4 research outputs found
Swelling and Surface Interactions of End-Grafted Poly(2-vinylpyridine) Layers in Acidic Solution: Influence of Grafting Density and Salt Concentration
In
previous studies, the authors found that end-grafted layers
of the weak polybase poly(2-vinylpyridine) (P2VP) in aqueous solutions
do
not only swell and collapse if the pH value and salt concentration
are varied but also exhibit a pH- and salinity-dependent adhesion
to microsized silica spheres. For a better understanding of these
effects, in situ force measurements using the AFM colloidal probe
technique were applied to end-grafted P2VP layers of different grafting
densities in NaCl solutions at pH 2.5. Although a mushroom-to-brush
transition could be seen in the dry state, the layers were in the
brush regime in aqueous solutions at all NaCl concentrations and grafting
densities. We observed an increase of the brush height with increasing
grafting density and a salinity-dependent collapse and reswelling
of the brushes. The adhesion between the P2VP layer and a silica sphere
depended on both grafting density and salinity. At low salt concentrations,
the adhesion reached its highest value at the intermediate grafting
density and disappeared with denser brushes. Maximum adhesion was
obtained for high NaCl concentrations and the lowest grafting density.
From a detailed analysis of the experiments, we gained insight into
chain stretching and density profiles under complex ionic conditions
and into the mechanism of adhesion of polyelectrolytes to solid surfaces
pH and Salt Response of Mixed Brushes Made of Oppositely Charged Polyelectrolytes Studied by in Situ AFM Force Measurements and Imaging
The response of mixed
brushes made of poly(acrylic acid) and poly(2-vinyl
pyridine) with a mixing ratio of about 60:40 was studied using atomic
force microscopy (AFM) force measurements with colloidal probes and
AFM imaging with a sharp tip in the pH range between 2.5 and 8 and
at varying KCl concentrations up to 1 M. It was found that under all
conditions a dense polyelectrolyte complex layer coexists with excess
polyelectrolyte chains in varying swelling states depending on pH
and salt concentration. The mixed brush thus combines typical features
of polyelectrolyte brushes and complexes. So, the increase of the
salt concentration not only led to a transition from osmotic to salted
brush regime but also to salt-induced softening or partial decomposition
of the complex layer. Attractive forces at high salt concentrations
indicated the presence of P2VP chains in the swollen layer even at
high pH values
Core–Shell Microgels with Switchable Elasticity at Constant Interfacial Interaction
Hydrogels
based on poly(<i>N</i>-isopropylacrylamide)
(pNIPAAm) exhibit a thermo-reversible volume phase transition from
swollen to deswollen states. This change of the hydrogel volume is
accompanied by changes of the hydrogel elastic and Young’s
moduli and of the hydrogel interfacial interactions. To decouple these
parameters from one another, we present a class of submillimeter sized
hydrogel particles that consist of a thermosensitive pNIPAAm core
wrapped by a nonthermosensitive polyacrylamide (pAAm) shell, each
templated by droplet-based microfluidics. When the microgel core deswells
upon increase of the temperature to above 34 °C, the shell is
stretched and dragged to follow this deswelling into the microgel
interior, resulting in an increase of the microgel surficial Young’s
modulus. However, as the surface interactions of the pAAm shell are
independent of temperature at around 34 °C, they do not considerably
change during the pNIPAAm-core volume phase transition. This feature
makes these core–shell microgels a promising platform to be
used as building blocks to assemble soft materials with rationally
and independently tunable mechanics
Tunable Hydrophilic or Amphiphilic Coatings: A “Reactive Layer Stack” Approach
Thin films with tunable properties
are very interesting for potential
applications as functional coatings with, for example, anti-icing
or improved easy-to-clean properties. A novel “reactive layer
stack” approach was developed to create covalently grafted
mono- and multilayers of poly(glycidyl methacrylate)/poly(<i>tert</i>-butyl acrylate) diblock copolymers. Because these copolymers
contain poly(glycidyl methacrylate) blocks they behave as self-cross-linking
materials after creation of acrylic acid functionalities by splitting
off the <i>tert</i>-butyl units. The ellipsometrically determined
coating thickness of the resulting hydrophilic multilayers depended
linearly on the number of applied layers. Amphiphilic films with tunable
wettability were prepared using triblock terpolymers with an additional
poly(methyl methacrylate) block. The mechanism of the formation of
the (multi)layers was investigated in detail by studying the acidolysis
of the surface-linked <i>tert</i>-butyl acrylate blocks
by infrared reflection absorbance spectroscopy, accompanied by surface
analysis using atomic force microscopy and contact angle measurements.
In the case of the amphiphilic and switchable terpolymer layers this
reaction was very sensitive to the used acidic reagent