THERMOMECHANICAL INVESTIGATION OF POLY(METHYLMETHACRYLATE) CONTAININGAN ORGANOBISMUTH RADIOPACIFYING ADDITIVE

Previously we demonstrated the feasibility of using up to 24% tripheny lbismuth (TPB) as a radiopaque, monomer-miscible additive for dental a crylic resins. In this study we examined the influence of TPB on therm omechanical properties of a representative polymethylmethacrylate (PMM A) ambient-cured resin used for temporary dental crowns and bridges. T PB (0%, 5%, 15% or 30% w/w) was dissolved in the monomer component, ad ded to the powder component, and allowed to cure in rectangular molds. After 1 h they were either stored at 23 degrees C for 23 h, or heated for 5 min at either 40 degrees C or 50 degrees C, and then stored for 23 h. They were then scanned from -10 degrees to 125 degrees C in a d ynamic mechanical thermal analyzer using the three-point bending mode of deformation at 1-Hz frequency. The onset to the glass-transition te mperature (T-g) is decreased by 13 degrees to 32 degrees C by addition of TPB, while the storage modulus (E') at 25 degrees C is either unch anged or is slightly increased. TPB did not interfere with the curing reaction, and postcure heating at 40 degrees C had no effect on either E' or T-g. However, heating at 50 degrees C generally increased T-g b ut had very little effect on E' throughout the 0-50 degrees C operatin g temperature range. TPB crystals were observed to have precipitated a t TPB levels above 8%. These crystals, dispersed throughout the PMMA, act as reinforcing fillers. This reinforcement can account for the lac k of a decrease in E', as would be expected if TPB had a plasticizing effect below T-g. However, even at 5%, a concentration at which all th e TPB remains dissolved in the solid polymer, no decrease in E' was ob served. This implies that TPB exerts an antiplasticizing effect at tem peratures below 50 degrees C, possibly by occupying free volume among the polymer chains. It is concluded that TPB, in amounts adequate to i mpart diagnostic levels of radiopacity, is unlikely to adversely affec t the clinical utility of PMMA-based dental acrylic resins. (C) 1996 J ohn Wiley & Sons, Inc.