Probing dispersion and re-agglomeration phenomena upon melt-mixing of polymer-functionalized graphite nanoplates

Smart Carbon Fiber-Reinforced Polymer Composites for Damage Sensing and On-Line Structural Health Monitoring Applications

High electrical conductivity, along with high piezoresistive sensitivity and stretchability, are crucial for designing and developing nanocomposite strain sensors for damage sensing and on-line structural health monitoring of smart carbon fiber-reinforced polymer (CFRP) composites. In this study, the influence of the geometric features and loadings of carbon-based nanomaterials, including reduced graphene oxide (rGO) or carbon nanofibers (CNFs), on the tunable strain-sensing capabilities of epoxy-based nanocomposites was investigated. This work revealed distinct strain-sensing behavior and sensitivities (gauge factor, GF) depending on both factors. The highest GF values were attained with 0.13 wt.% of rGO at various strains. The stability and reproducibility of the most promising self-sensing nanocomposites were also evaluated through ten stretching/relaxing cycles, and a distinct behavior was observed. While the deformation of the conductive network formed by rGO proved to be predominantly elastic and reversible, nanocomposite sensors containing 0.714 wt.% of CNFs showed that new conductive pathways were established between neighboring CNFs. Based on the best results, formulations were selected for the manufacturing of pre-impregnated materials and related smart CFRP composites. Digital image correlation was synchronized with electrical resistance variation to study the strain-sensing capabilities of modified CFRP composites (at 90° orientation). Promising results were achieved through the incorporation of CNFs since they are able to form new conductive pathways and penetrate between micrometer-sized fibers.

Nonmonotonic Strain Rate Dependence on the Strain Hardening of Polymer Nanocomposites

This Letter investigates the external deformation on modifying the polymer-nanoparticle (NP) and NP-NP interactions as well as their influences on the macroscopic properties of polymer nanocomposites (PNCs). Specifically, the applied uniaxial extension brings together the NPs along the transverse stretching direction and catalyzes the percolation transition from the initially well-dispersed NPs in the PNCs. The percolated NP network thus increases the strength of PNCs and leads to a strong surge in the elongation viscosity at very low strain rates, exhibiting the unexpected nonmonotonic strain rate dependence on the strain hardening. These results reveal explicitly the unconventional roles of the external deformation on modulating the relative strength of the polymer-NP and the NP-NP interactions, enabling advanced structures and properties control of PNCs.

Mechanisms of mechanical reinforcement by graphene and carbon nanotubes in polymer nanocomposites

Polymer nanocomposites reinforced with carbon-based nanofillers are gaining increasing interest for a number of applications due to their excellent properties. The understanding of the reinforcing mechanisms is, therefore, very important for the maximization of performance. This present review summarizes the current literature status on the mechanical properties of composites reinforced with graphene-related materials (GRMs) and carbon nanotubes (CNTs) and identifies the parameters that clearly affect the mechanical properties of the final materials. It is also shown how Raman spectroscopy can be utilized for the understanding of the stress transfer efficiency from the matrix to the reinforcement and it can even be used to map stress and strain in graphene. Importantly, it is demonstrated clearly that continuum micromechanics that was initially developed for fibre-reinforced composites is still applicable at the nanoscale for both GRMs and CNTs. Finally, current problems and future perspectives are discussed.

Percolation behaviors of model carbon black pastes

The percolation behaviors of a series of high-structured carbon black (CB) pastes (CB weight fractions 10-25 wt%, ethyl cellulose as the binder, α-terpineol as the solvent) were systematically investigated using analyses of rheology and impedance spectra together with characterization via small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM). When the CB concentration was near the static percolation threshold (∼20 wt%), the permittivity, ac conductance, and elastic modulus of the paste displayed notable increases, whereas the SAXS profile revealed the prevalence of isolated CB aggregates (mean radius of gyration ∼40 nm). Upon further aging at 25 and 40 °C (up to 6 h), two CB pastes near the static percolation threshold (i.e., 20 and 25 wt%) exhibited prominent temporally evolving responses, including more than tenfold increases in their ac conductance and elastic modulus, as well as a pronounced upturn in the low-q SAXS profile (q < 0.03 nm-1) and the formation of a (partially) interconnected cluster network in SEM observations of the morphologies of screen-printed films. In this case, we provide the first evidence of "(aging) Time-(relaxation) Time-Temperature-Concentration Superposition (TTTCS)" for the dynamic modulus data over a frequency range of seven orders of magnitude. This suggests that prolonged aging time imparted to CB aggregate interaction and restructuring (or gelation) may work in tandem with the known effects of the system temperature and concentration to further extend the accessible range of dynamic modulus data, in a similar way to recent reports on the effect of the curing (crosslinking) time on a carbon nanotube suspension and caramel. In combination with existing (three) master curves for two different colloidal materials, we show that there is a reasonable superposition of all the dynamic modulus data over a frequency range of 12 orders of magnitude.

Electrically Conductive Polyetheretherketone Nanocomposite Filaments: From Production to Fused Deposition Modeling

The present work reports the production and characterization of polyetheretherketone (PEEK) nanocomposite filaments incorporating carbon nanotubes (CNT) and graphite nanoplates (GnP), electrically conductive and suitable for fused deposition modeling (FDM) processing. The nanocomposites were manufactured by melt mixing and those presenting electrical conductivity near 10 S/m were selected for the production of filaments for FDM. The extruded filaments were characterized for mechanical and thermal conductivity, polymer crystallinity, thermal relaxation, nanoparticle dispersion, thermoelectric effect, and coefficient of friction. They presented electrical conductivity in the range of 1.5 to 13.1 S/m, as well as good mechanical performance and higher thermal conductivity compared to PEEK. The addition of GnP improved the composites’ melt processability, maintained the electrical conductivity at target level, and reduced the coefficient of friction by up to 60%. Finally, three-dimensional (3D) printed test specimens were produced, showing a Young’s modulus and ultimate tensile strength comparable to those of the filaments, but a lower strain at break and electrical conductivity. This was attributed to the presence of large voids in the part, revealing the need for 3D printing parameter optimization. Finally, filament production was up-scaled to kilogram scale maintaining the properties of the research-scale filaments.

