Effect of Sulfonate-, Phosphonate-, and Lipoate-Functional Modifiers on the Mechanical and Thermal Properties of Polymethyl Methacrylate Denture Base Resin

Objective: This study aimed to evaluate the mechanical and thermal properties of heat-polymerized polymethyl methacrylate (PMMA) denture base resin modified with different concentrations of sodium 4-styrenesulfonate (SO), 5,5-bis(diisopropoxyphosphoryl)pentyl methacrylate (PO), and 5,5-bis(diisopropoxyphosphoryl)pentyl lipoate (LIP). Materials and Methods: Heat-polymerized PMMA specimens were divided into a control group and experimental groups modified with 2%, 3%, or 5% SO, PO, or LIP (n = 8 per group). Maximum deflection, flexural strength, and Young’s modulus were determined using a three-point bending test. Surface hardness was evaluated using the Vickers hardness test, thermal behavior was assessed by thermogravimetric analysis (TGA), and fracture surfaces were examined by scanning electron microscopy (SEM). Data were analyzed using two-way ANOVA (modifier type × concentration) with Bonferroni-adjusted pairwise comparisons, and comparisons including the control group were performed using Welch’s or one-way ANOVA followed by Tamhane’s T2 or Bonferroni post hoc tests, depending on the homogeneity of variances (α = 0.05). Results: Modifier type significantly affected maximum deflection, flexural strength, Young’s modulus, and Vickers hardness of specimens. The LIP-containing groups showed the highest mean deflection values but significantly lower flexural strength, Young’s modulus, and hardness than the control group. In contrast, the SO-containing groups generally maintained mechanical properties closer to those of unmodified PMMA. TGA showed that all modifiers altered the thermal degradation profile of PMMA, with PO increasing the main degradation temperature by approximately 40 °C. Conclusions: The investigated modifiers produced distinct, modifier-dependent changes in the mechanical and thermal behavior of heat-polymerized PMMA. Although LIP increased deformation capacity, this occurred at the expense of flexural strength and stiffness. SO and PO modifications showed comparatively smaller changes in mechanical properties at selected concentrations. Further studies including direct fracture-toughness, fatigue, and clinically relevant aging assessments are required to determine whether these modifications provide functional advantages for denture base applications.