Daniele Nuvoli

High concentration few-layer graphene sheets obtained by liquid phase exfoliation of graphite in ionic liquid

In the present work, the use of a commercial ionic liquid as a convenient solvent medium for graphite exfoliation in mild and easy conditions without any chemical modification is presented. To confirm the presence of few layer graphene, its dispersion, which exhibits Tyndall effect, was characterized by Raman and UV spectroscopies, and atomic force and field emission electron microscopies. It is noteworthy that, by gravimetric analysis, a graphene concentration as high as 5.33 mg ml−1 was determined, which is the highest value reported so far in any solvent.

Graphene-containing thermoresponsive nanocomposite hydrogels of poly(N-isopropylacrylamide) prepared by frontal polymerization

Frontal polymerization has been successfully used to synthesize poly(N-isopropylacrylamide) nanocomposite hydrogels containing graphene. The latter was directly achieved by ultrasound treatment of a dispersion of graphite in N-methylpyrrolidone. The dispersion, having the concentration of 2.21 g L−1, was characterized by TEM analysis and mixed with suitable amounts of N-isopropylacrylamide for the synthesis of graphene-containing nanocomposite polymer hydrogels. The nanocomposite hydrogels were analyzed by SEM and Raman spectroscopy, and their swelling and rheological properties were investigated. It was found that graphene strongly influences the swelling ratio, dramatically increasing it, even if present in small amounts. Finally, the rheological properties of the hydrogels were correlated with the graphene content: G′ modulus and complex viscosity were found to increase with increasing nanofiller concentration, thus indicating the occurrence of good interactions between the two phases. Nevertheless, at a high concentration (i.e., 0.13 wt.%), graphene showed a lubrication effect, lowering the rheological parameters and approaching the same pseudoplastic behaviour of the unfilled material.

Stimuli Responsive Hydrogels Prepared by Frontal Polymerization

Frontal polymerization was used as an alternative method for the easy and fast preparation of polymer hydrogels prepared from N-isopropylacrylamide (NIPAAm) and N-vinylcaprolactam (VCL), the latter being less toxic and less expensive than NIPAAm. All samples were characterized in terms of their swelling behavior, and their thermal properties were investigated by DSC. It was found that VCL influences both pore size and shape distribution. Moreover, also the swelling ratio of the materials is dependent on the monomer ratio. Eventually, by a comparison with analogous samples prepared by the classical polymerization technique, it was found that the two methods give rise to hydrogels characterized by very diverse swelling capability; furthermore, swelling reversibility was also found to be different when temperature is allowed to cyclically vary between values that are below and above the lower critical solution temperature. In particular, samples prepared by frontal polymerization are characterized by lower swelling ratio and larger swelling recovery capability.

In situ production of high filler content graphene-based polymer nanocomposites by reactive processing

This work deals with the preparation of graphene dispersed in a monomer (tetraethylene glycol diacrylate) and the subsequent polymerization of the latter to the corresponding polymer nanocomposite, which is the first obtained so far by direct polymerization of the graphene-dispersing medium. The method used for its obtainment allows reaching the highest concentration of graphene reported until now in any medium (9.45 mg mL−1); besides, a certain amount of graphene nanoribbons is also well visible. Furthermore, this goal is achieved by directly sonicating graphite without any chemical manipulation, which generally results in a final material still containing a significant number of defects. Because of its obtainment in the monomer itself, no filtration of graphene is needed, thus avoiding the reaggregation process to graphite, which partially compromises any previous exfoliation process. The obtained graphene-based polymer nanocomposites, fully characterized by Raman and transmission electron microscopy, differential scanning calorimetry, thermogravimetry, and dynamic–mechanical thermal analysis, exhibit a very homogeneous distribution of the graphene sheets within the polymer matrix. In addition, the interactions between the polymer and nanofiller are very strong, as evidenced by a significant increase in the Tg values even in the presence of a very low graphene content, together with a strong increase in the mechanical features (flexural and storage moduli). Finally, the thermo-oxidative stability of the polymer matrix is not affected by the presence of graphene nanosheets.

The production of concentrated dispersions of few-layer graphene by the direct exfoliation of graphite in organosilanes

We report the formation and characterization of graphene dispersions in two organosilanes, 3-glycidoxypropyl trimethoxysilane (GPTMS) and phenyl triethoxysilane (PhTES) as new reactive solvents. The preparation method was mild and easy and does not produce any chemical modification. The dispersions, which exhibit the Tyndall effect, were characterized by TEM and Raman spectroscopy to confirm the presence of few-layer graphene. Concentrations as high as 0.66 and 8.00 mg/ml were found for PhTES and GPTMS, respectively. The latter is one of the highest values reported for a dispersion of graphene obtained by any method. This finding paves the way for the direct synthesis of polymer nanofiller-containing composites consisting of graphene and reactive silanes to be used in sol–gel synthesis, without any need for solvent removal, thus preventing graphene reaggregation to form graphite flakes.

