JOURNAL OF MATERIALS SCIENCE 35 (2 0 0 0 ) 3719 – 3726 Toughening of tetrafunctional (TGDDM) epoxy resins with telechelic extended perfluoroligomers E. MARTUSCELLI, P. MUSTO, G. RAGOSTA, F. RIVA Institute of Research and Technology of Plastics Materials, National Research Council, Via Toiano, 6, 80072 Arco Felice, Naples, Italy L. MASCIA ∗ Institute of Polymer Technology and Materials Engineering, Loughborough University, Loughborough, Leics LE11 3TU, UK E-mail: l.mascia@lboro.ac.uk Telechelic extended perfluoroether oligomers containing carboxylic acid terminal groups were found to be miscible in mixtures of a tetrafunctional epoxy resin (TGDDM) and methyl nadic anhydride. After curing the resin system remained transparent. SAXS examination revealed the presence of a two-phase morphology with domains of the order of 16–17 nm. The glass transition of the cured resin system showed a maximum at perfluoroether concentrations in the region of 5% and remained at about the same level as the unmodified system at concentrations as high as 20 wt %. The critical strain energy release rate (Gc) for these systems was found to increase very rapidly in the region of 5 to 15 wt % fluoroligomer to reach values about 8 times higher than those obtained for the control resin system. This was accompanied by a small reduction in modulus and yield strength. The main reason for the enhanced toughness, however, is attributed to the increased size of the yield zone at the crack tip. The fluoroligomer modified cured resin systems exhibited an unusual change in water diffusion characteristics. At about 15 wt % concentration of fluoroligomer modifier the diffusion coefficient increased by a factor of 70, while the equilibrium water absorption was reduced by more than 10 wt %. C  2000 Kluwer Academic Publishers 1. Introduction For more than thirty years it has been demonstrated that difunctional epoxy resins, particularly those based on bisphenol A (DGEBA) can be toughened by the addition of low molecular weight butadiene acryloni- trile copolymers containing functional terminal groups, such as carboxylic acid (CTBN) or primary amine (ATBN) [1–4]. The toughening component reacts with the resin prior to gelation, giving rise to the formation of high molecular weight species of reduced solubilty, which precipitate out into a multitude of particles as- suming final dimensions in the region of 1 to 10 µm [5]. It is widely accepted that the mechanism by which such rubbery inclusions increase the fracture toughness of glassy polymers, such as crosslinked epoxy resins, is the promotion of shear yielding in the surrounding ma- trix, and through the formation of microcavities within the particles and in the interfacial regions [6]. The use of CTBN type of oligomers, however, has not been found to be particularly effective with tetra- functional epoxy resins, such as tetraglycidyl diamino diphenyl methane (TGDDM), even though phase sep- aration of the oligomer takes place as in the case of di- functional DEGBA resins [7]. This has been attributed ∗ To whom correspondence should be addressed. to the excessive brittleness of the resin, which makes it difficult for the matrix surrounding the rubber particles to reach the yielding conditions. Since tetrafunctional epoxy resins are primarily used for high temperature applications, modifications with CTBN oligomers are not particularly appropriate owing to their low thermal oxidative stability [8]. Several attempts have been made to find alternative modifiers to CTBN to overcome the stated difficulties [9]. To this end several authors have used engineering thermoplastics, such as polysulphones and polyether- imides. Although phase separation through particle pre- cipitation takes place and interfacial adhesion can be in- creased with the use of functionalised polymers capable of reacting with the epoxy resin, a large concentration (more than 30 wt%) is normally required to achieve a substantial increase in toughness, as opposite to 10– 20% normally required for CTBN systems [10]. The drive to find suitable toughening agents for multifunctional epoxy resins, also capable of reduc- ing water absorption, has induced some authors to ex- amine polydimethylsiloxane and fuorinated polymers, particularly in the form of reactive oligomers [11]. Takahashi et al. have investigated the use of several 0022–2461 C  2000 Kluwer Academic Publishers 3719