Browsing by Author "CALLEJA, RD"

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    DIELECTRIC AND VISCOELASTIC RELAXATION OF POLY(2-CHLOROETHYL METHACRYLATE) AND POLY(3-CHLOROPROPYL METHACRYLATE)
    (ELSEVIER SCI LTD, 1994) PELISSOU, O; CALLEJA, RD; GARGALLO, L; RADIC, D
    Dielectric and viscoelastic relaxation measurements have been performed on poly(2-chloroethyl methacrylate) and poly(3-chloropropyl methacrylate) between -140-degrees-C and the glass transition temperatures. The splitting of the dipolar relaxations from those of conductive origin in the dielectric spectra was performed.
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    DIELECTRIC RELAXATIONS IN POLY(THIOCARBONATES)
    (JOHN WILEY & SONS INC, 1994) CALLEJA, RD; GARGALLO, L; RADIC, D
    The dielectric absorptions observed on six poly (thiocarbonates) (PTC) containing different substituents in the interphenylic carbon atom are subjected to a study. The a relaxation process associated with the glass transition temperature, which appears at the highest temperature studied, is overlapped with a strong conductive component. Between 80 and 100-degrees-C dielectric activity (beta) is observed, which is related to structural relaxation phenomena. Polymers with double substitution on the interphenylic carbon atom show a gamma relaxation with an activation energy of ca. 11 kcal mol-1 while those where the carbon atom is linked to a cyclohexyl group show higher activation energies and their respective characteristics. A cryogenic absorption around -120-degrees-C, for at least three polymers with an activation energy of about 5 kcal mol-1, is detected. (C) 1994 John Wiley & Sons, Inc.
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    DIELECTRIC-RELAXATION PROPERTIES OF POLY(DIMETHYLPHENYL METHACRYLATE)S
    (ELSEVIER SCI LTD, 1994) CALLEJA, RD; DEVINE, I; GARGALLO, L; RADIC, D
    The dielectric relaxations of poly(dimethylphenyl methacrylate)s were studied over a wide frequency and temperature range by two methods. The alpha relaxation, associated with the glass transition temperature, was studied by classical dielectric measurements. Thermal peak cleaning was used to deconvolute the global thermally stimulated current spectra over the range of low temperatures, corresponding to the beta relaxation. The a relaxation involves long-range, generalized and highly cooperative motions of the main chain. They are influenced by the steric hindrance and dipole-dipole interactions relative to the position of the CH3 group of the aryl ring in the polymer repeat unit. As a first approximation it was assumed that the beta process was related only to local motions of the side group, but the activation parameters are too high to be consistent with this assumption. Therefore it was concluded that intermolecular factors and local motions of the main chain could also be involved.
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    DYNAMIC-MECHANICAL BEHAVIOR OF POLY(BENZYL METHACRYLATE) AND POLY(CYCLOHEXYLMETHYL METHACRYLATE)
    (1993) CALLEJA, RD; GARGALLO, L; RADIC, D
    The dynamic mechanical properties of poly(benzyl methacrylate) (PBzMA), and poly(cyclohexylmethyl methacrylate) (PCHMMA) were studied. In order to show the beta relaxation in both polymers and the gamma relaxation in PBzMA, the derivative of the real part of the complex modulus dE'/dT was calculated, which is proportional to T-2E''. A detailed study of the gamma relaxation spectra of PCHMMA in terms of activation free energy distribution was performed.
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    ON THE SPLITTING OF DC CONTRIBUTION TO THE DIELECTRIC LOSS - CASE OF POLY(MONOCYCLOHEXYLITACONATE)
    (TAYLOR & FRANCIS AS, 1993) CALLEJA, RD; GARGALLO, L; RADIC, D
    The splitting of the d.c. conductivity on the low frequency side of the dielectric loss has been performed by using the complex dielectric polarizability alpha, instead of the permittivity. Data dealing with the dielectric loss for poly(monocvclohexylitaconate) have been used to compare both methods. The dielectric relaxation peaks in M'' and alpha'', are in general in good qualitative agreement. There is a small shift which is attributed to the fact that M is a modulus and alpha a compliance.
