Crosslinking in metallocene ethylene-co-5,7-dimethylocta-1,6-diene copolymers initiated by electron-beam irradiation
Graphical abstract
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.
Section snippets
Polymerisations
Methylaluminoxane, MAO (Witco), was purchased as a 10% (m/v) toluene solution. Me2Si(Cp)(Flu)ZrCl2 (Boulder), methanol and HCl (Merck) were used as received. Toluene (Petrogal), 5,7-dimethylocta-1,6-diene (DRT), nitrogen and ethylene (Air Liquide) were purified according to standard procedures.
The ethylene-co-5,7-dimethyl-1,6-octadiene copolymers were synthesised using a metallocene catalyst Me2Si(Cp)(Flu)ZrCl2 in association with MAO at several comonomer compositions. Polymerisations were
Results and discussion
Fig. 1 shows the gel content of the specimens, CEDMO0.0 homopolymer and the respective copolymers, versus irradiation dose. The extent of gel formation in the exposed samples is strongly dependent on DMO composition and irradiation dose. Higher gel content corresponds to a higher portion of the network structure in the amorphous region of the polymer, which is insoluble in the solvent. The sol content is related to the non-crosslinked portion of the polymer in both amorphous and crystalline
Conclusions
The existence of double bonds in the lateral chains incorporated by copolymerisation of ethylene with DMO makes easier the development of crosslinkings after application of irradiation doses. Therefore, at a given dose, the gel content is in the CEDMO copolymer higher than that found in the CEDMO0.0 homopolymer. The different polymeric materials synthesised are semicrystalline and the crystalline characteristics are strongly dependent on composition, mainly from CEDMO0.0 to CEDMO0.7. The
Acknowledgements
The authors are grateful for the financial support of Ministerio de Educación y Ciencia (projects MAT2005-00228 and MAT2007 65519-C02-01), Exchange Collaboration Program CSIC/GRICES (projects 2005PT0033 and Proc. 4-1-1 Espanha 2006/2007, respectively), and Fundação para a Ciência e a Tecnologia, Programa Operacional Ciência Tecnologia e Inovação (POCTI) do Quadro Comunitário de Apoio III e comparticipado pelo Fundo Comunitário Europeu FEDER (Project POCTI/CTM/41408/01). The synchrotron work was
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