Nanostructured Thermosetting Systems by Modification with Epoxidized Styrene-Butadiene Star Block Copolymers. Effect of Epoxidation Degree Elena Serrano, ² Agnieszka Tercjak, ² Galder Kortaberria, ² Jose A. Pomposo, ‡ David Mecerreyes, ‡ Nikolaos E. Zafeiropoulos, § Manfred Stamm, § and In ˜ aki Mondragon* ,² “Materials + Technologies” Group, Escuela UniV. Polite ´ cnica/Unibertsitate Eskola Politeknikoa (EUP-SS/UEP-D), Dpto. Ingenierı ´a Quı ´mica y M. Ambiente, UniVersidad Paı ´s Vasco/Euskal Herriko Unibertsitatea, Pza. Europa 1, 20018 Donostia-San Sebastia ´ n, Spain, Centre for Electrochemical Technologies (CIDETEC), Parque Tecnolo ´ gico de Miramo ´ n, P° Miramo ´ n 196, 20009 Donostia-San Sebastia ´ n, Spain, Leibniz Institute of Polymer Research Dresden, Department of Nanomaterials, Hohe Strasse 6, 01069 Dresden, Germany ReceiVed July 15, 2005; ReVised Manuscript ReceiVed NoVember 16, 2005 ABSTRACT: Novel epoxy-based blends containing 30 wt % star styrene-b-butadiene block copolymers epoxidized at several degrees (SepB) have been investigated in order to analyze the effect of epoxidation degree on the ability of these copolymers to produce nanostructures inside the epoxy matrix as well as their effect on the network structure of the matrix. For neat styrene-butadiene (SB) and SepB15-modified systems, macroscopic phase separation was observed. The SepB epoxidized at 40-76 mol %, however, yielded hexagonally ordered nanostructures formed by PS cylinders arranged in the matrix containing also the epoxidized and nonepoxidized butadiene units. DSC analysis indicates that the slight differences observed in self-assembling of the mixture containing the 40 wt % epoxidized block copolymer with respect to those for the blends with higher epoxidation degrees could be related with reactivity differences of the epoxidized copolymers with the curing agent. It is envisaged that these novel nanostructured blends may lead to novel materials with excellent optical properties and enhanced fracture toughness. Introduction Block copolymers (BC) are the focus of a great deal of research activity because of their intrinsic ability to self-assemble into different nanoscale structures. This intriguing ability can be used to design new polymeric nanostructures with potentially interesting properties. As has been reported by several authors, self-assembling of BC can be maintained in their blends with several homopolymers. 1-5 In this way, block copolymers are widely used as templates for generating nanostructured epoxy or phenolic matrixes with long-range order in both uncured and cured states. 6-18 One feasible pathway for generating self- assembled thermosetting nanostructures is the use of amphiphilic block copolymers, with one of the blocks miscible with the epoxy resin. Hillmyer et al. 7,8 reported the first nanostructured system obtained by modification of a network-forming formulation composed of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin and an aromatic amine with amphiphilic diblock copoly- mers such as poly(ethylene oxide)-block-poly(ethylethylene) (PEO-PEE) and poly(ethylene oxide)-block-poly(ethylene-alt- propylene) (PEO-PEP). Before curing, epoxy resin swells PEO chains, leading to a morphological behavior similar to those corresponding to nanoordered homopolymer/block copolymer blends. After curing, the nanostructures were retained and cure- induced phase transitions occurred because of the local expulsion of PEO chains from the epoxy matrix. Afterward, several works have been reported describing nanostructured thermosetting materials by modification of epoxy resins with PEO-based block copolymers. 9-11 Ritzenthaler et al., 12,13 on the other hand, blended polystyrene- block-polybutadiene-block-poly(methyl methacrylate) (SBM) copolymer with DGEBA by using 4,4′-diaminodiphenyl sulfone (DDS) and 4,4’-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA) as hardeners. By using MCDEA, different nanoor- dered morphologies were obtained before and after curing, showing an enhancement on fracture toughness of the epoxy matrix. They found that the requirement to obtain nanostructured thermosets with SBM triblocks copolymers is the solubility of the corresponding PMMA block with the growing thermoset during the whole reaction. A second generation of block copolymers has been developed for using the concept of chemical compatibilization. This approach incorporates reactive groups into one block in order to promote covalent bonding with the forming epoxy network without loss of ordering in the resulting blends. 6,16-19 This pathway could lead to an improvement in mechanical properties (notably fracture toughness) and in the stability of nanostructured materials. 16-18 Copolymers based in glycidyl methacrylate 16,18 and methacrylic acid 17 have been used by several authors. Rebizant et al. 16 obtained nanostructured thermosets, via DGEBA/DDS, by using polystyrene-block-polybutadiene-block- poly(glycidyl methacrylate) (SBGM) copolymers instead of the analogue SBM triblock copolymer, which macrophase-separates. Similar results were obtained by Grubbs et al. 18 with a polyisoprene-block-poly(methyl acrylate-co-glycidyl acrylate) (MG-I) copolymer by using 4,4′-methylenedianiline (MDA) as hardener. * Corresponding author. E-mail: iapmoegi@sc.ehu.es. Telephone: +34-943017271. Fax: +34-943017140. ² “Materials + Technologies” Group. Universidad del Paı ´s Vasco/Euskal Herriko Unibertsitatea. ‡ CIDETEC. § Leibniz Institute of Polymer Research Dresden. 2254 Macromolecules 2006, 39, 2254-2261 10.1021/ma0515477 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/22/2006