The influence of poly(acrylic) acid number average molecular weight and concentration in solution on the compressive fracture strength and modulus of a glass-ionomer restorative

ObjectivesThe aim was to investigate the influence of number average molecular weight and concentration of the poly(acrylic) acid (PAA) liquid constituent of a GI restorative on the compressive fracture strength (σ) and modulus (E).MethodsSeries of PAA solutions for eight experimental PAAs with molecular weights ranging from 5000 to 200,000 were prepared at concentrations ranging from 10 to 60%. The PAA number average molecular weights were determined using gel permeation chromatography and the viscosity at each concentration was determined using a digital viscometer. The PAA solutions were hand-mixed with a commercial GI restorative powder (Ionofil Molar; Voco, Cuxhaven, Germany). σ and E were determined at 24 h using cylindrical (6 mm height, 4 mm diameter) specimens (n = 20). Data was analyzed using regression analyses and two-way analyses of variance at p < 0.05.ResultsRegression analyses showed a significant reduction in PAA solution viscosity with reducing PAA number average molecular weight. Two-way ANOVAs for σ and E indicated a significant interaction of number average molecular weight × concentration (p < 0.001 and p < 0.001, respectively). Quadratic regression analyses showed significant curvature (n-shaped curves) within σ and E data for individual PAA number average molecular weights with increasing concentration. As a result, the optimum PAA concentration for each PAA number average molecular weight was determined from the curve vertexes.SignificanceThe choice of PAA molecular weight and concentration and the subsequent viscosity of the PAA solution have a significant influence on the σ and E of a GI restorative, however, the relationship is not a simple one.