Browsing by Author "Hormazabal, 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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    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.
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    Morphology and thermal properties of two polymethacrylates modified by a polymer liquid crystal
    (WILEY-BLACKWELL, 2004) Brostow, W; Gargallo, L; Hormazabal, A; Jaklewicz, M; Menard, KP; Opazo, A; Radic, D
    We have studied blends of a polymer liquid crystal (PLC) with poly(cyclohexylethyl methacrylate) (PCHEMA) or poly(cyclohexylpropyl methacrylate) (PCHPMA). The PLC is PET/0.6PHB where PET = poly(ethylene terephthalate), PHB = p-hydroxybenzoic acid and 0.6 is the mole fraction of the latter in the copolymer. The microstructure was studied by scanning electron microscopy (SEM). PCHEMA + PLC (20 wt% of the latter, blend E) has a fine texture with LC islands evenly distributed in the matrix and good adhesion between the phases resulting from their partial miscibility. The PCHPMA + PLC (20 wt% of the latter, blend P) shows only limited compatibility. The SEM results are confirmed by values of the glass transition temperatures T-g determined via thermal mechanical analysis. The T-g value of the blend E is shifted towards the T-g of PLC; T-g of blend P is practically equal to that of PCHPMA. The linear isobaric expansivity alpha(L) values for both blends are lower than the respective values for pure PCHPMA and PCHEMA. Thermal stabilities of the blends determined by thermogravimetry are also better than those of pure polymethacrylates. The temperature of 50% weight degradation for blend E is higher than that for pure PCHEMA by more-than 60K (C) 2004 Society of Chemical Industry.
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    Relaxational study of poly(2-chlorocyclohexyl methacrylate) by thermally stimulated current, dielectric, and dynamic mechanical spectroscopy
    (AMER CHEMICAL SOC, 1999) Sanchis, MJ; Calleja, RD; Gargallo, L; Hormazabal, A; Radic, D
    A comparative study of the mechanical and dielectric relaxation spectra of poly(2-chlorocyclohexyl methacrylate) (P2CCM) is reported. Two clear relaxational zones are found. The spectra present a moderate subglass gamma absorption, followed in increasing temperature order by a prominent a glass-rubber relaxation process. The last peak is not clearly observed in dielectric measurements due to conductivity effects which overlap the loss permittivity spectrum. Moreover, evidence of remnant mechanical as well as dielectric activity is observed near room temperature. This phenomenon has already been observed in poly(2-chlorocyclohexyl acrylate) (P2CCA). To describe in a suitable way these results, a deconvolution method has been proposed to get information about the relaxational behavior of this polymer. A more detailed analysis about the third relaxation called beta, between alpha and gamma relaxations, has been achieved by using the thermally stimulated depolarization current technique (TSDC). Special attention was devoted to the analysis of the molecular motion that produces dielectric and mechanical activity in glassy P2CCM as well as to link the elementary TSDC to the ac dielectric spectra. A comparative study of P2CCM and P2CCA is also reported.