Relaxation Phenomena in Chitosan-Au Nanoparticle Thin Films.

Chitosan-gold nanoparticle (CS/AuNP) thin films were synthesized through the chemical reduction of HAuCl in sodium citrate/chitosan solutions. The dielectric and dynamic mechanical behaviors of CS/AuNP films have been investigated as a function of moisture and HAuCl content. Two relaxation processes in the nanocomposites have been observed. The α-relaxation process is related to a glass transition in wet CS/AuNP films. However, in dry composites (with 0.2 wt% of moisture content), the glass transition vanished. A second relaxation process was observed from 70 °C to the onset of thermal degradation (160 °C) in wet films and from 33 °C to the onset of degradation in dry films. This relaxation is identified as the σ-relaxation and may be related to the local diffusion process of ions between high potential barriers in disordered systems. The α- and σ-relaxation processes are affected by the HAuCl content of the solutions from which films were obtained because of the interaction between CS, sodium succinate, and gold nanoparticles. With about 0.6 mM of HAuCl, the conductivity of both wet and dry films sharply increased by six orders, corresponding to the percolation effect, which may be related to the appearance of a conductivity pathway between AuNPs, HAuCl, and NaCl.