Rheological properties of microcrystalline chitosan gels

The procedure of thermocatalic processing of chitosan developed allows one to decrease its degree of polymerization to the so-called leveling-off degree and, after the precipitation from a solution and washing, to obtain hydrogels of microcrystalline chitosan (MCC.) The flow curves of the MCC gels showed that a spatial structure was formed in the gels even at low their concentrations. The measurements of vicoelastic properties of the dispersions verified the existence of such a structure. It is shown that, at MCC concentrations above 3%, a transition from the isotropic phase to a liquid-crystalline one occurs. For comparison, the flow properties of chitosan solutions in acetic acid were studied too. The measurements showed that, at the solution concentrations considered, they did not exhibit liquid-crystalline rheological properties. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 43, No. 3, pp. 385–398, May–June, 2007.     

The relative expansion of polymer gels in uniaxial stretching

In the stretching of undried polyacrylonitrile fibers in the gel state, an increase in the cross sectional area, calculated from the diameter, is observed as compared with the cross sectional area obtained from the fact that the volume of the sample remains constant during stretching. The effect of relative expansion of polymer gels during the stretching process is explained by a delay in the relaxation processes, related to the orientation of amorphous sections and the rearrangement of the microstructure on a supermolecular level.All-Union Scientific-Research Institute of Synthetic Fibers, Kalinin. Translated from Mekhanika Polimerov, No. 5, pp. 927–929, September–October, 1972.     

Effects of permeability and viscosity in linear polymeric gels

We propose and analyze a mathematical model of the mechanics of gels, consisting of the laws of balance of mass and linear momentum of the polymer and liquid components of the gel. We consider a gel to be an immiscible and incompressible mixture of a nonlinearly elastic polymer and a fluid. The problems that we study are motivated by predictions of the life cycle of body‐implantable medical devices. Scaling arguments suggest neglecting inertia terms, and therefore, we consider the quasi‐static approximation to the dynamics. We focus on the linearized system about stress‐free states, uniform expansions, and compressions and derive sufficient conditions for the solvability of the time‐dependent problems. These turn out to be conditions that guarantee local stability of the equilibrium solutions. We also consider non‐stress free equilibria and states with residual stress and derive an energy law for the corresponding time‐dependent system. The conditions that guarantee stability of solutions provide a selection criteria of the material parameters of devices. The boundary conditions that we consider are of two types, displacement‐traction and permeability of the gel surface to the fluid. We address the cases of viscous and inviscid solvent, assume Newtonian dissipation for the polymer component, and establish existence of weak solutions for the different boundary permeability conditions and viscosity assumptions. We present two‐dimensional, finite element numerical simulations to study stress concentration on edges, this being the precursor to debonding of the gel from its substrate. Copyright © 2015 John Wiley & Sons, Ltd.     

Studies in oxidation

The kinetics of the oxidation of toluene and its ring-substituted derivatives as well as of diphenyl methane by chromic acid have been studied in acetic acid-water systems. The reaction had to be in unbuffered solutions to get a measureable rate. Product analysis showed a step-wise reaction. Rate constants have been evaluated, using a graphical method, for a three-stage consecutive and concurrent reaction, the values being obtained for the first and last stages alone. Electron attracting substituents retard the reaction while those facilitating electron release accelerate the reaction. Diphenylmethane also undergoes only a step-wise reaction with the second step much slower than the first. A comparison of experimental activation energies indicates clearly that the entropy term of the rate equation exerts possibly an even greater influence on the reaction.