INTEMA   05428
INSTITUTO DE INVESTIGACIONES EN CIENCIA Y TECNOLOGIA DE MATERIALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Nanostructured materials obtained by self-assembly of block-copolymers in epoxy thermosets: evolution of local structure as a function of cure schedule
Autor/es:
HERNÁN E. ROMEO; ILEANA A. ZUCCHI; CRISTINA E. HOPPE; ROBERTO J. J. WILLIAMS
Lugar:
San Carlos de Bariloche
Reunión:
Workshop; II Workshop de Usuarios Argentinos de Luz Sincrotrón; 2013
Resumen:
Epoxy-amine systems are a class of important thermosetting polymers which have been widely employed as high-performance materials [1]. Over the past decades, considerable progress has been made on understanding the relationship between the morphologies and properties of multicomponent epoxy-based thermosetting blends. Specifically, the morphological control of this kind of polymers at the nanometer level is long a pursuit in the studies of polymer science, and it is consequently crucial to understand the mechanisms by which nanostructures can be accessed in these materials. There are basically two main approaches to nanostructure block copolymers(BCP) in epoxy blends. One of these methods, termed "self-assembly"(SA), requires a block immiscible with the initial solvent (thermoset precursors) and another block that is miscible in the reacting mixture up to high conversions. This allows the BCP to self-assemble into different nanostructures (depending on BCP concentration, effective volume ratio between copolymer blocks, among others), while the polymerization reaction leads finally to locking-in the preformed copolymer nanostructures [2]. The other approach, so called "reaction induced microphase separation"(RIMPS), requires that both blocks of the BCP are completely miscible with the reacting solvent at the beginning of the polymerization process. However, in this case, one of the blocks phase separates during the course of the polymerization, while the other one remains miscible in the thermoset matrix. This leads to different nanostructures that are obtained as a consequence of the phase separation process, which allows modulating the final morphologies by modifying the external parameters (reaction parameters) [3]. In this scenario, there is one concept that has not been paid attention when a RIMPS approach is employed to obtain nanostructured polymeric blends: the quality of the thermoset as a solvent of the block that remains miscible changes with both conversion and temperature. It shifts from a good solvent to a poor solvent, and eventually to a nonsolvent, along polymerization due to the increase in the average molar mass before gelation (decrease in the entropic contribution to the free energy of mixing) and the increase in cross-link density after gelation. However, the quality of the solvent can also be modified by the selected cure cycle due to variations of the BCP/solvent interaction parameter with temperature. Therefore, for a constant BCP concentration different nanostructures could be accessed in the course of polymerization, as shown in phase diagrams of solutions of BCP in selective solvents of different quality. The possibility of trapping one of the evolving nanostructures generated during polymerization by control of the cure cycle can be of interest to modulate final properties of the material (e.g., toughness, transparency, etc.). To the best of our knowledge, there are no previous studies in the vast literature of BCP/epoxy blends showing that morphologies generated during polymerization of a specific blend can be significantly varied by selecting different cure cycles. The selected specific blend to prove this concept was a homogeneous solution containing 20 wt% poly(styrene-b-methyl methacrylate) (PS-b-PMMA) in a stoichiometric mixture of diglycidyl ether of bisphenol A (DGEBA) and 4,4´-methylenebis(2,6-diethylaniline) (MDEA). TEM and in situ SAXS techniques demonstrated that different nanostructures, from diluted spherical micelles to hexagonally packed cylinders, were in fact generated and trapped in the BCP/epoxy blends by simply varying the cure cycle. [1] Zheng, S. In Epoxy Polymers, New Materials and Innovations; Pascault, J. P., Williams, R. J. J., Eds., Wiley-VCH: Weinheim, 2010, p. 81. [2] Rebizant, V.; Venet, A. S.; Tournillhac, F.; Girard-Reydet, E.; Navarro, C.; Pascault, J. P.; Leibler, L. Macromolecules 2004, 37, 8017. [3] Meng, F.; Zheng, S.; Li, H.; Liang, Q.; Liu, T. Macromolecules 2006, 39, 5072