- K. Saalwächter, W. Burchard.
Cellulose in New Metal-Complexing Solvents. 2. Semidilute Behavior
Macromolecules 34, 5587-5598 (2001).
A high molar mass cotton linters cellulose was investigated by static and
dynamic light scattering in semidilute solutions of the cadmium complexing
Cd-tren (tren = tris(2-aminoethyl)amine). In this solvent cellulose has a Kuhn
segment length of 15.3 nm and represents a semiflexible chain. Apparent molar
masses Mapp(c) and apparent radii of gyration
Rg,app(c) were measured by light scattering. The
influence of interchain interactions could be split off using either the
theoretically well-known interaction among flexible chains or thin rigid rods,
respectively. Estimations of the true molar mass Mw(c) and true radius of gyration Rg(c) at finite concentration c were obtained. Both
molecular parameters increased with concentration when the overlap concentration
was exceeded by a factor 4. Simultaneously, a slow mode of motion appeared in
dynamic light scattering. The observations were interpreted as a result of
association. The thermodynamic interaction could also be separated from the
mutual diffusion coefficients and yielded the self-diffusion coefficients. A
hydrodynamic radius Rh(c) was defined by
applying the Stokes-Einstein relationship to the self-diffusion coefficient.
This radius is interpreted as a correlation length for the influence of
hydrodynamic friction. The zero shear viscosity showed behavior common to other
linear chain molecules in solution. Shear thinning occurred in the semidilute
regime. The curves could be fitted by the Cross approximation. A scaled master
curve was obtained. The relaxation times for disengagement τd increased with a power of 3.6 as the concentration was
increased. This exponent is larger than 2.0 as predicted from the reptation
model. The reversibly associating bonds cause a significant disengagement
retardation of entangled chains.