LIVING POLYMERS IN ORGANIC-SOLVENTS - STRESS-RELAXATION IN BICOPPER TETRACARBOXYLATE TERT-BUTYL CYCLOHEXANE SOLUTIONS

Viscoelastic solutions of a bicopper tetracarboxylate complex in tert-butylcyclohexane have been studied by dynamic rheology in a wide range of concentrations (0.5-15.5% volume fraction). The zero shear viscosity, the elastic modulus, the terminal stress relaxation time and the height of the high-frequency dip, in a Cole-Cole representation of the complex elastic modulus, follow scaling laws. The related exponents are discussed in the context of the physics of ''living polymers'': a term used to describe worm-like species undergoing scission/recombination reactions competing mainly with the reptation motions of the chains. The current system, made up of molecular threads (17.5 angstrom diameter) of Cu2(O2C-CH(C2H5) C4H9)4 in the apolar solvent, is representative of a ''living polymer'' where, instead of mechanisms involving transient star polymeric crosslinks, a reversible scission mechanism prevails. The dynamics in the high-frequency range evolves from a regime where reptation is the dominant relaxation mechanism to a cross-over regime where ''breathing'' fluctuations and Rouse motions become important. Large modifications of the stress relaxation function occur for more concentrated systems. The binary system is the first example of a ''living polymer'' in an organic solvent and exhibits elastic moduli (G almost-equal-to ca. 120 Pa at phi = 1 %) which are at least 20 times larger than those found for the aqueous ''living polymer'' systems.