ELSEVIER Effect of gamma zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Polymer Degradation and Stability 61 (I 998) Wll :K; 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain Pll: .SOl41-3910(97)00131-6 0141-3910/98/$19.00 irradiation on the stress-relaxation of drawn LLDPE V. Djokovib,* Z. KaEarevi&Popovi& D. Dudib & D. Kostoski zyxwvutsrqponmlkjihgfedcbaZ The Institute of Nuclear Sciences ‘VinEh’, PO Box 522, 11001 Belgrade, Yugoslavia (Received 20 May 1997; accepted 12 July 1997) The stress-relaxation behaviour of undrawn and drawn linear low density poly- ethylene (LLDPE) samples has been studied as a function of radiation dose over the range e-500 kGy. It is shown that stress improves progressively with increas- ing radiation dose for all the samples, but this effect is greatest for undrawn samples. The crystallinity has been studied by wide-angle X-ray diffraction (WAXD). Crystallinity has been found to increase with increasing draw ratio and radiation dose. The stress-relaxation behaviour is discussed in terms of gamma radiation induced effects and increasing crystallinity. 0 1998 Elsevier Science Limited. All rights reserved 1 INTRODUCTION Orientation induced by cold drawing has marked effects on the improvement of some properties of polyethylene by increasing its modulus, toughness, chemical and light resistance.’ However, oriented fibres or tapes of oriented polyethylene obtained via solid-state drawing at elevated temperatures close to, but below, the melting temperatures have structures of reduced stability with respect to static loading for prolonged times. Previous attempts to improve the creep behaviour of oriented linear polyethylenes have suggested increasing the poly- mer molecular weight or introducing a small degree of branching. It was found that high levels of crystallinity or crosslinking also improved the time-dependent behaviour, indicating that stress relaxation and creep occur primarily by molecular relaxation in the amorphous phase. Radiation crosslinking of polyethylenes, after they have been formed to their final shape, is one of the most convenient methods for improving the long-term properties. Woods et al.* have shown a drastic reduction of creep by crosslinking oriented PE fibres using electron beam radiation with acetylene gas as a sensitizing agent. On the other hand, De Boer and Pennings3 have reported decreases in tenacity of gamma irradiated gel spun UHMW-PE *To whom correspondence should be addressed. fibres, which they have attributed to chain scission. It is well known that the efficiency of network for- mation in irradiated polyethylene will depend strongly on the polymer morphology. Crosslinking has been shown to predominate in isotropic poly- ethylene4 but chain scission becomes important in highly drawn fibres5 and films.6 In this work, attention has been focused on the study of the influence of gamma irradiation on the crystallinity and stress relaxation of drawn LLDPE. It could be of industrial importance to know at what stage of the drawing process the radiation crosslinking is most efficient and what radiation doses are the most convenient for application. 2 EXPERIMENTAL The polymer used for these studies was linear low density polyethylene (LLDPE) Union Carbide: A4, = 110 000 and d= 0.92 gcme3. Isotropic sheets 2 mm thick were obtained by compression molding at 150°C and 1.75 MPa pressure for 5 min, fol- lowed by quenching into water at 20°C. The rec- tangular samples were kept in air for 60min at 100°C and then oriented at the same temperature on a Zwick tensile testing machine, at a crosshead speed of 5mm/min. The drawing times for various samples were varied to obtain draw ratios of 6, 9 73