Polymer Testing 80 (2019) 106150 Available online 9 October 2019 0142-9418/© 2019 Published by Elsevier Ltd. Material Characterisation Nano-indentation as a tool for evaluating the rheological threshold in polymer composites Anesh Manjaly Poulose a, * , Ahmed Yagoub Elnour a , Ubair Abdus Samad b , Mohammad Asif Alam b , Justin George c , Ajit K. Sarmah c , Saeed M. Al-Zahrani a a Chemical Engineering Department, Sabic Polymer Research Centre (SPRC), King Saud University, P. O. Box 800, Riyadh, 11421, Saudi Arabia b Center of Excellence for Research in Engineering Materials (CEREM), Advance Manufacturing Institute, King Saud University, P. O. Box 800, Riyadh, 11421, Saudi Arabia c Civil and Environmental Engineering Department, University of Auckland, Auckland, 1142, New Zealand A R T I C L E INFO Keywords: Polymer composites Rheological threshold Nano-indentation ABSTRACT We investigated the variation of nano-mechanical properties of Poly (propylene) (PP)/Carbon Black (CB) com- posite using nano-indentation and evaluated rheological threshold of the composites. PP/CB composites were prepared by melt-mixing process with varying CB content (0–20 wt %) using a micro-compounder. The rheo- logical threshold of the composites was calculated using well known rheometer technique by power law relation. We also employed nano-indentation technique as an additional novel tool for evaluating rheological threshold in PP/CB composites. Rheological and nano-indentation measurements were correlated to assess the rheological threshold in PP/CB composites. The modulus of elasticity of PP/CB composites was increased from 0.8 GPa to 1.4 GPa with 20 wt % CB loading. There was a minor enhancement in the tensile strength of PP/CB composites with increase in CB content and the elongation at yield was decreased due to the brittleness imparted by CB. The thermal stability of PP/CB system also improved with CB content as the decomposition temperature was shifted from 450 C to 500 C. 1. Introduction The carbon based fillers such as carbon black [1–4], carbon fibers [5, 6], carbon nanotubes [1,7–9], graphite or graphene [10–12] have been widely used as a reinforcing and conducting fillers in polymer com- posites. These composites are lightweight, corrosion resistant, rigid and have high thermal and electrical conductivity. Applications of such composites can be found in electronics, electrical industries, electro- magnetic interference (EMI) shielding applications, antistatic and as electrostatic dissipation materials [13,14]. There are many other rein- forcing materials for polymeric matrix are reported in literature such as natural fibers, cellulose [15,16], minerals [17], bio-waste synthesized carbon [18] etc. The processing parameters such as filler aspect ratio, geometrical and surface characteristics of the filler, its orientation related to the material micro-structure, filler-filler and filler-polymer chain interactions have strong influences on the end properties of the resultant composites [19,20]. Based on the processing conditions and filler characteristics, the filler can disperse or agglomerate in the poly- mer substrate during the melt blending process [21]. The extent of filler dispersion and filler-matrix interactions can be indirectly assessed by rheological method [22,23] and are important for assessing the pro- cessing and application of the composites [24]. The filler-filler and filler-matrix network formation in composites can lead to concentration dependent changes in the viscoelastic properties especially at longer timescales and can be monitored from complex viscosity (η*), storage modulus (G 0 ), loss modulus (G 00 ) and loss tangent (tan δ) measurements [25,26]. On increasing filler concentration, a characteristic viscoelastic response corresponding to the onset of solid-like behavior due to the formation of heterogeneous network formation is known as rheological percolation threshold [27]. It is important to find out the percolation threshold value as it is related to filler dispersion and its interaction with the polymer chains [28,29]. Furthermore, the rheological properties of polymer composites are different before and after percolation value and are important for optimizing the preparation and shaping process by extrusion or injection molding [30]. Nano-mechanical behavior of the polymers and composites can be performed by different techniques using AFM [31], nano-indentation [32], micro-indentation [33] etc. Out of these, nano-indentation is a * Corresponding author. E-mail address: apoulose@ksu.edu.sa (A.M. Poulose). Contents lists available at ScienceDirect Polymer Testing journal homepage: http://www.elsevier.com/locate/polytest https://doi.org/10.1016/j.polymertesting.2019.106150 Received 28 January 2019; Received in revised form 1 September 2019; Accepted 8 October 2019