Giovanni Filippone

Viscoelasticity and Structure of Polystyrene/Fumed Silica Nanocomposites: Filler Network and Hydrodynamic Contributions

We investigate the relationships between structure and linear viscoelasticity of a model polymer nanocomposite system based on a mixture of fumed silica nanoparticles and polystyrene. Alterations in the viscoelastic behavior are attributed to the structuring of primary silica aggregates. Above a critical filler volume fraction, a space-filling network builds up as the result of cluster aggregation, and the complex frequency-dependence of the moduli is simplified by splitting the viscoelasticity of the composites into the independent responses of the suspending polymer melt and the filler network. Specifically, we present a refinement of a two-component model recently proposed for attractive colloidal suspensions, in which hydrodynamic effects related to the presence of the filler are properly taken into account using the concept of shear stress equivalent deformation. Our approach, validated through the building of a master curve of the elastic modulus for samples of different composition, allows the estimation of the elasticity of samples in which the filler network is too tenuous to be appreciated through a simple frequency scan. In addition, the structure of the filler network is studied using both the percolation and fractal approaches, and the reliability of the critical parameters is discussed. We expect that our analysis may be useful for understanding the behavior of a wide variety of complex fluids where the elasticity of the components may be superimposed.

Universal features of the melt elasticity of interacting polymer nanocomposites.

We study the structure and linear viscoelasticity of interacting polymer nanocomposites based on mixtures of poly(ethylene oxide) and fumed silica particles. The filler is dispersed within the polymer using two different techniques which lead to different dispersion states. The analysis of the dynamic response of our systems highlights the formation of a stress-bearing network above a critical volume fraction, Φ(c). Extending a two-phase model used to describe weakly interacting systems, we show that above Φ(c) the melt-state elasticity of the composites arises from the independent contributions of a polymer-particle network and a viscous matrix. We also find that, although Φ(c) depends on the initial state of dispersion, the network elasticity scales with volume fraction following a universal power-law, with an exponent ν ≈ 1.8. Such a scaling law has been recently predicted for the stress-bearing mechanism governed by polymer-mediated interactions.

Chitosan hydrogels embedding hyper-crosslinked polymer particles as reusable broad-spectrum adsorbents for dye removal

The removal of dye and toxic ionic pollutants from water is an extremely important issue that requires systematic and efficient adsorbent preparation strategies. To address this challenge, we developed composite chitosan (CS)-based hydrogels containing hyper-crosslinked polymer (HCP) particles to be used as broad-spectrum adsorbents. The goal is to efficiently combine the dye adsorption ability of chitosan and the capacity of the porous particles of trapping pollutant molecules. The HCP particles are well distributed and firmly embedded into the chitosan matrix and the composite hydrogels exhibit improved mechanical properties. Adsorption experiments reveal a synergistic effect between CS and HCP particles, and the samples are able to remove both anionic and cationic dyes (indigo carmine, rhodamine 6G and sunset yellow) from water. The maximum dye uptake is higher than that of comparable biosorbents. Moreover, the mechanical properties of the composite hydrogels are enhanced respect to pure CS, and the samples can be regenerated and reused keeping their adsorption ability unaltered over successive cycles of adsorption, desorption, and washing.

Functionalization of aliphatic polyesters by nitroxide radical coupling

Functionalized poly(butylene succinate) (PBS) samples were prepared by a post-polymerization method based on the coupling reaction between TEMPO derivatives bearing different functionalities and PBS macroradicals generated by H-abstraction using a peroxide. 4-Benzoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (BzO-TEMPO) and 4-(1-naphthoate)-2,2,6,6-tetramethylpiperidine-1-oxyl (NfO-TEMPO), a pro-fluorescent nitroxide, were successfully grafted on PBS, as revealed by MALDI TOF MS and UV-Vis spectroscopy. The functionalization degrees were accurately determined by UV-Vis analysis and confirmed by 1H-NMR spectroscopy. The grafting site was identified by combining theoretical calculations with experimental evidence. This evidence was collected by both EPR analysis of a functionalized sample subjected to controlled heating in the EPR cavity, and by 1H-NMR spectroscopy. Our functionalization method, which was also tested for poly(lactic acid) (PLA), preserves the original polymer structure. This avoids the crosslinking-branching side reaction, which generally affects the free radical treatment of biodegradable aliphatic polyesters. In addition, using a pro-fluorescent nitroxide to form functionalized samples is a significant step towards unambiguously demonstrating the radical grafting on these types of polymer. It also proves that well-defined fluorescently labeled biodegradable polyesters can be tailored.

Using matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry for the characterization of functionalized carbon nanotubes

RATIONALE Functionalization of carbon nanotubes (CNTs) generates complex systems that require the development of suitable characterization protocols. New techniques have been explored, and existing analytical and spectroscopic methods to characterize functionalized CNTs have been adapted. Presently, chemical characterization of functionalized CNTs (f-CNTs) remains a difficult task. METHODS Matrix-assisted laser desorption/ionization (MALDI) analysis is performed on f-MWCNT samples prepared via grafting or absorption of anti-oxidant (AO) molecules on both MWCNT-COOH and MWCNT-OH. Covalently functionalized MWCNTs were subjected to thermal degradation and/or hydrolysis reaction before analysis, whereas MWCNTs with a physical adsorption of the functionalizing molecules were directly spotted in the target sample. Noteworthy, in our approach f-MWCNTs constitute at the same time analyte and MALDI matrix. RESULTS The identification of functionalizing AO molecules is ascertained after degradation or hydrolysis reactions in both MWCNT-COOH and MWCNT-OH grafted samples. Absorbed AO molecules, as well as organic impurities derived from grafting reactions, are also revealed by MALDI MS without any preliminary cleavage reaction. CONCLUSIONS A simple MALDI-TOF mass spectrometry method permits to obtain the unambiguous discrimination between grafted or adsorbed functionalized molecules onto the surface of MWCNTs.

