Validity of predictive models of stress relaxation in selected dental polymers

The goal of this investigation was to assess validity of predictive models of stress relaxation in dental polymers when applied to extended master curves generated from short time experimental data by WLF time temperature superposition method.The stress relaxation modulus changes with time at three different temperatures near the ambient body temperature were determined for selected mono-methacrylate (PEMA and PMMA) and dimethacrylate (bis-acryl) dental polymers. WLF time-temperature superposition procedure of logarithmic shift of the data from other temperatures to those at 37°C was used to generate extended master curves of relaxation modulus changes with time. The extended data were analyzed for conformity to three different predictive models of stress relaxation including Maxwell, KWW stretched exponential function and Nutting's power law equation.Maxwell model was found to be a poor fit for the extended data in all polymers tested, but the data showed a much better fit for KWW (0.870<R(2)<0.901) and Nutting's (0.980<R(2)<0.986) models. The non-exponential factor β in the KWW function and the fractional power exponent n in Nutting's equation were both significantly different for PEMA based system when compared to that of PMMA and bis-acryl systems.The mean values of β in KWW function and power exponent n in Nutting's equation for PEMA resin is consistent with significant viscoplastic contribution to its deformation under stress unlike in PMMA and bis-acryl resin systems. This may have significant bearing for PEMA use in medium to longer term stress-bearing applications even as a temporization material.