Nanoindentation-derived viscoelastic creep and ageing stability of heat-cured PMMA denture bases using a three-element Voigt model.

Abstract

The viscoelastic creep behaviour and long-term stability of heat-cured poly(methyl methacrylate) (PMMA) denture bases under clinically relevant processing conditions remain poorly characterised, necessitating the optimisation of fabrication protocols. In this study, twelve PMMA specimens (Sets A-C; 4 specimens per set, 25 indents/specimen) were prepared by varying curing pressure (500-2000 psi), time (30-180 min), or temperature (80-140°C). Nanoindentation creep tests (20 mN peak load; 10 s load, 20 s hold, 10 s unload; 10 Hz) recorded depth-time data, which were fitted to a three-element Voigt model (R² > 0.99) to extract instantaneous modulus (E₁^e), delayed modulus (E₂^e), and viscosity (μ). Statistical analyses (one-way ANOVA with Tukey HSD, α = 0.05) revealed that extending curing time from 30 to 180 nearly doubled E₁^e (2.15 ± 0.12-4.26 ± 0.15 GPa; p < 0.001) and increased μ by 332 % (30 ± 3-129 ± 8 GPa·s; p < 0.001), with optimal properties at 120°C (E₁^e = 4.72 ± 0.10 GPa; μ = 133.6 ± 7 GPa·s). Ageing for 12 months induced significant stiffening in under-cured samples (ΔE₁^e = +12 %; p < 0.05), but did not affect well-cured specimens. These results show that curing time and temperature critically govern PMMA's short‑term creep resistance and ageing behaviour, and that a 120°C × 60 min protocol maximises mechanical stability and durability, offering predictive insight for tailoring denture fabrication, although the increased risk of pore formation at this temperature must be considered.