Nano-scaled carbon fillers and their functional polymer composites

The manufacturing of low density conductive plastics, that could replace metals in many applications, is a challenging target that has been pursued by several technological segments. The incorporation of carbon nanofillers, namely carbon nanotubes and graphene, into a polymer matrix is a recent and promising approach. The achievement of highly conductive nanocomposites, with low electrical percolation threshold, depends mostly on the intrinsic properties of the fillers and their state of dispersion throughout the matrix. In this system the macroscopic properties of the composite are undoubtedly governed by the nature and extension of the interactions between filler particles, as well as between filler and polymer matrix. This doctoral dissertation therefore examines the macroscopic electrical behavior of carbon fillers, at initial stage, still as a powder, and subsequently when inserted into polymer matrix to form conductive nanocomposites. The latex technology has proven to be efficient on promoting a homogenous incorporation of exfoliated CNTs into any kind of viscous polymer which can be artificially brought into latex form, or which can be synthetized via emulsion polymerization. One objective of this dissertation is henceforth to study the scope and limitations of multifunctional graphene-based nanocomposites, using the superior DPI-owned water-based latex concept developed for CNTs, for dispersing the two-dimensional graphene nanofiller in polymer matrices (PS, PP, PS/PPO). For comparison, zero-dimensional (statistically spherical) carbon black nanoparticles, one-dimensional MWCNTs, and three-dimensional graphite are also evaluated. We strive to understand differences observed for the different carbon allotropes, each with a high surface-to-volume ratio Special attention is given to graphene as nanofiller, whose properties may be highly variable depending on the preparation method. In order to assess a collective understanding of the main routes utilized in this work for preparation of graphene, Chapter 2 introduces a systematic literature review on the particularities of each preparation method. Techniques used for characterization of single sheet properties and its organization inside polymer composites are presented, with focus on atomic force microscopy and Raman spectroscopy. The conductive performance of a composite is directly related to the formation of a conducting network through the polymer matrix and its understanding depends critically on the knowledge of the electrical behavior of the agglomerated nanoparticles, e.g. in the form of a bulk powder or a paper film. In literature there is still a lack of information concerning these macroscopic bulk properties of carbon powders. Chapter 3 studies the electrical conductivity of the nanofillers MWCNTs, graphene, carbon black and graphite, using compacts produced by a paper preparation process and by powder compression. Powder pressing assays show that the bulk conductivity depends not only on the intrinsic material properties but is also strongly affected by the number of particle contacts and the packing density. Conductivities at high pressure (5 MPa) for the graphene, nanotube and carbon black show lower values (~102 S/m) as compared to graphite (~103 S/m). For nanotube, graphene and graphite particles, the conductive behavior during compaction is governed by mechanical particle arrangement/deformation mechanisms while for carbon black this behavior is mainly governed by the increasing particle contact area. The materials resulting from the paper preparation process for carbon black and graphite showed similar conductivity values as for the compacts, indicating a limited effect of the surfactant on the conductivity. The paper preparation process for the large surface area nanotube and graphene particles induces a highly preferred in-plane orientation, thereby yielding largely the single particle intrinsic conductivity for the in-plane direction, with values in the order of 103 S/m. In Chapter 4, the percolation thresholds and final conductivities of polypropylene (PP) composites, prepared with the fillers studied in Chapter 3, are evaluated and compared with powder and paper results. The latex technology concept is used for the incorporation of the carbon fillers in the polymer. The fillers are first dispersed in water (assisted by surfactants) using ultra-sonication, subsequently mixed with PP latex, then freeze-dried and, finally, hot-pressed into composite tablets. PP composites produced in this work showed well-dispersed fillers inside the polymer, with percolation thresholds as low as 0.3 wt.%. The maximum conductivity obtained for the composites is approximately ~1 S/m, not reaching the high value of ~103 S/m, which are obtained for graphene and nanotube-based paper films or graphite compacts. Chapter 5 focuses on the characterization of graphene layers via micro-Raman spectroscopy, tip-enhanced Raman spectroscopy (TERS) and tip-enhanced Raman spectroscopy mapping (TERM). In particular TERM allows for the investigation of individual graphene sheets with high Raman signal enhancement factors and allows imaging of local defects with nanometer resolution. Enhancement up to 560% of the graphene Raman bands intensity was obtained using TERS. TERM (with resolution better than 100 nm) showed an increase in the number of structural defects (D band) on the edges of both graphene and graphite regions. Continuing the investigation of graphene structures, Chapter 6 compares graphene sheets produced from graphite powder using the three best known water-based conversion approaches. The first two are based on chemical oxidation methods, only differing in the reduction process, either by the use of hydrazine or by thermal expansion, respectively. The third one is based on long-term ultrasonic exfoliation. Water/surfactant solutions were prepared with these three nanofillers and latex technology was applied for the preparation of conductive graphene/polystyrene composites with well-dispersed graphene platelets. The samples were characterized with respect to filler properties and morphology, and their influences on electrical conductive properties of the composites were compared. Microscopic studies showed that both reduction processes lead to agglomeration/wrinkling of the nanoplatelets, even though they yield composites with high conductivity and low percolation threshold. Although mechanical ultrasound exfoliation of graphite produces less defective multi-layer graphene, these platelets have a smaller lateral size and their composites exhibit a higher percolation threshold. As a final attempt, in Chapter 7, the concept of liquid-phase dispersion, inspired on the latex technology, was applied for the preparation of well-dispersed suspensions of multi-wall carbon nanotubes and graphene in chloroform, using long-time ultra-sonication, without the use of surfactants. The dispersions with pre-defined filler concentration (0.5 mg/ml) were monitored via UV-Vis until the achievement of optimum exfoliation (6 h). The mixture of the filler suspensions with a PS/PPO solution, both using chloroform as solvent, subsequent drying and hot pressing, yielded for most of the samples a visually homogeneous and shiny black composite tablet. The well-dispersed organization of the fillers inside the polymer matrix, visualized with scanning electron microscopy, resulted in ultimate conductivities and percolation thresholds of 57 S/m and 0.2 wt.% for nanotubes composites, and 0.9 S/m and ~1 wt.% for graphene composites, respectively. Dynamic mechanical analysis showed that an increase in the storage moduli of the PS/PPO matrix could be gradually obtained by the insertion of fillers, e.g. reaching ~30% of enhancement by the addition of 3 wt.% of graphene filler. The same trend in improvement, at lower augmentation, was observed for the corresponding nanotubes-based composites.