Vincenzo Villani

On melt-crystallization of polytetrafluoroethylene and of random fluorinated copolymers of tetrafluoroethylene

Through differential scanning calorimetry, isothermal crystallization from the melt of polytetrafluoroethylene (PTFE) has been investigated. PTFE was regarded as one of the polymers for which crystallization is so rapid that the samples crystallize during the cooling from the melt to the selected crystallization temperature. By contrast, we now report that a stochastic behavior is observed for isothermal melt-crystallization of PTFE. In fact, on cooling very quickly the samples from the molten state to the selected crystallization temperature, crystallization during the cooling is randomly observed. Therefore, repeating the experiments until crystallization on cooling was absent, it was possible to investigate isothermal melt-crystallization of PTFE. However, crystallization is very fast; in fact, crystallization kinetics can be followed just for very low undercoolings, while as the undercooling becomes as large as about 15°C, only secondary crystallization is observed. In both cases, the data have been examined through the well-known Avrami analysis, taking into account the different physical meaning of the obtained parameters. For the first cases (actual crystallization kinetics) very low, noninteger Avrami exponents have been obtained. They have been related to the fractal dimension of the crystallites and their values to the morphological observations on PTFE. For the second cases, the typical low values of Avrami exponents of secondary crystallization are obtained. Moreover, isothermal melt-crystallization of random fluorinated copolymers of tetrafluoroethylene with either hexafluoropropylene or perfluoromethylvinylether as comonomers has been studied and compared with that of PTFE.

Phase Behavior of Blends of Poly(ϵ‐Caprolactone) and a Modified Montmorillonite‐Poly(ϵ‐Caprolactone) Nanocomposite

Blends between high‐molecular weight poly(ϵ‐caprolactone) (PCL) and an intercalated nanocomposite of poly(ϵ‐caprolactone)‐organophilic montmorillonite (OMONT), containing 30% by weight of the nanofiller (NPCL30) have been investigated. Such blends have been shown to have much improved mechanical properties with respect to the parent polymer. The detailed phase behavior, crystallization kinetics, and morphology of neat PCL and of blends containing 2, 9, and 15% by weight of OMONT have been studied by using differential scanning calorimetry (DSC). The thermal properties and the overall crystallization rate of pure PCL are strongly influenced by the presence of the montmorillonite nanoparticles. The study of the solid‐state dynamic‐mechanical properties revealed an enhancement of the storage modulus in particular above Tg and upon increasing the clay content. The enhancement of the modulus can be attributed to confined orientation of the polymer chains among the silicate galleries (intercalation). This is confirmed by the appearance in the sample with the highest clay content, of a second Tg peak at higher temperature indicative of a less mobile amorphous fraction.

Melting and solid phase transitions of isothermally crystallized copolymers of tetrafluoroethylene and hexafluoropropylene

Abstract The melting and the solid-solid transition behaviours of isothermally crystallized samples of FEP copolymers of various compositions are described. The results suggest that a significant contribution to the increases of the melting and transition temperatures, observed upon isothermal crystallization, is due to a reduction of the concentration of the perfluoromethyl groups in the crystallites, with partial segregation of them in the melt. Nevertheless a suitable extrapolation of the experimental melting temperatures leads to a reasonable value for the equilibrium melting temperature of poly(tetrafluoroethylene).

Calorimetric study of the room-temperature transitions : of tetrafluoroethylene-hexafluoropropylene copolymer : thermal history and crystalline state

Abstract Differential scanning calorimetry reveals at least three very clear room-temperature transitions for native tetrafluoroethylene-hexafluoropropylene (TFE-HFP) copolymer, with a composition of 0.3 mol.% HFP, from aqueous dispersion polymerization. The influence of the thermal history on the room-temperature transitions was studied. The enthalpies and the temperatures of each transition were evaluated vs. the annealing temperature. The results are compared with those previously obtained for polytetrafluoroethylene (PTFE). As for the homopolymer, it is suggested that the native copolymer and the copolymer annealed at T ann

PIDOTIMOD, a new immunostimulating dipeptide, studied by molecular mechanics, normal mode analysis and dynamics calculations☆

Abstract A theoretical study of PIDOTIMOD, a new immunostimulating dipeptide, using a combination of molecular mechanics, normal mode analysis and dynamics methods, has been carried out. The results have been compared with the experimental ones obtained using NMR spectroscopy. Conformational calculations and NMR spectra identify two classes of conformers, trans and cis, around the peptide bond between the rings, with relative conformational populations of 0.55 and 0.45, respectively. The best agreement is obtained using a dielectric constant ϵ = 7 in the calculations. The conformational features show that the path connecting the trans and the cis forms may involve the rotation of the bridged peptide bond alone. From a dynamic point of view the conformers appear rather rigid, and only the oxo-prolyl ring and the carboxyl group show significant rigid body-like motions. Interesting correlations occur in the fluctuations of the torsion angles.