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    RELAXATION BEHAVIOR OF SEMIRIGID CHAINS IN THE AMORPHOUS STATE
    (1994) CALLEJA, RD; SAIZ, E; RIANDE, E; GARGALLO, L; RADIC, D
    Dielectric and mechanical behaviour of two polymers having severe restrictions on the conformational versatility of the backbone, namely poly(monocyclohexylmethylene itaconate) (PMCMI) and poly(dicyclohexylmethylene itaconate) (PDCMI), have been studied. Both polymers present a weak glass-rubber relaxation named alpha and two stronger absorptions in the glassy state that, in increasing temperature order, are called beta- and gamma- relaxations. Both mechanical and dielectric beta-peaks are located in the same region of the spectra and have the same activation energy, which suggest that both relaxations are caused by the same molecular mechanisms. The analysis of the dipole moments of both polymers suggest that they behave as freely joint chains as far as their polarity is concerned. The dipolar correlation coefficients in the sub-glass region were calculated by assuming that the dielectric activity is only produced by reorientations of the side groups with respect to the frozen main chains. The fact that the values of the coefficients thus calculated are somewhat larger than the experimental values suggests that dipolar intermolecular interactions cannot be neglected in the analysis of the dielectric behaviour of these polymers.
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    RELAXATIONS IN POLY(DI-N-ALKYL AND DIISOALKYL ITACONATES)
    (AMER CHEMICAL SOC, 1995) CALLEJA, RD; GARGALLO, L; RADIC, D
    A systematic study about the viscoelastic and dielectric relaxations was performed for a family of poli(di-n-alkyl and diisoalkyl itaconates): poly(dimethyl itaconate) (PDMI), poly(diethyl itaconate) (PDEI), poly(di-n-propyl itaconate) (PDPI), poly(di-n-butyl itaconate) (PDBI), poly(diisopropyl itaconate) (PDIPI), and poly(diisobutyl itaconate) (PDIBI). Three dielectric relaxation processes, labeled as alpha, beta, and gamma were observed in all cases. Nevertheless, in some polymers a poor resolution of the peaks is observed and only small indications of the phenomena can be seen. In the case of PDPI and PDBI measurements at very low temperatures were performed and dielectric absorption processes are found at those temperatures. Behavior similar to that observed for dielectric relaxations is found for the mechanical one. The temperature at which the a relaxation, associated with the glass transition temperature, is present diminishes as the length of the n-alkyl side group increases. This behavior is similar to that reported for poly(n-alkyl methacrylates). a relaxation is only detected at some frequencies, and it is more easily detected in terms of E'' or epsilon'' than in terms of tan delta. This relaxation is less pronounced than in the case of poly(n-alkyl methacrylates). An important difference between these polymers and poly(n-alkyl methacrylates) is the existence of a new relaxation process labeled as gamma relaxation between -40 and -120 degrees C. This relaxation is very sensitive to the dryness of the sample or to the presence of molecules of low molecular weight in the matrix.
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    THERMAL EFFECTS ON THE STRUCTURE AND RELAXATION PROPERTIES OF POLY(MONOCYCLOPENTYL ITACONATE)
    (HUTHIG & WEPF VERLAG, 1995) CALLEJA, RD; SANCHIS, MJ; GARGALLO, L; RADIC, D
    The effect of thermal treatment and subsequent chemical structural modifications on the viscoelastic and dielectric properties of PMCPI (poly(monocyclopentyl itaconate)) was studied. The low temperature relaxation (gamma-relaxation) is unaffected by the thermal history or chemical modifications. The intermediate relaxation (beta-relaxation) is shifted by about 40 degrees C to higher temperature after thermal treatment. The alpha-relaxation (related to the glass transition temperature T-g) suffers more striking changes when moving to higher temperatures.
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    VISCOELASTIC RELAXATIONS IN POLY(DIMETHYLPHENYL METHACRYLATES)
    (1994) CALLEJA, RD; GARGALLO, L; RADIC, D
    Dynamic mechanical properties of poly(dimethylphenyl methacrylates) were studied considering the different positions of the methyl groups on the phenyl ring. In all cases a clear a relaxation can be observed which is associated to the polymer glass transition temperature (Tg). Nevertheless, at lower temperatures, viscoelastic activity is negligible. A correlation between the steric hindrance due to the methyl groups and the temperature at which the relaxation is detected, was established.