Liberata Guadagno

Chemical and morphologial modifications of irradiated linear low density polyethylene (LLDPE)

Films of linear low density polyethylene (LLDPE) produced with the spherilene technology were exposed to accelerated weathering through UV irradiation at 60°C for increasing times. A different series of films were only annealed at 60°C, to differentiate the thermal effects from those due to the UV irradiation. FT–IR analysis was used to investigate the change of the chemical structure. It was found that, following the initial production of hydroperoxides, the degradation is dominated by the formation of carbonyl and vinyl species. This process proceeds slowly up to 150 h of irradiation and afterwards it accelerates, completely degrading the material. The samples exposed more than 150 h are mechanically very fragile and soon fail. Structural analysis of the irradiated samples showed that there is a perfection of the elementary cell of polyethylene, principally along the a axis, and a consistent increase of crystallinity. The first effect was found also in thermally treated samples, whereas the second is more consistent in the irradiated samples. Also the dimension of the crystals increase much more for the irradiated samples than for the annealed ones. All the effects are mostly evident for samples after 150 h of irradiation, that is when the degradation is accelerated. The chain scission due to the photo-oxidation makes the amorphous chains more mobile and free for further crystallization, and this happens mainly when the degradation occurs simultaneously in many chains.

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.

Influence of the crystallinity on the transport properties of isotactic polypropylene

Samples of polypropylene of varying crystallinity were obtained by blending isotactic with atactic polypropylene, and the crystallinity determined by X-ray diffraction and differential scanning calorimetry. Crystallinity ranged between 20 and 75%. The transport properties of dichloromethane were analysed varying the activity of the vapour. We observed that the sorption decreases, as the crystallinity increases, proportionally to the decrease of the amorphous fraction. As matter of the fact, the specific sorption, normalized by the amorphous fraction, does not depend on the crystallinity. At variance, a simple correlation between the thermodynamic diffusion coefficient and the crystallinity was not obtained; at low values of this parameter, up to 40%, the zero concentration diffusion coefficient is independent of it. A sharp transition separates a range of crystallinities, in which the diffusion parameter decreases, increasing the crystallinity, due to the tortuosity of the path, and shows that the presence of the impermeable crystals is important only for values higher than 50%.

Mechanical and transport properties of irradiated linear low density polyethylene (LLDPE)

Abstract Films of a new linear low density polyethylene (LLDPE) were exposed to accelerated weathering in a UV chamber at 60°C for increasing times. The changes in the mechanical properties, due to the degradation were followed by determining the elastic modulus ( E ), the stress at the yield point ( σ y ), the post yield stress drop (PYSD), the stress at the breaking point ( σ b ), the strain at the breaking point ( ϵ b ), and the toughness ( t ). They were correlated to the exposure time and to the carbonyl index, previously determined on the same samples. All the mechanical parameters were found to be very sensitive to the chemical degradation and to the morphological changes. Furthermore the curves of the mechanical parameters as a function of the exposure time allowed the determination of the point beyond which the material becomes useless. This point corresponds to 150 h of irradiation and to a carbonyl index of 0.3. The transport properties, diffusion and sorption, of a non polar molecule, n -pentane and a more polar molecule, dichloromethane were also correlated to the changes in the molecular structure.

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.

Carbon‐Supported Copper Nanomaterials: Recyclable Catalysts for Huisgen [3+2] Cycloaddition Reactions

Highly disperse copper nanoparticles immobilized on carbon nanomaterials (CNMs; graphene/carbon nanotubes) were prepared and used as a recyclable and reusable catalyst to achieve Cu(I) -catalyzed [3+2] cycloaddition click chemistry. Carbon nanomaterials with immobilized N-heterocyclic carbene (NHC)-Cu complexes prepared from an imidazolium-based carbene and Cu(I) show excellent stability including high efficiency at low catalyst loading. The catalytic performance evaluated in solution and in bulk shows that both types of Cu-CNMs can function as an effective recyclable catalysts (more than 10 cycles) for click reactions without decomposition and the use of external additives.

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.

Electrical properties and memory effects of field-effect transistors from networks of single- and double-walled carbon nanotubes

We study field-effect transistors made of single- and double-walled carbon nanotube networks for applications as memory devices. The transfer characteristics of the transistors exhibit a reproducible hysteresis which enables their use as nano-sized memory cells with operations faster than 10 ms, endurance longer than 10(+4) cycles and charge retention of a few hours in air. We propose water enhanced charge trapping at the SiO(2)/air interface close to the nanotubes as the dominant mechanism for charge storage. We show that charge storage can be improved by limiting exposure of the device to air.