Crosslinking of Metallocenic Polyethylene And Ethylene - 1-Hexene Copolymers by Gamma Radiation

A. SATTI; N. ANDREUCETTI; R. QUIJADA; J. M. PASTOR; E. M. VALLÉS

San Sebastián, España

Congreso; Fourth International Conference on Polymer Modification, Degradation and Stabilization. MODEST 2006; 2006

Ethylene/á-olefin copolymers obtained by metallocene catalysts show well defined structures, homogeneous comonomer distribution and narrow molecular weight distribution. On the other hand, cross-linking is a broadly used method for the modification of polymers. Polyethylene (PE) when irradiated with high energy radiation predominantly undergoes cross-linking. In this work we study the effect of gamma irradiation on several metallocene ethylene – 1-hexene (PEH) copolymers with different comonomer content. The polymers were irradiated in vacuum and subsequently characterized by FTIR, DSC, RAMAN, SEC-MALLS and rheological techniques. The ethylene homopolymer and three PEH  (containing 3,3, 9,2 and 16,1% of hexene) were obtained by polymerization in toluene at 60◦C and 2 bar employing Et[Ind]2ZrCl2, and methylaluminoxane (MAO) as catalyst for the reaction. The irradiation of the different polymers was carried out in vacuum at room temperature using gamma rays from a 60Co source. Various doses ranging from 7 to 100 kGy were applied to the PEH. The amount of crystalline, amorphous and interfacial contents was determined from the analysis of the internal mode region of the Raman spectra (960<Äí< 1550 cm−1) from a confocal Dilor Labram spectrometer. It is well known that tertiary carbons and unsaturations are two important factors that affect the reactivity of the regular -CH2-CH2- sequences of the polyethylene chains.  Both groups are present in different proportions on the copolymers employed in this work, modifying the reactivity of the crosslinking and scission reactions that occur in the irradiation of polyethylene. The evolution of the concentration of these species with the applied radiation doses was studied by FTIR. A decrease of terminal vinyl groups and an increase of trans double bonds with the radiation dose was observed. The latter is present in irradiated PE, although not in virgin PE, which means that this unsaturation is radioinduced. No significant changes in the melting temperatures and degree of crystallinity were observed by DSC in the different PEH copolymers as a consequence of the irradiation process. Only irradiated PE showed a slight decrease in these properties. A detailed structural analysis was accomplished by RAMAN spectroscopy. Curve analysis in the 1250 to 1350 cm-1 region showed that there are two important component bands in the spectra of polyethylenes samples. The main one near 1298 cm-1 corresponds to the crystalline phase and interphase. The less stable peak position of the amorphous band is located between 1303 and 1307 cm-1. The CH2 bending vibration centered near 1419 cm-1 was used to measure the relative number of units present in the orthorhombic crystalline phase. Changes in the crystallinity of the irradiated PE and no significant changes in irradiated PEH were confirmed by this technique. An important conclusion of this work is that an increase of the interfacial content as a function of radiation dose was observed in most cases. It has been reported by many authors that great part of the radioinduced reactions take place on the nearby region to the surface of the crystals, by means of radical migration, terminal double bond reactions, etc. SEC analysis allowed to determine the evolution of the molecular weight distribution with the radiation doses and the critical gel dose for the different PEH. The changes of the relaxation behavior of the PEH copolymers as a function of the radiation doses was followed by sinusoidal dynamic oscillations experiments at different temperatures. A detailed discussion of the structural and morphological and changes induced by radiation on this family of ethylene – hexene metallocene copolymers will be given in this presentation.