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, 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 α 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 γ relaxation, centered in the vicinity of -50°C at 1 Hz and, in some cases, a subglass β absorption is detected at higher temperature partially masked by the glass-rubber relaxation. The mean-square dipole moments per repeating unit, 〈μ2〉/x, measured at 25°C in benzene solutions, are 2.5 D², 1.9 D², and 5.0 D² 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°C and 32°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.