Ultra-high Molecular Weight Polyethylene Samples Research Articles

Melt drawing of ultra-high molecular weight polyethylene: Comparison of Ziegler- and metallocene-catalyzed reactor powders

The drawing behavior of the ultra-high molecular weight polyethylene (UHMW-PE) melts has been studied by comparing the stress/strain curves for two types of samples as polymerized using conventional Ziegler and newer metallocene catalyst systems. Two UHMW-PE samples, having the same viscosity average molecular weight of 3.3 × 106, but different molecular weight distribution, have been drawn from melt at special conditions. The sample films for drawing were prepared by compression molding of reactor powders at 180°C in the melt. Differences in the structural changes during drawing and resultant properties, ascribable to their broad or narrow molecular weight distribution, were estimated from tensile tests, SEM observations, X-ray measurements and thermal analyses. The metallocene-catalyzed sample having narrower molecular weight distribution, could be effectively drawn from the melt up to a maximum draw ratio (DR) of 20, significantly lower than that obtained for the Ziegler-catalyzed sample, ∼ 50. The stress/strain curves on drawing were remarkably influenced by draw conditions, including draw temperature and rate. However, the most effective draw for both was achieved at 150°C and a strain rate of 5 min−1, independent of sample molecular weight distribution. The efficiency of drawing, as evaluated by the resultant tensile properties as a function of DR, was higher for the metallocene-catalyzed sample having narrower molecular weight distribution. Nevertheless, the maximum achieved tensile modulus and strength for the Ziegler sample, 50–55 and 0.90 GPa, respectively, were significantly higher than those for the metallocene sample, 20 and 0.65 GPa, respectively, reflecting the markedly higher drawability for the former than the latter. The stress/strain behavior indicated that the origin of differences during drawing from the melt could be attributed to the ease of chain relaxation for the lower molecular weight chains in the melt. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1921–1930, 1999

Graft polymerization of methyl methacrylate onto radiation-peroxidized ultrahigh molecular weight polyethylene in the presence of metallic salt and acid

Using a radiation peroxide grafting technique, a ultrahigh molecular weight polyethylene (UHMWPE) stored at room temperature for 10 days after irradiation in air was graft copolymerized with methyl methacrylate (MMA) in the presence of metallic salt and acid. The MMA-grafted UHMWPE samples were analyzed by measuring Fourier transform infrared spectroscopy in attenuated total reflectance (FTIR–ATR) and by electron spectroscopy for chemical analysis (ESCA). The 1,1-diphenyl-2-picrylhydrzyl (DPPH) technique was utilized to evaluate the concentration of peroxide formed in the peroxidized UHMWPE samples by counting the quantity of DPPH consumed from the reaction of peroxide radicals with DPPH. It was shown that the inclusion of an FeSO4 ·7H2O and sulfuric acid in MMA grafting solutions was extremely beneficial and led to a most unusual synergistic effect in the radiation-peroxidized grafting. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 659–666, 1999

The role of long lived free radicals in the ageing of irradiated ultra high molecular weight polyethylene

γ irradiation of ultra-high molecular weight polyethylene (UHMWPE) results in both immediate and time dependent property changes. The time dependent changes are believed to arise from long lived free radicals reacting with oxygen and causing chain scission. Samples of UHMWPE have been irradiated in air and in vacuum at ambient temperatures to various dose levels, and subjected to a series of ageing and heat treatment regimes, following which the electron spin resonance spectra have been recorded. Material irradiated in air initially shows a seven peak spectrum, and by fitting hyperfine splitting values, and comparison with literature this is identified as arising primarily from alkyl radicals with smaller contribution from allyl and peroxy radicals. There is some evidence that with time the relative proportion of allyl radicals increases, but comparison of observed and theoretical spectra shows that the allyl radical is less significant than the alkyl radical in the first few months of ageing. Material irradiated to a received dose of 11.25 Mrad and stored under vacuum for 24 h had a radical concentration of 2.03×10 19/g. The number of radicals decreases with time when the irradiated samples are exposed to air. After 70 days post-irradiation air exposure all alkyl radicals had reacted, leaving only the more stable peroxy radicals. Following reaction of the alkyl radicals there is little further reduction in the number of spins during ageing suggesting that there is constant regeneration of peroxy radicals throughout the cycle of oxidative ageing. Heat treatment or ‘annealing’ at 85°C is effective in reducing the number of radicals within an irradiated specimen. Air annealing of 50 μm thick irradiated specimens for 24 h is sufficient for removal of all alkyl radicals, leaving only peroxy radicals. A further 100 h annealing is necessary for complete removal of all radicals and the material is then fully stabilised. When annealing in vacuum, there is still evidence of alkyl radicals after 24 h of treatment. Vacuum annealing causes radicals to react without oxidation, presumably to form crosslinks. Annealing under this condition requires a longer period for complete elimination of radicals present, indicating that the rate of crosslinking is slower than the rate of oxidation.

