310 research outputs found
Viscoelasticity and shear flow of concentrated, non-crystallizing colloidal suspensions: Comparison with Mode-Coupling Theory
We present a comprehensive rheological study of a suspension of
thermosensitive particles dispersed in water. The volume fraction of these
particles can be adjusted by the temperature of the system in a continuous
fashion. Due to the finite polydispersity of the particles (standard deviation:
17%), crystallization is suppressed and no fluid-crystal transition intervenes.
Hence, the moduli and in the linear viscoelastic regime as well as
the flow curves (shear stress as the function of the
shear rate ) could be measured in the fluid region up to the
vicinity of the glass transition. Moreover, flow curves could be obtained over
a range of shear rates of 8 orders of magnitude while and could be
measured spanning over 9 orders of magnitude. Special emphasis has been laid on
precise measurements down to the smallest shear rates/frequencies. It is
demonstrated that mode-coupling theory generalized in the integration through
transients framework provides a full description of the flow curves as well as
the viscoelastic behavior of concentrated suspensions with a single set of
well-defined parameters
Linear Viscoelasticity of Complex Coacervates
Rheology is a powerful method for materials characterization that can provide detailed information about the self-assembly, structure, and intermolecular interactions present in a material. Here, we review the use of linear viscoelastic measurements for the rheological characterization of complex coacervate-based materials. Complex coacervation is an electrostatically and entropically-driven associative liquid-liquid phase separation phenomenon that can result in the formation of bulk liquid phases, or the self-assembly of hierarchical, microphase separated materials. We discuss the need to link thermodynamic studies of coacervation phase behavior with characterization of material dynamics, and provide parallel examples of how parameters such as charge stoichiometry, ionic strength, and polymer chain length impact self-assembly and material dynamics. We conclude by highlighting key areas of need in the field, and specifically call for the development of a mechanistic understanding of how molecular-level interactions in complex coacervate-based materials affect both self-assembly and material dynamics
Rheological Characterization of Liquid-to-Solid Transitions in Bulk Polyelectrolyte Complexes
Polyelectrolyte complexation has long been known to result in both liquid and solid complexes. However, the exact nature of the liquid-to-solid transition remains an open question. We have used rheology to explain this phenomenon for the model system of poly(4-styrenesulfonic acid, sodium salt) (PSS) and poly(diallyldimethyl ammonium chloride) (PDADMAC) in the presence of potassium bromide (KBr). The use of a time-salt superposition allows for a detailed analysis of changes in the linear viscoelastic response for both liquid complex coacervates and solid polyelectrolyte complexes as a function of salt concentration, and facilitates unambiguous determination of the mechanism for this phase transition. Decreasing salt concentration, and the commensurate decrease in the water content of PSS/PDADMAC/KBr complexes is shown to lead to the formation of a physical gel due to the development of a network with trapped electrostatic crosslinks that percolates the sample at a critical salt concentration
- …