Biomedical subjects

S Kalachandra

Publications and source records attributed to S Kalachandra.

23 records · Page 2Linked to original sources

PubMed1989

Influence of fillers on the water sorption of composites.

The main objectives of this work were to characterize the water sorption of dental composites in terms of water uptake, diffusion coefficients (D), and polymer content, and to study how these parameters are influenced by the nature of the filler and the presence of 4-META (4-methacryloxy ethyl trimellitic anhydride). Four anhydrous composites--(1) tribasic calcium phosphate (TCP 50%) and triethylene glycol dimethacrylate (TEGDM), (2) silanated lithium aluminum silicate (SS, 75%) and TEGDM, (3) barium sulfate [BaSO4 (70%) and poly(methyl methacrylate) (PMMA)], and (4) silane-coated barium glass (SBG) (75%)--and PMMA were employed in the water-sorption studies at 37 degrees C. The effect of 5% 4-META on the diffusion and uptake of water was studied at 37 degrees C. The data conformed approximately to Fick's laws of diffusion. The values of D were found to be significantly smaller with SS, suggesting an effective coupling. Polymer contents in the latter two composites were determined by incineration of the samples to constant weight. The uptake of water by the filled specimens was about two-fold that which would be expected on the basis of the PMMA content. These filled specimens took nearly twice as much water as unfilled PMMA. An additional amount of water is perhaps accommodated at the interface between the filler and PMMA matrix. The D values for water in filled specimens were considerably larger than those in the unfilled specimens. It appears that the filler-matrix interface provides paths of facile diffusion similar to grain boundary diffusion.

Acrylates↗

PubMed1987

Water sorption of polymethacrylate networks: bis-GMA/TEGDM copolymers.

Polymethacrylate networks were made by copolymerization of a range of compositions of bis-GMA and triethylene glycol dimethacrylate (TEGDM). Polymerization was initiated both by heating with benzoyl peroxide or by photopolymerization (lambda greater than 400 nm) using camphoroquinone as sensitizer. The uptake of water increased from 3 to 6% as the proportion of TEGDM increased from 0 to 1.0. Intermediate compositions took up less water than would be predicted from the law of mixtures. Volumetric changes were determined and clinical significance discussed. A copolymer prepared by photopolymerization took up more water as the temperature was increased from 24-60 degrees C. In this range, values of the diffusion coefficient (D) conformed to the Arrhenius equation, D = Do exp (-E/RT), giving E = 42-46 kJ/mol and Do = 0.13 cm2 s-1.

Bisphenol A-Glycidyl Methacrylate↗

PubMed1985

Lubrication and surface chemical properties of ophthalmic solutions.

Surface chemical and lubrication parameters such as contact angle, surface tension, and coefficient of friction, together with viscosity, were measured for commercial ophthalmic solutions. A sensitive friction-testing instrument was employed to measure the coefficient of friction between low-energy polymeric surfaces, e.g., polymethylmethacrylate (PMMA). The viscosity of ten commercial tear substitutes employed in the present study ranged from 2 to 25 cp. The results showed that there is no correlation between the coefficient of friction and viscosity, surface tension, or contact angle of these tear substitutes. The coefficient of friction of tear substitutes appears to depend upon the structure, conformation, and adsorption characteristics of polymer as well as surface characteristics of sliding surfaces. A systematic study was undertaken to evaluate the effect of speed and load on the coefficient of friction between different sliding surfaces, namely PMMA/nylon and PMMA/PMMA. The analysis of data established that our system was operating in the region of "boundary lubrication." In general, a PMMA/PMMA system exhibited a lower coefficient of friction as compared with the PMMA/nylon system.

Lubrication↗

PubMedDate not supplied

Relationship between filler and matrix resin characteristics and the properties of uncured composite pastes.

The relationships between filler type, filler content, matrix resin composition and viscosity and the flow characteristics of composite paste formulations have been investigated. BIS-GMA and 10 experimental BIS-GMA analogues were diluted as needed with TEGDMA to produce 1000 and 2000 cp solutions corresponding to the viscosity range of commercial composite matrices. All of these resins were mixed with a silanated hybrid-filler and a silanated micro-filler. For each combination the maximum filler content was determined, as well as the paste consistencies (plasticities) at a series of filler contents. Paste consistencies were measured by a modification of the ADA specification methods developed for determining standard test consistencies for zinc phosphate and silicate cements. For each monomer-filler combination increasing filler contents resulted in monotonically reduced plasticities. The maximum filler contents appeared to be an attribute of the particular filler rather than matrix resin characteristics. For the materials used, the maximum contents were approximately 86 wt% (83 vol%) filler for the hybrid filler, and 36% wt% (24 vol%) for the microfilled material. At lower filler contents, plasticity differences unexpectedly existed even at equal filler contents and matrix viscosities. Evidence was found that the plasticity varied both with the base monomer composition and the amount of TEGDMA present. The results imply that compositional interactions between the filler and matrix influence the results when all known mechanical factors are controlled.

Absorption↗

PubMedDate not supplied

Water uptake of soft lining materials from osmotic solutions.

OBJECTIVES: The water uptake characteristics of soft lining materials are of obvious importance in that they are expected to function in the oral environment. Results for Novus (Hygenic Corp., Akron, OH, USA) show a very high uptake from distilled water. Despite this high uptake, Novus appears to function satisfactorily in the mouth. High water uptake of soft lining materials has been attributed to the presence of water soluble impurities that, on immersion, form solution droplets; the driving force for the uptake being the osmotic gradient between the droplets and the external solution. Uptake should therefore be less from ionic solutions. The object of this study was to test the applicability of this theory to Novus and two experimental soft lining materials. METHODS: Water uptake of two experimental materials and Novus has been determined from distilled water and two saline solutions (0.45 and 0.9 M). After 196 days specimens were desorbed to constant weight and then subjected to a second sorption cycle. RESULTS: Novus had the highest uptake from distilled water at approximately 18%, the experimental materials having an uptake approximately 7%. Desorptions were all rapid, minimum weight being reached within 1-2 days. Uptakes of the second sorptions from water were all higher. Uptake from saline solutions was approximately 12% for all materials, uptake from 0.9 M saline being the lowest. Second sorption results from solution were similar to the first. CONCLUSION: The results obtained support the theory that the high water uptake of elastomeric materials is osmotically driven.

Adsorption↗