RESUMEN
The thermal degradation of a graphene-like two-dimensional honeycomb membrane with bonds undergoing temperature-induced scission is studied by means of Molecular Dynamics simulation using Langevin thermostat. We demonstrate that at lower temperature the probability distribution of breaking bonds is highly peaked at the rim of the membrane sheet whereas at higher temperature bonds break at random everywhere in the hexagonal flake. The mean breakage time τ is found to decrease with the total number of network nodes N by a power law τ â N(-0.5) and reveals an Arrhenian dependence on temperature T. Scission times are themselves exponentially distributed. The fragmentation kinetics of the average number of clusters can be described by first-order chemical reactions between network nodes n(i) of different coordination. The distribution of fragments sizes evolves with time elapsed from initially a δ-function through a bimodal one into a single-peaked again at late times. Our simulation results are complemented by a set of 1st-order kinetic differential equations for n(i) which can be solved exactly and compared to data derived from the computer experiment, providing deeper insight into the thermolysis mechanism.
RESUMEN
We consider the fracture of a free-standing two-dimensional (2D) elastic-brittle network to be used as protective coating subject to constant tensile stress applied on its rim. Using a molecular-dynamics simulation with a Langevin thermostat, we investigate the scission and recombination of bonds, and the formation of cracks in the 2D graphenelike hexagonal sheet for different pulling force f and temperature T. We find that bond rupture occurs almost always at the sheet periphery, and the first mean breakage time <τ> of bonds decays with membrane size as <τ> âN(-ß), where ß≈0.50±0.03 and N denotes the number of atoms in the membrane. The probability distribution of bond scission times t is given by a Poisson function W(t)ât(1/3)exp(-t/<τ>). The mean failure time <τ(r)> necessary to rip off the sheet declines with growing size N as a power law <τ(r)>âN(-φ(f)). We also find <τ(r)>âexp(ΔU(0)/k(B)T), where the nucleation barrier for crack formation ΔU(0)âf(-2), in agreement with Griffith's theory. <τ(r)> displays an Arrhenian dependence of <τ(r)> on temperature T. Our results indicate a rapid increase in crack spreading velocity with growing external tension f.
Asunto(s)
Membranas Artificiales , Modelos Químicos , Modelos Moleculares , Polímeros/química , Simulación por Computador , Módulo de Elasticidad , Modelos Estadísticos , Estrés MecánicoRESUMEN
Two groups of probands have been studied: experimental and control. After the clinically performed oral hygiene all probands stopped the brushing of the teeth for a 7-day period. The oral cavity was twice daily rinsed with 10 ml solution for 30 s--with "Antiplaque C" for the experimental and with placebo--for the control. All probands were taken bilateral probe-impressions according to Dzemileva, Stojanova and Popov in the area of 31 tooth, stained by crystal-violet and observed under light microscope and immersion. The cytological finding in the experimental group did not reveal an inflammatory process in the gingival sulcus whereas increased gingival exudate was established in the placebo group as well as intensified leukocyte migration and single erythrocytes (subclinical phase of inflammation). The bacterioscopic finding in the group, systematically used "Antiplaque C", was reduced to poor dental plaque, preserved "fresh" till the end of the 7-day period being predominantly coccoid-rod-like. In the placebo group the dental plaque "ages"--from coccoid-rod-like it turns into mainly fibril with multiple vibrios and spirillae. The cytological and bacterioscopic data objectivize the inhibitory effect of "Antiplaque C".
Asunto(s)
Placa Dental/prevención & control , Antisépticos Bucales/uso terapéutico , Adulto , Bacterias/efectos de los fármacos , Placa Dental/microbiología , Líquido del Surco Gingival/microbiología , Humanos , Pruebas de Sensibilidad Microbiana , Antisépticos Bucales/farmacologíaRESUMEN
The authors propose a model complex of methods for objectivization of the effect of oral-hygienic agents. Attention is drawn to the selection and preparation of the probands for the clinical experiment, describing also the conditions of the experiment. The most appropriate indices for the evaluation of the effect of the oral-hygienic agents also presented: 1. Doublet plaque-index according to T. Dzemileva and M. Drjankova--enabling the parallel differential evaluation and recording of the incidence and thickness of the dental plaque. 2. Index of Fjodorov and Volodkina--particularly adequate for investigation of the agents for mechanical removal of dental plaque. 3. PMA-index--giving an idea of the therapeutic-prophylactic action of the oral-hygienic agents on gingival status. 4. Supragingival and bilateral probe-imprint--according to the well known method of T. Dzemileva et al. The proper selection of the methods for objectivization of the effect of the oral-higienic agents will guarantee the reliability of the results.