Epoxy polymers and composites for advanced applications

R.J.J. WILLIAMS

Granada

Congreso; European Polymer Congress 2011; 2011

European Polymer Federation

Introduction: Due to their excellent properties epoxy polymers are extensively used in traditional applications such as adhesives, coatings and composites. In recent years there has been an increasing use of epoxies in the development of advanced functional materials.1 Some examples will be given in this presentation. Reversible Epoxy Networks: The reaction of diglycidylether of bisphenol A (DGEBA) with an n-alkylamine leads to a linear epoxy polymer. However, linear chains can be assembled by tail-to-tail associations between n-alkyl groups leading to a network with physical crosslinks. The strength of the physical bonds decreases with temperature and with the length of the n-alkyl group.2 By increasing temperature the material reversibly transforms from a gel to a liquid, a fact that may used to develop self-healing epoxies. Shape Memory Epoxies: When epoxies are heated above their glass transition temperature (Tg) they can be deformed to a temporary shape by applying a relatively small stress. By fixing the deformation and cooling below Tg, a glass is obtained that stores elastic energy in chain conformations removed from their equilibrium values. When the material is released from any constraint and is heated again above Tg, a rapid recovery of the initial shape is obtained as chains recuperate their equilibrium conformations. But if the heating step is performed keeping the initial deformation, the material develops a recovery stress. Actuators based on epoxies can make use of the shape recovery or the stress recovery. They can be designed for large tensile elongations (e.g., 75 % or higher) or large recovery stresses (e.g. 3 MPa or higher). However, meeting both requirements simultaneously is a difficult task because changes in the crosslink density affect both variables in opposite ways. An epoxy formulation based on the reaction of DGEBA with n-dodecylamine (DA) and m-xylylenediamine (MXDA) gives a network with both chemical and physical crosslinks that can be used for a shape memory material verifying both requirements. Epoxy Networks Containing Silver Nanoparticles (NPs): Ag NPs can be introduced into epoxy coatings to obtain specific electrical or antimicrobial properties. Different methods can be employed to obtain a uniform dispersion of NPs. A DGEBA/DA gel was swollen with a 6mM solution of AgNO3 in THF/H2O (90:10). After rinsing with THF and drying in vacuum, the epoxy network was heated at 100 ºC for 1 h, generating a uniform dispersion of Ag NPs with an average size close to 10 nm.3 The reduction was performed by the secondary alcohols present in the epoxy backbone. The Tg increased from 20 ºC (neat epoxy) to 28 ºC (nanocomposite). A different strategy to obtain a uniform dispersion of Ag NPs in an epoxy matrix was to introduce reactive groups in the organic chains stabilizing the NPs. Silver NPs with an average size of 4nm and stabilized with an organic group containing secondary hydroxyls, were synthesized and dissolved in DGEBA. The epoxy was polymerized using a tertiary amine as initiator. Ag NPs were covalently bonded to the epoxy network through chain transfer reactions to the secondary hydroxyls of the organic ligands. The nanocomposites were strongly colored and showed a dependence of Tg on the concentration of NPs.4 Epoxy Networks Containing Single-Wall Carbon Nanotubes (SWCNT): Epoxy nanocomposites containing SWCNT exhibit significant improvements in mechanical, thermal or electrical properties. Surface functionalization of SWCNT enables to obtain an adequate dispersion of the nanotubes in the epoxy formulation. A problem that is present in some formulations is the different partition of the epoxy monomer and the hardener in the interphase, a process favored by the large specific area per unit volume. This leads to a heterogeneous network characterized by two relaxation peaks and a significant decrease of the glass transition temperature. This problem was avoided by a convenient functionalization of the SWCNT, producing a localized formation of bundles of nanotubes in the course of polymerization. The Tg of the nanocomposite was the same as the one of the neat epoxy.5 Epoxy Networks Containing POSS Crystalline Platelets: The dispersion of intercalated/exfoliated clays in polymers decreases their permeability due to geometrical effects. However, processing is difficult due to the initial high viscosity. An alternative is to produce the crystallization of an initially soluble precursor during polymerization. The feasibility of this idea was proved using a polyhedral oligomeric silsesquioxane (POSS) dissolved in the epoxy precursors.6      References 1. Pascault, J.P.; Williams, R.J.J.; Eds., Epoxy Polymers: New Materials and Innovations, Wiley-VCH, Weinheim, 2010. 2. Puig, J.; Zucchi, I.A.; Hoppe, C.E.; Pérez, C.J.; Galante, M.J.; Williams, R.J.J.; Rodríguez-Abreu, C. Macromolecules 2009, 42, 9344-9350. 3. Ledo-Suárez, A.; Puig, J.; Zucchi, I.A.; Hoppe, C.E.; Gómez, M.L.; Zysler, R.; Ramos, C.; Marchi M.C.; Bilmes S. A.; Lazzari, M.; López-Quintela, M.A.; Williams, R.J.J. J. Mater. Chem. 2010,20, 10135-10145. 4. dell’Erba, I.E.; Hoppe, C.E.; Williams, R.J.J. Langmuir 2010, 26, 2042-2049. 5. Auad, M.L.; Mosiewicki, M.A.; Uzunpinar, C.; Williams, R.J.J. Polym. Eng. Sci. 2010, 50, 183-190. 6. Di Luca, C.; Soulé, E.R.; Zucchi, I.A.; Hoppe, C.E.; Fasce, L.A.; Williams, R.J.J. Macromolecules 2010, 43, 9014-9021.