystallinity from ∼89 to ∼96 wt % and in chain orientation, while the long period of fibrils and the break load of fibers surprisingly decrease. The decrease in the long period wit...

Investigation of the drawing mechanism of UHMWPE fibers

The influence of draw ratios (DR) of gel-spun ultrahigh molecular weight polyethylene (UHMWPE) fibers on resultant morphologies, tensile, degrees of orientation, and crystal phase transition properties were investigated using wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The anisotropic crystalline structure with full concentric circular rings originally shown on the WAXD patterns of the as-prepared and drawn UHMWPE fibers gradually transform into oriented fibers with azimuthal spots on the equator as their DR values increase from 1 to 20, in which their orthorhombic crystals, percentage crystallinity, crystalline orientation, and the birefringence values increase significantly. As evidenced by SEM and WAXD analysis, the chain-folded molecules originally present in kebab crystals of the as-prepared UHMWPE fiber specimens gradually transformed into shish-like crystals with relatively high orientation as their DR values increase from 1 to 20. In contrast, the crystallinity and crystal orientation values of the drawn UHMWPE specimens increase only slightly, as their DR values increase from 20 to 40, wherein both crystallinity values of orthorhombic and monoclinic crystals increase slightly. In fact, barely any oriented kebab but only shish crystals were observed on the surfaces of drawn UHMWPE fiber specimens with DR values higher than 20. The birefringence values increase only slightly with further increasing DR values, while crystallinity and crystal orientation values of the drawn UHMWPE fiber specimens remained relatively unchanged as their DR values increase from 40 to 150. In the meantime, the monoclinic crystals gradually grow at the expense of the orthorhombic form crystals as the DR values of drawn UHMWPE fiber specimens increase from 40 to 150. Possible reasons accounting for these interesting properties found for the drawn UHMWPE fibers with varying draw ratios are proposed in this study.

Wetting of oriented and etched ultrahigh molecular weight polyethylene

Highly oriented gel-spun ultrahigh molecular weight polyethylene (UHMWPE) fibers possess many outstanding properties desirable for composite materials but their adhesion to such matrices as epoxy is poor. This article describes the combined effects of drawing and surface modification on the bulk and surface properties of gel-cast UHMWPE films emphasizing the effects of etching on both undrawn and drawn films. Drawing the films yields a fibrillar structural hierarchy similar to UHMWPE fibers and a significant increase in orientation, melting point, modulus, and strength. The effects of drawing on bulk properties were more significant than those of etching. The poor adhesion of epoxy to the smooth, fibrillar, and relatively nonpolar drawn film surface improves significantly with oxidization and roughening on etching. The interlaminar shear failure occurred cohesively in the UHMWPE, and thus the interlaminar shear failure strength was greater for the drawn UHMWPE with its greater tensile strength. Nitrogen plasma etching yielded the best results, both removing any low molecular weight surface layer and etching the UHMWPE beneath. Oxygen plasma etching enhanced wetting but was too harsh, causing extensive surface degradation and a significant reduction in mechanical properties.

Crystallite Sizes and Lattice Distortions of Gel-Spun Ultra-High Molecular Weight Polyethylene Fibers

A detailed method is reported for deriving the lattice distortions and crystallite sizes of ultra-high molecular weight polyethylene (UHMWPE) fibers employing a full wide angle X-ray diffraction (WAXD) pattern. The predominant crystals are orthorhombic form and the refined unit cell dimensions for a and b are 7.40(±0.03) and 4.94(±0.03) A, respectively. The average crystallite size normal to 110 planes was found to be in the range of 163–182 A. The microstrain lattice distortion was predominant in the gel-spun UHMWPE fibers, with the lattice constant variation of 0.5%.

Surface modification of drawn gel-cast ultra-high molecular weight polyethylene films

Gel-spun ultra-high molecular weight polyethylene (UHMWPE) fibers have superior properties but their use in composite material applications is limited by their poor adhesion to polymer matrices. Previous studies have shown that etching improves the adhesion of epoxy to the fibers, but leads to a reduction in mechanical properties. The purpose of this research was to use uniaxially drawn gel-cast UHMWPE films as a model system since both films and fibers have a highly oriented fibrillar structural hierarchy. Etching has detrimental effects on the mechanical properties and crystallinity of these very thin films. The small amount of carbonyl and carboxyl groups added to the surface through etching raises the film's surface tension and enhances wetting by epoxy. Even though the unmodified film cannot be bonded with epoxy, the interlaminar shear strength between epoxy and the etched films approaches the cohesive strength of the epoxy. A combination of interfacial and UHMWPE cohesive failures is observed. The increase in adhesion is attributed to the slight increase in surface oxygen.

Solid-state n.m.r. studies of crystalline phases in gel-spun ultrahigh molecular weight polyethylene

Gel-spun ultrahigh molecular weight polyethylene (UHMWPE) was examined by advanced solid-state 13C n.m.r. techniques to extract detailed information on the morphology and nature of the crystalline phases. Two partially overlapping resonances separated by 1.5 ppm in the 13C magic angle spinning n.m.r. spectrum have been assigned to monoclinic and orthorhombic crystalline phases. To analyse better the small amount of monoclinic phase, a 13C chemical shift filter was employed to suppress partially the dominant orthorhombic signal. The monoclinic phase exhibits a slightly smaller chemical shielding anisotropy of 27 ppm with chemical shift principal tensor values of 50, 36.5 and 16 ppm as compared to a shielding anisotropy of 30 ppm and principal tensor values of 50, 35.5 and 13 ppm for the orthorhombic phase. Additionally, a minor fraction of interface was detected arising from heterogeneous molecular packing. Both crystalline phases reveal an extremely high degree of order, with 〈 p 2〉 around 0.94. Whereas the conformations of the chains in the non-crystalline regions of both gel-spun and melt-spun polyethylene are alike, the mobility of the former is largely reduced. The morphology of UHMWPE is compared with that of melt-spun polyethylene.

The effect of fibre diameter on the drawing behaviour of gel-spun ultra-high molecular weight polyethylene fibres

In the continuous drawing of gel-spun UHMWPE fibres, the diameter of the undrawn fibre appears to have a pronounced effect on its drawing behaviour and on the mechanical properties of the resulting hot-drawn fibres. A highly oriented structure is developed more efficiently upon drawing of thinner fibres, which may be attributed to differences in the deformation mechanism between as-spun fibres of various diameters. By drawing of thin fibres, UHMWPE filaments having a strength of 6.0 GPa and a Young's modulus of 222 GPa can be obtained at a relatively low draw ratio of λ=70.

Two-dimensional 13C n.m.r. studies of the morphology and orientational order in gel-spun ultrahigh molecular weight polyethylene fibres

In this communication we demonstrate the feasibility of determining the orientational distribution functions of various morphological components in ultrahigh molecular weight polyethylene fibres by two-dimensional rotor-synchronized 13C cross-polarization/magic angle spinning nuclear magnetic resonance (n.m.r.) techniques. For comparative purposes we have also included a drawn polyethylene fibre of normal molecular weight. The results showed that these fibres contain two crystalline components, designated C 1 and C 2, and an amorphous (non-crystalline) component, A. Order parameters and orientational distribution functions of these components can be obtained through analysis of the two-dimensional 13C n.m.r. spectra.

Cross-linking of porous gel-spun ultra-high molecular weight polyethylene by means of electron beam irradiation

Porous gel-spun ultra-high molecular weight polyethylene (UHMWPE) fibres, consisting of large folded chain lamellae linked to each other by means of tie molecules, have been Irradiated at room temperature by means of high energy electrons.