1,721,011 research outputs found
Viscoelasticity of Nanobubble-Inflated Ultrathin Polymer Films: Justification by the Coupling Model
No full textThe nanobubble inflation method is the only experimental technique that can measure the viscoelastic creep compliance of unsupported ultrathin films of polymers over the glassrubber transition zone as well as the dependence of the glass transition temperature (Tg) on film thickness. Sizeable reduction of Tg was observed in polystyrene (PS) and bisphenol A polycarbonate by the shift of the creep compliance to shorter times. The dependence of Tg on film thickness is consistent with the published data of free-standing PS ultrathin films. However, accompanying the shift of the compliance to shorter times, a decrease in the rubbery plateau compliance is observed. The decrease becomes more dramatic in thinner films and at lower temperatures. This anomalous viscoelastic behavior was also observed in poly(vinyl acetate) and poly (n-butyl methacrylate), but with large variation in the change of either the Tg or the plateau compliance. By now, well established in bulk polymers is the presence of three different viscoelastic mechanisms in the glassrubber transition zone, namely, the Rouse modes, the sub-Rouse modes, and the segmental a-relaxation. Based on the thermorheological complexity of the three mechanisms, the viscoelastic anomaly observed in ultrathin polymer films and its dependence on chemical structure are explained in the framework of the Coupling Mode
Polarization fluctuations near the glass transition
No full textMeasurements of polarization fluctuations were performed for an epoxy glass former. The voltage noise produced by polarization fluctuations of the sample filling a capacitor was acquired via a home-made very high input impedance current-voltage converter, in series with an ultra low noise pre-amplifier, achieving high sensitivity and accuracy in the range 0.1-1000 Hz. The temperature and frequency dependence of polarization noise was investigated above and below the glass transition temperature T-g. The sample was driven to the glassy state with different cooling rate and then isothermally aged, while the noise spectral density was measured at different times and compared with the Johnson Nyquist noise determined by the sample impedance measured by conventional dielectric spectroscopy. At thermodynamic equilibrium the polarization noise agreed with the predictions of the fluctuation-dissipation theorem linking noise spectral density to susceptibility. On the contrary, a strong violation of the theorem was observed after a fast cooling below Tg: an intense polarization noise was detected, with a power spectral density following an inverse power law frequency behavior, whose intensity was decreasing with aging time. At the same time, the amplitude of polarization fluctuations showed a non-Gaussian distribution, whose width reduced during the aging process, up to recover the Gaussian statistics on approaching the equilibrium. (c) 2006 Elsevier B.V. All rights reserved
The Johari-Goldstein beta-relaxation of glass-forming binary mixtures RID A-8503-2012
No full textThe present paper shows, by means of broadband dielectric measurements, that the primary alpha- and the secondary Johari-Goldstein (JG) beta-processes in binary mixtures are strongly correlated. This occurs for different polar rigid probes dissolved in apolar glass-forming solvents, over a wide temperature and pressure range. reserved. We found that the coupling parameter n = 1 - beta(KWW) and the ratio between alpha- and beta-relaxation time reduce on increasing the size of the solute solved within the same apolar matrix. Moreover, such a ratio is invariant when calculated at different combinations of P and T maintaining either the primary or the JG relaxation times constant. Dielectric spectra measured at different T-P combinations but with an invariant alpha-relaxation time are well superposed in both the alpha- and beta-frequency ranges. Experimental results can be rationalized by Coupling Model equation. (C) 2010 Elsevier B.V. All rights reserved
Pressure and temperature dependence of structural relaxation dynamics in polymers: a thermodynamic interpretation
No full textWe analyse the slowing down of the structural relaxation dynamics of polymers in terms of the Adam and Gibbs theory. We consider a previously derived general relation between the configurational and the excess entropy, which was used to derive an analytical equation for the dependence of the structural relaxation time from the pressure and temperature. The model proved to successfully fit the relaxation dynamics of poly(methyl methacrylate), poly(propylene glycol) and poly(propylene glycol dimethylether), of different molecular weights, over a wide region of temperature and pressure values above the glass transition
Effect of the isobaric and isothermal reductions in excess and configurational entropies on glass-forming dynamics
