NON-DEBYE DIELECTRIC-RELAXATION AROUND THE LIQUID-GLASS TRANSITION OF A GLASS-FORMING POLYMER

The dynamics of glass-forming liquids shows universal features like non-Debye behavior, scaling, and non-Arrhenius temperature dependence of the characteristic rates. These features have been mainly established in the time range between 10(-9) and 10 s. It is well known that when the temperature of a glass-forming liquid is lowered and the characteristic relaxation time becomes larger than hundreds of seconds, the system falls out of equilibrium and a glass is obtained. However, whether or not the typical features of the glass-forming-liquid dynamics extend to the glass phase through the experimental liquid-glass transition, T(g) is still unknown. Trying to address this question better we have performed a study of the dielectric relaxation of a glass-forming polymer, poly(bisphenol-A, 2-hydroxypropylether), around T(g). By using both frequency-domain and time-domain dielectric techniques, we have studied the dynamical processes in the range 10(-5)-10(5) s. We have also carried out calorimetric measurements for comparison. In the supercooled-liquid state (T > T(g)) we have observed all the universal features expected. However, although in the glassy state ( T less than or similar to T(g)) the non-Debye character is preserved the scaling is broken, i.e., we observed a relaxation shape depending on the temperature and the state of the glass. Moreover, the temperature dependence of the characteristic time scale of the dielectric relaxation below T(g) is Arrhenius-like albeit the apparent activation energy is too high to be a truly activated process. All the features observed have been discussed in the framework of several theoretical approaches.