A fractional moisture-dependent viscoelasticity model for thermoplastic polymers

The viscoelastic behavior of thermoplastic polymers is significantly influenced by external conditions such as temperature and moisture. High temperatures accelerate polymer chain mobility, reducing stiffness, while moisture absorption acts as a plasticizer, altering key thermomechanical properties, including the glass transition temperature. The time- and temperature dependence for thermorheologically simple materials is effectively described by the time-temperature superposition principle. This study aims to describe the material properties of the thermoplastic polymers continuously under varying temperatures and moisture levels within the glass transition range. Fractional models are attractive for describing moisture-dependent material properties due to the few required parameters. Furthermore, in contrast to standard discrete rheological models, fractional approaches can accurately capture the distinct asymptotic behavior of the loss factor of the thermoplastic polymers. We validate the performance of a fractional model using a new data set that incrementally resolves the influence of moisture on mechanical properties and evaluate it against the well-established generalized Maxwell model. The results demonstrate that the fractional model accurately predicts the moisture-dependent thermoviscoelastic material behavior and requires significantly fewer model parameters compared to standard discrete rheological models.