Influence of the irradiation conditions on the effect of radiation on polyethylene

Two types of polyethylene, low density (LDPE) and high density (HDPE), as well as low density polyethylene containing an antioxidant were subjected to gamma-irradiation in the presence of air and in water. The irradiated polymers were studied using IR spectrophotometric analysis. The radiation induced oxidative degradation was followed through the formation of oxygen containing groups by the development of bands in the 1850-1650 cm(-1) region and double bonds formation by the development of bands in the 1050-850 cm(-1) region. The crosslinking efficiency was determined by measuring the gel content by extraction with xylene. The radiation induced changes in the molecular structure, evolution of oxygen containing species and formation, of vinyl double bonds as well as of the crosslinking efficiency are discussed in terms of the properties of the polymers in an electric field of low strength

The high temperature secondary crystallisation of aged isotactic polypropylene

The secondary crystallization of aged isotactic polypropylene (iPP) of different morphologies, which occurs during annealing in the vicinity of the melting temperature (140 degreesC), was studied using differential scanning calorimetry (DSC) and density measurements. The results of calorimetric and density measurements were correlated. Kinetics of the high temperature secondary crystallization (HTSC) was studied by using Avrami relations. Avrami exponents, obtained according to the increase in crystallinity after annealing at 140 degreesC, were considered as the measure of the ability of iPP to crystallize secondarily. Depending on the initial morphology of the samples, accelerated ageing influences in different ways the ability of iPP to subsequently crystallize upon annealing. The aged samples with larger amorphous content show a reduced secondary crystallization at high temperature compared with the unaged samples. On the other hand, aging of medium crystalline samples leads to more pronounced HTSC. (C) 2004 Elsevier Ltd. All rights reserved

Viscoelastic behavior of semicrystalline polymers at elevated temperatures on the basis of a two-process model for stress relaxation

Viscoelastic behavior at elevated temperatures of high-density polyethylene and isotactic polypropylene was investigated by using the stress relaxation method. The results are interpreted from the view of an established two-process model for stress relaxation in semicrystalline polymers. This model is based on the assumption that the stress relaxation can be represented as a superposition of two thermally activated processes acting in parallel. Each process is associated either with the crystal or amorphous phase of a polymer sample. It was found that the temperature dependence of viscosity coefficients and elastic moduli of these two fractions are similar in the two materials. The experimental data was correlated with literature data of alpha and beta processes in polyethylene and polypropylene obtained from dynamic mechanical thermal analysis. (C) 2000 John Wiley and Sons, Inc

Charge trapping and dielectric relaxations of gamma irradiated radiolytically oxidized highly oriented LDPE

The influence of gamma irradiation in air on the effects of charge trapping in highly oriented LDPE has been studied through dielectric loss (tan delta) analysis in the temperature range from 25 to 325 K and using thermally stimulated discharge current (TSDC) measurements. The radiation induced oxidation was observed using IR spectroscopy. The intensity and the position of the gamma and beta dielectric relaxation maxima were correlated with maxima of TSDC measurements. It was found that the positions of the relaxation peaks are strongly dependent upon the changes in the microstructure of the amorphous phase and on the surface of the crystallites induced by orientation and gamma irradiation. (C) 2003 Elsevier Ltd. All rights reserved

Viscoelastic properties of polyethylene at elevated temperatures on the basis of two-process model for stress relaxation

Stress relaxation at room and elevated temperatures of high density (HDPE) and linear low-density polyethylene (LLDPE) has been investigated. Viscoelastic parameters (elasticity moduli and viscosity coefficients) of amorphous and crystal fraction at temperatures 18, 40, 60, 80 and 100 degreesC are determined by using two-process model for stress relaxation of polyethylene. The results show that amorphous viscosities of both polyethylene types clearly satisfy the Arrhenius dependence on temperature. Therefore, in the amorphous fraction of examined polyethylenes at elevated temperatures, there is a process with temperature independent activation energy. The estimated activation energies DeltaH approximate to 62.5 kJ/mol for LLDPE and DeltaH approximate to 59 kJ/mol for HDPE of that process are compared with activation energies determined from the time temperature superposition of experimental relaxation curves and with literature data.Trends in Advanced Materials and Processes, 3rd Yugoslov-Materials-Research-Society Conference, Sep 20-24, 1999, Herceg Novi, Yugoslavi

Stress relaxation in high density polyethylene. Effects of orientation and gamma radiation

The stress relaxation of unoriented (lambda = 1) and oriented (lambda = 9) high density polyethylene (HDPE) was studied for different levels of radiation (0, 100, 300, and 500 kGy). The relaxation behavior is analyzed in terms of a formerly established two-process model. In that model, one process is related to the crystal and the other to the amorphous fraction of a polyethylene sample. The modified viscoelastic properties of HDPE, due to effects of orientation and irradiation, are reflected in changes in viscosity coefficients and elasticity moduli of these two fractions. This made possible better comprehension of the nature of changes in the relaxation behavior of HDPE induced by irradiation and orientation

Formation and behaviour of low-temperature melting peak of quenched and annealed isotactic polypropylene

The process of low-melting point (LMP) alpha-phase formation in a quenched and annealed isotactic polypropylene (iPP) was studied by means of differential scanning calorimetry. The influence of preparation conditions (quenching and annealing temperatures, annealing times and sample thickness), together with the measurement parameters (heating and cooling rates), on the iPP melting behaviour are analysed. The results reveal a constant yield of LMP crystals over a wide range of quenching temperatures. This study also suggests that the LMP alpha-crystals originate from the crystallization of polymer portions, which are somewhere between the amorphous and the smectic phase on the macromolecular scale of order. (C) 2001 Society of Chemical Industry

Effects of gamma irradiation on the stress relaxation of drawn ultrahigh molecular weight polyethylene

Effects of high energy radiation on stress relaxation behavior of unoriented (lambda = 1) and oriented (lambda = 9) ultrahigh molecular weight polyethylene (UHMWPE) were investigated. As in a previous study, the stress relaxation is interpreted in terms of a model that consists of two thermally activated processes acting parallel. The first process is related to crystal and the second to the amorphous fraction. Application of the two-process model enabled separate studying of the influence of irradiation on viscoelastic behavior of these two fractions. (C) 1999 Elsevier Science Ltd. All rights reserved.9th Tihany Symposium on Radiation Chemistry, Aug 29-Sep 03, 1998, Tatabanya, Hungar