Research paperOrganoclay-epoxy nanocomposites modified with polyacrylates: The effect of the clay mineral dispersion method
Introduction
Nanocomposites constituted by Layered silicate and epoxy resin have been extensively studied as a strategy for developing new materials with improved stiffness, thermal stability, barrier properties, etc. (Hackman and Hollaway, 2006, Le Baron et al., 1999). The most popular layered silicate for such systems is montmorillonite (Mt), which is naturally available, thermally inert, inexpensive and presents high expandable capacity between the layers. Moreover, the cations naturally located within the interlayer space of the clay mineral can be easily exchanged by bulky organic cations, increasing its organophilic characteristic and compatibility with monomers and polymers (Bergaya et al., 2013). Epoxy resin also constitutes an efficient matrix for clay mineral-based nanocomposites because of its low viscosity before curing, which favors the diffusion of the epoxy chains into the clay mineral interlayer. As a consequence, intercalated/exfoliated structures may be achieved, contributing for the unique macroscopic properties of the corresponding nanocomposites with the addition of low amount of the organo-modified clay mineral. Although the strength and modulus of the epoxy networks are generally improved by the presence of clay mineral, the fracture toughness is reduced in some extent (Chen and Curliss, 2001). The incorporation of reactive liquid rubber has been addressed as an efficient approach for improving toughness of epoxy network. Some of them include carboxyl-terminated butadiene – acrylonitrile (CTBN) (Thomas et al., 2004, Williams et al., 1997), amine-terminated butadiene-acrylonitrile (ATBN) (Chikhi et al., 2002), isocyanate-terminated polybutadiene (ITPB) (Barcia et al., 2003, Soares et al., 2011) and carboxyl-functionalized polyacrylates (Zaioncz et al., 2007).
Recently, some researchers have been addressed their studies on the incorporation of a third phase, thermoplastic or rubber, in the clay mineral - epoxy nanocomposites with the aim of combining the benefits of both components in the epoxy network. Balakrishnan and Raghavan (2004) and Balakrishnan et al. (2005) employed pre-formed acrylic rubber particles in mineral clay/epoxy systems and observed an improvement of ductility of the corresponding ternary systems. Similar behavior has been also observed by using different polymers as the third phase, including CTBN (Lee et al., 2009, Lee et al., 2010, Liu et al., 2004), hydroxyl terminated poly(ether ether ketone) oligomer (Asif et al., 2007), core-shell rubber particles (Gam et al., 2003), acrylic rubber (Wang et al., 2014), acrylic triblock copolymers (Bashar et al., 2012) and acrylonitrile – butadiene – styrene copolymers (ABS) (Mirmohseni and Zavareh, 2010a, Mirmohseni and Zavareh, 2010b), among others. Moreover, the effect of clay mineral on the reaction induced phase separation and morphology of the epoxy resin modified with polyetherimide (Peng et al., 2007), poly (methyl methacrylate) (PMMA) (Hernandez et al., 2007, Hernandez et al., 2010) CTBN (Vijayan et al., 2012) and PMMA – grafted natural rubber (Yuhana et al., 2012) was also investigated.
Recently, the toughening efficiency of low molar mass random copolymers based on methyl methacrylate (MMA) and 2-ethyl-hexyl acrylate (PMMA-co-PEHA) in epoxy networks cured with triethylenetetramine (TETA) was investigated (Zaioncz et al., 2007). The addition of 10 phr of these copolymers with different compositions resulted in an improvement of the impact resistance without decreasing the strength of the corresponding epoxy networks. Also recently the in situ interlamellar polymerization of MMA inside the clay mineral was performed in the presence of acrylic acid and thioglycolic acid (Silva et al., 2010). The presence of low amount of acrylic acid provided better dispersion of the montmorillonite in the polymer matrix. In this context, the aim of the present work is to investigate the nanostructure of the ternary epoxy – based nanocomposite systems containing organic modified montmorillonite and acrylate polymers which were obtained using two different methodologies. The first one was based on the conventional dispersion of the clay mineral into the epoxy/polyacrylate copolymer binary blends. The second approach consisted of the previous preparation of polyacrylate nanocomposites by the in situ lamellar polymerization of MMA or MMA/2-ethyl-hexyl acrylate (EHA) mixture inside the swollen organoclay. The acrylic acid was also employed in low amount to improve the dispersion of the clay mineral and to provide active sites for attaining better compatibilization with the epoxy matrix. PMMA was chosen because of its well-known miscibility with epoxy resin before the curing process (Woo and Wu, 1996). Also PMMA-co-PEHA copolymer was employed aiming to improve the toughening of the ternary nanocomposite systems as previously observed for epoxy-acrylate binary blend (Zaioncz et al., 2007).
Section snippets
Materials
The organoclay (OMt) used in this work is Cloisite 30B, an organophilic clay mineral modified with methyl, tallow, bis-2-hydroxyethyl quaternary ammonium salt, supplied by Southern Clay Products Inc. Diglycidyl ether of bisphenol A (DGEBA) – based epoxy resin (ER), trade name = Epon 828, was purchased by Shell Company with epoxide equivalent = 180–190 g/eq. The hardener used for the curing process was triethylenetetramine (TETA) supplied by Shell Chemical Co (EPICURE 3140). The epoxy resin and the
Morphological investigation
XRD patterns corresponding to the OMt/epoxy-acrylate nanocomposites prepared by different procedures were compared with that of pure Cloisite 30B in Fig. 2. The EPMMA/Clo30B sample, prepared by the method 1, where the clay mineral had been dispersed in the polyacrylate-modified epoxy resin, presented a very small and broad reflection at around 2θ ≈ 2°, indicating the presence of intercalated and disordered structure and another almost imperceptible deflection in similar region as that observed
Conclusions
The objective of this work was to investigate the nanostructure of OMt/epoxy-acrylate ternary nanocomposites obtained by two different methods: a dispersion of OMt in the epoxy matrix previously modified with acrylate polymers or a previous dispersion of clay mineral in the acrylate polymers through the in situ intercalative polymerization of the acrylate monomers followed by the dispersion of the clay-acrylate composite in the epoxy resin. The last procedure provides higher degree of
Acknowledgments
We thank the Brazilian funding agency Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) for the financial support. We also acknowledge the Brazilian Synchrotron Light Laboratory (LNLS) for opportunity of SAXS measurements.
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