# Preparation of Piezo-Resistive Materials by Combination of PP, SEBS and Graphene
## 1 Introduction  
Polyolefins (polyethylene and polypropylene) are the most widely used synthetic polymers due to their easy preparation, low cost, good processability, recyclability, and biocompatibility that make them attractive candidates for very different areas like packaging, consumer goods, biomedical, or automotive sectors. During the last decades the incorporation of nanofillers has expanded the versatility of polyolefin-based materials to more advanced and engineering applications [1; 2]. Graphene is nowadays one of the most interesting nanomaterials incorporated into polymer matrices due to its exceptional properties [3; 4; 5; 6; 7]. The combination of graphene with polyolefins provides materials with additional functionalities like electrical conductivity as well as enhanced mechanical, thermal, or barrier properties, thus broadening their spectrum of applications [8; 9; 10; 11; 12; 13; 14; 15].  
Polypropylene (PP) is increasingly used in the automotive sector. In this particular industry, stiffness and toughness have to be simultaneously improved for structural components that, in the case of PP, is an important challenge [16; 17]. One of the main strategies used to improve the impact resistance of PP is based on blending it with thermoplastic elastomers such as styrene-ethylene-butylene-styrene (SEBS) because of its excellent toughness, at the expense of other mechanical properties like tensile strength [18; 19]. In order to achieve simultaneous improvements in stiffness and toughness in isotactic polypropylene (iPP), the addition of a third component such as inorganic nanofillers, other crystalline polymers, or additives has been widely explored [20; 21; 22].  
Thus, the preparation of ternary systems by incorporating SEBS and graphene into PP has emerged as a very promising alternative, not only for structural materials but also for electrically conductive materials which are able to sense changes in the components during operation. In immiscible polymer blends, the distribution and location of the nanofiller is critical which can affect the blends properties, especially the electrical conductivity [23; 24]. A selective localization of the nanofiller can tune the performance of the material. In the particular case of simultaneous increments in stiffness and toughness in these ternary systems, the aim is to disperse the rigid filler preferentially in the iPP phase without altering the elastomeric phase.  
It has been observed that in ternary PP/SEBS/montmorillonite nanocomposites the rigid filler locates preferentially in the SEBS domains, counteracting the positive effect on the toughness of the polymer [21]. Moreover, the incorporation of graphene into elastomers has led to a significant improvement in Young's modulus and a decrease in the elongation at break with increasing graphene content [25].  
The chemical functionalization of the filler has been demonstrated to be a powerful tool to enhance the filler/polymer physical interaction to attain good filler dispersion in the matrix [5; 6; 26]. Very recently, we demonstrated that graphene can be selectively located into specific SEBS domains by specific functionalization of graphene with short brushes of polyethylene [27; 28] or polystyrene [28]. Furthermore, we have also developed a protocol to functionalize graphene with short polypropylene brushes that improves the load sharing with the iPP matrix [10; 29].  
In this paper, we studied the preparation of iPP/SEBS/graphene nanocomposites by evaluating two different preparation procedures. In addition, two different nanofillers (non-functionalized and polypropylene-modified graphene) were evaluated in order to investigate whether the selective location of graphene had an effect on the final properties of the blends.