Effect of large dosage irradiation in air on polyethylene

The large dosage irradiation effect on polyethylene in air was examined by the method of double irradition. Polyethylene was first irradiated with γ-rays to a dosage of a few hundred Mrad in air at 298°K. The formed radicals were destroyed, and the polymer, changed chemically and morphologically, was irradiated again in vacuo at 77°K with a dose of 12.7 Mrad. The thermal decay of radicals produced in polyethylene by the second irrdiation was investigated. The results were compared with the sample not exposed to the first irradiation. The results were also compared with previous data of polyethylene irradiated in vacuo to a great extent and then treated in the same manner as that in the present experiment. The results show that scission of the polymer chain by oxidation makes the radical decay faster. Simultaneously, crosslinks formed in polyethylene irradiated in air to a great extent slow down the radical decay. Almost the same number of double bonds was formed in polyethylene irradiated in air as in polyethylene irradiated in vacuo. These double bonds react with alkyl radicals to form allyl radicals.     

Conversion of alkyl radicals to allyl radicals in irradiated single crystal mats of polyethylene

The decay of alkyl radicals, the conversion of alkyl radicals to allyl radicals and the trapping of allyl radicals in irradiated single crystal mats of polyethylene have been studied by electron spin resonance (e.s.r.). It has been suggested that in the crystal core alkyl radicals react with trans-vinylene double bonds and are converted into trans-vinylene allyl radicals; at the crystal surface, alkyl radicals react with vinyl end groups and are converted into allyl radicals with vinyl end groups. The decay of radical pairs and the formation of trans-vinylene double bonds are discussed.     

Chlorination of irradiated polyethylene single crystals: 1: Analysis of alkyl radical decay kinetics

The chlorination of electron beam-irradiated polyethylene (PE) single crystals was studied for a range of irradiation doses, temperatures and chlorine interaction times. The results of this work are presented in two parts: (1) Electron paramagnetic resonance (e.p.r.) analysis of irradiated PE without chlorination and (2) characterization of chlorine uptake in irradiated PE by use of X-ray energy of spectroscopy (XES) and electron spectroscopy for chemical analysis (e.s.c.a.). Spectra were obtained for 75 – 600 Mrad doses over 136 to 333 K temperature range. Alkyl radical concentrations and decay kinetics were derived from e.p.r. spectra and provided a basis for the chlorination analysis in Part II. A first-order alkyl radical decay constant of 4.3 × 10^?5s^?1 at 298 K was determined in agreement with previous research. Alkyl radical migration was modelled as a one, two or three-dimensional unsteady-state Fickian diffusion process. E.p.r. alkyl radical concentration data was used to estimate alkyl radical diffusion coefficients yielding values of 10 to 4 × 10^?18cm^2?1 at 298 K for the three models evaluated, respectively. These values are consistent with a diffusivity estimated by Seguchi and Tamura based on a 3-D model. A three-dimensional diffusion model seems to be the most realistic since it permits a zig-zag radical trajectory due to the staggered arrangement of ?CH₂- units in crystalline PE. Although a 3-D model would allow diffusion in all directions, it is still consistent with the observation that consecutive migration steps are most likely intermolecular.     

Irradiation of oriented LDPE films

Low-density polyethylene (LDPE) films deformed uniaxially at 70°C and to low draw ratios have been irradiated on a ⁶⁰Co source with a dose rate of 0.33 Mrad/h in air. Their elastic modulus has been determined at room temperature and at 145°C, and their crystallinity has been studied by wide-angle X-ray scattering (WAXS). A two-stage process has been proposed for the influence of irradiation on the structure and properties of the films. The first stage is characterized by enhanced chain scission in the strained regions, as a result of which structural reorganization takes place there and the crystallinity increases. This increase is greater for higher draw ratios and higher doses. The second stage is characterized by no structural changes in the material. The value of D, which separates the two stages, shifts toward higher doses for higher draw ratios. Under the conditions of the present experiment, the orientation of the films decreases the efficiency of network formation by the irradiation.     

Effect of electron beam radiation on the mechanical and thermal properties of poly(4-methylpentene-1)

The changes in the mechanical and thermal properties of electron-beam-irradiated PMP of two different molecular weights (〈M_w〉 = 9.2 × 10⁵, 〈M_w〉 = 1.8 × 10⁶) have been studied. Electron beam (EB) irradiation was performed either in a nitrogen or air atmosphere to a maximum dosage of 40 Mrad. Stress–strain behavior of the irradiated materials show that the lower molecular weight polymer is more affected within this dose range than the higher molecular weight material. The modulus of both PMP materials (at 23°C), however, was not affected by EB. Moreover, it was observed that by increasing radiation dose up to 10 Mrad the occurrence of yielding disappeared in the case of the lower molecular weight system but was still found in the high molecular weight material up to 20 Mrad. The elongation at break of both PMP materials was systematically decreased by increasing the dose level. The rate of stress–relaxation of irradiated samples increased as dosage increased. It is believed that oxidative degradation is promoted as a result of irradiation which induces chain scission. This result was confirmed by GPC analysis which showed that, by increasing radiation dose, the molecular weight systematically decreased. DSC measurements used to investigate the changes in thermal properties showed that the melting temperature and heat of fusion decreased as the dose increased. An interesting feature of the DSC studies was the presence of an endothermic doublet in the melting behavior that transformed into a single peak following irradiation.     

Radical yields in irradiated poly(ethylene terephthalate) fibres

The formation of free radicals and their subsequent decay in irradiated polyethylene terephthalate monofilaments was studied by e.s.r. under a variety of conditions. A yield of 0.023 radicals per 100 eV was found, in agreement with the literature values. Reduced values were found in the presence of water, methylene chloride and chlorine. The decay followed a two-stage second order pattern presumably due to radicals in the crystalline and amorphous regions. At ?196°C, G values of 0.7 of trapped electrons were found, rapidly decaying as the temperature was raised. At about ?135°C the signal changed to the characteristic signal found at room temperature with a G value of 0.42. On adding methylene chloride to irradiated PET, comparatively rapid decay of the trapped radicals took place, the rate corresponding closely to the rate of sorption of the methylene chloride into the monofilaments. This indicated very rapid decay even from the crystalline regions, once the solvent is present. A number of other observations are presented and interpreted in relation to, for example, the radiation grafting yields in the presence of solvents.     

Urethane-based stabilizers for radiation-crosslinked polyethylene

Unsaturated urethane-based stabilizers for use in radiation-crosslinked polyethylene were synthesized. Aromatic amine moieties were attached to allylic and acrylic monomers by means of aromatic or aliphatic diisocyanates. The synthesized stabilizers were incorporated in high-density polyethylene films which were subjected to electron beam irradiation. The oxidative stability of the films prior to and after extraction was determined by DTA in the temperature range 185–210°C and compared with samples treated with commercial amine-bearing antioxidants. Tensile strength and gel content were also determined. Best results were obtained with a stabilizer prepared from equimolecular amounts of allyl alcohol, tolylene-2,4-diisocyanate and N-phenyl-1,4-phenylenediamine. Estimated lifetimes at 70°C of stabilized irradiated polyethylene samples were calculated.     

Radiation crosslinking of polyethylene with electron beam at different temperatures

The effect of temperature over the range ?196 to 150°C on the crosslinking of polyethylenes irradiated by electron beam has been investigated on the basis of gel content determination and Fourier transform infra-red (FTIR) spectroscopy. The crosslinking efficiency increases significantly with increasing irradiation does and at elevated irradiation temperature. The crosslinking rates of high density polyethylene (HDPE) and low density polyethylene (LDPE) samples above the melting point (T_M) are much higher than those below T_m. The FTIR data give positive evidence: (i) that trans-vinylene double bonds in cross linked HDPE and LDPE samples increase with increasing irradiation dose temperature (ii) that vinyl double bonds in HDPE decrease rapidly with increasing irradiation dose and temperature, and (iii) vinylidene groups in LDPE decrease slowly with increasing temperature at the lower dose and are almost independent of the irradiation temperature at above room temperature and the higher dose of more than 100 kGy. Gas bubbles are observed in LDPE samples irradiated at 100 and 150°C with high dose (200 to 250 kGy). The size of the bubbles increases gradually at high temperatures.     

