1. Introduction Increasing efforts are devoted to the research of thermoplastic nanocomposites exhibiting improved and novel properties. Most of these studies are focused on the investigation of correlations between structural features and mechanical properties. A large body of research has been developed on polar nanofillers (such as silicas, metal oxides, metal salts, layered silicates, etc.) which have been successfully added to thermoplastic matrices in order to improve their thermal, mechanical and rheological perform- ances [1–8]. On the other hand, these nanofillers are generally poorly dispersed in apolar thermoplastics (such as polyolefins), thus limiting the beneficial effects of nanofiller addition on the thermo-mechani- cal properties. Different strategies have been adopted in order to improve the dispersability of polar nano- fillers, such as the direct incorporation of the filler during the in-situ synthesis of the polymer [9], the addition of the filler during melt mixing [10, 11] or the dispersion of the filler by solution techniques [12]. However, in order to attain a qualitatively fine dispersion of the nanofiller within the matrix, a sur- face treatment of the filler should be considered [13–15], or a polymeric compatibilizer should be added during melt mixing [5, 16–19]. Due to its combination of low cost, high chemical resistance and relatively good mechanical proper- 652 Viscoelastic behaviour and fracture toughness of linear-low-density polyethylene reinforced with synthetic boehmite alumina nanoparticles D. Pedrazzoli 1 , R. Ceccato 1 , J. Karger-Kocsis 2,3 , A. Pegoretti 1* 1 University of Trento, Department of Industrial Engineering and INSTM Research Unit, Via Mesiano 77, 38123 Trento, Italy 2 MTA–BME Research Group for Composite Science and Technology, Muegyetem rkp. 3., H-1111 Budapest, Hungary 3 Department of Polymer Engineering, Faculty of Mechanical Engineering, Budapest University of Technology and Economics, Megyetem rkp. 3., H-1111 Budapest, Hungary Received 1 March 2013; accepted in revised form 14 April 2013 Abstract. Aim of the present study is to investigate how synthetic boehmite alumina (BA) nanoparticles modify the vis- coleastic and fracture behaviour of linear low-density polyethylene. Nanocomposites containing up to 8 wt% of untreated and octyl silane-functionalized BA nanoparticles, were prepared by melt compounding and hot pressing. The BA nanoparticles were finely and unformly dispersed within the matrix according to scanning electron microscopy inspection. The results of quasi-static tensile tests indicated that nanoparticles can provide a remarkable stiffening effect at a rather low filler content. Short term creep tests showed that creep stability was significatively improved by nanofiller incorporation. Concurrently, both storage and loss moduli were enhanced in all nanocomposites, showing better result for surface treated nanoparticles. The plane-stress fracture toughness, evaluated by the essential work of fracture approach, manifested a dramatic increase (up to 64%) with the BA content, with no significant differences among the various types of BA nanoparticles. Keywords: nanocomposites, fracture and fatigue, mechanical properties, thermal properties eXPRESS Polymer Letters Vol.7, No.8 (2013) 652–666 Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2013.62 * Corresponding author, e-mail: alessandro.pegoretti@unitn.it © BME-PT