Shear mechanical relaxation and resonance data are reported for 20 wtthinsp;percnt; hydration of diethylamine melanins with differentpH values ranging from 2 to 7. Shear impedance measurements in the temperature range from 60 to minus;80thinsp;deg;C, with frequencies from 1 to 362 MHz, indicate a Davidsonndash;Cole spectrum in the primary relaxation and a single relaxation with a characteristic frequency off= 4 MHz at 60thinsp;deg;C referred to the molecular motions of a group composed of an average molecular weightMnquest;1.4times;104. A resonance at 250 MHz is found in the specific melanin called No. 1, associated with stacking of indole monomer units, and a single relaxation atf= 270 MHz at 60thinsp;deg;C is found in the hydrate of melanin No. 2. The nature of the stacking is surveyed on account of Fitzgeraldrsquo;s theory. The limiting shear modulus in the primary relaxation related to the molecular motion of backbone chains obeys the modified Hiraindash;Eyring equation. All the hydrations have an amorphous phase composed of a threehyphen;dimensional network structure.
展开▼
