Influence of molecular weight of polymethyl(methacrylate) beads on the properties and structure of cross-linked denture base polymer.

OBJECTIVES Denture base polymers are multiphase polymers made of polymethyl(methacrylate) (PMMA) beads and monomers containing methylmethacrylate and cross-linking agent. The cross-linking agent is typically dimethacrylate but methacrylated dendrimers have been tested. The aim of this study was to investigate the influence of the molecular weight of the PMMA beads on the mechanical properties of cross-linked denture base polymers. METHODS Resin powder with three different molecular weights (Mw 120,000, 350,000, and 996,000) and a commercial autopolymerizing denture base resin (Palapress, Mw 220,000) were tested. The resin monomer liquid was applied with a methylmethacrylate mixture containing 4.6 vol% dendrimer (DD1, VTT Processes). To investigate the flexural properties, the surface microhardness of the PMMA beads, and the thickness of the swollen interpenetrating polymer network (IPN) layer on the PMMA beads, test specimens (3.3×10×65 mm) were fabricated from autopolymerizing resin using a powder/liquid ratio of 10 g/7 ml. The specimens were polymerized in distilled water maintained at 55 °C under 0.4 MPa pressure for 20 min. The flexural strength and flexural modulus (n=8/group) were measured with a three-point bending test at a crosshead speed of 5 mm/min. The Vickers hardness of the area of the polymer with PMMA beads (n=10/group) was tested using a load of 98.12 mN for 5 s. In addition, the thickness of the swollen layer on the PMMA beads was measured (n=10/group). Statistical analysis was performed using a one-way ANOVA and Tukey's test. RESULTS The flexural strength of the specimens with Mw 220,000 and 350,000 PMMA beads was significantly higher than the strength of specimens with beads having other molecular weights. The flexural modulus of specimens with Mw 120,000 PMMA beads was the lowest. There was no difference in the surface microhardness among all groups. The thickness of the swollen IPN layer on specimens with 120,000 Mw PMMA beads was significantly higher than in the other groups. CONCLUSIONS The molecular weight of the PMMA beads of multiphase denture base polymers considerably influences their flexural properties and formation of IPN layer between the matrix polymer and the PMMA beads.