DSC ofγ-irradiated ultra-high molecular weight polyethylene and high density polyethylene of normal molecular weight

The melting and the crystallization of-irradiated (doses: 0–6Mrad) ultra-high molecular weight nascent polyethylene (UHMWPE) and high density nascent polyethylene with normal molecular weight (NMWPE) were investigated by DSC. The heat of melting of the nascent UHMWPE (DSC degree of crystallinity, respectively) increases up to a dose of 3 Mrad, after which it slightly decreases. The heat of the second melting of UHMWPE and of the first and second melting of NMWPE increases slightly up to a dose of 3 Mrad, after which it does not change. The X-ray degree of crystallinity of the nascent non-irradiated and irradiated polymers was 0.62±0.02. The calorimetric crystallinity was compared to the X-ray one. The results show that radiation does not affect the polymer crystallinity, but influences the thermodynamic heat of melting. The increase ofH _m vs. dose in UHMWPE is explained in terms of processes of tie molecule scission within the amorphous regions and on the surface of the crystals, which predominate over crosslinking up to a dose of 3 Mrad. That leads to an increase in the conformational mobility of the molecules and to an increase in the enthalpy, according to Peterlin's formula. The scission of the chains at the points of entangling of the tie molecules leads to a decrease in the temperature and to an increase in the enthalpy of crystallization of UHMWPE vs. dose. In NMWPE these effects are considerably weaker.     

Pecularities of the thermomechanical behaviour of ultra-high molecular weight linear polyethylene and its blends with linear polyethylene of normal molecular weight

Ultra-high molecular weight polyethylene UHMWPE (M _w=4 · 10⁶,I _s=O g/ 10 min), high density polyethylene of normal molecular weight NMWPE (I _s= 4.8 g/10 min) and their blends have been investigated by means of thermomechanical loading in constant and impulse regime. It has been established that after melting, NMWPE passes to a viscous-liquid state. After melting at 138 °C UHMWPE passes to a high-elastic state. The transition of UHMWPE to a viscous-liquid state takes place at temperatures higher than 180 °C and is accompanied by a high-elastic reversible deformation. The blends of UHMWPE with 10 and 20 mass % of NMWPE show a plateau on the thermomechanical curves, corresponding to a high-elastic state, in a shorter temperature range where the deformation is greater. The blends containing the higher percent of NMWPE show thermomechanical curves lacking such a plateau. All blends are characterized by a singular thermomechanically defined temperature of melting, which increases with increase of UHMWPE content. The existence of the high-elastic state in the curves of UHMWPE and its blends containing NMWPE less than 30 mass % above their melting temperatures is explained by the high degree of physical crosslinking of UHMWPE.     

Blends of normal high density and ultra-high molecular weight polyethylene, γ irradiated at a low dose

The kinetics of a nonisothermal crystallization and melting of irradiated with dose of 6 Mrad blends of an ultra-high molecular-weight polyethylene (UHMWPE) and a high-density polyethylene with normal molecular weight (NMWPE) is investigated by means of DSC. The blends have been prepared at temperature below the flow temperature of UHMWPE: The enthalpies of melting of the polyethylenes increase, while those of their blends decrease after irradiation. The enthalpies of crystallization of the pure polyethylenes are higher, while those of their blends almost do not change or are a bit higher after irradiation. The rates of a nonisothermal crystallization and melting of the polyethylenes increase, while those of the polyethylenes in the blends decrease after irradiation. Thermomechanical measurements under constant load in wide-temperature interval of irradiated polyethylenes and their blends have been made. A high-elastic plateau in viscous-liquid state is established on the thermomechanical curves of UHMWPE, and the blends with high content of UHMWPE. On the basis of results obtained assumptions have been made about the processes taking place in the blends under the action of irradiation, as well as about the character of the mutual influence between the components in the process of irradiation.     

Temperature-dependence of mechanical and morphological properties of ultra-high molecular weight polyethylene cross-linked by electron beam irradiation

Cross-linking of ultra-high molecular weight polyethylene was performed with electron-beam irradiation in the range of radiation dose from 12 to 96 Mrad under nitrogen. Dry gel films and melt films were used as specimens. Two kinds of cross-linked specimens could be kept at 200°C for a prolonged time in an undeformed state and this tendency was independent of radiation dose. The elongation of the gel films hampered the heat-resistant effect and the drawn specimens were broken at temperatures lower than 175 °C. The elongation of the melt films could not be realized, because of a marked fixation of chains in the fiber network, even at a dose of 12 Mrad.     

