Anelastic phenomena associated to water loss and collagen degradation in human dentin

This work describes the anelastic and dynamic Young modulus behaviour of human dentin from room temperature up to 673 K. Human molars, extracted from individuals (males 55-70 years old) as part of their dental treatment, were cut to obtain bar-shaped samples subsequently used for mechanical spectroscopy experiments. In addition, thermo-gravimetric analysis (TGA) has been performed to assess a possible weight loss occurring in the same temperature range of mechanical spectroscopy tests. A broad and asymmetric internal friction (Q-1) maximum at 500 K has been observed during the heating of the as prepared samples. This maximum is absent during the following cooling down to room temperature. It is therefore due to the occurrence of an irreversible transformation in the sample. TGA shows a remarkable weight loss in the same temperature range. This effect has been related to loss of fluids and degradation of collagen. Another set of samples, previously kept for 36 hours under a vacuum of 10-2 Pa, were submitted at room temperature to tests at increasing strain from 6 x 10-6 to 7 x 10-4. The results show transient and fully recoverable Q-1 increase and dynamic modulus (E) decrease. The phenomenon has been ascribed to the breaking of weak H-bonds between polypeptide chains forming the triple-helix with consequent increase of the mean length of vibrating chain segments.