9 research outputs found
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
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
How do immobilised cell-adhesive Arg–Gly–Asp-containing peptides behave at the PAA brush surface?
<p>Bio-engineered surfaces that aim to induce normal cell behaviour <i>in vitro</i> need to ‘mimic’ the extracellular matrix in a way that allows cell adhesion. In this computational work, several model cell-binding peptides with a minimal cell-adhesive Arg–Gly–Asp sequence are investigated in the bulk as well as immobilised on a soft surface. For this reason, a combination of density functional theory and all-atom MD simulations is applied. The major goal of the modelling is to characterise the accessibility of the cell-recognition motif on the functionalised soft polymer surface. As a reference system, the behaviour of three peptide sequences is preliminarily studied in explicit water simulations. From the analysis of the MD trajectories, the solvent accessible surface area, the distribution of water molecules around peptide groups, the secondary structure and the thermodynamics of hydration are evaluated. Furthermore, each peptide is immobilised on the surface of a homopolymer poly(acrylic acid) brush. During MD simulations, all three peptides approach closely toward PAA brush, and their surface accessibility is characterised. Although the peptides are adsorbed onto the brush, they are not hidden by the polymer strands, with RGD unit accessible on the surface and available for guided cell adhesion.</p
Facile Approach to Grafting of Poly(2-oxazoline) Brushes on Macroscopic Surfaces and Applications Thereof
This study reports on a facile and versatile approach
for modification
of macroscopic surface via grafting of multifunctional poly(2-oxazoline)
molecules in brush-like conformation. For this purpose, carboxyl-terminated
poly(2-isopropyl-2-oxazoline) molecules have been synthesized by ring-opening
cationic polymerization and subsequently grafted on underlined substrates
by exploiting the ”grafting to” approach. A systematic
variation in thickness of the grafted poly (2-isopropyl-2-oxazoline)
brushes has been demonstrated. Polymer-modified surfaces have been
characterized by means of a number of analytical tools including ellipsometry,
X-ray photoelectron spectroscopy, ultraviolate spectroscopy, attenuated
total reflection infrared spectroscopy
and atomic force microscopy. Interestingly, poly(2-isopropyl-2-oxazoline)
molecules have been found to retain their physical properties even
after grafting on macroscopic surfaces. Finally, fabricated polymer
brushes have been used as platform for stabilization of inorganic
nanoparticles on macroscopic surfaces
Molecular Dynamics of Swollen Poly(2-vinylpyridine) Brushes
The influence of
swelling on the molecular dynamics of poly(2-vinylpyridine)
(P2VP) brushes is measured by broadband dielectric spectroscopy (BDS)
in a broad temperature (350–420 K) and spectral (0.1 Hz–1
MHz) range with nanostructured, highly conductive silicon electrodes,
separated by 35 nm high insulating silica spacers. A “grafting-to”
method is applied to prepare P2VP brushes with a grafting density
σ = 0.12 nm<sup>–2</sup> and a film thickness <i>d</i> = 7.3 nm as measured by ellipsometry. Swelling of the
P2VP brushes is realized with tetrahydrofuran (THF) vapor using a
flow cell. In the dry state, the segmental dynamics of the P2VP brushes
coincides with the dynamic glass transition of the bulk system while
in the swollen state it becomes faster by up to 1–2 decades
due to the plasticizing effect of THF
Salt Sensitivity of the Thermoresponsive Behavior of PNIPAAm Brushes
We report investigations
on the salt sensitivity of the thermoresponsive
behavior of PNIPAAm brushes applying the quartz crystal microbalance
coupled with spectroscopic ellipsometry technique. This approach enables
a detailed study of the optical and mechanical behavior of the polymer
coatings. Additional conclusions can be drawn from the difference
between both techniques due to a difference in the contrast mechanism
of both methods. A linear shift of the phase-transition temperature
to lower temperatures with the addition of sodium chloride was found,
similar to the behavior of free polymer chains in solution. The thermal
hysteresis was found to be decreased by the addition of sodium chloride
to the solution, hinting to the interaction of the ions with the amide
groups of the polymer, whereby the formation of hydrogen bonds is
hindered. The results of this study are of relevance to the application
of PNIPAAm brushes in biological fluids and demonstrate the additional
potential of the ion sensitivity besides the better known thermosensitivity
Biofunctionalization of Titanium Substrates Using Nanoscale Polymer Brushes with Cell Adhesion Peptides
The grafting of polymer brushes to substrates is a promising method
to modify surface properties such as wettability and the affinity
toward proteins and cells for applications in microelectronics, biomedical
devices, and sensors. Poly(acrylic) acid (PAA) brushes are of high
interest because of their stimuli-responsive behavior and the presence
of carboxy (COOH) groups, which allow for immobilization of bioactive
molecules. The “grafting-to” approach results in homogeneous
and well-defined polymer brushes, but, although grafting-to has been
demonstrated with PAA brushes on silicon (Si) substrates, it has not
been performed on biocompatible materials such as titanium (Ti). Here,
we have described a facile method to modify biocompatible Ti substrates
with PAA brushes to amplify their substrate functionality. The grafting-to
PAA “pseudo” brushes were successfully grafted to Ti
substrates and retained their pH-dependent swelling behavior. An RGD
peptide was covalently bound to COOH groups of the PAA brushes (PAA–RGD)
as a model bioactive group. While NIH/3T<sub>3</sub> cell adhesion
was significantly decreased on PAA-functionalized Ti substrates, PAA–RGD
on Ti had cell adhesion comparable to that of flat Ti at 24 and 48
h, with significantly more cells adhered to PAA–RGD compared
to PAA on Ti at 48 h
Adsorption of Poly(vinylformamide-<i>co</i>-vinylamine) Polymers (PVFA-<i>co</i>-PVAm) on Copper
The adsorption of poly(vinylformamide) (PVFA) and its
derivative
statistical copolymer poly(vinyl-formamide-<i>co</i>-vinylamine)
(PVFA-<i>co</i>-PVAm) on metallic copper and copper oxide
particles as well as planar copper surfaces was studied as a function
of the degree of hydrolysis of PVFA, the pH, and the polymer concentration
in solution. The chemical composition and molecular structure of the
PVFA-<i>co</i>-PVAm layers were investigated by surface-sensitive
spectroscopic methods such as XPS, DRIFT spectroscopy, and ellipsometry.
The findings allowed us to explain the adsorption mechanisms and the
forces driving the PVFA-<i>co</i>-PVAm adsorption. It was
shown that PVFA-<i>co</i>-PVAm layers thicker than 30 nm
are able to protect the planar copper surface against corrosive attack
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