5 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
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
On the Stability and Morphology of Complex Coacervate Core Micelles: From Spherical to Wormlike Micelles
We present a systematic study of the stability and morphology
of
complex coacervate core micelles (C3Ms) formed from polyÂ(acrylic acid)
(PAA) and polyÂ(<i>N</i>-methyl-2-vinylpyridinium)-<i>b</i>-polyÂ(ethylene oxide) (PM2VP-<i>b</i>-PEO). We
use polarized and depolarized dynamic and static light scattering,
combined with small-angle X-ray scattering, to investigate how the
polymer chain length and salt concentration affect the stability,
size, and shape of these micelles. We show that C3Ms are formed in
aqueous solution below a critical salt concentration, which increases
considerably with increasing PAA and PM2VP length and levels off for
long chains. This trend is in good agreement with a mean-field model
of polyelectrolyte complexation based on the Voorn–Overbeek
theory. In addition, we find that salt induces morphological changes
in C3Ms when the PAA homopolymer is sufficiently short: from spherical
micelles with a diameter of several tens of nanometers at low salt
concentration to wormlike micelles with a contour length of several
hundreds of nanometers just before the critical salt concentration.
By contrast, C3Ms of long PAA homopolymers remain spherical upon addition
of salt and shrink slightly. A critical review of existing literature
on other C3Ms reveals that the transition from spherical to wormlike
micelles is probably a general phenomenon, which can be rationalized
in terms of a classical packing parameter for amphiphiles
Ultralow Adhesion and Friction of Fluoro-Hydro Alkyne-Derived Self-Assembled Monolayers on H‑Terminated Si(111)
New fluorine-containing terminal alkynes were synthesized
and self-assembled
onto Si(111) substrates to obtain fluorine-containing organic monolayers.
The monolayers were analyzed in detail by ellipsometry, X-ray photoelectron
spectroscopy (XPS), Fourier transform infrared reflection absorption
spectroscopy (FT-IRRAS), static water contact angle measurements (CA),
and atomic force microscopy (AFM). The SAMs exhibit excellent hydrophobicity,
with static water contact angles of up to 119° and low critical
surface tensions of 5–20 mN/m depending on the number of F
atoms per molecule. IRRAS confirmed the formation of highly ordered
monolayers, as indicated by the antisymmetric and symmetric stretching
vibrations of the CH<sub>2</sub> moieties at 2918–2920 and
2850–2851 cm<sup>–1</sup>, respectively. Upon increasing
the number of fluorine atoms in the alkyne chains from 0 to 17, the
adhesion of bare silica probes to the SAMs in air decreases from 11.6
± 0.20 mJ/m<sup>2</sup> for fluorine-free (F0) alkyne monolayers
to as low as 3.2 ± 0.03 mJ/m<sup>2</sup> for a heptadecafluoro-hexadecyne
(F17)-based monolayer. Likewise, the friction coefficient decreases
from 5.7 × 10<sup>–2</sup> to 1.2 × 10<sup>–2</sup>. The combination of high ordering, excellent hydrophobicity, low
adhesion, and low friction makes these fluoro-hydro alkyne-derived
monolayers highly promising candidates for use in high-performance
microelectronic devices
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