Browsing by Author "Garcia Bernabe, A"

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    Dielectric relaxation behaviour of poly(cyclobutyl methacrylate)s
    (WILEY, 2002) Diaz Calleja, R; Garcia Bernabe, A; Sanchez Martinez, E; Hormazabal, A; Gargallo, L; Radic, D
    Dielectric behaviour of two poly(methacrylate)s with cyclobutyl groups in the side-chains has been studied. This investigation was performed by determining the dielectric permittivity and loss in terms of frequency and temperature for poly(cyclobutyl. methacrylate) (PCBuM) and poly(cyclobutylmethyl methacrylate) (PCBuMM). Dynamic dielectric measurements show sub-glass relaxations, probably due to motions in the lateral chain, including the cyclobutyl ring. The effect of the insertion of a flexible spacer group into the side-chain is also analyzed. The alpha relaxations were analyzed in terms of the Havriliak-Negami equation and the free volume theory tested according to the Vogel-Fulcher-Tamman-Hesse equation. The conductive and interfacial phenomena were studied by hopping and MacDonald-Coelho models. (C) 2002 Society of Chemical Industry.
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    Dielectric relaxational behavior of poly(diitaconate)s containing cyclic rings in the side chain
    (WILEY, 2003) Diaz Calleja, R; Garcia Bernabe, A; Sanchez Martinez, E; Sanchis, MJ; Hormazabal, EA; Gargallo, L; Radic, D
    Dielectric relaxations of several poly(diitaconate)s with cyclobutyl, cycloheptyl, and cyclooctyl groups in the side chain were investigated. The study was performed by determining the dielectric permittivity and loss, depending on the frequency and temperature. Dynamic dielectric measurements indicated several relaxations according to the chemical structure of the polymers. The dielectric behavior of these polymers was compared with those of poly(dicyclohexyl itaconate), previously reported. The a relaxations were analyzed with the Havriliak-Negami equation. Significant differences in the subglass relaxations were observed. A tentative explanation of the molecular origin of each absorption was proposed in terms of the number of carbon atoms of the ring and their conformational versatility. Strong conductive processes were observed in these polymers at low frequencies and high temperatures. (C) 2003 Wiley Periodicals, Inc.
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    Dielectric relaxations of poly(2-norbornyl methacrylate) and poly(3-methyl-2-norbornyl methacrylate)
    (ELSEVIER SCI LTD, 2001) Diaz Calleja, R; Garcia Bernabe, A; Pagueguy, C; Gargallo, L; Radic, D
    Dielectric behaviour of poly(2-norbornyl methacrylate) (P2NBM) and poly(3-methyl-2-norbornyl methacrylate) (P3M2NBM) was studied. The effect of the methyl substituent on the norbornyl ring is to decrease the temperature of the dynamic glass transition about 50 K. Moreover, weak secondary relaxations are observed in the two polymers under study. The secondary process of P3M2NBM has been conveniently analysed in terms of the empirical Fuoss-Kirkwood (F-K) equation. (C) 2001 Elsevier Science Ltd. All rights reserved.
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    Dynamic mechanical and dielectric relaxational behavior of poly(cyclohexylalkyl methacrylate)s
    (AMER CHEMICAL SOC, 2001) Calleja, RD; Garcia Bernabe, A; Sanchez Martinez, E; Hormazabal, A; Gargallo, L; Gonzalez Nilo, F; Radic, D
    The dynamic mechanical and dielectric behavior of three poly(cyclohexylalkyl methaerylate)s with different spacer groups has been studied. The study was performed by determining the components of the complex relaxation modulus E* and the complex dielectric permittivity epsilon*. Results are discussed in terms of the effect of the side chain structure and the insertion of flexible spacer groups. Molecular dynamic calculations for the repeating unit of the polymers under study are analyzed. Close to room temperature the interconversion between axial and equatorial conformations is not observed whitin the total time of 5 ns. The analysis was then performed from 1000 to 1500 K, and the results were extrapolated to lower temperatures. By this way the fraction of axial and equatorial conformations was calculated. The free energy change against the number of carbon atoms in the side chain is in excellent agreement with the experimental data. This means that the gamma relaxation associated with the chair-to-chair conformational change in the cyclohexyl group is also influenced by the length of the spacer group. The molecular dynamics approach allows to observe the relative incidence of the two conformations of the cyclohexyl group which can be related to the motions responsible for the gamma relaxation.