7 research outputs found
Linear Viscoelasticity of Polyelectrolyte Complex Coacervates
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 polyÂ(<i>N</i>,<i>N</i>-dimethylaminoethyl methacrylate), PDMAEMA,
and polyÂ(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
DataSheet1_Quantitative coarse graining of laminar fluid flow penetration in rough boundaries.PDF
The interaction between a fluid and a wall is described with a certain boundary condition for the fluid velocity at the wall. To understand how fluids behave near a rough wall in a completely laminar flow regime, the fluid velocity at every point on the rough surface may be provided. This approach requires detailed knowledge of, and likely depends strongly on the roughness. Another approach of modelling the boundary conditions of a rough wall is to coarse grain and extract a penetration depth over which on average the fluid penetrates into the roughness. In this work, we examine the impact of well-defined patterned surfaces on the fluid flow behaviour. We considered two extreme cases: one with horizontal ridges and another with vertical ridges on the wall and an intermediate case with ridges at an angle on the wall. We show that for a broad range of periodic roughness patterns and relative flow velocities, a universal penetration depth function can be obtained. We obtain these results with experiments and complementary numerical simulations. We evaluate how this penetration depth depends on the various roughness parameters such as ridge depth, ridge spacing and ridge angle. Our results present a novel approach to investigating wall roughness boundary conditions by considering the penetration depth δ that captures the spatially averaged behaviour of the decaying velocity profile between the asperities. We find that this penetration depth δ can be rescaled into a simple exponential master curve δ = δ∞(1 − e−kD/S) for horizontal ridges with varying depth D and spacing S. A similar variation of δ with D and S is observed for vertical ridges, but with a smaller magnitude δ∞, while for ridges at an angle, the penetration depth lies between the two extreme cases.</p
Direct Measurement of the Strength of Single Ionic Bonds between Hydrated Charges
The strength of ionic bonds is essentially unknown, despite their widespread occurrence in natural and man-made assemblies. Here, we use single-molecule force spectroscopy to measure their strength directly. We disrupt a complex between two oppositely charged polyelectrolyte chains and find two modes of rupture: one ionic bond at a time, or cooperative rupture of many bonds at once. For both modes, disruption of the ionic bonds can be described quantitatively as an activated process. The height of the energy barrier is not only lowered by added salt, but also by the applied force. We extract unperturbed ionic bond lifetimes that range from milliseconds for single ionic bonds at high salt concentration to tens of years for small complexes of five ionic bonds at low salt concentration
Synergistic Stiffening in Double-Fiber Networks
Many
biological materials are composite structures, interpenetrating
networks of different types of fibers. The composite nature of such
networks leads to superior mechanical properties, but the origin of
this mechanical synergism is still poorly understood. Here we study
soft composite networks, made by mixing two self-assembling fiber-forming
components. We find that the elastic moduli of the composite networks
significantly exceed the sum of the moduli of the two individual networks.
This mechanical enhancement is in agreement with recent simulations,
where it was attributed to a suppression of non-affine deformation
modes in the most rigid fiber network due to the reaction forces in
the softer network. The increase
in affinity also causes a loss of strain hardening and an increase
in the critical stress and stain at which the network fails
The Weak Interaction of Surfactants with Polymer Brushes and Its Impact on Lubricating Behavior
We
study the weak interaction between polymers and oppositely charged
surfactants and its effect on the lubricating behavior and wettability
of polymer brush-covered surfaces. For cationic (PMETAC) and anionic
(PSPMA) brushes, a gradual transition from ultralow friction to ultrahigh
friction was observed upon adding oppositely charged surfactant as
a result of the electrostatic and hydrophobic interactions between
surfactant and polymer. The surfactant exchange led to a strong dehydration
of the brush and a concomitant increase in friction. Upon adding surfactant
above the CMC, we find a reduction in friction for the anionic brushes,
while the cationic brushes maintain a high friction. This difference
between the two lubrication systems suggests a different interaction
mechanism between the polymers and the surfactants. For zwitterionic
(PSBMA) and neutral (POEGMA) polymer brushes, where electrostatic
and hydrophobic interactions could be negligible, there is nearly
no surfactant uptake and also no effect of surfactant on lubrication
Physical and mechanical properties of thermosensitive xanthan/collagen-inspired protein composite hydrogels
<div><p>ABSTRACT</p><p>Functionalization of xanthan hydrogels is of interest for biomaterial applications. The authors report characterization of electrostatic complexation of xanthan with a recombinant collagen-inspired triblock protein polymer. This polymer has one charged polylysine end-block that can bind to xanthan by electrostatic interactions, and another end-block that can self-assemble into thermosensitive collagen-like triple helices; the end-blocks are connected by a neutral, hydrophilic, mostly inert random coil. The protein modifies the xanthan/protein composite hydrogels in three ways: (a) a significant increase in storage modulus, (b) thermosensitivity, and (c) a two-step strain softening in nonlinear rheology.</p></div
Physical Gels Based on Charge-Driven Bridging of Nanoparticles by Triblock Copolymers
We have prepared an aqueous physical gel consisting of
negatively
charged silica nanoparticles bridged by ABA triblock copolymers, in
which the A blocks are positively charged and the B block is neutral
and water-soluble. Irreversible aggregation of the silica nanoparticles
was prevented by precoating them with a neutral hydrophilic polymer.
Both the elastic plateau modulus and the relaxation time increase
slowly as the gel ages, indicating an increase both in the number
of active bridges and in the strength with which the end blocks are
adsorbed. The rate of this aging process can be increased significantly
by applying a small shear stress to the sample. Our results indicate
that charge-driven bridging of nanoparticles by triblock copolymers
is a promising strategy for thickening of aqueous particle containing
materials, such as water-based coatings