8 research outputs found
Salt-Induced Control of the Grafting Density in Poly(ethylene glycol) Brush Layers by a Grafting-to Approach
In
this work, a method to obtain control of the grafting density
during the formation of polymer brush layers by the grafting-to method
of thiolated poly(ethylene glycol) onto gold is presented. The grafting
density of the polymer chains was adjusted by adding Na<sub>2</sub>SO<sub>4</sub> in concentrations between 0.2 and 0.9 M to the aqueous
polymer solution during the grafting process. The obtained grafting
densities ranged from 0.26 to 1.60 chains nm<sup>–2</sup>,
as determined by surface plasmon resonance. The kinetics of the grafting
process were studied in situ by a quartz crystal microbalance with
dissipation, and a mushroom to brush conformational transition was
observed when the polymer was grafted in the presence of Na<sub>2</sub>SO<sub>4</sub>. The transition from mushroom to brush was only observed
for long periods of grafting, highlighting the importance of time
to obtain high grafting densities. Finally, the prepared brush layer
with the highest grafting density showed high resistance to the adsorption
of bovine serum albumin, while layers with a lower grafting density
showed only limited resistance
Fully Biobased Adhesive from Chitosan and Tannic Acid with High Water Resistance
In
this study, we developed a fully biobased adhesive using chitosan
and tannic acid, drawing inspiration from natural mussel adhesive
mechanisms. Our goal was to address the technological challenges associated
with creating environmentally friendly adhesives that can perform
comparably to their synthetic counterparts, particularly in terms
of water resistance. The methodology involved the molecular complexation
of chitosan, a polysaccharide rich in amine groups, with tannic acid,
which is known for its galloyl moieties. This complexation mimics
the dopa-lysine synergy found in mussel adhesives, leading to a cohesive
and adhesive material under the right pH conditions. Notably, after
mild thermal curing (70 °C for 2 h), our biobased adhesive demonstrated
outstanding water resistance and long-term durability. It maintained
a lap shear strength of approximately 3.5 MPa even after immersion
in water for 2 months and could be stretched and released more than
1000 times before breaking. These performance metrics surpassed those
of a commercial water-resistant adhesive, Gorilla Glue, and a well-known
strong dry adhesive, Loctite Super Glue (both cured under their recommended
conditions). The success of this adhesive underscores the importance
of leveraging nature’s mechanisms, such as molecular complexation
and bioinspired chemistry, in developing next-generation biobased
adhesives that overcome the challenge of poor water resistance. This
biobased adhesive has potential applications as a sustainable alternative
to petroleum-based adhesives, while the price and availability of
the biobased raw materials need continuous evaluation
The development of violinism in the Slovene lands until the beginning of the Second World War
Composite
polyelectrolyte multilayers of chitosan and low molecular
weight poly(acrylic acid) (PAA) have been assembled by sequential
adsorption as a first step toward building a surface anchored chitosan
gel. Silane chemistry was used to graft the first chitosan layer to
prevent film detachment and decomposition. The assembly process is
characterized by nonlinear growth behavior, with different adsorption
kinetics for chitosan and PAA. In situ analysis of the multilayer
by means of surface sensitive total internal reflection Raman (TIRR)
spectroscopy, combined with target factor analysis of the spectra,
provided information regarding composition, including water content,
and ionization state of weak acidic and basic groups present in the
thin composite film. Low molecular weight PAA, mainly in its protonated
form, diffuses into and out of the composite film during adsorption
and rinsing steps. The higher molecular weight chitosan shows a similar
behavior, although to a much lower extent. Our data demonstrate that
the charged monomeric units of chitosan are mainly compensated by
carboxylate ions from PAA. Furthermore, the morphology and mechanical
properties of the multilayers were investigated in situ using atomic
force microscopy operating in PeakForce tapping mode. The multilayer
consists of islands that grow in lateral dimension and height during
the build-up process, leading to close to exponentially increasing
roughness with deposition number. Both diffusion in and out of at
least one of the two components (PAA) and the island-like morphology
contribute to the nonlinear growth of chitosan/PAA multilayers
A Biomimetic Water-Resistant Adhesive Based on ε‑Polylysine/Tannic Acid Complexation
This study investigates the potential of combining ε-polylysine
and tannic acid to develop bio-based adhesives with enhanced water
resistance. The two biomolecules exhibited complexation/precipitation
in aqueous solutions in the pH range of 5–9. Water-based adhesives
were prepared using complexes formed at pH 5, 7, and 9, followed by
evaluation of their adhesion properties in both dry and wet lap shear
tests. The complexes prepared at higher pH values showed a larger
adhesion strength and improved water resistance. To further enhance
the adhesive properties, an epoxide-based reagent was utilized to
double-cross-link the complexes, resulting in a lap shear strength
of ∼2 MPa after being submerged in water for 7 days
Direct Measurement of Colloidal Interactions between Polyaniline Surfaces in a UV-Curable Coating Formulation: The Effect of Surface Hydrophilicity/Hydrophobicity and Resin Composition
The
interactions between polyaniline particles and polyaniline
surfaces in polyester acrylate resin mixed with 1,6-hexanediol diacrylate
monomer have been investigated using contact angle measurements and
the atomic force microscopy colloidal probe technique. Polyaniline
with different characteristics (hydrophilic and hydrophobic) were
synthesized directly on spherical polystyrene particles of 10 μm
in diameter. Surface forces were measured between core/shell structured
polystyrene/polyaniline particles (and a pure polystyrene particle
as reference) mounted on an atomic force microscope cantilever and
a pressed pellet of either hydrophilic or hydrophobic polyaniline
powders, in resins of various polymer:monomer ratios. A short-range
purely repulsive interaction was observed between hydrophilic polyaniline
(doped with phosphoric acid) surfaces in polyester acrylate resin.
