Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. II. Structural changes occuring in low-density polyethylene at an oxygen content less than 0.0005%

Abstract

Samples of low-density polyethylene, free from additives, were heated at temperatures between 284° and 355°C under high-purity nitrogen. Changes in molecular weight distribution (MWD), molecular weight averages, and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). At 284° and 315°C, the MWD's were shifted toward higher molecular weights and the Mw values increased. At 333° and 355°C, the MWD's shift toward lower molecular weight, but the high molecular weight, tail is largely retained. Mw decreases slowly at 333°C. At 355°C, Mw undergoes a rapid initial drop which levels off. Mw/Mn and the degree of LCB increase with heating time and temperature. Olefinic unsaturation increases. The vinyl groups show a larger relative increase than do the trans-vinylene and vinylidene groups. At 355°C, the peak of the unimodal DSC thermogram is shifted to ∼3°C higher temperature. A lower melting peak then develops, and after 72 and 90 min the two peaks are about equal in size. The density increases from 0.922 g/cm3 to 0.930 g/cm3 for samples heated at 355°C, and the weight loss was 1.5% after 90 min. A reaction scheme for the thermal degradation of polyethylene is discussed. Initiation is suggested to be accomplished by scission of allylic CC bonds. Propagation proceeds by both intra- and intermolecular hydrogen abstraction, followed by β-scission. Termination can occur by both combination and disproportionation. Combination reactions are suggested to account for the observed formation of LCB and high molecular weight material. Due to changes in the degree of LCB during the degradation, viscometry alone will not give a proper measure of the changes in molecular weight.