ESR and FTIR studies on electron beam‐irradiated low‐density polyethylene blends

Low‐density polyethylene (LDPE) composites modified with a resin based on ethylene/methacrylic acid copolymer (surlyn) and/or citric acid were electron beam‐irradiated and investigated by electron spin resonance (ESR) at room temperature. ESR studies were carried out directly after irradiation and after various time intervals up to 72 h postirradiation. The irradiated samples showed the ESR spectrum of seven lines that was assigned to the formation of allyl radical. The nature and yield of the allyl radical of the different LDPE samples were analyzed as a function of time after irradiation. Also, the radical concentration, decay, decay rate, and kinetics of radical decay were evaluated. Fourier transform infrared (FTIR) analysis at a series of different temperatures upon cooling from room temperature to ?175°C and the reverse heating to +125°C was also carried out. The structural changes while cooling and heating of LDPE samples were investigated using FTIR spectrometry. The results showed that cooling of unirradiated LDPE samples to ?175°C results in a decrease of the intensities of IR bands. However, heating the samples from ?175°C up to +125°C led to a consequence increase in the intensities of the IR bands. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3461–3469, 2007     

Radiation-crosslinked polypropylene: Physical and dielectric properties

Irradiation of two stereoregular polypropylenes and a polyallomer via ⁶⁰Co in the presence of 3–8% allyl methacrylate (AMA) monomer, to does up to 5 Mrad, lead to an improvement in mechanical properties. With irradiation, heat resistance, tensile strength, and gel fractions increased, and creep compliance decreased. The decrease in creep compliance was revealed by measurements of irradiated samples at 185⁰C (at constant load and monomer level), a temperature above the T_m of polypropylene. Dielectric properties (dielectric constant K and dissipation factor tan δ) were virtually unaffected by irradiation of 0.3 to 5.0 Mrad in nitrogen when AMA was absent. In the presence of monomer, small but generally tolerable increases in tan δ resulted if the monomer concentration was not too great. To obtain improvements in mechanical properties while not simultaneously altering the dielectric losses, it was necessary to keep the AMA concentration to a maximum of 4%.     

Effects of gamma radiation on two aromatic polysulfones. II. A comparison of irradiation at various temperatures in air-vacuum environments

The aromatic polysulfone poly(oxy-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1,4-phenyleneiso-propylidene-1,4-phenylene) (I) showed no change in flexural yield strength after doses of γ-radiation up to 600 Mrad in vacuum at 35,80, and 125°C (T_g = 190°C)). However, the flexural strength decreased markedly with doses above 100 Mrad on irradiation in air, to 40–60% of the initial value after 200–400 Mrad, depending on the sample and the irradiation conditions. Chain crosslinking was predominant over scission for irradiation in vacuum at all temperatures; (G(X), G(S), and G(S)/G(X) increased with the irradiation temperature, but G(S)/G(X) decreased to zero above T_g. Poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene) (II) behaved similarly, except that the flexural strength was found to be very dependent on the thermal treatment of the sample. This polymer showed a remarkable retention of its mechanical properties on irradiation up to 200°C (T_g = 230°C) in the absence of air, the flexural strength being retained up to 500 Mrad. Radiation annealing occurred at 35°C in vacuum and air and combined radiation and thermal annealing at 125 and 220°C. Progressive removal of surface layers from flexural test bars of I irradiated in air showed that the decrease in flexural strength with dose could be explained by a decrease in the molecular weight towards the surface resulting from radiation-oxidation reactions.     

Dialysis membranes from polyethylene films grafted with acrylic acid

Low‐density polyethylene‐g‐poly(acrylic acid) membranes were prepared by the direct radiation grafting of aqueous acrylic acid solutions (containing Mohr's salt) onto low‐density polyethylene films and were irradiated at two different irradiation doses (2 and 3 Mrad) at a dose rate of 0.02 Mrad/h. Two series of polyethylene‐g‐poly(acrylic acid) membranes with 100 and 150% grafting were obtained. The free carboxylic acid groups in the grafted films were converted into the corresponding acrylates by reactions with different metal salts. The swelling (water uptake) and dialysis permeability of glucose and urea through the grafted membranes in different metal‐ion forms were investigated. The prepared membranes showed good permeability to both solutes, which increased as the hydrophilicity of the membrane increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 10–14, 2004     

