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Rheology and gel point of the enzymatic hydrolysis of urea in the presence of urease
R. Serrato-Millán,L. Medina-Torres,F. Calderas,B.L. España-Sánchez,M. Estevez,A.R. Hernandez-Martínez,M. Cruz-Soto,I.C. Sánchez,R. Gómez-García,I. Sánchez-Betancourt,M.C. Velasquillo-Martínez,G. Luna- 한국유변학회 2017 Korea-Australia rheology journal Vol.29 No.1
This study reports on the rheology of the gelation kinetics of raw chitosan (CTS) solutions (2% w/v) produced by enzymatic hydrolysis of urea at different urea concentrations (40, 50, 60, 80, and 100 mM) in the presence of urease at 1 U/mL. Viscoelastic parameters and pH values were evaluated during gelation process and the rheological properties of CTS hydrogels produced were monitored after 24 h at 37°C to simulate human body temperatures. pH measurements suggest that above some critical urea concentration (50 mM) the time required (tgel) to reach the critical pH gelation shows no dependence on urea concentration (tgel was ca. 70 minutes). Above 50 mM of urea concentration, CTS hydrogels exhibit an elastic modulus G' higher than the viscous modulus G'' with no frequency dependence characteristic of a gel behavior. Gelation kinetics analyzed by rheology suggest that the G' (i.e., structure) development depends on urea concentration during solution neutralization.
C. G. Garay-Reyes,S. E. Hernández-Martínez,J. L. Hernández-Rivera,J. J. Cruz-Rivera,E. J. Gutiérrez-Castañeda,H. J. Dorantes-Rosales,J. Aguilar-Santillan,R. Martínez-Sánchez 대한금속·재료학회 2017 METALS AND MATERIALS International Vol.23 No.2
According to Lifshitz, Slyozov, and Wagner (LSW) and Trans-Interface Diffusion-Controlled (TICD) theoreticalmodels, this paper reports the microstructure and its coarsening behavior of γ' metastable-coherentprecipitates in concentration gradient of Ni-13.75Ti (at%)/Ni generated by diffusion couple. The coarseningof precipitates was evaluated in two different Ti contents (R1-11.4Ti (at%) and R2-13Ti (at%)) generated alongthe concentration gradient and includes average size, size distributions and growth rate. The solvus andmetastable-coherent bimodal lines as determined at 850 °C of 9.16 (at%) and 9.92Ti (at%) respectively byscanning electron microscopy. This paper suggests that elastic strains produced by the matrix/precipitate latticemismatch caused significant deviations between the experimental results and those predicted by the LSW or TIDCtheories. Activation energies for TIDC (Qi) and LSW (Qr) are Qr: 219.69 and 172.61 kJ mol-1for R1 and R2regions, respectively, and Qi: 218.46 and 164.56 kJmol-1for R1 and R2 regions, respectively. A concentration gradientallows the study of various alloys with different concentration and volume-fraction in a single sample.