Degree of Conversion of Bis-acrylic Based Provisional Crown and Fixed Partial Denture Materials

Monomers in the polymer-based provisional crown and fixed partial denture materials form polymer network as carbon-carbon double bonds are converted to carbon-carbon single bonds during polymerization. This reaction is not complete. There will be a certain amount of monomer remaining in the resin materials. This is influenced by several factors, such as type of monomer, curing temperature, exposure to oxygen, and the amount of initiator, etc. The degree of conversion is a measure of the carbon double bonds converted into single bonds. The degree of conversion may influence the ultimate mechanical and physical properties of the materials. The high levels of the residual monomer may cause deleterious effect on properties, such as low hardness, low strength, low resistance to wear, and low color stability. In addition, the residual monomer may irritate the surrounding tissues, including pulp, and oral mucosa. A number of reports have appeared in the literature regarding hypersensitivity to the residual monomer, such as pulpitis, burn, glossitis, stomatitis, generalized erythematous rash, although its possibility was low. The residual monomer may elute from the polymer-based provisional crown and fixed partial denture materials and diffuse into pulp and saliva resulting in redness and a burning sensation of the oral mucosa. Polymerization takes place as external energy, such as heat, light and chemicals, forms free radicals from suitable initiator molecules, and a free radical attacks on the double bond (π-bond) of a vinyl group in a monomer, resulting in the new formation of a single bond (σ-bond) to another carbon and an unpaired electron. As polymerization progresses, the amount of aliphatic carbon double bond decreases. Infrared spectroscopy has been used widely for measuring the unreacted carbon double bond. Spectroscopically, the decrease of aliphatic carbon double bond causes a decrease in the absorbance at 1637 cm. BisGMA has a functional group which does not participate in polymerization. The aromatic double bond of this group (phenyl group) has an absorbance at 1608 cm. This absorbance remains constant before and after polymerization. This method is based on comparing the intensities of the absorbance of carbon-carbon double bonds in methacrylate group before and after polymerization. In aromatic moieties, the absorbance of aromatic double bonds can be used as an internal standard. The purpose of this study was to measure the degree of conversion of three bis-acrylic based provisional crown and fixed partial denture materials by using an infrared spectroscopic method. The null hypothesis to be tested is that there is no significant difference in the degree of conversion among bis-acrylic based provisional crown and fixed partial denture materials.