Dynamic mechanical and dielectric relaxations of poly(difluorobenzyl methacrylates)

This work reports the mechanical and dielectric relaxation spectra of three difluorinated phenyl isomers of poly(benzyl methacrylate), specifically, p oly(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, respecti vely, 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 w hich 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 partia lly masked by the glass-rubber relaxation. The mean-square dipole moments p er 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 methac rylate), respectively. These results, in conjunction with Onsager type equa tions, permit to conclude that auto and cross-correlation contributions to the dipolar correlation coefficient may have the same time-dependence. On t he 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) 2 000 John Wiley & Sons, Inc.