No full textDielectric relaxation times tau, over a broad range of temperatures and pressures for simple molecular glass-forming systems reveal good agreement with the pressure-extended Adam-Gibbs equation derived from the Adam Gibbs model, relating directly the dynamics to the thermodynamics through the configurational entropy S-c, assuming it to be proportional to the excess entropy S-exc of the melt with respect to the crystal. In the present study by making use of both calorimetric and expansivity measurements it was possible to make an accurate determination of the pressure and temperature dependences of S-exc over the whole investigated range, and the expected proportionality between Sc and Sexc was experimentally checked. Results show that different proportionality factors g(P) and g(T) exist between S-c and S-exc in isothermal and isobaric conditions and the ratio g(T)/g(P) of these is nearly 0.7 for all the systems
Applications of the rheo-dielectric technique
No full textAn apparatus for dynamical mechanic measurements was modified in order to allow broadband dielectric spectroscopy measurements (10 mHz to 10 MHz) on systems under shear stress. Shear oscillatory deformation was applied to different systems under conditions of controlled deformation amplitude, choosing different shear frequencies and amplitudes, including the range of the non-linear viscoelastic response. The plate-plate geometry was used, with the two metallic sample holders serving both as mechanical tools and as electrodes of a capacitor. The rheo-dielectric technique was used to study the effect of oscillatory shear on two classes of systems: nematic liquid crystals and polymeric melts. (c) 2007 Published by Elsevier B.V
Unravelling the Detailed Microstructure of a Semiconducting (Quasi-Metal) Soluble Polymer Incorporating Conjugated Thienylene Methine Sequences
No full textThis article reports a thorough spectroscopic characterization and the complete microstructural unravelling of a novel soluble poly(thienylene methylene) recently obtained by a straightforward process based on the methanesulfonic acid-catalyzed self-condensation of 2-acetoxymethyl-3,4-dimethylthiophene. These macromolecules were found to generate both in situ (in the acidic reaction medium) and ex situ (by the addition of appropriate dopants) conjugated sequences consisting of alternating aromatic-and quinoid-like thiophene rings, that is, conjugated thienylene methine sequences. The effect of different types of dopants on the electronic features of these polymers was assessed by the extent of bathochromic shifts they induced in the macromolecule UV-vis spectra. Doped films obtained by solution casting were characterized by dielectric spectroscopy to evaluate their electronic conductivity. The observed low values of conductivity were explained on microscopic basis evidencing the presence of polymer branching. All the structures arising from side reactions responsible for branching were determined by an exhaustive NMR study, which led to the formulation of the corresponding mechanisms. Remarkably, all these polymers, irrespective of their specific structural features and molecular weights, retained complete solubility in common solvents. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5227-5238, 201
Dynamics of supercooled and glassy dipropyleneglycol dibenzoate as functions of temperature and aging: Interpretation within the coupling model framework
No full textDielectric relaxation measurements of a typical small molecular glassformer, dipropyleneglycol dibenzoate show the presence of two secondary relaxations. Their dynamic properties differ in the equilibrium liquid and glassy states, as well as the changes during structural recovery after rapid quenching the liquid to form a glass. These differences enable us to identify the slower secondary relaxation as the genuine Johari-Goldstein (JG) beta-relaxation [G. P. Johari and M. Goldstein, J. Chem. Phys. 53, 2372 (1970)], acting as the precursor of the primary alpha-relaxation. Agreement between the JG beta-relaxation time and the independent relaxation time of the coupling model leads to predicted quantitative relations between the JG beta-relaxation and the alpha-relaxation that are supported by the experimental data. (C) 2004 American Institute of Physics
Effect of temperature and volume on structural relaxation time: Interpretation in terms of decrease of configurational entropy
No full textWe analyze the slowing down of structural relaxation dynamics of two small molecular glass formers and one polymer: o-terphenyl, triphenylchloromethane, and poly(methylmethacrylate). Considering the literature data of expansivity and heat capacity we calculate configurational entropy using a previously proposed relation between the configurational and the excess entropy, and we directly check the Adam and Gibbs theory for glass transition. In particular, we clearly show that using such expression for configurational entropy the predicted linear dependence between the logarithmic of structural relaxation time and the product of temperature with configurational entropy is well satisfied and it does not depend on pressure. Moreover, we also derive an equation for calculating the pressure dependence of the glass transition temperature, which in these systems is in good accordance with the experimental values. (c) 2005 Elsevier B.V. All rights reserved
Influence of molecular characteristics on dielectric relaxation of propylene glycol oligomers
No full textThe relaxation dynamics of polyropylene glycol oligomers having different end groups was studied by broadband dielectric spectroscopy (10 mHz to 10 GHz) in the temperature interval 110-350 K. above and below the glass transition. The effects produced on structural and secondary relaxations by the different end groups are discussed
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