Melt-Mixed 3D Hierarchical Graphene/Polypropylene Nanocomposites with Low Electrical Percolation Threshold

Abstract

Graphene-based materials are a family of carbonaceous structures that can be produced using a variety of processes either from graphite or other precursors. These materials are typically a few layered sheets of graphene in the form of platelets and maintain some of the properties of pristine graphene (such as two-dimensional platelet shape, aspect ratio, and graphitic bonding). In this work we present melt mixed graphene-based polypropylene systems with significantly reduced percolation threshold. Traditionally melt-mixed systems suffer from poor dispersion that leads to high electrical percolation values. In contrast in our work, graphene was added into an isotactic polypropylene matrix, achieving an electrical percolation threshold of~1 wt.%. This indicates that the filler dispersion process has been highly efficient, something that leads to the suppression of the β phase that have a strong influence on the crystallization behavior and subsequent thermal and mechanical performance. The electrical percolation values obtained are comparable with reported solution mixed systems, despite the use of simple melt mixing protocols and the lack of any pre or post-treatment of the final compositions. The latter is of particular importance as the preparation method used in this work is industrially relevant and is readily scalable.

Nonlinear “oddities” at the percolation of 3D hierarchical graphene polymer nanocomposites

“…In this study, we present an industrially relevant melt mixed system where a novel 3D hierarchical reduced graphene oxide (HrGO) filler has been successfully dispersed in isotactic polypropylene. Consequently, the electrical percolation threshold was observed in the vicinity of ϕ c ≈ 0.94 wt% and ranks lowest among similar systems in similar preparation conditions (Gkourmpis et al 2019). Such percolation thresholds are comparable to graphene-based systems prepared using solution mixing techniques (Kim and Macosko 2008).…”

“…Thus, while overall filler dimensions exceed nanoscale, the filler structure involves the presence of features at nanoscale level, i.e., the rGO sheets, and can be thus classified as a 3D nanofiller (Ashby et al 2009). We note that the hierarchical structure is a direct result of the proprietary exfoliation process (Gkourmpis et al 2019). The HrGO was produced via a modified Staudenmaier method (Staudenmaier 1898) and was provided by Cabot Corporation (Boston, MA, USA).…”

“…The critical exponent t is also conductivity-dependent with universal values of t ≈ 1.33 and t ≈ 2 for two and three dimensions respectively (Stauffer and Aharony 1987;Sahimi 1994). Despite the universality of the critical exponent, a wide range of values as high as t ≈ 10 have been reported (Gkourmpis 2014;Bauhofer and Kovacs 2009;Gkourmpis et al 2019), but this divergence can mainly be associated with complex tunneling transport phenomena (Balberg 1987;Balberg 2009;Grimaldi and Balberg 2006;Johner et al 2008) without disregarding more practical limitations such as filler variability in terms of production, entanglement, interconnections, and surface chemistry (Mutiso and Winey 2012). The percolated network properties are expected to depend on the type of filler, e.g., some fillers are flexible (CNT) whereas certain morphological types favor inter-connectivity compared to others, e.g., 2D vs. 0D nanofillers (Hassanabadi and Rodrigue 2013).…”

Barrier Properties of GnP–PA-Extruded Films

“…The low apparent density of commercially available GnP powders leads to complications in extruder feeding [19].The incorporation of nano additives has a significant effect on crystallization behavior. In particular, reports of GnP showing enhanced crystallinity can be found for polypropylene [21] and polyamide-6 [22]. Furthermore, it is known that crystallinity has beneficial effects on the permeation rate, where two aspects of crystallinity have to be considered: degree of crystallization and crystalline structure [23,24].Considering these aspects, the objective of the present work is the development of a method for the incorporation of graphene nanoplatelets into the semi-crystalline polymer matrix by means of melt mixing.Secondly, the barrier properties will be investigated as a function of filler content and crystallinity.…”

“…The incorporation of nano additives has a significant effect on crystallization behavior. In particular, reports of GnP showing enhanced crystallinity can be found for polypropylene [21] and polyamide-6 [22]. Furthermore, it is known that crystallinity has beneficial effects on the permeation rate, where two aspects of crystallinity have to be considered: degree of crystallization and crystalline structure [23,24].…”