Immobilization of natural anti-oxidants on carbon nanotubes and aging behavior of ultra-high molecular weight polyethylene-based nanocomposites

The use of natural antioxidants is an attractive way to formulate nanocomposites with extended durability and with potential applications in bio-medical field. In this work, Vitamin E (VE) in the form of α-tocopherol and Quercetin (Q) are physically immobilized on the outer surface of multi-walled carbon nanotubes (CNTs). Afterward, the CNTs-VE and CNTs-Q are used to formulate thermally stable ultra high molecular weight polyethylene based nanocomposites. The obtained results in the study of the thermo-oxidation behavior suggest a beneficial effect of the natural anti-oxidant carbon nanotubes systems. The unexpected excellent thermo-resistance of the nanocomposites seems to be due to a synergistic effect of the natural anti-oxidant and carbon nanotubes, i.e. strong interaction between CNT surface and anti-oxidant molecules. Particularly, these interactions cause the formation of structural defects onto outer CNT surfaces, which, in turn, increase the CNT radical scavenging activity.

ELASTICITY AND DYNAMICS OF PARTICLE GELS IN NON‐NEWTONIAN MELTS

We investigate the relation between structure and viscoelasticity of model polymer nanocomposite systems based on a mixture of spherical nanoparticles and different polymer matrices. These composites exhibit a strong time‐dependence of the linear elastic and viscous moduli for filler volume fractions above a critical threshold. Despite the complexity of the rheological response, we can scale the viscoelastic properties of the hybrids by splitting their elasticity and dynamics into the independent responses of the suspending polymer melt and that of an elastic particle network. We show that the elasticity of the particle network exhibits critical behavior at the percolation threshold. Our analysis is expected to be useful for understanding the behavior of other complex fluids where the elasticity of the components may be superimposed.

Insight on mendable resin made by combining Diels-Alder epoxy adducts with DGEBA

Formation of micro-cracks is a critical problem in polymers and polymer composites during their service in structural applications. In this context, materials endowed with self-healing features would lead to the next polymers generation. In the present paper, an epoxy system integrating Diels-Alder epoxy adducts is investigated by thermal and spectroscopic analysis. The direct and retro D-A reaction have been studied by FTIR and specific absorption bands have been identified. Finally, mechanical tests have been performed on the system. The polymer is able to heal fracture and micro-cracks recovering its stiffness after a thermal treatment.

Study of the morphology and texture of poly(ε-caprolactone)/polyethylene oxide blend films as a function of composition and the addition of nanofillers with different functionalities

Films of a blend of semi-crystalline polymers, namely poly(e-caprolactone) (PCL) and poly(ethylene oxide) (PEO), are prepared by casting drops of ethyl lactate (EL) polymer solutions onto a glass substrate. The goal of this work is to assess how the surface structure of the blend films can be controlled by: (i) varying the relative amounts of the polymeric components, and (ii) adding inorganic nanoparticles (NPs) with different functionality. Specifically, four types of NPs are used here: bare and silanized titanium dioxide, hydroxyapatite and aluminum–magnesium layered double hydroxide. All the films exhibit a segregated surface morphology, characterized by either PEO-rich domains dispersed in a PCL continuous phase or discrete PCL domains embedded in a PEO-rich phase, depending on the composition of the blend. Phase inversion occurs within the 30–40% range of PEO weight fraction. The incorporation of NPs to the starting polymeric solution is found to significantly affect the final blend morphology, leading to either a coarsening or a refinement of the polymer phases mainly according to the chemical affinity between the NPs and the suspending medium.

Nanoparticle Dynamics in Polymer Melts

Adding solid particles to polymeric materials is a common way to reduce the costs and to impart desired mechanical and transport properties. This makes polymers potential substitutes for more expensive non-polymeric materials. The advantages of filled polymers are normally offset by the increased complexity in the rheological behaviour of the resulting composite. Usually, a compromise has to be made between the benefits ensured by the filler, the increased difficulties in melt processing, the problems in achieving a uniform dispersion of the solid particulate, and the economics of the process due to the added step of compounding [Shenoy, 1999]. Filled polymers can be described as a suspension of particles and particle aggregates dispersed in the polymer matrix. Interactions between individual particles or aggregates and the matrix, as well as between particles, hinder the material deformability modifying both the solidand melt-state behaviour of the host polymer. In polymer-based microcomposites, these effects only become significant at relatively high filler contents, i.e. when the filler particles are sufficiently close to each other to form a network that spans large sections of the polymer matrix. Over the last fifteen years, the same reinforcing and thixotropic effects have been observed with the use of very small amounts of inorganic nanoparticles, which has resulted in extensive research in the field of polymer-based nanocomposites (PNCs) [Usuki et al., 1993; Kojima et al., 1993]. In order to fully understand the exceptional properties of PNCs, the morphological and structural implications stemming from the nanometric sizes of the filler have to be taken into account. With respect to traditional microcomposites, nanocomposites show very high specific interface area, typically of order of ~102 m2 g-1. The matrix properties are significantly affected in the vicinity of the reinforcement, varying continuously from the interface towards the bulk polymer. As a consequence, the large amount of reinforcement surface area means that a relatively small amount of nanoscale reinforcement can have remarkable effects on the macroscale properties of the composite material.