Structure–property relationships in biaxially deformed polypropylene nanocomposites Rund Abu-Zurayk * , Eileen Harkin-Jones, Tony McNally, Gary Menary, Peter Martin, Cecil Armstrong, Marion McAfee School of Mechanical & Aerospace Engineering, Queen’s University Belfast, BT9 5AH, UK article info Article history: Received 11 November 2009 Accepted 2 April 2010 Available online 14 April 2010 Keywords: D. Dynamic mechanical thermal analysis (DMTA) D. Transmission electron microscopy (TEM) D. X-ray diffraction (XRD) D. Differential scanning calorimetry (DSC) abstract Semi-solid forming processes such as thermoforming and injection blow moulding are used to make much of today’s packaging. As for most packaging there is a drive to reduce product weight and improve properties such as barrier performance. Polymer nanocomposites offer the possibility of increased mod- ulus (and hence potential product light weighting) as well as improved barrier properties and are the subject of much research attention. In this particular study, polypropylene–clay nanocomposite sheets produced via biaxial deformation are investigated and the structure of the nanocomposites is quantita- tively determined in order to gain a better understanding of the influence of the composite structure on mechanical properties. Compression moulded sheets of polypropylene and polypropylene/Cloisite 15A nanocomposite (5 wt.%) were biaxially stretched to different stretching ratios, and then the structure of the nanocomposite was examined using XRD and TEM techniques. Different stretching ratios produced different degrees of exfoliation and orientation of the clay tactoids. The sheet properties were then inves- tigated using DSC, DMTA, and tensile tests .It was found that regardless of the degree of exfoliation or orientation, the addition of clay has no effect on percentage crystallinity or melting temperature, but it has an effect on the crystallization temperature and on the crystal size distribution. DMTA and tensile tests show that both the degree of exfoliation and the degree of orientation positively correlate with the dynamic mechanical properties and the tensile properties of the sheet. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Polymer/clay nanocomposites have received much attention over the last decade [1]. A small amount of nanodispersed filler leads to an improvement in material properties, such as modulus, strength, heat resistance, flame retardancy and lower gas perme- ability. Nanofillers are being used to replace other fillers and pro- vide an improved balance of stiffness and toughness while reducing weight. In order to understand how the nano filler en- hances the properties of the polymer, many studies have been per- formed to find a link between the structure and the properties of the polymer/clay nanocomposites. Most of those studies use XRD and TEM techniques to study the structure of the nanocomposites [2–5]. Linking the structure to tensile properties of PP/clay nano- composite, Wang et al. [2] found that the orientation of the clay tactoids has a greater effect than their dispersion degree on the tensile properties and Li et al. [3] found that the presence of agglomerates decreased the tensile properties. In a study con- ducted on cross-linked polyester/clay nanocomposites, it was found that the tensile and storage modulus decreased with an in- crease in clay content especially at the content that gave the high- est degree of exfoliation due to a decrease in cross-linking [4]. Similar results regarding the relation between degree of exfoliation and storage modulus were presented by Ray et al. [5]. They found that the degree of exfoliation is not the most important factor in producing the enhancement in storage modulus, and that a system with high aspect ratio and a well ordered intercalated structure produced better storage modulus compared to other systems with low aspect ratio and a better degree of exfoliation. Fan et al. [6] studied the microstructure dependent properties of PP/clay nano- composite. They found that the storage modulus was higher for 1 wt.% clay compared to 5 wt.% clay due to the higher degree of exfoliation in the 1 wt.% clay system; however, the storage moduli of both systems became the same at high temperature due to crys- tallization occurring in the polymer matrix at this high tempera- ture. They also found that a higher degree of exfoliation resulted in higher crystallinity and higher crystallization onset tempera- ture. This was attributed to the superior nucleation efficiency of the more exfoliated system. On the issues of the relationship be- tween structure and glass transition temperature, it has been found that where good interaction between the nanoparticles 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.04.011 * Corresponding author. E-mail address: rabuzurayk01@qub.ac.uk (R. Abu-Zurayk). Composites Science and Technology 70 (2010) 1353–1359 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech