Behavior of viscoelastic glass bead suspensions under superposed flows

Parallel superposed flows may be useful in order to explain the rheological behavior of viscoelastic suspensions. It has been shown that relaxation times of molecules are affected by a combined flow, and the rheological behavior of glass bead suspensions in a viscoelastic medium is explaned in terms of such variations.     

Nanocomposites by melt intercalation based on polycaprolactone and organoclay

Nanocomposites based on biodegradable polycaprolactone (PCL) and organically modified layered silicates (organoclay) were prepared by melt mixing. Their structures and properties were characterized by wide‐angle X‐ray diffraction, thermal analysis, and rheological measurements. The exfoliation of the organoclay was achieved via a melt mixing process in an internal mixer and showed a dependence on the type of organic modifier, the organoclay contents, and the processing temperature. The addition of the organoclay to PCL increased the crystallization temperature of PCL, but a high content of the organoclay could show an inverse effect. The PCL/organoclay nanocomposites showed a significant enhancement in their mechanical properties and thermal stability due to the exfoliation of the organoclay. The nanocomposites showed a much higher complex viscosity than the neat PCL and significant shear‐thinning behavior in the low frequency range. The shear storage modulus and loss modulus of the nanocomposites also exhibited less frequency dependence than the pure PCL in the low frequency range, and this was caused by the strong interactions between the organoclay layers and PCL molecules and by the good dispersion of exfoliated organoclay platelets in the PCL. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 670–678, 2003     

Effect of the segmental mobility restriction on the thermoset chemical kinetics

The cure kinetics of an epoxy–amine commercial thermoset system have been investigated with the isothermal differential scanning calorimetry technique. In particular, a kinetic study has been performed in the glass–transition zone, in which diffusion phenomena compete with the chemical transformations and the overall reaction rate is partially slowed by the reduced segmental chain mobility. A generalized form of the Vogel equation has been formulated to account for the effect of the increasing glass–transition temperature on the chain mobility and, therefore, on the overall reaction rate. The kinetic model has been expressed with two factors representing the chemical reaction rate and the segmental mobility reduction. As the main result, the activation energy relative to the diffusion phenomena has been found to be very low, having a value of 42.5 K ≈ 0.356 kJ/mol, which is compatible only with the small‐angle rotation of the reactive unit. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3757–3770, 2002     

A predictive approach based on the Simha–Somcynsky free‐volume theory for the effect of dissolved gas on viscosity and glass transition temperature of polymeric mixtures

The gas concentration and pressure effects on the shear viscosity of molten polymers were modeled by using a unified approach based on a free volume theory. A concentration and pressure dependent “shift factor,” which accounts for free volume changes associated with polymer‐gas mixing and with variation of absolute pressure as well as for dilution effects, has been herein used to scale the pure polymer viscosity, as evaluated at the same temperature and atmospheric pressure. The expression of the free volume of the polymer/gas mixture was obtained by using the Simha and Somcynsky equation of state for multicomponent fluids. Experimental shear viscosity data, obtained for poly(ε‐caprolactone) with nitrogen and carbon dioxide were successfully predicted by using this approach. Good agreement with predictions was also found in the case of viscosity data reported in the literature for polystyrene and poly(dimethylsiloxane) with carbon dioxide. Free volume arguments have also been used to predict the T_g depression for polystyrene/carbon dioxide and for poly(methyl methacrylate)/carbon dioxide mixtures, based on calculations performed, again, with the Simha and Somcynsky theory. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1863–1873, 2006     

Quality control of the cure process of thermosetting resins by means of differential scanning calorimetry

Calorimetric and rheological characterizations of thermosetting resin formulations containing a novalac epoxy resin hardened with 60 to 120 parts per hundred of methylated maleic acid adduct anhydride are described. The calorimetrically determined epoxy conversion has been related to the gelation limits theoretically calculated from Flory's non-linear copolymerization theory and experimentally verified in rheological tests.

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Zusammenfassung Die kalorimetrische und rheologische Charakterisierung der Ansätze von hitzhärtbarem Harz wird beschrieben. Die Ansätze enthalten ein mit 60–120% methyliertem Maleinsäure-Adduktanhydrid gehärtetes Novolackepoxidharz. Die kalorimetrisch bestimmte Epoxid-konversion wurde auf die theoretisch aus der nicht-linearen Flory'schen Kopolymerisationstheorie berechneten und experimentell durch rheologische Tests bestätigten Erstarrungsgrenzen bezogen.

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Financial and scientific support by the CNR Progetto Chimica Fine e Secondaria and the Societa Italiana Resine (SIR) is gratefully acknowledged.     

Spectroscopic Study of the Growth Mechanism of Silver Microclusters

Polymer-protected silver microclusters have been obtained by room-temperature alcoholic reduction of metal ions in presence of a polymeric stabilizer (i.e., poly(N-vinyl-2-pyrrolidone) (PVP)). The method allows one to prepare bulk quantities of the material with a control of size from a molecular level. Spectrophotometric monitoring technique has been employed to investigate the influence of the aging of the protective PVP layer on the microcluster growth rate, under different reaction temperatures and PVP/ethylene glycol weight ratios. The aging time of the polymeric stabilizer solution has been found to play a fundamental role in the reproducibility of the microcluster growth kinetic. In addition, the microcluster growth process has resulted governed by a diffusion-controlled mechanism.     

Effect of Molecular Modification on PCL Foam Formation and Morphology of PCL

Poly(ϵ-caprolactone) was chemically modified by using dicumyl peroxide from 0.25 to 2 % (w/w) and the effects of molecular architecture on the density and morphology of PCL foams were examined. The polymer was first blended with dicumyl peroxide at a low temperature (80°C), to prevent premature peroxide decomposition. The peroxide modification was then performed at different temperatures, from 110°C to 150°C. The reaction kinetic was followed by measuring the dynamical rheological properties of the melt in isothermal experiments by using a parallel plate rheometer. The evolution of the macromolecular structure during the chemical reaction was followed by analyzing the time evolution of the complex viscosity. Foams were prepared from the peroxide modified PCL with a batch foaming process using nitrogen as the foaming agent under different process conditions. As expected, the increase of the molecular modification led to a shift towards higher temperatures of the foaming window and, moreover, influenced the viscoelastic behavior of the expanding polymeric matrix so that the final foam properties are affected.     

Impedance spectroscopy of reactive polymers. 1

Dielectric measurements were utilized to follow the advancement of cure in an epoxy/amine formulation. In contrast to earlier studies, complex impedance was measured during cure and used to calculate ionic resistivity. By using complex impedance were able to separate according to their frequency dependence the contributions to overall polarization from electrode blocking layers, migrating charges, and dipole relaxations. At any stage of cure, there is a unique frequency at which ionic resistivity can be singularly measured. Our approach does not involve trial-and-error frequency search and is conducive to the development of phenomenological models based on equivalent circuits. Excellent agreement was reported between the calculated values of normalized degree of cure obtained by dielectric and calorimetric measurements. © 1994 John Wiley & Sons, Inc.     

Mutual information and correlation measures in atomic systems

Mutual information is introduced as an electron correlation measure and examined for isoelectronic series and neutral atoms. We show that it possesses the required characteristics of a correlation measure and is superior to the behavior of the radial correlation coefficient in the neon series. A local mutual information, and related local quantities, are used to examine the local contributions to Fermi correlation, and to demonstrate and to interpret the intimate relationship between correlation and localization.