ABSTRACT
In this study the isothermal kinetic behavior of two resin modified glass ionomer cements (RMGIC) and a dental composite have been compared by differential scanning calorimetry (DSC) and thermo-mechanical analysis (TMA). The simultaneous evolution of the multiple reactions occurring in RMGIC has been analyzed not only by DSC and TMA but also by ultrasonic wave propagation using the pulse-echo technique. The propagation of ultrasonic waves, acting as a dynamic mechanical deformation at high frequencies, is proportional to the longitudinal bulk moduli of the material and may be used to measure the changes of mechanical properties induced by a chemical reaction as occurs in RMGIC. TMA and ultrasonic analysis have been used to monitor the acid-base reaction of RMGIC in dark conditions. Moreover an RMGIC presenting a double reactive mechanism in dark conditions, a thermally activated radical polymerization and an acid-base reaction are studied using these experimental techniques. Finally DSC and TMA results obtained during photopolymerization of an RMGIC and of a dental composite have been compared.
ABSTRACT
Hydrogels have been prepared by a freezing-thawing procedure and investigations made of the effect of both number of freezing-thawing cycles and different content of hyaluronic acid (HA) on the mechanical properties of the PVA-HA hydrogels using non-destructive testing. The bulk elastic modulus K of hydrogels has been determined by pulse-echo measurements. It is noted that hydrogel elastic properties improve with the number of the cycles in PVA-HA 100/0; on the other hand samples with a high HA (1,000,000 molecular weight) content, beyond the third cycle, seem to be unaffected by the number of cycles. A bulk elastic modulus fall-off is then observed in samples submitted to an additional overnight freezing between two subsequent cycles. K increases in hydrogels with the highest HA content, when samples undergo pulse-echo measurements soon after their preparation. When hydrogels reach equilibrium, after having been kept in deionized water for 12 h, K values are lower, showing a nearly constant behaviour with different PVA-HA ratios and cycles. Furthermore, by means of scanning laser acoustic microscopy (SLAM) defects have been detected in the hydrogels. In samples which have reached equilibrium, SLAM images show that these defects disappear in PVA-HA hydrogels.
