Irradiation of ultrahigh-molecular-weight polyethylene

An ultrahigh-molecular-weight polyethylene material covering a range of crystallinity from 42 to 79% increased in calorimetric crystallinity as a result of chain scission following ionizing irradiation. Carbonyl was formed by a diffusion-limited reaction of oxygen with long-lived free radicals. Trans-vinylene production was linear with radiation dose and was highest for the sample of highest crystallinity but was not sensitive to environment.     

Properties of oriented film tapes prepared via solid-state processing of a nascent ultrahigh-molecular-weight polyethylene reactor powder synthesized with a postmetallocene catalyst

The correlation between the molecular mass of a nascent ultrahigh-molecular-weight polyethylene reactor powder synthesized with a postmetallocene catalyst and the specific features of plastic deformation during the orientational drawing of the material compacted and consolidated under laboratory conditions is studied. In the range 5 × 10⁶?7 × 10⁶, molecular mass is controlled via a change in the polymerization conditions. Under comparable conditions of orientational drawing, the highest values of strength (2.65 GPa) and the elastic modulus (100 GPa) are found for samples with M = 6.3 × 10⁶.     

The melt anisotropy of ultrahigh-molecular-weight polyethylene

In the course of melt-flow crystallization studies with ultrahigh-molecular-weight polyethylene (UHMWPE), we observed that the melt of UHMWPE is highly anisotropic above its equilibrium melting point and has a tendency to fibrillate. An examination of the melt anisotropy of UHMWPE by optical, Thermal, and x-ray analysis indicates that the melt anisotropy persists at 345°C, i.e., the temperature at which the polymer degrades under nitrogen, and appears similar to a smectic liquid-crystalline phase.     

Coextrusion drawing of reactor powder of ultrahigh molecular weight polyethylene

Certain nascent polymers have been shown to have unusual thermal and morphological properties that are irretrievably lost once the polymer has been melted or otherwise reduced to the isotropic state. We show further that nascent ultrahigh molecular weight polyethylene “reactor powder” exhibits a remarkable ductility when uniaxially drawn by coextrusion techniques after initial compaction in film form at 100°C. When drawn at a temperature of 110°C, draw ratios of 56 have been obtained, resulting in an enhanced tensile modulus of 58 GPa. Thermal analyses and dynamic mechanical measurements were also made towards understanding the initial and final morphologies.     

Single-crystal-like orientation of ultrahigh-molecular-weight polyethylene by uniaxial stretching

Dried gel film of ultrahigh-molecular-weight polyethylene (UHMW PE) can be drawn to 370 times its original length at 135°C. Single-crystal mats and dried gel film of UHMW PE develop double orientation despite uniaxial stretching. This is unique for preferentially oriented UHMW PE systems. To elucidate the origin of this single-crystal-like orientation, precursors with different aspect ratios were prepared and drawn uniaxially. The degree of double orientation was measured by infrared spectroscopy. The origin of single-crystal-like orientation seems to reside in the necking region. The stacked lamellar structure is transformed into a fibrillar structure in a two-dimensional fashion. This condition is easily provided when UHMW PE single-crystal mat or dried gel film is drawn uniaxially. A draw ratio of 40 and aspect ratio of 40 are the optimal conditions to obtain a doubly oriented structure from UHMW PE single-crystal mat or gel film at 135°C.     

Drawing of ultrahigh-molecular-weight polyethylene single-crystal mats: The crystallinity

Single-crystal mats of ultrahigh-molecular-weight polyethylene can be drawn uniformly to high draw ratios, more than 20χ at the highest, after the necking process is completed. The dynamic mechanical modulus of the drawn mats increases markedly during the uniform drawing stage. The structural changes induced by the uniform drawing at 100°C have been followed by wide-angle and small-angle x-ray scattering, infrared absorption, differential scanning calorimetry, and birefringence. The crystallinity is estimated from the x-ray amorphous scattering intensity, the IR absorbance of gauche bands, the heat of fusion from DSC, and the density. The estimated crystallinities of the drawn mats are all very high and increase slightly and monotonically with increased drawing after necking, though the values of the crystallinity depend on the method of estimation. IR gauche bands and the SAXS peak due to the long period disappear at a draw ratio of about 80χ. All the results suggest that the uniform drawing after necking destroys the two-phase structure made up of alternately stacked crystalline and amorphous regions and then reorganizes it into a single-phase crystalline structure.     

Modification of ultrahigh-molecular-weight polyethylene by low-temperature plasma (review)

Published data appeared in the last decade on the modification of ultrahigh-molecular-weight polyethylene (UHMWPE) films and plates with the use of low-temperature plasma are considered. Setups for sample processing by discharges of various types and methods for studying the changes occurring on the polymer surface are described. The results of the examination of contact, adhesion, strength, and tribological properties are presented. Changes in the chemical composition and structure of UHMWPE films are discussed, and data on the metabolic activity of the modified films are presented.     

