Marialuigia Raimondo

The role of carbon nanofiber defects on the electrical and mechanical properties of CNF-based resins

Heat treatment of carbon nanofibers has proven to be an effective method in removing defects from carbon nanofibers, causing a strong increase in their structural perfection and thermal stability. It affects the bonding states of carbon atoms in the nanofiber structure and causes a significant transformation in the hybridization state of the bonded carbon atoms.Nanofilled resins made of heat-treated CNF show significant increases in their electrical conductivity even at low concentrations. This confirms that enhancement in the perfection of the fiber structure with consequent change in the morphological features plays a prominent role in affecting the electrical properties. Indeed heat-treated CNFs display a stiff structure and a smooth surface which tends to lower the thickness of the unavoidable insulating epoxy layer formed around the CNF which, in turn, plays a fundamental role in the electrical transport properties along the conducting clusters. This might be very beneficial in terms of electrical conductivity but might have negligible effect on the mechanical properties.

Effect of incorporation of POSS compounds and phosphorous hardeners on thermal and fire resistance of nanofilled aeronautic resins

The aim of this work is the identification of the best strategy for improving thermal, fire resistance and electrical conductivity of an epoxy resin for aeronautic applications. The effect of DodecaPhenyl POSS (DPHPOSS), Epoxycyclohexyl POSS (ECPOSS), Glycidyl POSS (GPOSS) and TriglycidylCyclohexyl POSS (TCPOSS) to act as flame retardants of the resin was evaluated. Flame retardancy tested by the limiting oxygen index (LOI) indicated that GPOSS has meaningful effects on the flame retardancy of the epoxy mixture. The incorporation of 5 wt% of GPOSS into the epoxy matrix resulted in a LOI value of 33 with respect to 27 of the pure epoxy mixture. The trend observed by LOI tests was confirmed by mass loss calorimetry measurements: a decrease from 540 kW m−2 down to 327 kW m−2 was observed in the peak of heat release rate (PHRR). LOI and PHRR values were compared with those obtained for the same resin replacing the 4,4′-diaminodiphenyl sulfone (DDS) with the bis(3-aminophenyl) phenylphosphineoxide (BAPPO) and the bis(3-aminophenyl) methyl phosphine oxide (BAMPO). BAMPO and BAPPO proved to be more effective than POSS compounds to increase LOI values. Carbon nanotubes (CNTs), embedded inside the epoxy resin to enhance electrical conductivity, are found to affect significantly fire properties of epoxy systems mainly by preventing the epoxy systems from forming intumescent charring.

Optimization of graphene-based materials outperforming host epoxy matrices

The degree of graphite exfoliation and edge-carboxylated layers can be controlled and balanced to design lightweight materials characterized by both low electrical percolation thresholds (EPT) and improved mechanical properties. So far, this challenging task has been undoubtedly very hard to achieve. The results presented in this paper highlight the effect of exfoliation degree and the role of edge-carboxylated graphite layers to give self-assembled structures embedded in the polymeric matrix. Graphene layers inside the matrix may serve as building blocks of complex systems that could outperform the host matrix. Improvements in electrical percolation and mechanical performance have been obtained by a synergic effect due to finely balancing the degree of exfoliation and the chemistry of graphene edges which favors the interfacial interaction between polymer and carbon layers. In particular, for epoxy-based resins including two partially exfoliated graphite samples, differing essentially in the content of carboxylated groups, the percolation threshold reduces from 3 wt% down to 0.3 wt%, as the carboxylated group content increases up to 10 wt%. Edge-carboxylated nanosheets also increase the nanofiller/epoxy matrix interaction, determining a relevant reinforcement in the elastic modulus.

Effective formulation and processing of nanofilled carbon fiber reinforced composites

This work describes a successful approach toward the development of a carbon fiber-reinforced composite based on an optimized nanofilled resin for industrial applications. The epoxy matrix is prepared by mixing a tetrafunctional epoxy precursor with a reactive diluent which allows reduction of the viscosity of the epoxy precursor and facilitation of the dispersion of 0.5% wt multiwall carbon nanotubes. The proper choice of the viscosity value and the infusion technique allow improvement of the electrical properties of the panels. The obtained in-plane electrical conductivity is about 20 kS m � 1 , whereas a value of 3.9 S m � 1 is achieved for the out of plane value. Such results confirm that the fibers govern the conduction mechanisms in the direction parallel to the fibers, whereas the percolating path created by the effective distribution of carbon nanotubes achieved by resin formulation and adopted processing approach lead to a significant enhancement of the overall electrical performance of the composites.

Healing efficiency and dynamic mechanical properties of self-healing epoxy systems

Several systems to develop self-repairing epoxy resins have recently been formulated. In this paper the effect of matrix nature and curing cycle on the healing efficiency and dynamic mechanical properties of self-healing epoxy resins were investigated. We discuss several aspects by transferring self-healing systems from the laboratory scale to real applications in the aeronautic field, such as the possibility to choose systems with increased glass transition temperature, high storage modulus and high values in the healing functionality under real working conditions.