Free radical formation in oriented polybutadiene during mechanical deformation below Tg

The tensile behavior of oriented polybutadiene at 83°K is systematically studied as a function of strain rate and pre-test orientation. Electron spin resonance studies of radical formation are made in conjunction with the mechanical tests. Three different modes of mechanical behavior are observed (brittle, crazing and a second ductile behavior without crazing), depending on test conditions. Radical formation is observed in association with the two ductile modes of behavior. The ESR spectra obtained are attributed to a combination of allyl radicals formed by chain scission between the α-methylene groups and peroxy radicals. The relative quantity of the two radical species present is thought to be related to the ratio of cis/trans-isomerism. Stability of the radicals observed with time and with an increase in temperature is studied. Further studies are made of the quantity of environmental test gases absorbed during crazing.     

Radiation-induced graft copolymerization of butadiene on polyethylene by trapped radicals

The low-temperature grafting of butadiene on polyethylene (PE) preirradiated in air is compared with that on PE preirradiated in vacuo for the effect of several factors on the grafting. Little fundamental difference between the two cases was observed. In both cases the graft conversion curve is trapped radical type and the overall activation energies are 9.7 and 8.4 kcal/mol for 0.947-density PE and 0.916-density PE, respectively. Percent grafting is proportional to the square root of the dose rate (= total dose). The retarding effect of oxygen at irradiation or storage after irradiation on the grafting scarcely appears when the irradiation temperature or storage temperature is below 15°C. For thinner film and lower density of PE this effect appears clearly at the higher temperature. Therefore, it may be concluded that the low-temperature grafting of butadiene onto PE preirradiated in air is initiated predominantly by nonoxidized radical (allyl radical) trapped in PE and that the grafting reaction occurs in the semicrystalline part of PE.     

Electron beam‐induced piezoelectric phase in poly(vinylidene fluoride) nanohybrid: effect at the molecular level

A nanohybrid has been synthesized by incorporating organically modified layered silicate in a poly(vinylidene fluoride) (PVDF) matrix. Molecular‐level phenomena have been explored after exposing PVDF and its nanohybrid to an electron beam of varying doses. The electron beam interacts with polymer chains and thereby generates different free radicals, the number of which is quite high in nanohybrid as compared to pure PVDF. The stability of free radicals has been confirmed through density functional theory energy minimization, predicting stable β‐phase free radicals in the nanohybrid. Quantitative analyses of chain scission, crosslinking and double bond formation are reported and compared after irradiation for both PVDF and its nanohybrid using UV‐visible and Fourier transform infrared spectroscopies, sol–gel analyses and gel permeation chromatography, revealing both chain scission and crosslinking phenomena in irradiated PVDF and its nanohybrid, but at higher dose (>90 Mrad) crosslinking dominates in the nanohybrid due to more free radicals and proximity of radical chains on top of templated system in the nanohybrid as compared to pure PVDF. The enhanced crosslinking alters the nanostructure causing disappearance of the peak at 2θ ≈ 3°. Moreover, the electron beam induces significant piezoelectric β‐phase in the nanohybrid against only α‐phase in pure PVDF at a similar dose and raises the possibility for the use of electron‐irradiated nanohybrid as an electromechanical device. β‐Phase formation is also supported through solid‐state NMR, scanning electron microscopy and differential scanning calorimetry studies. The thermal properties in terms of heat of fusion and degradation temperature have been verified indicating steady decrease of melting point and heat of fusion for pure PVDF while considerably less effect is observed for the nanohybrid. The combined effect of chain scission and crosslinking makes both PVDF and its nanohybrid brittle, but with greater stiffness with respect to unirradiated specimens. © 2014 Society of Chemical Industry     

Electron beam irradiation induced grafting of acrylonitrile to polyethylene powder

