Epoxy-matrix composites ﬁlled with surface-modiﬁed SiO 2 nanoparticles Jelena Macan 1 • Klaudia Paljar 1 • Bruno Burmas 1,2 • Goran S ˇ pehar 1 • Mirela Leskovac 1 • Andreja Gajovic ´ 3 Received: 29 October 2015 / Accepted: 19 November 2016 / Published online: 28 November 2016 Ó Akade ´miai Kiado ´, Budapest, Hungary 2016 Abstract Composites based on an epoxy resin, diglycidyl ether of bisphenol A and SiO 2 nanoparticles, unmodiﬁed and surface-modiﬁed with a coupling agent 3-glycidy- loxypropyltrimethoxysilane, were prepared. Successful modiﬁcation of the nanoparticles was conﬁrmed by infra- red spectroscopy and combined differential scanning calorimetry and thermogravimetric analysis (DSC–TG). Composite materials were prepared by adding 0.5–5 phr (parts per hundred parts of resin) of modiﬁed and unmodiﬁed nanoparticles into the epoxy resin which was then cured with a poly(oxypropylene) diamine. Curing was followed by DSC, and cured materials were characterised by tensile and hardness testing. Morphology of fractured surfaces after tensile testing was investigated by scanning electron microscopy (SEM). Thermal stability of cured materials was studied by TG and their glass transition temperature determined by DSC. The presence of the ﬁller was found not to inﬂuence the curing mechanism of the epoxy resin nor the degradation mechanism of the cross- linked epoxy–amine matrix. Glass transition exhibits a small shift to higher temperatures in composites, which also exhibit increased char formation—at lower ﬁller content for the composite with unmodiﬁed nanoparticles and at higher for the one with modiﬁed nanoparticles. All composites show improved mechanical properties in comparison with the neat epoxy. For the unmodiﬁed par- ticles, strength and modulus are particularly improved at lower nanoﬁller content due to their better dispersion, as observed by SEM. The modiﬁed nanoparticles contributed to a signiﬁcant increase in elongation at break, increasing the toughness of the cured resin while retaining its strength and without having an adverse inﬂuence on the modulus. Keywords Epoxy Á Silica nanoparticles Á Surface modiﬁcation Á Thermal properties Á Toughening Introduction Epoxy resin is widely used as a matrix for high-perfor- mance composite materials due to its excellent mechanical and thermal properties. Highly crosslinked structure of epoxy polymer also makes the material fairly brittle, leading to intensive research in improving its toughness without an adverse inﬂuence on other properties [1–6]. The addition of nanoﬁller, especially at low loading content, can lead to signiﬁcant improvement in thermal, optical and mechanical properties of the crosslinked epoxy [3, 6–8]. It is necessary to obtain good dispersion of nanoﬁller within the matrix; otherwise, the ﬁller can form agglomerates which act as weak points in the structure, usually resulting in a very small improvement or even worsening of the properties. In order to improve the dispersion of nanoﬁller within the matrix and improve the ﬁller–matrix interac- tions, nanoﬁller surface is usually modiﬁed with coupling agents [9, 10]. Although we found a number of articles investigating the inﬂuence of nanosilica ﬁller on thermal and mechanical properties of the crosslinked epoxy [9, 11–15], few papers & Jelena Macan jmacan@fkit.hr 1 Faculty of Chemical Engineering and Technology, University of Zagreb, Marulic ´ev trg 19, 10000 Zagreb, Croatia 2 Present Address: Faculty of Natural Sciences and Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway 3 Rud¯er Bos ˇkovic ´ Institute, Bijenic ˇka 54, 10000 Zagreb, Croatia 123 J Therm Anal Calorim (2017) 127:399–408 DOI 10.1007/s10973-016-5976-7