Processing, thermal stability and morphology of chiral sensing syndiotactic polystyrene films

Processing conditions for the specific δ-form syndiotactic polystyrene (s-PS) films, being suitable to detect amplify and memorize the chirality of several volatile non-racemic organic guests, have been investigated. The occurrence of induced circular dichroism (ICD) phenomena is critically dependent not only on the presence of the nanoporous δ phase but also on the solvent and process used for the film preparation. The dependence of the ICD phenomena on the guest enantiomeric excess has also been studied. The observed ICD phenomena, and hence the memory of the non-racemic guest molecules, remain stable not only after chiral-guest removal but also after thermal treatments up to the polymer melting temperature (≈270 °C). In particular, the maintenance of intense ICD phenomena also after the thermal transition toward the trans-planar α crystalline phase indicates that the chiral memory is not associated with some molecular structure but with the formation of non-racemic supramolecular structures. These supramolecular structures could be possibly associated with the large rod-like crystallites observed only for δ-form s-PS chiral-sensing films (by atomic force microscopy). In fact, these crystallites are clearly re-crystallized, as a consequence of guest sorption, and then remain essentially unaltered for thermal treatments, at least up to 240 °C.

Influence of carbon nanoparticles/epoxy matrix interaction on mechanical, electrical and transport properties of structural advanced materials

The focus of this study is to design new nano-modified epoxy formulations using carbon nanofillers, such as carbon nanotubes, carbon nanofibers and graphene-based nanoparticles (CpEG), that reduce the moisture content and provide additional functional performance. The chemical structure of epoxy mixture, using a non-stoichiometric amount of hardener, exhibits unique properties in regard to the water sorption for which the equilibrium concentration of water (C eq) is reduced up to a maximum of 30%. This result, which is very relevant for several industrial applications (aeronautical, shipbuilding industries, wind turbine blades, etc), is due to a strong reduction of the polar groups and/or sites responsible to bond water molecules. All nanofillers are responsible of a second phase at lower glass transition temperature (Tg). Compared with other carbon nanofillers, functionalized graphene-based nanoparticles exhibit the best performance in the multifunctionality. The lowest moisture content, the high performance in the mechanical properties, the low electrical percolation threshold (EPT) have been all ascribed to particular arrangements of the functionalized graphene sheets embedded in the polymeric matrix. Exfoliation degree and edge carboxylated groups are responsible of self-assembled architectures which entrap part of the resin fraction hindering the interaction of water molecules with the polar sites of the resin, also favouring the EPT paths and the attractive/covalent interactions with the matrix.

Graphene-based structural adhesive to enhance adhesion performance

This paper proposes the design of a new graphene nano-modified formulation to enhance the mechanical performance of structural adhesives. Well-characterized graphene platelets, produced through an effective approach for bulk production and morphology control, were embedded at different contents inside an epoxy adhesive based on tetraglycidylmethylene dianiline (TGMDA). The adhesive formulations were used to manufacture bonded joints, according to ASTM 2095, to analyze the effect of graphene platelets on the tensile strength of the joints. The effect of incorporating graphene in the adherents was also considered. Epoxy adhesives filled with graphene at a concentration of 1 wt% significantly enhanced the mechanical behavior of the bonded joints. Only in the case of unfilled adherents, the inclusion of 4 wt% graphene did not have a significant effect on the mechanical performance. This is likely due to the agglomeration of nanofillers causing heterogeneity in large domains at the interface between adherents and adhesives. The effect of graphene incorporation in the adherents, acting on the chemical compatibility between adhesives and adherent surfaces, led to a considerable increase in tensile strength in comparison with the corresponding joints with unfilled adherents. This beneficial effect is most probably due to the cumulative effects of intermolecular interactions between the graphene platelets and resin networks.

Healing agent for the activation of self-healing function at low temperature

The development of smart composites capable of self-repair in aeronautical structures is still at the planning stage owing to complex issues to overcome. It is critical that self-healing activity functions at low working temperatures which can reach values as low as −50 °C. Also, another problem concerns the components’ stability of the proposed composites which are compromised at the cure temperatures necessary for good performance of the composite. Here, we show a multifunctional autonomically healing composite with self-healing function capable at very low temperatures (−50 °C). The self-repair function in this self-healing system is based on the metathesis polymerization of 5-ethylidene-2-norbornene/dicyclopentadiene (ENB/DCPD) blend activated by Hoveyda-Grubbs’1st generation (HG1) catalyst, dispersed at molecular level in the matrix. The formulated material shows a self-healing efficiency of about 72%.

Influence of carbon nanofillers on the curing kinetics of epoxy-amine resin

The cure kinetics of an epoxy resin based on the tetrafunctional epoxy precursor N,N′-tetraglycidyl methylene dianiline-(TGMDA) hardened with 4,4-diaminodiphenyl sulfone is investigated. The influence of carbon nanofillers (carbon nanotubes, carbon nanofibers, and graphene based nanoparticles) on the cure kinetic is studied. Kinetic analysis is performed by dynamic and isothermal differential scanning calorimetry (DSC). In dynamic experiments, the activation energy was computed using an advanced isoconversional method while under isothermal conditions, the Kamal’s model of diffusion control was applied to simulate the systems throughout the curing process. The isothermal analysis shows that the introduction of the diluent decreases, particularly the activation energy of secondary amine-epoxy reaction. A similar effect was obtained by the dynamic DSC analysis that shows a decrease in the activation energy for α > 0.7, a value of conversion for which it is considered that the reaction of secondary amines is active. The inclusion in the resin of one-dimensional fillers does not lead to big differences in the curing kinetics behaviour with respect to the raw epoxy. An increase in the activation energy is found in the case of highly exfoliated graphite. This is likely due to a reduction of free molecular segments of the epoxy network trapped inside the self-assembly structures.