Structural studies of thermally stable, combustion-resistant polymer composites

Composites of the industrially important polymer, poly(methyl methacrylate) (PMMA), were prepared by free-radical polymerization of MMA with varying amounts (1–30 wt. %) of sodium dioctylsulfosuccinate (Aerosol OT or AOT) surfactant added to the reaction mixture. The composites with AOT incorporated show enhanced resistance to thermal degradation compared to pure PMMA homopolymer, and micro-cone combustion calorimetry measurements also show that the composites are combustion-resistant. The physical properties of the polymers, particularly at low concentrations of surfactant, are not significantly modified by the incorporation of AOT, whereas the degradation is modified considerably for even the smallest concentration of AOT (1 wt. %). Structural analyses over very different lengthscales were performed. X-ray scattering was used to determine nm-scale structure, and scanning electron microscopy was used to determine μm-scale structure. Two self-assembled species were observed: large phase-separated regions of AOT using electron microscopy and regions of hexagonally packed rods of AOT using X-ray scattering. Therefore, the combustion resistance is observed whenever AOT self-assembles. These results demonstrate a promising method of physically incorporating a small organic molecule to obtain a highly thermally stable and combustion-resistant material without significantly changing the properties of the polymer

Coronal displacement of cementum: Correlation between age and coronal movement of cementum in impacted teeth

This study was conducted on 48 impacted and 51 erupted maxillary permanent canine teeth extracted from healthy patients aged 13-73 years. Longitudinal buccolingual ground sections were prepared. The distance between the edges of cementum and enamel in each specimen was measured with an eyepiece micrometer. A correlation test was applied between age and the distance between enamel and cementum measurements. in impacted teeth, depending on age, cementum had a tendency to overlap the enamel; there was a linear correlation between age and coronal displacement of cementum (n-2 = 46, r = 0.69, distance (mu m) = 21.7* age (years) -440, p0.05). Results of the study indicated that the cementum in impacted teeth migrated coronally during the ageing process. This may be related to continually erupting forces which affect the impacted teeth and may be a mechanism by which the teeth are protected at the cemento-enamel junction (CEJ). This phenomenon could be used in forensic dentistry to determine age. The absence of correlation in erupted teeth could be attributed to the masking effects of extrinsic factors