Radiation-induced degradation of polyethylene: Role of amorphous region in the formation of oxygenated products and the mechanical properties

Quenched samples of linear low density, medium density and two kinds of high density polyethylene films were irradiated with γ-rays from a ⁶⁰Co source in vacuum and in air at room temperature with irradiation doses ranging from 0 to 100 Mrad. On irradiation in vacuum the extent of crosslinking was about one-and-a-half times greater in the linear low density polyethylene (LLDPE) than in the high density polyethylene (HDPE). On the other hand, irradiation in air produced more crosslinking in high density polyethylene (HDPE). Growth of trans-vinylene unsaturation was found around 10 Mrad in all the samples. Initial increase in elongation and breaking strength (below 5 Mrad) occurred, which was followed by a decrease with increasing dose. LLDPE showed some elongation even at 50 Mrad, while the other samples became brittle and broke at doses far below this value. The mechanism of oxidative degradation is discussed.     

Kinetics of nonisothermal crystallization and melting of normal high density and ultra-high molecular weight polyethylene blends

The kinetics of nonisothermal crystallization and melting of blends of ultra-high molecular weight polyethylene (UHMWPE) and polyethylene high density with normal molecular weight (NMWPE) are investigated by means of differential scanning calorimetry (DSC). Mixing the components at a temperature below the flow temperature of UHMWPE (215 °C) results in increased crystallization/melting rates of the individual components in the blends above the corresponding additive values. The morphological observations of the blends, carried out by means of polarization microscopy, show that a strong boundary of both types of structures (UHMWPE non-flowing aggregates and NMWPE spherulite structures) does not exist. The NMWPE spherulites' dimensions decrease on increasing the UHMWPE concentration in the blends, but their number increases. The facilitation of the crystallization/melting of the components in the blends is explained in terms of mutual influence exhibited by the components with respect to each other. It is due to the inner stresses in nonflowing UHMWPE characterized with a lot of entangled tie molecules and to the partial co-crystallization of NMWPE molecules with the flowing part of UHMWPE. At mixing temperatures above 215 °C the melting/crystallization integral kinetic curves have only one linear part in contrast to these of the same blend (11 ratio of components), prepared at 190 °C. The rates of melting/crystallization remain almost constant with the increase of the mixing temperatures.     

Morphology and mechanical properties of ultrahigh molecular weight polypropylene prepared by gelation/crystallization at various temperatures

Films of ultrahigh molecular weight (5.4×10⁶) polypropylene were produced by gelation/crystallization at various temperatures from dilute decalin solutions according to the method of Smith and Lemstra. The temperatures chosen were 20°, 30°, 50°, and 60°C. With increasing the temperature, the long period and crystallinity of the resultant gel film increased. By contrast, when the films were stretched up to 50 } 60 times, the increases in Young's modulus and crystallinity become more significant, as the temperature of the gelation/crystallization became lower. This interesting phenomenon is thought to be due to the dependence of the number of entanglements on the temperatures concerning gelation/crystallization and evaporation of solvent from the gel to form a film.     

Surface studies of ultra‐high molecular weight polyethylene irradiated with high‐energy pulsed electron beams in air

Ultra‐high molecular weight polyethylene (UHMWPE) was irradiated in air with high‐energy (9 MeV), pulsed electron beams to doses ranging from 2.5 to 100 Mrad and subsequently heat treated at 120°C for a time period of 120 min. Surface characterization of the target side of irradiated UHMWPE samples was carried out both before and after the heat treatment by means of attenuated total reflection Fourier‐transform infrared (FTIR/ATR) spectroscopy and microhardness measurement. The obtained results provided further evidence supporting our earlier observation (Tretinnikov, O. N.; Ogata, S.; Ikada, Y. Polymer 1998, 39, 6115) that thermal decomposition of hydroperoxides formed upon irradiation of UHMWPE with high‐energy, pulsed electron beams in air leads to surface crosslinking, and the subsequent surface hardening of the irradiated polymer. Importantly, we found that this phenomenon has the highest contribution to the surface hardness enhancement of the polymer when the radiation dose is in the range of 10–30 Mrad. In addition, we found that this irradiation and subsequent heat treatment of UHMWPE in air does not lead to formation of carbonyl‐containing products unless the radiation dose exceeds 20 Mrad. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1503–1512, 1999     

