A joint viscoelastic model for full-spectrum creep prediction: application to PMMA/TiO 2 nanocomposites

Abstract Polymer nanocomposites are widely used in applications ranging from biomedical implants to aerospace components and emerging terahertz (THz) devices. However, their long-term mechanical performance is often limited by time-dependent viscoelastic deformation. This study investigates the nanoindentation creep behaviour of polymethyl methacrylate (PMMA) reinforced with varying titanium dioxide (TiO 2 ) filler contents (0%, 25%, and 60%) under different loading rates (200, 100, 50, and 20 μN s −1 ). While conventional viscoelastic models such as Kelvin-Voigt and Burgers capture either transient or steady-state behaviour of the creep, they fail to represent the full deformation spectrum. To address this limitation, we propose a novel hybrid viscoelastic framework that combines the Kelvin-Voigt and Burgers models into a unified joint model. This approach provides superior predictive accuracy across the entire creep curve and enables precise estimation of key mechanical parameters, including instantaneous and delayed elastic moduli and viscosity. The transition time (t c ) increases with higher TiO 2 content and varies strongly with loading rate, indicating reduced chain mobility and enhanced resistance to viscoelastic deformation. These insights offer a valuable pathway toward engineering durable, high-performance polymer nanocomposites for advanced structural and functional applications.