Manifestation of the monoclinic phase in IR spectra of ultrahigh-molecular-weight polyethylene

A new band situated in the vicinity of 1056 cm⁻¹ has been observed in the IR spectrum of UHMW PE. This band appears during compaction of the reactor powder, pressing of pellets, and drawing of the resulting films at room temperature. The intensity of the band increases with the time of staying under pressure in proportion to logt with a simultaneous reduction in the intensity of absorption bands corresponding to the orthorhombic phase of PE. Upon staying of the samples at room temperature, the intensity of this band decreases for a long time with a concomitant increase in the intensity of the bands due to the orthorhombic phase of PE. An increase in temperature accelerates this process, and annealing at 100°C for 30 min brings about disappearance of the 1056-cm⁻¹ band and cessation of an increase in the intensity of absorption bands corresponding to the orthorhombic phase of PE. On the basis of the above data, it is inferred that the band at 1056 cm⁻¹ may be attributed to the monoclinic phase of PE.     

Nanocomposites and high-modulus fibers based on ultrahigh-molecular-weight polyethylene and silicates: Synthesis, structure, and properties

Nanocomposites based on ultrahigh-molecular-weight polyethylene and inorganic fillers—such as organomodified layered aluminosilicates, aerosil, and diatomite—are prepared via polymerization filling. The polymerization of ethylene was conducted in the suspension mode with the use of a conventional Ziegler-Natta catalyst, TiCl₄ + Al(i-Bu)₃, under mild conditions (a temperature of 30°C and a pressure of 0.1MPa). The structure and properties of the composites are studied via X-ray diffraction analysis and DSC. The polyethylene matrix features a high enthalpy, a high melting temperature (up to 143°C), a crystallinity of 70–80%, a content of the monoclinic phase of 12–15%, and a bulk density of 0.05–0.15 g/cm³; the molecular mass is (1.5–1.6) × 10⁶. High-modulus, high-strength fibers with an elastic modulus of 25–28 GPa and a strength of 0.65–0.70 GPa are prepared via direct solvent-free molding of nascent reactor powders based on ultrahigh-molecular-weight polyethylene filled (7 wt %) with aerosol or montmorillonite modified with vinyltrimethoxysilane.     

On the fracture of composites based on ultrahigh-molecular-weight polyethylene and fine aluminum particles

Mechanical characteristics of polymerization filled composite materials based on ultrahigh-molecular-weight polyethylene and fine aluminum particles are studied. The prepared composites preserve their ability for high plastic deformations even when the volume filler content is φ = 0.57. For the tensile drawing of the composite material with randomly distributed particles, an equation describing the dependence of breaking stress on the volume filler content is derived. For the model of the composite with regularly ordered particles, the Nielsen equation is the approximation of the equation proposed in this work for a material with randomly distributed particles.     

Functionalized hyperbranched polyethylene powder supports

[reaction: see text] Polyethylene powders with useful loadings of functional groups are accessible by hyperbranched grafting chemistry. Using a 200 microm diameter high-density powder, loadings of up to 0.4 mmol/g of CO(2)H groups are attained. The carboxylic acid groups in the resulting powders can be further functionalized covalently or ionically and resulting powders have the physical durability and solvent resistance associated with polyethylene itself.     

Effect of molecular weight distribution on the structure and mechanical properties of ultradrawn,ultrahigh-molecular-weight polyethylene cast from solution. I. Thermoluminescence and NMR

Ultradrawing of ultrahigh-molecular-weight polyethylene (UHMW-PE) films produced by gelation/crystallization from solution was investigated using thermoluminescence (TL), DSC, and pulsed NMR. The TL from an x-irradiated UHMW-PE gel film is considerably affected by ultradrawing. The TL glow peak is correlated with the α_c dispersion of the crystalline phase in the UHMW-PE gel film. The TL integrated intensity is weak for folded-chain crystals but strong for extended-chain crystals. An extra higher TL peak, observed for samples drawn 150x, is related to the appearance of some highly extended tie-chains in the interfacial region in the process of ultradrawing. On the basis of the above observations, the effects of molecular weight distribution on the morphology of UHMW-PE gel film are examined. It is concluded that B-PE (Broad MWD) having a broader MWD has better drawability than N-PE; but at a given draw ratio, higher modulus and tensile strength are realized for N-PE (Narrow MWD).     

Effect of molecular weight distribution on the structure and mechanical properties of ultradrawn,ultrahigh-molecular-weight polyethylene cast from solution. II. Drawability and mechanical properties

The effect of molecular weight distribution (MWD) on the ultradrawability and mechanical properties of solution-cast films of ultrahigh-molecular-weight polyethylene (UHMW-PE) has been investigated using tensile and dynamic mechanical measurements. The MWD has a marked effect on ultradrawability and thus on the ultimate mechanical properties such as the tensile modulus. It is proposed that UHMW-PE with a narrow MWD(N-PE) attains the ultimate structure at a lower draw ratio than UHMW-PE with a broad MWD(B-PE) because of the existence in the latter of less fully extended intercrystalline tie chains. It is found that, at the same drawing temperature (100°C), N-PE shows a higher modulus than B-PE at draw ratios up to 150 x, which is assumed to be the ultimate value for N-PE.