Browsing by Author "Riande, E"

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    Deconvolution of the relaxations associated with local and segmental motions in poly(methacrylate)s containing dichlorinated benzyl moieties in the ester residue
    (AMER INST PHYSICS, 2005) Dominguez Espinosa, G; Diaz Calleja, R; Riande, E; Gargallo, L; Radic, D
    The relaxation behavior of poly(2,3-dichlorobenzyl methacrylate) is studied by broadband dielectric spectroscopy in the frequency range of 10(-1)-10(9) Hz and temperature interval of 303-423 K. The isotherms representing the dielectric loss of the glassy polymer in the frequency domain present a single absorption, called beta process. At temperatures close to T-g, the dynamical alpha relaxation already overlaps with the beta process, the degree of overlapping increasing with temperature. The deconvolution of the alpha and beta relaxations is facilitated using the retardation spectra calculated from the isotherms utilizing linear programming regularization parameter techniques. The temperature dependence of the beta relaxation presents a crossover associated with a change in activation energy of the local processes. The distance between the alpha and beta peaks, expressed as log(f(max;beta)/f(max;alpha)) where f(max) is the frequency at the peak maximum, follows Arrhenius behavior in the temperature range of 310-384 K. Above 384 K, the distance between the peaks remains nearly constant and, as a result, the alpha onset temperature exhibited for many polymers is not reached in this system. The fraction of relaxation carried out through the alpha process, without beta assistance, is larger than 60% in the temperature range of 310-384 K where the so-called Williams ansatz holds. (c) 2005 American Institute of Physics.
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    Dynamic mechanical and dielectric relaxations of poly(difluorobenzyl methacrylates)
    (WILEY-BLACKWELL, 2000) Diaz Calleja, R; Sanchis, MJ; Saiz, E; Martinez Pina, F; Miranda, R; Gargallo, L; Radic, D; Riande, E
    This work reports the mechanical and dielectric relaxation spectra of three difluorinated phenyl isomers of poly(benzyl methacrylate), specifically, poly(2,4-difluorobenzyl methacrylate), poly(2,5-difluorobenzyl methacrylate) and poly(2,6-difluorobenzyl methacrylate). The strength of the dielectric glass-rubber relaxation of the 2,6 difluorinated phenyl isomer is, respectively, nearly three and two times larger than the strengths of the 2,5 and 2,4 isomers. The 2,4 isomer presents a mechanical ct peak the intensity of which is nearly two times that of the other two isomers. Both the mechanical and dielectric relaxation spectra display a subglass process, called gamma relaxation, centered in the vicinity of -50 degrees C at 1 Hz and, in some cases, a subglass beta absorption is detected at higher temperature partially masked by the glass-rubber relaxation. The mean-square dipole moments per repeating unit, [mu 2]/x, measured at 25 degrees C in benzene solutions, are 2.5 D-2, 1.9 D-2, and 5.0 D-2 for poly(2,4-difluorobenzyl methacrylate), poly(2,5-difluorobenzyl methacrylate) and poly(2,6-difluorobenzyl methacrylate), respectively. These results, in conjunction with Onsager type equations, permit to conclude that auto and cross-correlation contributions to the dipolar correlation coefficient may have the same time-dependence. On the other hand, dipole intermolecular interactions, rather than differences in the flexibility of the chains, seem to be responsible for the relatively high calorimetric glass-transition temperature of the 2,6 diphenyl isomer, which is, respectively, nearly 36 degrees C and 32 degrees C above the T-g's of the 2,4 and 2,5 isomers. Molecular Mechanics calculations give a good account of the differences observed in the polarity of the polymers. (C) 2000 John Wiley & Sons, Inc.
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    Influence of the fine structure on the response of polymer chains to perturbation fields
    (AMER CHEMICAL SOC, 2006) Dominguez Espinosa, G; Diaz Calleja, R; Riande, E; Gargallo, L; Radic, D
    The relaxation behavior of poly(3-methylbenzyl methacrylate), poly(3-fluorobenzyl methacrylate), and poly(3-chlorobenzyl methacrylate) was thoroughly studied by broadband dielectric spectroscopy with the aim of investigating the influence of slight differences in chemical structure on the response of polymers to electric perturbation fields. Retardation spectra calculated from dielectric isotherms utilizing linear programming regularization parameter techniques were used to facilitate the deconvolution of strongly overlapped absorptions. Above the glass transition temperature, the spectra of the two halogenated polymers present a secondary gamma process well separated from a prominent peak resulting from the overlapping of the alpha and beta relaxations. The spectra of poly(3-methylbenzyl methacrylate) exhibit at long times a well-developed alpha absorption followed in decreasing order of time by two weak absorptions, named beta and gamma, whose intensities increase with temperature. The temperature dependence of the distance of the a peak from the beta and gamma peaks, expressed in terms of log(f(max,beta)/f(max,alpha)) and log(f(max,gamma)/f(max,alpha)), respectively, is studied. The Williams ansatz and the extended ansatz give a fairly good account of the relaxation behavior of the polymers. The stretch exponent associated with the a relaxation increases with temperature from ca. 0.2 at low temperatures to the vicinity of 0.5 at high temperatures. At low temperatures, the alpha relaxation is described by a Vogel-type equation, but at high temperature the beta and alpha processes are roughly described by the same Arrhenius equation. In the whole temperature range, the activation energy they relaxation is significantly lower than that of the beta absorption. The mechanisms involved in the development of the secondary relaxations are qualitatively discussed.