Thermomechanical investigation of poly(methylmethacrylate) containing an organobismuth radiopacifying additive

Previously we demonstrated the feasibility of using up to 24% triphenylbismuth (TPB) as a radiopaque, monomer-miscible additive for dental acrylic resins. In this study we examined the influence of TPB on thermomechanical properties of a representative polymethylmethacrylate (PMMA) ambient-cured resin used for temporary dental crowns and bridges. TPB (0%, 5%, 15% or 30% w/w) was dissolved in the monomer component, added to the powder component, and allowed to cure in rectangular molds. After 1 h they were either stored at 23°C for 23 h, or heated for 5 min at either 40°C or 50°C, and then stored for 23 h. They were then scanned from 10° to 125°C in a dynamic mechanical thermal analyzer using the three-point bending mode of deformation at 1-Hz frequency. The onset to the glass-transition temperature (T(g)) is decreased by 13° to 32°C by addition of TPB, while the storage modulus (E') at 25°C is either unchanged or is slightly increased. TPB did not interfere with the curing reaction, and postcure heating at 40°C had no effect on either E' or T(g). However, heating at 50°C generally increased T(g) but had very little effect on E' throughout the 0-50°C operating temperature range. TPB crystals were observed to have precipitated at TPB levels above 8%. These crystals, dispersed throughout the PMMA, act as reinforcing fillers. This reinforcement can account for the lack 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 the TPB remains dissolved in the solid polymer, no decrease in E' was observed. This implies that TPB exerts an antiplasticizing effect at temperatures below 50°C, possibly by occupying free volume among the polymer chains. It is concluded that TPB, in amounts adequate to impart diagnostic levels of radiopacity, is unlikely to adversely affect the clinical utility of PMMA-based dental acrylic resins.