Influence of branched poly(acrylic acid) on setting behaviour and mechanical properties of dental glass ionomer cementsTuesday (09.05.2017) 11:40 - 12:00 Part of:
Glass ionomer cements (GIC) are typically obtained by an acid-base reaction between a linear polymeric acid, e.g. poly(acrylic acid) (PAA), and an acid degradable fluoro-aluminosilicate glass. These cements are used in dentistry e.g. as temporary restoratives. One factor controlling cement mechanical performance is the molecular weight of the used PAA: typically, higher values result in improved mechanical properties through more effective chain entanglement. However, this also increases viscosity and renders homogenous mixing more challenging. By combining linear and branched PAA, the viscosity of the mixture can be adjusted as in general branched polymers exhibit lower viscosities at comparable molecular weights. This would allow an improved processing of GIC mixtures with favorable mechanical properties. The aim of this study was to evaluate the influence of branched PAA on mixing characteristics and mechanical properties of dental GIC. Commercial linear PAA (Mn = 35000; Advanced Healthcare, UK) was combined with three branched PAA with varying molecular weight (Mn = 7000, 10800 or 15500) and fluoro-aluminosilicate glass (4.5SiO2-3Al2O3-1.5P2O5-3CaO-2CaF2) to obtain stable cements. To determine working time (tw) and setting time (ts), rheological properties (storage modulus, loss modulus, loss tangent) were investigated by time sweep analysis using an oscillating rheometer (TA Instruments AR1500ex) in plate-plate mode. Compressive strength test samples (6 mm x 4 mm) were stored in de-ionised water (24h; 37°C) and subsequently tested (Universal Testing machine Zwick/Roell Z005, cross-head speed: 1 mm min-1). Using increasing amounts of branched PAA, tw and ts increased significantly, thereby facilitating mixing. Compressive strength increased with increasing molecular weight of branched polymers. However, when incorporating too high percentages of branched PAA, compressive strength dropped too much, owing to a lack of mechanical interlocking between polymer chains. This suggests only small percentages of branched polymers are necessary to tailor viscosity and setting.