Morphological and structural characterization of polypropylene/conductive graphite nanocomposites

Abstract

The structure and morphology of polypropylene/conductive graphite (PP/CG) composites were studied by wide angle X-ray diffraction, small-angle X-ray scattering and scanning electron microscopy. An effect of graphite on the crystallization behavior was observed and the opposite influences of enhanced thermal conductivity and hinder of chain mobility on the formation of the γ-phase of PP were discussed.

Introduction

Polyolefins can be compounded with a variety of common and special fillers, reinforcements, and modifiers to yield specific properties in a wide range of applications. Among these additives are electrically and thermally conductive modifiers that can provide protection against static accumulation, electrostatic discharge and molding cycle reduction. Various fillers, some of them metallic, are used to produce the modification of conductive properties of the neat polymer. However, high concentrations of filler (e.g. 30% carbon black), are needed, that can take a toll on physical and esthetical properties of the polymer: selection of conductive material has often involved compromises. The preparation of polymer composites by dispersion of small loadings of nanosized fillers in a polymeric matrix has lately attracted much attention in the academia and the industry for its potential in improving the performance of macromolecular materials [1]. Nanoparticles may confer to the matrix physical–mechanical properties much improved with respect to traditional micron-sized fillers, due mainly to a maximization of the polymer-filler interfacial region [2], [3]. Although considerable success was achieved with polymer-layered silica composites [1], reports on the use of graphite, another common type of layered filler with a high aspect ratio, are comparatively rare [4], [5], [6], [7]. Natural occurring graphite consists of graphene layers formed by sp² hybridized carbon atoms. Van der Waals forces bind the layers one to each other. Single graphite sheets are stacked in aggregates, in which they have a spacing of 0.335 nm [5], [8]. The electrical conductivity of graphite arises from the delocalized π bonds, resulting from the sp² hybridization. The weak van der Waals forces that keep neighboring layers together allow the possibility of intercalating molecules in the interlayer space, thus increasing the interplanar spacing and achieving a thorough mixing of the filler in the matrix. The production of polypropylene/graphite composites is especially difficult because hardly any interaction occurs between polypropylene (PP) and chemically inert graphite layers [7]. In order to do so, conductive expanded graphite (CG) can be used instead of naturally occurring graphite. A number of methods have been reported [9], [10], [11], [12], [13], [14], [15] to intercalate atoms and molecules between graphene layers, increasing the interlayer spacing by a distance determined by the size of the guest. Intercalation is known to occur along with a charge transfer between the intercalate species and the graphene layers [10]. The obtained compounds are called graphite intercalation compounds (GIC) and can exhibit the staging phenomenon [10], [11], [13], in which each intercalated layer is separated by a definite number of graphene layers. The stage number n designates the number of graphene layers that separate adjacent intercalate layers.

The aim of this work was the study of the structure of PP/CG composites as a function of the filler content, using small-angle X-ray scattering (SAXS), wide angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM).