Browsing by Author "Dominguez Espinosa, G"

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    Comparative study of localized side group in poly(2,3 and 4 methyl cyclohexyl methacrylate)s. TSDC measurements
    (ELSEVIER SCI LTD, 2005) Dominguez Espinosa, G; Sanchis, MJ; Diaz Calleja, R; Pagueguy, C; Gargallo, L; Radic, D
    By means of thermal sampling techniques, the fine structure of the gamma-relaxation zone of poly(methyl cyclohexyl methacrylate)s (P2MCHMA), (P3MCHMA), (P4MCHMA) were analyzed. Results reveal that in this relaxation zone, at least two peaks are present. These peaks are attributed to the cis and trans isomers. Loss permittivity of the polymers under study in this relaxation zone has been reproduced from the partial depolarization data by using the elementary relaxation times and activation energies. Results are in relatively good agreement with the experimental data previously obtained. Molecular mechanic calculations have been carried out in order to elucidate the characteristics and molecular origin of the relaxations observed in this zone. An interpretation of the height of the peaks associated to the cis- and trans-isomer in terms of strain energy (SE) have been carried out. (c) 2005 Elsevier Ltd. All rights reserved.
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    Comparative study of poly(2,3 and 4 methyl cyclohexyl methacrylate)s. Dielectric relaxation spectroscopy (DRS)
    (ELSEVIER SCI LTD, 2005) Dominguez Espinosa, G; Sanchis, MJ; Diaz Calleja, R; Pagueguy, C; Gargallo, L; Radic, D
    The relaxation properties of poly(methyl cyclohexyl methacrylate)s (P2MCHMA), (P3MCHMA), (P4MCHMA) were analyzed. Dielectric spectroscopy (DS) techniques were used for this purpose. These polymers exhibit prominent alpha peaks, associated to the dynamic glass transitions, and then poorly defined beta-relaxations and two other relaxation zones labelled as gamma and delta-relaxation in order of decreasing temperatures. The alpha relaxation processes were analyzed by means of the free volume theory, using the Vogel-Fulcher-Tammann-Hesse (VFTH) equation and in terms of the Havriliak-Negami (HN) empirical equation. Previously to these analyses, conductive contributions to the loss permittivity were subtracted. The apparent activation energies for the secondary processes under study have been calculated by means of Arrhenius. Comparison with previous relaxation results of poly(cyclohexyl methacrylate) (PCHMA), i.e. the polymer without substituents in the cyclohexyl ring, has been also carried out. In this way, changes in the glass transition absorptions as well as in the secondary processes have been discussed. (c) 2005 Elsevier Ltd. All rights reserved.
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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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    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.