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

This topical review discusses the theoretical progress made in the field of polymer nanocomposites, i.e., hybrid materials created by mixing (typically inorganic) nanoparticles (NPs) with organic polymers. It primarily focuses on the outstanding issues in this field and is structured around five separate topics: (i) the synthesis of functionalized nanoparticles; (ii) their phase behavior when mixed with a homopolymer matrix and their assembly into well-defined superstructures; (iii) the role of processing on the structures realized by these hybrid materials and the role of the mobilities of the different constituents; (iv) the role of external fields (electric, magnetic) in the active assembly of the NPs; and (v) the engineering properties that result and the factors that control them. While the most is known about topic (ii), we believe that significant progress needs to be made in the other four topics before the practical promise offered by these materials can be realized. This review delineates the most pressing issues on these topics and poses specific questions that we believe need to be addressed in the immediate future.

Effects of Particle Size and Surface Chemistry on the Dispersion of Graphite Nanoplates in Polypropylene Composites

Carbon nanoparticles tend to form agglomerates with considerable cohesive strength, depending on particle morphology and chemistry, thus presenting different dispersion challenges. The present work studies the dispersion of three types of graphite nanoplates (GnP) with different flake sizes and bulk densities in a polypropylene melt, using a prototype extensional mixer under comparable hydrodynamic stresses. The nanoparticles were also chemically functionalized by covalent bonding polymer molecules to their surface, and the dispersion of the functionalized GnP was studied. The effects of stress relaxation on dispersion were also analyzed. Samples were removed along the mixer length, and characterized by microscopy and dielectric spectroscopy. A lower dispersion rate was observed for GnP with larger surface area and higher bulk density. Significant re-agglomeration was observed for all materials when the deformation rate was reduced. The polypropylene-functionalized GnP, characterized by increased compatibility with the polymer matrix, showed similar dispersion effects, albeit presenting slightly higher dispersion levels. All the composites exhibit dielectric behavior, however, the alternate current (AC) conductivity is systematically higher for the composites with larger flake GnP.

The chemical functionalization of graphene nanoplatelets through solvent-free reaction

Graphene nanoplatelets (GNPs) were functionalized through 1,3-dipolar cycloaddition of azomethine ylide using a solvent-free approach and under different reaction conditions. The yield and the functionality of the carboxyl-terminated pyrrolidine ring attached on the surface of GNPs could be affected by varying the reaction temperature as well as the reactant to GNP weight ratio. The functionalized GNPs were characterized extensively using a range of spectroscopic and microscopy techniques.

Carboxyl-terminated pyrrolidine functionalized graphene nanoplatelets through a solvent-free reaction.

Development of Dispersion during Compounding and Extrusion of Polypropylene/Graphite Nanoplates Composites

Carbon-based nanoparticles have unique electrical, thermal, barrier and mechanical properties. When incorporated into polymer matrices, the resulting nanocomposites are potentially suitable for a wide scope of advanced applications. In practice, the properties of the nanocomposites are strongly determined by the level of dispersion achieved and by the degree of polymer/particle interfacial bonding. Production and processing of nanocomposites are often carried out in successive thermo-mechanical cycles. These may change the state of nanoparticle dispersion. This work analyzes the evolution of the dispersion of graphite nanoplates (GnP) in a polypropylene matrix during compounding in a co-rotating twin screw extruder and subsequent processing in a single screw extruder, aiming at a better understanding of the kinetics and stability of dispersion. Dispersion was evaluated along the compounding and processing stages and correlated with the composite electrical conductivity, an important engineering property. Two commercial GnP were used as received and chemically modified to graft PP-g-MA (fGnP-PP). Compositions with 2 or 10 wt.% of GnP and fGnP-PP were studied.

Effect of Graphite Nanoplate Morphology on the Dispersion and Physical Properties of Polycarbonate Based Composites

The influence of the morphology of industrial graphite nanoplate (GNP) materials on their dispersion in polycarbonate (PC) is studied. Three GNP morphology types were identified, namely lamellar, fragmented or compact structure. The dispersion evolution of all GNP types in PC is similar with varying melt temperature, screw speed, or mixing time during melt mixing. Increased shear stress reduces the size of GNP primary structures, whereby the GNP aspect ratio decreases. A significant GNP exfoliation to individual or few graphene layers could not be achieved under the selected melt mixing conditions. The resulting GNP macrodispersion depends on the individual GNP morphology, particle sizes and bulk density and is clearly reflected in the composite’s electrical, thermal, mechanical, and gas barrier properties. Based on a comparison with carbon nanotubes (CNT) and carbon black (CB), CNT are recommended in regard to electrical conductivity, whereas, for thermal conductive or gas barrier application, GNP is preferred.

Synergistic effect on static and dynamic mechanical properties of carbon fiber-multiwalled carbon nanotube hybrid polycarbonate composites

The synergistic effect of reinforced non-functionalized and functionalized MWCNTs accompanied by CF is observed on the mechanical properties in a polycarbonate matrix.

Unique method to improve the thermal properties of bisphenol A tetraacrylate by graphite oxide induced space confinement

Improvement of the thermal properties of bisphenol A tetraacrylate by graphite oxide induced space confinement.