Synthesis and Characterization of Functionally Gradient Materials Obtained by Frontal Polymerization

Functionally gradient materials (FGMs) with gradual and continuous changes of their properties in one or more dimensions are useful in a wide range of applications. However, obtaining such materials with accurate control of the gradient, especially when the gradient is nonlinear, is not easy. In this work, frontal polymerization (FP) was exploited to synthesize polymeric FGMs. We demonstrated that the use of ascending FP with continuous feeding of monomers with computer-controlled peristaltic pumps provided an excellent method for the preparation of functionally gradient materials with programmed gradients. To test the effectiveness of the method, copolymers made from triethylene glycol dimethacrylate/hexyl methacrylate with linear and hyperbolic gradient in composition were synthesized. Differential scanning calorimetry (DSC), Shore A hardness measurements, compression tests, and swelling studies were performed along the length of the materials to assess the relationship between the gradients and the material properties. Glass transition temperatures, determined by DSC, showed a linear dependence on the composition and were in agreement with theoretical values. The other properties showed different and specific behaviors as a function of the compositional gradient.

Synthesis and characterization of poly(2-hydroxyethylacrylate)/β-cyclodextrin hydrogels obtained by frontal polymerization.

For the first time, the synthesis of polymeric hydrogels containing cyclodextrins (CDs) obtained by frontal polymerization (FP) is reported. In particular, the effects of CDs on poly(2-hydroxyethylacrylate) hydrogel properties are investigated. In a first series of materials, β-cyclodextrin is dispersed into the polymer matrix, while in the second one acryloyl-β-cyclodextrin is grafted to poly(2-hydroxyethylacrylate) chains. FP parameters (front velocity and maximum temperature), swelling properties, glass transition temperatures and mechanical properties of the hydrogels are studied. Results show that both types of cyclodextrin influence the above properties, and the major effects are found for concentration higher than 1mol% of acryloyl-β-cyclodextrin. Namely, a significant increase of glass transition temperature and of compression moduli are found. Finally, this study demonstrates that FP is a convenient technique to obtain CD-containing hydrogels, in which the type and amount of cyclodextrin can be suitably modulated to tune polymer properties, in function of the desired hydrogel applications.

β-Cyclodextrin-based supramolecular poly(N-isopropylacrylamide) hydrogels prepared by frontal polymerization

Frontal polymerization (FP) was successfully applied to the synthesis of poly(N-isopropylacrylamide)-grafted-acryloyl-β-cyclodextrin supramolecularly crosslinked hydrogels. It was established that acryloyl-β-cyclodextrin (AβCD) allowed performing successful frontal polymerizations with N-isopropylacrylamide even in the absence of any covalent crosslinker, which is something generally required. It was found that the swelling properties of the resulting hydrogels can be tuned by varying the amount of AβCD. Namely, when little amounts of this non-covalent crosslinker were used, superabsorbent hydrogels were obtained. Hydrogels containing also a covalent crosslinker were also prepared for comparison. These latter exhibited swelling ratios that are much lower than the others.

Multistimuli-responsive hydrogels of poly(2-acrylamido-2-methyl-1-propanesulfonic acid) containing graphene

Nanocomposite hydrogels of poly(2-acrylamido-2-methyl-1-propanesulfonic acid) containing graphene were prepared by radical polymerization. Their swelling properties in response to ionic strength and electrical stimuli were assessed. Graphene was obtained through an easy and convenient method lately developed by our research group, which consists in the exfoliation of graphite by sonicating it in a proper solvent medium. It was found that the graphene content influences the swelling properties of hydrogels; in particular, those containing graphene swell more than the filler-free ones; graphene content influences also the swelling ratio variation between the swollen and deswollen states.

Study of polymeric nanocomposites prepared by inserting graphene and / or Ag, Au and ZnO nanoparticles in a TEGDA polymer matrix, by means of the use of dielectric spectroscopy

Polymericnanocomposites were prepared by inserting graphene and/or Ag,Au and ZnOnanoparticles in a TEGDA (tetraethyleneglycol diacrylate) polymer matrix. The polymeric films were characterized in terms of their dielectric properties by electrochemical impedance spectroscopy. The impedance data were fitted by generalized relaxation functions in order to determine conductivity, dielectric response and molecular relaxation time of the nanocompositefilms. In particular, a stretched exponential function, Kohlrausch-Williams-Watts function (KWW), was used to investigate polymer/graphene/metal nanocomposites.