Network Structure and Thermomechanical Properties of Hybrid DGEBA Networks Cured with 1-Methylimidazole and Hyperbranched Poly(ethyleneimine)s Xavier Fern andez-Francos, 1 David Santiago, 2,3 Francesc Ferrando, 2 Xavier Ramis, 3 Josep M. Salla, 3 A ` ngels Serra, 1 Marco Sangermano 4 1 Department of Analytical and Organic Chemistry, Universitat Rovira i Virgili, C/ Marcellı ´ Domingo s/n, 43007 Tarragona, Spain 2 Department of Mechanical Engineering, Universitat Rovira i Virgili, C/Paı ¨sos Catalans 26, 43007 Tarragona, Spain 3 Thermodynamics Laboratory, ETSEIB, Universitat Polite ` cnica de Catalunya, Av. Diagonal 647, 08028 Barcelona, Spain 4 Department of Applied Science and Technology, Politecnico di Torino, Corso Duca Degli Abruzzi 24, 10129 Torino, Italy Correspondence to: X. Fern andez-Francos (E-mail: xavier.fernandez@urv.cat) Received 29 June 2012; accepted 30 July 2012; published online 11 September 2012 DOI: 10.1002/polb.23145 ABSTRACT: The use of commercially available hyperbranched poly(ethyleneimine)s (Lupasol TM , BASF) as polymeric modifiers in diglycidyl ether of bisphenol A thermosetting formulations using 1-methylimidazole (MI) as anionic initiator has been studied. Poly(ethyleneimine)s can get incorporated into the network structure by condensation of amine and epoxy groups. The excess, over-stoichiometric epoxy groups can undergo ani- onic homopolymerization initiated by MI. The thermal, dyna- momechanical, and mechanical properties of the resulting materials have been determined using DSC, thermomechanical analysis (TMA), dynamomechanical analysis (DMA), and me- chanical testing. The effect of the different amine modifiers on the MI networks, determined by their structure, is complex. Low initiator content and high molecular weight modifiers cre- ate significant mobility restrictions, which have a strong effect on the glass transition temperature and the apparent crosslink- ing density of the cured materials. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 1489–1503, 2012 KEYWORDS: epoxy thermosets; hyperbranched; impact strength; mechanical properties; network structure; thermal properties; thermomechanical properties; thermosets INTRODUCTION Epoxy resins are widely used in applications such as coatings, adhesives, structural applications, or elec- tronics because of their mechanical properties, relatively low shrinkage, and high chemical and thermal resistance. 1–3 The wide range of curing agents available for their cross linking makes them extremely versatile. However, their inherent brittleness, which arises from their high cross-linking den- sity, can limit their use in certain applications. Hyperbranched polymers (HBPs) are a type of dendronized polymers, which can be used as effective polymer modifiers of thermosetting materials because (1) their high degree of branching makes them less viscous than their linear counter- parts with a similar molecular weight and (2) they possess a high concentration of surface groups that can be modified to fine-tune their physical compatibility with the matrix or make possible their covalent linkage to the matrix. The prop- erties of the final material can thus be tailored as a function of the core structure, the degree of branching, and the type of functional end-groups. 4 One of the most relevant applica- tions in thermosets is their use as toughening agents that phase-separate during curing 5–11 or else get incorporated into the network structure. 12 Although many studies were published on the modification of epoxy-amine systems with aliphatic polyester-ether HBPs such as Boltorn H30 and related structures, some reports have also appeared on ther- mal curing of epoxy-HBP formulations 13 and on cationic cur- ing. 12,14 We reported the successful incorporation of Boltorn H30 and modified H30 in diglycidyl ether of bisphenol A (DGEBA) using a variety of initiators or and curing agents. 15–18 Hydroxyl-terminated poly(amino-ester)s or poly(ester-amide)s were also used in DGEBA anionic formulations 19 or in DGEBA- anhydride formulations, 20 respectively. In contrast, hyper- branched poly(ethyleneimine)s have been scarcely used as curing agents 21,22 or tougheners for thermosets. 23,24 Recently, we have studied in detail the curing kinetics of a commercially available hyperbranched poly(ethyleneimine) (LP2000) in comparison with a linear aliphatic amine, dieth- ylenetriamine (DETA). 25 In this work, we study the use of LP2000 as reactive modifier in DGEBA formulations cured with 1-methylimidazole (MI). The effect of the molecular V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50, 1489–1503 1489 JOURNAL OF POLYMER SCIENCE WWW.POLYMERPHYSICS.ORG FULL PAPER