This study focuses on the influence of various parameters on the post-irradiation grafting of acrylonitrile monomer to a low density polyethylene powder having an average particle size of 20 μm diameter. The type of radiation used was 0.8 MeV electrons with a dose rate of either 0.1 Mrad/min or 4.0 Mrad/min. Irradiations were carried out in vacuum, air, and dry oxygen environments at room temperature to total doses ranging from 1 Mard to 15 Mrad. Several experimental techniques were used to investigate various interrelated aspects of both the graft copolymer product and the graft process, including infrared spectroscopy to measure percent graft, electron spin resonance to measure radical concentrations, scanning electron microscopy to investigate surface morphology, and differential scanning calorimetry to measure crystalline content. Comparisons of the percent graft determined from different probes were found to be consistent. The dose dependence of grafting exhibited a linear relationship below 8 Mrad. Initiation by both alkyl and peroxy radicals at 77°C was supported by results of decreased grafting with decreased temperature, increased dose rate or vacuum irradiation. Decreased grafting after oxygen irradiation suggested a unique change in grafting mechanism. Higher grafting yields were found for quenched relative to annealed samples. The amount of extractable homopolymer formed was found to be ～ 10% at a dose of 10 Mrad and a dose rate of 4 Mrad/min.     

Mechanism and kinetics of polyethylene crosslinking by α,α′-bis(tert-butylperoxy)-p-diisopropylbenzene

The study of the mechanism of polyethylene crosslinking is realized by a kinetic analysis of the α,α-bis(tert-butylperoxy)-p-diisopropylbenzene decomposition, as well as by the determination of its decomposition products and crosslink formation in the polymer. The experiments were carried out in a temperature range of 118°–148°C in both polyethylene and its low-molecular model, n-octane. From the results obtained it follows that the peroxide decomposition in both hydrocarbon media is kinetically a unimolecular reaction with an activation energy of 36 ± 2 kcal/mole and with an equivalent participation of both peroxidic groups, whereby a biradical formation is improbable. Macroradicals arise by a dehydrogenization reaction in which mainly primary oxyradicals of various types take part and methyl radicals are also formed by a transformation process of the former. Both types of radicals decay exclusively in a substitution reaction with polymer chains. The whole process is terminated by macroradical recombination so leading to crosslink formation in polyethylene.     

Effect of double bonds on the γ-radiation-induced crosslinking of polyethylene

Various low-density polyethylenes ranging in initial weight-average molecular weight (M?_w) from 7600 to 589000 having a ratio of M?_w to number-average molecular weight (M?_n) of about 5 were irradiated by γ-rays in vacuo at 30°C. Gel fractions were determined and analyzed by using the equation derived by Charlesby and Pinner. The following relationships were obtained when M?_w was used as the molecular weight: where r_g represents the gel point dosage (Mrad), [C?C]₀ is the sum of the initial contents of terminal vinyl and vinylidene unsaturations (mole/g polyethylene), and q₀ and p₀ are the probabilities of crosslinking and main-chain scission per monomer unit for a unit radiation dose in Mrad, respectively. Similar relationships to the equations described above were also obtained when M?_n was used. From the results, it was concluded that terminal vinyl and vinylidene unsaturations play an important role for the gel formation in the γ-radiation-induced crosslinking of polyethylene in vacuo at room temperature.     

Dielectric relaxations of high and low density irradiated polyethylenes

A systematic study of dielectric spectrum on a series of high and low density commercial polyethylenes has been performed. Every polyethylene was characterized by determination of molecular weight distribution, the number of functional groups: ketone, aldehyde, vinyl, vinylidene, trans-vinylene, the fraction crystalline, and the degree of branching. We have observed that both γ and α dielectric relaxations zones are formed by two overlap relaxations which have been labeled γ_II, γ_I, and α′, and α respectively, in order of increasing temperatures. The intensity of the dielectric spectrum of all polyethylene depends over and above other parameters on the number of the carbonyl groups formed as a result of irradiation. This number is different for every polyethylene even when they receive the same dose of irradiation (20 Mrad). However, the participation of carbonyl groups in the γ_I and α′ dielectric relaxations decreases with the total crystalline content of each polyethylene. The α dielectric relaxation position in the temperature axis is governed by the most probable crystallite thickness.     

Effects of irradiation on PTC performance of LDPE/EPDM blends filled with carbon blacks

Low-density polyethylene/ethylene–diene terpolymer/carbon black blends were irradiated in air with a ⁶⁰Co γ ray or an electron beam accelerator. The absorbed dose ranged from 1–400 Mrad. The influences of irradiation on positive temperature coefficient behavior of the blends were analyzed. © 1996 John Wiley & Sons, Inc.