Substrate interaction with 5α-reductase enzyme: influence of the 17β-chain chirality in the mechanism of action of 4-azasteroid inhibitors

Abstract A series of steroidal compounds were synthesized in order to evaluate the possible influence of the configuration of a stereocenter in the 17β-side chain on the inhibitory activity on the enzyme 5α-reductase (5AR). For this purpose diastereomerically pure 4-azasteroids epimers at C-22 were prepared (compounds 1–11) and tested as inhibitors of 5AR in ‘in vitro’ tests. The obtained data showed that in most cases the couples of epimers possess a significant difference in their biological activity. We also considered, for the tested molecules, a series of chemico-physical parameters in order to find a possible correlation with their biological activity. The findings allowed us to propose a model of the binding site of 5AR which comprises also, for 4-azasteroid inhibitors, the configurational aspect of the 17β-side chain.

Rheology of Modified Wood by Means of Pure Solvent or Solution Treatments

Viscoelasticity of chemically modified Cedrus deodara by means of pure solvent or solution treatments has been evaluated using dynamic-mechanical analysis in bending mode at 30 °C. The samples are treated up to equilibrium conditions in solvents with different polarity (water, ethanol and acetone), and corresponding solutions with bifunctional chemicals (acyl dichloride and aliphatic-diamine). Then, in all cases the samples are air-dried and temperature equilibrated before to be characterized. Mechanical properties have been explored by means of stress–strain plots at different strain rate, creep and step-strain experiments. Dynamic-mechanical properties have been studied by frequency-strain spectra and E1, E2 and tanδ have been investigated. Outcomes have been compared with those of a previous paper determined for seasoned or thermocured wood. The samples treated with the pure solvents exhibit an improvement of mechanical properties, due to the extraction of low molecular weight compounds acting as plasticizers of the lignin-based matrix: an increase of the elastic modulus, a reduction of the compliance and longer stress-relaxation time are observed. For the samples treated with the solutions of dichloride or diamine, the viscoelasticity behaviour is the resultant of plasticizers extraction and cross-linking of the polymeric matrix by means of the bifunctional reagents.

Molecular dynamics simulations of the conformational behaviour of the synthetic dipeptide PIDOTIMOD, a new immunostimulating agent

Abstract Molecular dynamics (MD) simulations of the conformational behaviour of the conformers of PIDOTIMOD, an interleukin-2 lymphocytes T receptor agonist dipeptide, have been carried out. MD simulation at constant temperature of 300 and 400 K and constant total energy runs at high temperatures have been performed, and trajectories and statistical properties of motions investigated. At 300 K two trans and one cis conformer were found to be dynamically stable. Two puckered forms of the oxo-prolyl ring in rapid equilibrium have been detected. The equilibrium conformations appear rather rigid and poor in H-bonds. At 400 K both trans and cis conformers are thermally mixed. At high temperature large fluctuations of the peptide bond between the oxo-prolyl and thyazolidin rings are observed. Nevertheless in the short time scale trans—cis interconversion does not occur. Suggestions on the nature of the structure—activity relationship are made

Catalysis design: from Ti-bis(enolato-imino) to Ti-bis(enolato-azo) in living olefin polymerization at DFT level

Abstract Reaction mechanisms of both propagation and termination steps in homogeneous polyolefin catalysis are investigated at DFT level. Starting from existent catalysts, we propose the design of a new Ti-bis(enolato-azo) catalytic family. We considered a number of hydrogenated or fluorinated models in aromatic group and chelating ring. Fluorination in the aromatic rings is able to control the propagation steps by activating the metal site, while hydrogenation of the ligand can inhibit the termination transfers. So, we have optimized the structure of the catalyst to give it the highest reactivity towards polymerization and the lowest one towards termination, by means of the meta-F substitutions in the aromatic group, and –CH2CF3 on the chelating ring. Thus, the developed bis(enolato-azo) models seem to be interesting for experimental investigations as new living catalysts.