Compendium Supplement Seopodent May 2010 : Page 3
Fisher, www.thermofisher.com). A linometer (Academisch Centrum Tandheelkunde Amsterdam, www.acta.nl) was employed to measure the polymerization volumetric shrink- age. Dynamic polymerization stress was measured with a cantilever beam tensometer (American Dental Association Health Founda tion, www.ada.org), and flexural strength and modulus were obtained in three-point bending with 2 mm x 2 mm x 25 mm specimens on a 20-mm span at a crosshead speed of 1 mm/min (Mini Bionix®, MTS Systems, www.mts.com). RESULTS AND DISCUSSION The DADMA monomers have molecular weights of 673 g/mol to 849 g/mol with initial methacrylate group con- centrations of 2.4 mol/L to 2.7 mol/L as compared with the values for TEGDMA of 286 g/mol and 7.5 mol/L, re- spectively. The DADMA homopolymers have been shown to undergo rap id, high conversion photopolymerization to produce ex tremely hydrophobic rubbery polymers.7 Mon omer I has no hydrogen bond donor functionality and presents the lowest viscosity in the DADMA series. It has limited miscibility with Bis-GMA or UDMA but is compatible with Bis-EMA. Conversely, DADMA mono- mers II and III, which do form hydrogen bonds through the hy droxyl or urethane functionality, respectively, are compatible with Bis-GMA or UDMA but only partially miscible with Bis-EMA. Therefore, while selected binary comon omer mixtures involving the DADMA monomers and conventional di meth acrylates could be prepared as ho- mogeneous mixtures, ternary compositions that combined one DADMA monomer with both Bis-EMA and UDMA Table 1: Resin Properties Resin Bis-GMA/TEGDMA UDMA/I/Bis-EMA UDMA/I/Bis-EMA Bis-EMA/II/UDMA Bis-EMA/II/UDMA Bis-EMA/III/UDMA allowed formulations to be prepared in which the degree of thermody namic compatibility could be precisely tuned. Formulations near the thermodynamic stability boundary as monomers were found to undergo a polymerization- induced phase separation (PIPS) to generate heterogeneous polymer networks. The extent of phase separation depends on the resin composition and reaction conditions with greater sensitivity apparently related to reaction temperature as opposed to reaction kinetics. The final heterogeneous polymers obtained through the PIPS process were slightly to moderately hazy in appearance. The optical transmission efficiency passes through a minimum during polymeriza- tion (data not shown here) primarily because of refractive in dex changes associated with different reaction rates in the different phases. Conversion values of all the experimental resins were significantly greater than the control (Table 1). The flex- ural strength of the experimental resins was less than that of Bis-GMA/TEGDMA with the exception of the resin based on the urethane-containing monomer III, which notably contained the highest concentration of DADMA monomer used in this study. The experimental resins were generally less brittle than the control. The polymerization shrinkage results for various resin formulations are shown in Figure 2. It is evident that despite the greater limiting conversion val ues associated with the DADMA-based res- ins, polymerization shrinkage is significantly reduced. In addition, the shrinkage profiles for some of the heteroge- neous experimental materials demonstrate a modest volume recovery during the postcure observation interval. This type of novel behavior has been reported for other systems Mass Ratio 70/30 60/20/20 55/20/25 70/20/10 70/17/13 60/25/15 Conversion, % 64.1 ± 2.8a 75.1 ± 0.4b 70.1 ± 1.1c 80.1 ± 1.8d,e 77.6 ± 1.9b,d 82.9 ± 1.1e Flex Strength, MPa Strain at Break, % 93.2 ± 8.3a 65.5 ± 7.9b,c 62.2 ± 3.4b 73.7 ± 0.8c 74.9 ± 4.0c 85.3 ± 2.3a 7.1 ± 2.4a 10.1 ± 4.1a,b 13.1 ± 2.3b 9.5 ± 0.4a,b 6.8 ± 0.6a 9.5 ± 1.4a,b Within each column, the letters indicate statistically significant differences (P < .05) as determined by a one-way ANOVA and Tukey post-hoc pairwise comparison test. Compendium—Volume 31 (Special Issue 2) 3
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