Morphology and mechanical properties of ultra-high molecular weight polyethylene-poly(tetrafluoroethylene) blend films

This paper deals with the morphology and mechanical properties of blend films for polytetrafluoroethylene (PTFE) and ultra-high molecular weight polyethylene (UHMWPE) prepared by kneading techniques. This experiment was carried out for blend films, prepared with different compositions of PTFE and UHMWPE to improve thermal properties of PE. In spite of the incompatibility of the two polymers, the blend film with the PTFE/UHMWPE composition =75/25 was maintained under the measurement of complex modulus at temperature higher than 300°C. This indicates that the UHMWPE chains dispersed in PTFE fibrous texture were not separated by the melting flow of UHMWPE at 300°C. To check the origin of this interesting phenomenon, the morphology of the blend films was investigated by using scanning electron microscopy, X-ray diffraction, and¹³C nuclear magnetic resonance.     

Thermogravimetry and differential scanning calorimetry of γ-irradiated i-polypropylene films

Isotactic polypropylene films, Buplen Type, 40m thick, irradiated by a⁶⁰Co source to doses 0.37–37 Mrad, are investigated by means of optical microscopy, WAXS, thermogravimetry, DSC and DTA. The original film exhibits a paracrystal structure. Irradiation does not change the films' structure. The kinetic parameters of the non-isothermal destruction and the thermodynamic parameters of melting are obtained. The samples irradiated to small doses (up to 3 Mrad) are thermally more stable; the activating energy of the destruction is higher than that of the original film. The temperature of melting slightly increases, while the enthalpy of melting decreases. For the range of doses of 3.7–37 Mrad, the films show low thermal stability and the destruction proceeds with low activating energy. From the results of the data obtained, the following assumptions are made: the-irradiation causes simultaneous crosslinking and chain scission at random sites along the chains. Fragments of partially crosslinked molecules and fractions of low molecular linear segments are formed. The destruction caused by radiation prevails above 3 Mrad.     

Effect of chemical crosslinking on mechanical and electrical properties of ultrahigh-molecular-weight polyethylene-carbon fiber blends prepared by gelation/crystallization from solutions

In an attempt to improve the mechanical property of polyethylene composite at high temperature, crosslinking of ultrahigh-molecular-weight polyethylene (UHMWPE) and carbon fiber (CF) blends was carried out by using dicumyl peroxide (DCP). The specimens were prepared by gelation/crystallization from solutions. The effect of chemical crosslinking on mechanical and electrical properties of UHMWPE/CF blends with composition of 1/0, 1/0.25, and 1/1 (w/w) were investigated in detail. Electrical conductivity and thermal mechanical properties of the blends with the 1/1 composition were greatly improved by incorporation of enough content of CF and adequate crosslinking network formation. Surprisingly, the Young’s modulus of the 1/1 blend reached 20 GPa at room temperature (20 °C). On the other hand, heat treatment at 135 °C played an important role for obtaining a high PTC effect for the UHMWPE-CF blend in which the PTC intensity reached 10⁷.     

Electrical and dielectric properties in carbon fiber‐filled LMWPE/UHMWPE composites with different blend ratios

Carbon fiber (CF) filled low‐molecular‐weight polyethylene (LMWPE) and ultra‐high molecular weight polyethylene (UHMWPE) composites were prepared by the gelation from solution and the kneading in the melting state. The content of carbon fibers was fixed to be 23.5 vol %. The resistivity, positive temperature coefficient (PTC), and dielectric behaviors of the composites became more pronounced with increasing content of LMWPE with much higher thermal expansion than that of UHMWPE. The PTC effect became most significant, when the blend ratio of LMWPE to UHMWPE was 9/1. Beyond 9/1, the PTC effect was less pronounced. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) revealed that the UHMWPE and LMWPE chains within the composite crystallized independently by gelation from solution and were virtually unaffected by the presence of carbon fibers. Consequently, it was confirmed that carbon fibers selectively were localized in the mixed region of LMWPE and UHMWPE for the composite (3/1 and 6/1) and mainly in the region of LMWPE for the 9/1, 12/1, and 15/1 composites. This indicated that the content of carbon fibers within LMWPE region was the highest for the 9/1 composite and the 9/1 composite provides the most significant PTC effect. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 359–369, 2008     

