Browsing by Author "Miranda, R"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Synthesis, dilute solution properties, and chain flexibility of poly(difluorobenzyl methacrylates)
    (MARCEL DEKKER INC, 2000) Gargallo, L; Miranda, R; Radic, D
    The synthesis and dilute solution behavior of poly(2,5-difluorobenzyl methacrylate) (P2,5-DFBM) and poly(2,6-difluorobenzyl methacrylate) (P2,6-DFBM) are described. The polymers were characterized by viscosity and classical lightscattering and size exclusion chromatography (SEC) measurements. The Kuhn-Mark-Houwink-Sakurada (KMHS) relationships were established. Unperturbed dimensions (1/2), the rigidity factor sigma, the characteristic ratio C-infinity and the thermodynamic parameters were determined using the Stockmayer-Fixman treatment. The double substitution in the 2,5 and 2,6 positions by fluorine atoms increases the conformational parameters. The presence of substituents such as -CH3 or fluorine atoms close to the main chain (2,6 position) affects the internal rotation potential in the same way.