Publications (7)6.97 Total impact
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Article: Temperature dependence of a glass transition cooperativity
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ABSTRACT: The size of glass transition cooperativity for 11 polymers and glycerol is determined from heat capacity spectroscopy (HCS) and differential scanning calorimetry (DSC) by using a calorimetric fluctuation formula. The data cover in total the 0.25 ≤ x ≤ 0.95 part of the reduced temperature interval between the Vogel temperature and a cooperativity onset temperature. A steep cooperativity increase at lower temperature is observed. The data are consistent with the hypothesis that the temperature dependence can be described by the enlargement of a minimal cooperativity at the onset.Acta Polymerica 03/2003; 48(9):369 - 378. -
Article: Dynamic Glass Transition above the Cooperativity Onset in Poly(n-octyl methacrylate)
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ABSTRACT: Experimental heat capacity, dielectric, and shear spectroscopy data are reported on the αβ splitting region of poly(n-octyl methacrylate). New facets of a major difference between the high-temperature a relaxation at frequencies above the αβ splitting region and the ordinary α relaxation below are obtained: An α cooperativity onset in the splitting region and dominance of Rouse−Zimm-like modes R for the a relaxation. The α and a relaxations are not related by temperature−time superposition.12/1997; -
Article: Glass-Transition Cooperativity Onset in a Series of Random Copolymers Poly(n-butyl methacrylate-stat-styrene)
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ABSTRACT: Dielectric spectroscopy, heat capacity spectroscopy (HCS), and differential scanning calorimetry (DSC) investigations in the αβ relaxation splitting region of a series of random copolymers of n-butyl methacrylate with styrene are reported. A separate onset of the α relaxation is dielectrically observed, about one frequency decade below a continuous local aβ component in the Arrhenius diagram. This splitting scenario shifts to higher frequencies and temperatures for increasing styrene content and does not qualitatively change from homo PnBMA up to 54 mol % styrene. The logarithms of onset frequency, log ωon, and of WLF asymptotic frequency, log Ω, change linearly with the styrene content, but their ratio is constant and remains large, log10(Ω/ωon) = 3.8 ± 1. The log Ω ≈ 7 (rad/s) values for small styrene content are unusually low. Ω is explained as the frequency of local cooperativity chances in the concept of kinetic molecular randomness for the dynamic glass transition. The α dielectric intensity, Δεα, the caloric intensity, Δcp, and the square root of cooperativity from a fluctuation formula, Nα1/2, are linearly proportional to the temperature difference to the onset, e.g. Δεα (Ton − T). The dielectric activities of the α process and a hypothetical γ process (beyond the Johari Goldstein β process) increase with increasing styrene content although the styrene unit is almost nonpolar. This is interpreted by dipole decompensation for the α and γ relaxation modes caused by the random styrene units.11/1997; -
Article: Heat Capacity Spectroscopy Compared to Other Linear Response Methods at the Dynamic Glass Transition in Poly(vinyl acetate)
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ABSTRACT: Heat capacity spectroscopy measurements in the frequency range from 1.25 to 12 600 rad/s at the dynamic glass transition in poly(vinyl acetate) are reported. The data are compared to results of dielectric and shear spectroscopy in a comparable frequency range on the same sample. The peaks of the different imaginary parts do not have the same position across the main transition zone. The sequence of the peaks with increasing frequency is as follows: dielectric compliance, entropy compliance, and shear modulus. The distances between the different peaks are 0.3 ± 0.4 and 1.0 ± 0.4 frequency decades, respectively.07/1996; -
Article: Fine Structure of the Main Transition in Amorphous Polymers: Entanglement Spacing andCharacteristic Length of the Glass Transition. Discussion of Examples
Macromolecules 01/1996; 29:6589-6600. · 5.17 Impact Factor -
Article: Broad band heat capacity spectroscopy in the glass-transition region of polystyrene
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ABSTRACT: The combination of results from temperature modulated DSC (TMDSC) and 3 omega-method (HCS) in the glass-transition region allows broad band heat capacity spectroscopy in a frequency range of seven orders of magnitude. In an Arrhenius diagram, the curve for calorimetric data is close to that of dielectric data, except from some remaining problems of temperature calibration. The dynamic glass-transition temperature and the width of the transition interval are determined in the frequency range 10(-4) < nu < 10(3) Hz and compared with DSC measurements on linear cooling (vitrification). The dynamic calorimetric data are obtained in the liquid equilibrium state, where the dispersion of the glass-transition interval is related to the mean temperature fluctuation of cooperatively rearranging regions (CRR). As expected from the fluctuation approach to glass transition, this temperature fluctuation increases with increasing temperature. This indicates decreasing size of CRRs with increasing temperature. The a = q/delta T*omega parameter is obtained as a = 6+/-2 from a comparison of thermal and dynamic glass transition. (C) 1997 Elsevier Science B.V. [References: 11]Thermochimica Acta 305:251-255. · 1.80 Impact Factor -
Article: Small Characteristic Length at the Glass Transition Cooperativity Onset
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ABSTRACT: A fluctuation theory approach to a cooperativity onset of the dynamic glass transition is described. Recent dielectric and heat capacity spectroscopy (HCS) experiments for several random copolymers of n-butyl methacrylate with styrene indicate a steep linear increase of relaxation intensities ($\Delta_\varepsilon,\;\Delta C_{\rm p}$) and of square root of cooperativity $(N_\alpha^{1/2})$ as function of temperature below the onset. A quasi continuous description is derived from kinetic molelcular randomness. This description can be applied to small cooperativity near the onset. The experimental indications can analytically be reproduced by means of a Landau order parameter expansion adapted to dominance of fluctuation in a free volume approach to the dynamic glass transition. An important parameter of the approach is the minimal cooperativity of order $N_\alpha^{\rm min}\approx 1$. The sharp onset obtained in the extrapolation is associated with the construction of a large conditionality raster. Far below the onset, the size of cooperativity at the glass temperature is theoretically estimated to be of order $N_\alpha(T_{\rm g})\approx 100$ molecules. A new interpretation of the WLF asymptote lg $\Omega$ is suggested.http://dx.doi.org/10.1051/jp1:1997177.
