Crosslinking in metallocene ethylene-co-5,7-dimethylocta-1,6-diene copolymers initiated by electron-beam irradiation

Abstract

The synthesis of ethylene-co-5,7-dimethylocta-1,6-diene copolymers (CEDMO) with different molar DMO contents has been performed to incorporate double bonds in the lateral and make easier the development of crosslinkings in the resulting polymer material after application of electron-beam irradiation. The gel content as well as a comprehensive evaluation of the changes in the crystalline characteristics have been carried out. Both features are strongly dependent on DMO composition and irradiation doses. Variation in the mechanical response of the different specimens has been checked by microhardness measurements.

Introduction

Crosslinking is a broadly used method for the modification of polymer properties. This process involves the formation of tridimensional structures causing substantial changes. Crosslinked polyolefins, especially PE, are of significant practical interest as well as crosslinked rubbers and thermosetting resins. All commercially utilised processes for the initiation of polyolefin crosslinking are based on the formation of polyalkene macroradicals at some stage of the process. Common ways of initiating crosslinking involve macroradical formation via thermal decomposition of organic peroxides [1], high energy irradiation (gamma or electron beam) [2], [3], [4] and grafting of silane groups, which form crosslinks via hydrolysis of silanole moieties [5]. Other procedures for initiation of polyolefin crosslinking are less frequently used or have only been investigated in the laboratory. These include high-frequency heating, initiation by thermal decomposition of azo-esters or ethers, UV irradiation, redox initiation and free radical initiated grafting of various moieties onto polyalkene chains which can react under various conditions leading to the formation of crosslinks often physical or physico-chemical in nature [6]. Several interesting modifications of peroxide initiated crosslinking were suggested. A detailed description of the various initiation procedures together with mechanisms for these processes and specific features of each procedure was given in a comprehensive review by Lazar et al. [7]

Polyethylene, PE, is reported to be widely used in medical devices and pharmaceutical packaging [8]. Each type (high density, HDPE, low density, LDPE and linear low density, LLDPE) offers different benefits and functionalities: HDPE provides stiffness, chemical resistance and barrier properties while LDPE offers resistance to stress cracking and excellent impact properties. In addition, its capability of being sterilized is a favourable and an important aspect. The objective of sterilization is to prevent the introduction of pathogenic organisms into the body. Sterilization can be achieved through a variety of methodologies, the electron-beam irradiation being commonly utilized. As aforementioned, upon irradiation PE predominantly undergoes crosslinking in the amorphous regions or at the boundaries of crystallites [6]. The crosslinking takes place in the solid state and changes its morphological characteristics and, accordingly, its mechanical properties at ambient temperature. This variation on the mechanical behaviour depends very much on the type of PE, molecular weight, processing and crosslinking conditions, and other factors, irradiation dose being one of the most important parameters. Moreover, functional monomers, such as multifunctional acrylate, methacrylate and allylic reactive molecules, can be additionally blended with the base PE to promote crosslinking at a reduced radiation level [1]. A decrease of the degree of crystallinity can be observed when crystallisation proceeds from a crosslinked melt.

The current investigation is focused on another strategy, not explored up to now to the best of our knowledge, and aims to facilitate the development of crosslinking in PE based samples. This approach consists of the preparation of unsaturated polyolefins through the copolymerisation of ethylene with non-conjugated dienes. This synthetic route takes advantage of the use of metallocene catalysts [9], [10], [11], [12], [13] leading to the synthesis of copolymers with narrow molecular weight distributions and with the side branches randomly distributed along the polymer backbone. 5,7-Dimethyl-1,6-octadiene was chosen as comonomer since it has a vinylic double bond that can be polymerised with a metallocene catalyst, while the presence of substituents on the second double bond inhibits its polymerisation leading this way to pendant double bonds on the ethylene-co-5,7-dimethylocta-1,6-diene copolymers.

Therefore, the goals of this research are, on one hand, the synthesis of ethylene-co-5,7-dimethyl-1,6-octadiene copolymers (CEDMO) with pendant unsaturations in low molar composition to avoid a significant loss of crystallinity [14] and, on the other hand, the further evaluation of the effect of different irradiation doses on their structure and the mechanical behaviour, comparing the results with those found in an also metallocene HDPE. The structural changes have been checked by wide and small angle X-ray scattering, WAXS and SAXS respectively, Fourier transform infrared spectroscopy, FTIR, and differential scanning calorimetry, DSC, whereas microhardness measurements have been performed as starting point in evaluating the mechanical response.