Viscoelastic properties of denture base resins obtained by underwater test

The viscoelastic properties of denture base polymers, such as poly(methylmethacrylate) (PMMA), polycarbonate (PC), polysulfone (PSF) and polyethersulfone (PES) which have been used in current clinical dentistry were investigated. In addition, water absorption, diffusion coefficient of water into material, cubical expansion, specific volume and residual monomer were also determined. From the results of this study, the viscoelastic behaviours of the denture base polymers in water have been discussed. The relaxation modulus (Er(5)) which was calculated using a stress at 5 s from the start of measurement for the specimen which reached an equilibrium moisture content (wet specimen), decreased from 23.1 to 25.5% compared with that of PMMA stored in a desiccator (37 degrees C) for 1 week (dry specimen). In contrast, this decreasing ratio was 7.0 to 10.0% for PC, PSF and PES. It is suggested that this phenomenon is caused by the increase of specific volume (cm3/g) with water absorption in each material. The water absorption of the PMMA group was relatively large in comparison with other material and was 1.81-1.85%. This value was within the range from 0.38 to 1.74% for PC, PSF and PES. The diffusion coefficients of water for PC, PSF and PES increased by approximately 1.5-2.7 times that of PMMA. Furthermore, the coefficient of cubical expansion of the wet specimen was larger than that of the dry specimen. The increasing ratio was 1.03-1.10 times for the PMMA group and 1. 18-2.38 times for PC, PSF and PES. The variation of Er(5) of the PMMA group with temperature in water was larger than those of PC, PSF and PES.