Linear Viscoelasticity
of Polyelectrolyte Complex
Coacervates
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Abstract
Two flexible, oppositely charged polymers can form liquid-like
complex coacervate phases with rich but poorly understood viscoelastic
properties. They serve as an experimental model system for many biological
and man-made materials made from oppositely charged macromolecules.
We use rheology to systematically study the viscoelastic properties
as a function of salt concentration, chain length, chain length matching,
and mixing stoichiometry of model complex coacervates of poly(<i>N</i>,<i>N</i>-dimethylaminoethyl methacrylate), PDMAEMA,
and poly(acrylic acid), PAA. The dynamics of making and breaking ionic
bonds between the oppositely charged chains underlie all linear viscoelastic
properties of the complex coacervates. We treat (clusters of) ionic
bonds as sticky points and find that there is a remarkable resemblance
between the relaxation spectra of these complex coacervates and the
classical sticky Rouse model for single polymer systems. Salt affects
all relaxation processes in the same way, giving rise to a widely
applicable time–salt superposition principle. The viscoelastic
properties of the complexes are very different from those of the individual
components. In the complexes with a chain length mismatch, the effect
of the mismatch on the viscoelastic properties is not trivial: changing
the length of the polycation affects the relaxation behavior differently
from changing the length of the polyanion