SAXS Investigations on Relationships between Synthesis Conditions and Solid State Drawability of High Molecular Weight Polyethylene Nascent Reactor Powders. Sintering and Annealing of Powders

High and ultrahigh molecular weight polyethylene samples have been investigated by small angle X-ray scattering (SAXS) methods to study the structural changes induced by solid state drawing of nascent reactor powders. The results are correlated with prior reports of differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The powders were prepared by two different Ziegler-Natta synthesis processes : polymerization in a slurry and in the gas phase. The synthesis temperature range was 30-85 °C. Monoclinic crystals were identified in samples synthesized at 30 °C. SAXS patterns have been recorded at different stages of the solid state processing. Results obtained on nascent reactor powders and on sintered and annealed powder specimens are discussed. No interference peak was ever detected. SAXS profiles obtained from all samples were consistent with the scattering produced by a polydispersed ensemble of particles having approximately globular shape. Values of the radii and of the weight fractions of these gobular aggregates were obtained by Gaussian analysis of their SAXS patterns, according to the iterative generalized Guinier approximation. Results obtained from nascent powders were consistent with five dimensional groups of globular particles. The dependence of weight fractions and globule dimensions on synthesis parameters was investigated. Values of the radii, R i , of the S185 samples are quasi-multiples of 25 A, up to the top value of 450 A of the R 5 group. Such a multiplicity in the sizes of the globular aggregates can be attributed either to the clustering of small subunits into larger particles or to lamellar thickening by preferred doubling during synthesis. Volume contractions of the globular aggregates produced by annealing are more important in samples synthesized at low temperature. Sintering reduces differences among the radii and the weight fractions of different samples. It is proposed that mobility of polymer chain segments is higher for samples synthesized by a slurry process at 85 °C, since modifications induced by sintering and annealing are more important for these samples. Thus the higher chain mobility and the higher ductilies obtained by slurry synthesis at 85 °C suggest that these conditions may lead to nascent powders having lower degrees of entanglement.

On the structure of ultra-high molecular weight polyethylene gels

Gel-crystallized ultra-high molecular weight polyethylene samples have been studied by transmission electron microscopy (TEM) and differential scanning calorimetry. The microstructure of the gels was investigated by substitution of the solvent by methacrylate monomers which were subsequently cured by ultraviolet radiation at low temperature. TEM images of RuO 4-stained thin sections showed a network of lamella crystals in which the links were formed by local contacts of the crystal surfaces. Electron spectroscopic imaging yielded carbon concentration profiles across the width of isolated crystals. Annealing of the dried gels above T α illustrated a high mobility of the polyethylene chains within the crystal above the α-transition and well below the melting point. Long lamellae became decorated by shorter and thinner crystallites which appear to be grown perpendicular on the surface of the long ones.

Use of a sequential extraction technique to determine the MWD of bulk UHMWPE

AbstractThe molecular weight distribution (MWD) of bulk ultrahigh molecular weight polyethylene (UHMWPE) was determined by an increasing temperature sequential extraction technique. Using decahydronapthalene (decalin) as the solvent, 5 g samples of UHMWPE were fractionated stepwise from room temperature to 190°C in 10°C increments. Results via high temperature capillary viscometry show that intrinsic viscosity ([η]) ranges from 12.0 to 37.0 dL/g or that the viscosity average molecular weight (M̄v) ranges from approximately 1.6 to 7.4 × 106. When the integral distribution is plotted against M̄v on logarithmic probability graph paper, a straight line (i.e., a Wesslau distribution) results. Although the present methodology is tedious, many previous problems which have led to degradation are overcome by this technique.