Molecular mobility and fragility in n-ethylene glycol dimethacrylate monomers

Di-, tri- and tetra-ethylene glycol dimethacrylate monomers were investigated by dielectric relaxation spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature region comprised between -120 and +25 °C. A rather intense relaxation process associated with the glass transition is found for all the monomers in the temperature region between -80 and -50 °C. The mobility of the cooperative process decreases with the molecular weight increase. The temperature dependence of the relaxation times reveals some curvature, which increases with the size of the ethylene glycol moiety (di≅tri<tetra). In spite of this behavior the di- and tri-ethylene monomers could be treated as Arrhenius type, presenting similar activation energies. The fragility indexes estimated by both dielectric relaxation spectroscopy and differential scanning calorimetry techniques agree for the di- and tri-ethylene monomers only when Arrhenius behavior is assumed. Despite the different values estimated, the fragility is comparable to a quite strong glass former as glycerol.