In contrast, interactions between hydrophobic polyaniline (doped with
n-decyl phosphonic acid) were dominated by attractive forces, suggesting
less compatibility and higher tendency for aggregation of these particles
in liquid polyester acrylate compared to hydrophilic polyaniline.
Both observations are in agreement with the conclusions from the interfacial
energy studies performed by contact angle measurements
Marketing strategy challenges for the brand Lisca in the German market
Responsive
biomaterial hydrogels attract significant attention
due to their biocompatibility and degradability. In order to make
chitosan based gels, we first graft one layer of chitosan to silica,
and then build a chitosan/poly(acrylic acid) multilayer using the
layer-by-layer approach. After cross-linking the chitosan present
in the polyelectrolyte multilayer, poly(acrylic acid) is partly removed
by exposing the multilayer structure to a concentrated carbonate buffer
solution at a high pH, leaving a surface-grafted cross-linked gel.
Chemical cross-linking enhances the gel stability against detachment
and decomposition. The chemical reaction between gluteraldehyde, the
cross-linking agent, and chitosan was followed in situ using total
internal reflection Raman (TIRR) spectroscopy, which provided a molecular
insight into the complex reaction mechanism, as well as the means
to quantify the cross-linking density. The amount of poly(acrylic
acid) trapped inside the surface grafted films was found to decrease
with decreasing cross-linking density, as confirmed in situ using
TIRR, and ex situ by Fourier transform infrared (FTIR) measurements
on dried films. The responsiveness of the chitosan-based gels with
respect to pH changes was probed by quartz crystal microbalance with
dissipation (QCM-D) and TIRR. Highly cross-linked gels show a small
and fully reversible behavior when the solution pH is switched between
pH 2.7 and 5.7. In contrast, low cross-linked gels are more responsive
to pH changes, but the response is fully reversible only after the
first exposure to the acidic solution, once an internal restructuring
of the gel has taken place. Two distinct p<i>K</i><sub>a</sub>’s for both chitosan and poly(acrylic acid), were determined
for the cross-linked structure using TIRR. They are associated with
populations of chargeable groups displaying either a bulk like dissociation
behavior or forming ionic complexes inside the hydrogel film
Hydrophobic Surfaces: Topography Effects on Wetting by Supercooled Water and Freezing Delay
Hydrophobicity, and in particular
superhydrophobicity, has been
extensively considered to promote ice-phobicity. Dynamic contact angle
measurements above 0 °C have been widely used to evaluate the
water repellency. However, it is the wetting properties of <i>supercooled</i> water at subzero temperatures and the derived
work of adhesion that are important for applications dealing with
icing. In this work we address this issue by determining the temperature-dependent
dynamic contact angle of microliter-sized water droplets on a smooth
hydrophobic and a superhydrophobic surface with similar surface chemistry.
The data highlight how the work of adhesion of water in the temperature
interval from about 25 °C to below −10 °C is affected
by surface topography. A marked decrease in contact angle on the superhydrophobic
surface is observed with decreasing temperature, and we attribute
this to condensation below the dew point. In contrast, no significant
wetting transition is observed on the smooth hydrophobic surface.
The freezing temperature and the freezing delay time were determined
for water droplets resting on a range of surfaces with similar chemistry
but different topography, including smooth and rough surfaces in either
the Wenzel or the Cassie–Baxter state as characterized by water
contact angle measurements at room temperature. We find that the water
freezing delay time is not significantly affected by the surface topography
and discuss this finding within the classical theory of heterogeneous
nucleation
Sustained Frictional Instabilities on Nanodomed Surfaces: Stick–Slip Amplitude Coefficient
Understanding the frictional properties of nanostructured surfaces is important because of their increasing application in modern miniaturized devices. In this work, lateral force microscopy was used to study the frictional properties between an AFM nanotip and surfaces bearing well-defined nanodomes comprising densely packed prolate spheroids, of diameters ranging from tens to hundreds of nanometers. Our results show that the average lateral force varied linearly with applied load, as described by Amontons’ first law of friction, although no direct correlation between the sample topographic properties and their measured friction coefficients was identified. Furthermore, all the nanodomed textures exhibited pronounced oscillations in the shear traces, similar to the classic stick–slip behavior, under all the shear velocities and load regimes studied. That is, the nanotextured topography led to sustained frictional instabilities, effectively with no contact frictional sliding. The amplitude of the stick–slip oscillations, σ<sub><i>f</i></sub>, was found to correlate with the topographic properties of the surfaces and scale linearly with the applied load. In line with the friction coefficient, we define the slope of this linear plot as the stick–slip amplitude coefficient (SSAC). We suggest that such stick–slip behaviors are characteristics of surfaces with nanotextures and that such local frictional instabilities have important implications to surface damage and wear. We thus propose that the shear characteristics of the nanodomed surfaces cannot be fully described by the framework of Amontons’ laws of friction and that additional parameters (<i>e.g.</i>, σ<sub><i>f</i></sub> and SSAC) are required, when their friction, lubrication, and wear properties are important considerations in related nanodevices