Adsorption of polyethylene glycol from aqueous solution on montmorillonite clays

Adsorption rates and capacities of polyethylene glycol (PEG) were investigated for five montmorillonite clays. The adsorption of PEG for all the montmorillonite clays was rapid, and equilibrium was attained within 30 min. The adsorption isotherms of PEG for all the montmorillonites conformed to the Freundlich equation. The adsorption heats were 7.3 and 11.6 kJ · mol^–1(mw.:2000), and 8.7 and 14.2 kJ · mol^–1(mw.:20000) for the montmorillonite and the bentonite II-Ca, respectively. Adsorption capacities for all the clay samples approached constants for the molecular weight of PEG over 2000, though they increased with the increase of molecular weight under 2000. The adsorption capacities were slightly influenced by a nearly neutral pH. The montmorillonite clays which had different interlayer cations showed quite different adsorption capacities. The bentonite II-Ca, the acid clay, and the activated clay showed large adsorption capacities that were 30–50 % of that of an activated carbon.     

Morphology and electric conductivity of cross-linked polyethylene–carbon black blends prepared by gelation/ crystallization from solutions

 Ultra-high-molecular-weight polyethylene (UHMWPE) – carbon black (CB) blends were prepared by gelation/ crystallization from PE dilute solutions containing CB particles. The UHMWPE/CB composition chosen were 1/0.15, 1/0.25, 1/0.5, 1/0.75, 1/1, 1/3, 1/5, and 1/9, etc. The cross-linking of PE chains was performed by chemical reaction of dicumyl-peroxide at 160 ^°C. X-ray diffraction patterns indicate that the crystallinity of PE within the blends decreased drastically through the chemical reaction at high temperature. The sample preparation method by gelation/crystallization provided the UHMWPE–CB system with various CB contents up to 90% and the conductivities for the resultant specimens were in the range from 10^-9 to 1 Ω^-1 cm^-1 corresponding to the electric conductivity range of semiconductors. The blends assured thermal stability of electric conductivity by cross-linking of PE chains, although the mechanical property such as the storage and loss moduli were very sensitive to temperature. The conductivity of the blends with CB content ≥20% were almost independent of temperature up to 220 ^°C and the values in the heating and cooling processes were almost the same. On the other hand, for the UHMWPE–CB blends with 13% CB content corresponding to the critical one, temperature dependence of electric resistivity showed positive temperature coefficient (PTC) effect. The PTC intensities for non-cross-linked and cross-linked materials were lower than that of the corresponding low-molecular-weight-polyethylene (LMWPE)–CB blend but the maximum peak appeared at 160 ^°C which is higher than the peak temperature of LMWPE–CB blend. Received: 10 December 1997 Accepted: 9 April 1998     

Radiation-induced changes in the microdestruction mechanisms in high-density polyethylene

High–density polyethylene (HDPE) samples irradiated by Co⁶⁰ gamma source have been investigated. The integration between the macromolecular structural units building the supermolecular structural organization is changed and different microdestruction mechanisms are achieved in the irradiation interval (0–20 Mrad). These microdestruction mechanisms are a result of specific radiation-induced crosslinking for the different irradiation doses. The crosslinking unites the primary structural units into macromolecular networks and lattices, which have a different effect upon the micro- and macrocharacteristics of the polymer, i.e., local micro-Brownian mobility, elongation at break, etc. © 1996 John Wiley & Sons, Inc.     

The effect of particle concentration on zeta potential in extremely dilute solutions

The electroporetic mobility of hexadecane particles in water and in very dilute CTAB solutions was measured. The technique of microelectrophoresis was applied. Zeta potential was calculated according to the Hückel formula, applying Henry's correction factors. Electrokinetic charge density was calculated according to the formula used by Delgado et al., and previously discovered empiricaly by Loeb et al. and derived mathematicaly by Ohshima et al. It was found that the particle concentration in emulsion limits their charge in solutions of very low ion concentration (10^–7:10^–6 M), because the greater the particle concentration, the smaller the extent of the ions adsorbed at the surface of one particle. The zeta potential was found to be independent of particle concentration if the ratio of the number of bulk ions to particle number is not lower than 1.6×10⁶. This ratio depends on particle size, and the value 1.6×10⁶ relates to particles of diameter 1.6 m.     

