Activation Energy for the Relative Stability of the Residual Monomer and the Morphologies of Cryogenically Fractured Surfaces of Acrylic Denture-Base/PVC Composites

The intensity of the overtone infrared absorption band at 1.62 μm due to the monomeric methyl methacrylate was used to estimate the relative monomer concentration of acrylic denture base resins. It was found that the amount of residual monomer decreases at first very rapidly and then followed, by a rather sluggish decline, but did not diminish to zero concentration before reaching an apparent equilibrium during the period （10 h） of cure investigated. Curing temperature affected the residual monomer concentration of the cold-cured resins. The apparent equilibrium concentration was attained with higher residual monomer concentration in the sample cell cured at 22 ℃ than that cured at 60℃. However, there was no significant difference in the level of the residual monomer concentration between opened and closed cell samples for the period （10 hours） of cure. Assuming a limiting value of （H^-h）,where （h） is the overtone IR peak height at 1.62μm, and （H） being the normalized adjacent IR peak at 1.672μm, the temperature-dependence of the activation energy for the relative stability of the residual monomer was determined using the Arrhenius plots. Materials cured in air under open conditions, needed larger energy （3.297 and 4.137 KJK^-1, respectively for Denstsply Maxum and Pour-n-Cure） to achieve some level of monomer relative stability than those stored under closed conditions （2.376 and 3.656 KJK^-1, respectively for Denstsply Maxum and Pour-n-Cure）. The morphology of the cryogenically fractured acrylic denture resin showed some fibrillar texture for the pure PMMA and micro-domains as a function of specific composition in the blend of PMMA/PVC, suggesting a possible cohesive failure in the matrix.