1. College of Dentistry, The Ohio State University, Columbus, Ohio, USA

1. Effects of N-vinyl caprolactam containing polyacids on fracture toughness and flexural strength of Glass-ionomer Cements (GICs) Alireza Moshaverinia1, Fengyuan Zheng1, Nima Roohpour2, Sahar Ansari1 and Scott Schricker1 1. College of Dentistry, The Ohio State University, Columbus, Ohio, USA 2. Department of Materials, Queen Mary University of London, Mile End Road, London E1 4NS (UK) Introduction Fracture Toughness and Biaxial Flexural Strength Test Plane strain fracture toughness (KIc) was measured in accordance with ASTM Standard 399-05.33. Minicompact specimens (Figure 2) with a precrack were formulated. Biaxial flexural strength (BFS) tests were conducted using disc shaped samples (10 mm diameter and 1 mm thickness). The specimens were conditioned in distilled water at 37oC for one week (N=7). Fracture toughness and flexural strength testing were conducted on a screw-driven universal testing machine using a cross speed rate of 0.5 mm/min. Micro-Hardness Measurements Cylindrical shaped samples (6 mm diameter and 3 mm thickness) were prepared and used (N=10). After initial setting, samples were immersed and stored in distilled water at 37oC for 24 hrs and 1 wk. The hardness of the specimens was determined using a microhardness tester ( MVK-E, M 400, Leco, MI, USA) with a diamond indenter (100 g load and dwell time of 10 s) [2]. The mechanical strength values and hardness numbers were evaluated and compared with the control group (Fuji IX GIC). The mean data obtained was subjected to one way analysis of variance (ANOVA) at α = 0.05. * Glass-ionomer cements (GICs) have been successfully used as a dental restorative material since their invention by Wilson and Kent in early 1970. GICs are well-known for their unique properties such as direct adhesion to tooth structure, anticariogenic properties, low shrinkage and biocompatibility [1]. Their disadvantages compared to other restorative materials are brittleness and low mechanical properties. NVC (N-vinyl caprolactam) containing polymers have attracted much attention due to their thermo sensitivity and biocompatibility and they have wide range of biomedical applications. In this study it is hypothesized that copolymers of itaconic, acrylic acid and NVC would enhance the mechanical properties of GICs by increasing the chain mobility. Therefore, the main aim of this study was to investigate the effect of NVC containing terpolymer on the clinically related mechanical properties such as fracture toughness, microhardness and flexural strength of conventional GIC. * Figure 3. Vickers Hardness (VHN) results for NVC modified GIC and control groups. Discussion Carboxylic acid containing polymers with non-reactive hydrophilic spacers in the back-bone will allow for greater mobility of the carboxylic acid groups and decrease the steric hindrance [3]. As a result, the modified acid polymers have a greater freedom to react with Ca2+ and Al3+ ions from the glass resulting in greater filler loading, polysalt formation and homogeneity. Moreover, as NVC exhibits strong hydrophilic domains due to presence of amide and carbonyl groups. These hydrophilic domains can inhibit the separation of the planes of atoms which affects the response of the material during fracture toughness study, hardness and BFS (biaxial flexural strength) tests. The experimental cements show significantly higher fracture toughness and BFS in comparison to the Fuji IX control. Considerable improvements in mechanical properties of experimental cement may indicate increased homogeneity and degrees of polysalt bridge formations of the final set glass cement. Conclusion NVC containing polymers are capable of enhancing the fracture toughness, flexural strength and hardness of conventional GIC. This type of glass ionomers are promising restorative dental materials with improved mechanical properties. Materials and Methods Polymer synthesis & characterization FORCE Figure 2. Schematic representation of minicompact specimen prepared for fracture toughness study. An acrylic acid- itaconic acid- NVC terpolymer (Figure 1) with 8:1:1 (AA:IA:NVC) molar ratio was synthesized by free radical polymerization and characterized using FTIR, and 1H-NMR. Results Results of fracture toughness and biaxial flexural study are shown in Table 1 and hardness test in Figure 3. NVC modified GICs exhibited significantly higher fracture toughness and flexural strength in comparison to commercially available Fuji IX GIC. Table 1. Fracture toughness and biaxial flexural strength values of experimental and control GIC samples. Figure 1. Chemical structure of the synthesized AA-IA-NVC terpolymer Sample Preparation The experimental GIC samples were made from a 50% solution of the synthesized terpolymer with Fuji IX powder in a 3.6: 1 P/L ratio. Specimens were mixed and fabricated in room temperature, based on the manufacturer's instructions. For microhardness test five samples were made from each material for each time interval (24 hrs and 1 week). * p< 0.05 † The large standard deviation (SD) for the experimental sample is due to one data point which might have been a defective sample but it could not be excluded. After the elimination of the outlier the SD would be 0.08. References: 1. Wilson AD et al. Glass-ionomer cement. Quintessence Pub. Co. 1988. 2. Hammouda IM. J Mech Behav Bomed Mater 2009; 2:73-81. 3. Culbertson BM. Prog Polym Sci 2001; 26: 577-604