Viscosity reduction and disentanglement in ultrahigh molecular weight polyethylene melt: Effect of blending with polypropylene and poly(ethylene glycol)

A significant reduction in melt viscosity of ultrahigh molecular weight polyethylene (UHMWPE) was obtained by blending with polypropylene (PP) and poly(ethylene glycol) (PEG). The mechanism of viscosity reduction was investigated from the view of disentanglement effect. Dynamic mechanical analysis indicated that the pseudoequilibrium modulus (E′) of UHMWPE/PP(80/20) blend in the rubbery plateau was much lower than that of UHMWPE. Accordingly, the calculated entanglement density (ν_e) of UHMWPE/PP (80/20) blend was smaller than that of UHMWPE. Further reduction in E′ and ν_e of the blend was obtained by the incorporation of 1 phr PEG. Slow DSC analysis showed that the high temperature endotherm and exotherm for UHMWPE at slow temperature ramp diminished and increased, respectively when 5 phr PEG was added. It also revealed that the entanglement level of UHMWPE decreased with the addition of a small amount of PEG.     

Mechanical properties,surface morphology and failure mode of γ-ray irradiated blends of polypropylene and ethylene-vinyl acetate rubber

The effect of ⁶⁰Co γ-radiation on the mechanical properties, surface morphology and failure characteristics of blends of polypropylene [PP] and ethylene-vinyl acetate rubber [EVA] has been studied with specific reference to the effect of blend ratio, dynamic crosslinking of the rubber phase and absorbed radiation doses. Samples were subjected to radiation in the dose range of 1 to 100 Mrad in air at room temperature at the rate of 0·321 Mrad/h. Both chain scission and crosslinking occur simultaneously in the blend samples. PP and blends containing higher proportions of PP (≥50%) undergo predominant chain scission at lower doses (≤50 Mrad), which causes a drastic drop in tensile strength, followed by a levelling out at higher doses of 100 Mrad. EVA undergoes crosslinking at lower doses resulting in an increase in tensile strength in the dose range 1 to 10 Mrad followed by a decrease in the range 10–25 Mrad. Further increase in radiation dose has little effect on tensile strength. The effect of radiation on stress-strain behaviour, elongation at break, energy at rupture and hardness was also studied. The morphology of the irradiated surfaces after an absorbed dose of 100 Mrad has been examined by scanning electron microscopy (SEM). In order to understand the effect of γ-radiation on the failure mechanism, tensile failure surfaces of both unirradiated and irradiated samples have also been examined by SEM.     

Radiation-induced crosslinking: III. Effect on the crystalline and amorphous density fluctuations of polyethylene

Small-angle x-ray scattering (SAXS) was used to determine density fluctuation in radiation-induced crosslinked polyethylene of varying degrees of crystallinity. Density fluctuation FL decreases with increasing crystallinity, while it increases linearly with increasing radiation dose or degree of crosslinking. By means of extrapolation, density fluctuations in the crystalline and the amorphous phasesFL _c andFL _a were obtained. At a given dose,FL _a is greater thanFL _c . The increase inFL _a with radiation is found to be much greater than that ofFL _c compared with the initial values at 0 Mrad,FL _c showing only a negligible increase event at 312 Mrad. The present findings suggest that crosslinks are not introduced within the crystalline phase; they take place primarily in the noncrystalline phase, in agreement with the conclusions reached previously on the basis of changes in crystalline and amorphous densities in irradiated polyethylene.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday     

HYSCORE and Davies ENDOR study of irradiated ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) has been studied with different magnetic resonance techniques to elicit information on the nature and the location of radicals generated during high energy irradiation. Field swept electron paramagnetic resonance, pulsed Davies electron nuclear double resonance and hyperfine sublevel correlation spectroscopic measurements allowed extracting for the first time the full ¹H hyperfine coupling tensors of the most abundant radical, i.e. a secondary alkyl radical and to ascertain the formation of allyl radicals in the first stages of the irradiation process. The ¹H hyperfine coupling tensors are analogous to those reported for single crystal irradiated polyethylene, suggesting that radicals generated in UHMWPE are located in the crystalline region of the polymer. Copyright © 2012 John Wiley & Sons, Ltd.