Dielectric properties of polymers at low temperatures

Abstract

Dielectric spectroscopy is a powerful method that allows the study of the dynamics of polymers in a wide frequency range. The different regimes of the dielectric function can be observed and the dynamics of the primary and secondary relaxations can be found. In fact, to obtain a complete characterization, a large range of frequencies and temperatures must be used. In this work, the investigation was focused in poly(lactid acid), PLA, in two forms, industrial and purified. This polymer is an aliphatic polyester, and one of the most important biocompatible and biodegradable material that has received increasing attention in the last 10 years. The β relaxation was observed between -150 and -30 °C, in frequency domain measurements between 1 Hz and 100 kHz, and was assigned to the secondary relaxation in the glassy state. The changes in the structure, which are connected with the water penetration in the polymer, directly affect that relaxation process. Water molecules confined by the polymer chains and in the polymer networks itself play an important role in the degradation of the material. We studied the evolution of that degradation during 4 weeks, in a controlled humidity environment. It is accepted that water preferentially enters in the amorphous zones, but also affects the crystalline regions. It is observed a clear evolution of the relaxation activation energy during the degradation of the polymer. The dielectric relaxation studies are complemented with water permeability measurements during the degradation process with time.