Two kinds of polyethylene samples, low-density polyethylene and high-density polyethylene, have been studied using positron lifetime measurements in the temperature range from 96 to 370 K. Glass transitions and other secondary transitions are observed and the glass transition temperatures are determined. It has been found that the orthopositronium lifetime and its intensity are sensitive to the transitions and the number of free-volume holes reaches a minimum near the glass transition temperature Tg. The molecular motions, the structural transitions and the crystallinity effects are discussed in terms of the properties of the free volume.

Differential thermal analysis (DTA) and infrared (IR) spectroscopy have been used to investigate the effects of an antioxidant (neozone D) and an oxidation catalyst (dispersed copper) on polyethylene melt oxidation under nonisothermal conditions (samples were heated at a constant rate). An increase in the content of the antioxidant or the oxidation catalyst gives similar results: a decrease in the thickness of the oxidized surface layer and accordingly in the total anaotmt of oxidation in polyethylene samples. This is due to an increase in the rate of oxidation of the polymer.Most papers concerning investigations into the oxidation of polyethylene melts deal with samples oxidized under isothermal conditions [1][2][3][4][5][6][7][8][9][10]. Of greater interest, however, is the investigation of polyethylene melt oxidation under nonisothermal conditions, for in the course of polyethylene item production (injection molding, powder coating, extrusion plating, etc.) oxidation takes place at varying temperatures. The derivatograph allows observation of oxidation in polymers under conditions similar to those applied in practice to the material. DTA curves for polyethylene free of antioxidant show a distinct peak in the range of 425 to 525 K, due to oxidation reactions in the polymer [11 ]. Thermogravimetric (TG) curves exhibit a peak in the same temperature range as a consequence of two competing processes: a mass increase due to oxygen absorption by the polyethylene, and a mass decrease in the oxidized polyethylene due to evaporation of the products of oxidative destruction of macromolecules. DTA and TG do not reveal peaks in the curves for preliminarily oxidized polyethylene in the above-mentioned temperature range [12,13]. When a layer of 50 to 150/tin is removed from polyethylene oxidized under isothermal and nonisothermal conditions, DTA and TG curves again show peaks [13]. These and some other data [14,15] indicate that polyethylene resists oxidation into the depth of the sample. The distribution of the degree of oxidation (optical density of carbonyl groups in the IR spectrum of the polyethylene) through the sample thickness is described by an exponential function [13]. Polypropylene follows a similar law [16]. For isothermal oxidation of a polyethylene melt at 423 K, IR spectroscopy shows the thickness of the oxidized surface layer to be about 1.6 mm [13]. Increasing the oxidation temperature decreases the thickness of the oxidized surface layer [13,15]. Filling polyethylene with substances which cause variations in the rate of the oxidation reactions may

SYNOPSISThe paper considers the problem of solid-phase oxidation for cases of polyethylene coatings applied onto active (copper) and passive (aluminum) substrates. Oxidation of PE coatings on copper, unlike that of PE coatings on aluminum, is shown to develop in two stages separated by a time period of autoinhibition. During this time period the concentration of carboxyl groups does not in fact change. In a similar manner copper-containing compounds accumulate in the coating, which fact supports the relation between metal transfer and polymer oxidation processes.

Oxidation of polyethylene (PE) melts in contact with metals (Cu, Pb, Au, Al, Zn, Ag) has been studied by infrared spectroscopy and differential thermal analysis (DTA). These metals may be divided into two groups, depending on their activity for oxidizing PE: namely, high‐activity metals (Cu, Pb, Ag, Zn) and low‐activity metals (Al, Au). During the oxidation of PE in contact with high‐activity metals dissolution of the surface layer of metal is observed with accumulation of metal‐containing compounds (salts of carboxylic acids) in the bulk of the polymer. With low‐activity metals these phenomena are not observed. The rate of oxidation of PE on low‐activity metals approaches the oxidation rate of nonmetals (polytetrafluoroethylene and inorganic glass). With certain high‐activity metals (Cu, Pb) the process of oxidation is accelerated only in the early stage of oxidation; then the oxidation rate slows down and the oxidation process ceases. PE films separated from metal after being oxidized on it possess chemical memory, i.e., their oxidation rate depends on the nature of the metal with which they had been in contact, and on the duration of the contact oxidation. The effect of salts of carboxylic acids (metal stearates) on the oxidation of PE melts was also studied. Based on the data obtained, it is concluded that the rate of oxidation of PE melts on high‐activity metals is controlled by metal‐containing compounds which are the products of contact reactions.

Studies were done to learn the role of some factors (nature of substrate, thickness of polymeric layer, presence of ller in the polymer) in the oxidation (the consumption rate of the antioxidant, the induction period) and adhesion strength (resistance to separation of coating from the substrate) of stabilized PE coatings.These factors lead to local zones in the coatings (in the polymer layer in contact with catalytically active substrates, near the ller particles, and in the outermost surface layer of the coatings oxidized under the conditions where oxygen was supplied by the diffusion mechanism) in which the antioxidant was consumed at an increased rate. The antioxidant consumed in these zones is replenished diffusively from the remaining part of the coating. A correspondence was observed between the induction period for oxidation of the polymer layer in contact with the substrate and the induction period for attaining adhesion (bonding) of the coating.