11 research outputs found
Influence of Surface Charge Density and Morphology on the Formation of Polyelectrolyte Multilayers on Smooth Charged Cellulose Surfaces
To
clarify the importance of the surface charge for the formation
of polyelectrolyte multilayers, layer-by-layer (LbL) assemblies of
polydiallyldimethylammonium chloride (pDADMAC) and polystyrenesulfonate
(PSS) have been investigated on cellulose films with different carboxylic
acid contents (20, 350, 870, and 1200 μmol/g) regenerated from
oxidized cellulose. The wet cellulose films were thoroughly characterized
prior to multilayer deposition using quantitative nanomechanical mapping
(QNM), which showed that the mechanical properties were greatly affected
by the degree of oxidation of the cellulose. Atomic force microscopy
(AFM) force measurements were used to determine the surface potential
of the cellulose films by fitting the force data to the DLVO theory.
With the exception of the 1200 μmol/g film, the force measurements
showed a second-order polynomial increase in surface potential with
increasing degree of oxidation. The low surface potential for the
1200 μmol/g film was attributed to the low degree of regeneration
of the cellulose film in aqueous media due to increasing solubility
with increasing charge. The multilayer formation was characterized
using a quartz crystal microbalance with dissipation (QCM-D) and stagnation-point
adsorption reflectometry (SPAR). Extensive deswelling was observed
for the charged films when pDADMAC was adsorbed due to the reduced
osmotic pressure when ions inside the film were released, and the
1:1 charge compensation showed that all the charges in the films were
reached by the pDADMAC. The multilayer formation was not significantly
affected by the charge density above 350 μmol/g due to interlayer
repulsions, but it was strongly affected by the salt concentration
during the layer build-up
Tailoring Soft Polymer Networks Based on Sugars and Fatty Acids toward Pressure Sensitive Adhesive Applications
The present work describes the synthesis
and characterization of
fully biobased soft polymer networks for pressure sensitive adhesives
applications. The incorporation of different sugars into fatty-acid-based
monomers, made it possible to tailor the viscoelastic properties of
the materials. Lipase catalysis allowed to afford monomers with varying
hydroxyl content and epoxy-functionalities. Step-growth polymerization
catalyzed by DBU resulted in soft-polyester networks through combination
of the monomers with a biobased diacid. Rheological and adhesion studies
were performed to elucidate the different viscoelastic and adhesive
properties of the materials as a function of their composition
Direct Adhesive Measurements between Wood Biopolymer Model Surfaces
For the first time the dry adhesion was measured for
an all-wood
biopolymer system using Johnson–Kendall–Roberts (JKR)
contact mechanics. The polydimethylsiloxane hemisphere was successfully
surface-modified with a Cellulose I model surface using layer-by-layer
assembly of nanofibrillated cellulose and polyethyleneimine. Flat
surfaces of cellulose were equally prepared on silicon dioxide substrates,
and model surfaces of glucomannan and lignin were prepared on silicon
dioxide using spin-coating. The measured work of adhesion on loading
and the adhesion hysteresis was found to be very similar between cellulose
and all three wood polymers, suggesting that the interaction between
these biopolymers do not differ greatly. Surface energy calculations
from contact angle measurements indicated similar dispersive surface
energy components for the model surfaces. The dispersive component
was dominating the surface energy for all surfaces. The JKR work of
adhesion was lower than that calculated from contact angle measurements,
which partially can be ascribed to surface roughness of the model
surfaces and overestimation of the surface energies from contact angle
determinations
Adhesive Layer-by-Layer Films of Carboxymethylated Cellulose Nanofibril–Dopamine Covalent Bioconjugates Inspired by Marine Mussel Threads
The preparation of multifunctional films and coatings from sustainable, low-cost raw materials has attracted considerable interest during the past decade. In this respect, cellulose-based products possess great promise due not only to the availability of large amounts of cellulose in nature but also to the new classes of nanosized and well-characterized building blocks of cellulose being prepared from trees or annual plants. However, to fully utilize the inherent properties of these nanomaterials, facile and also sustainable preparation routes are needed. In this work, bioinspired hybrid conjugates of carboxymethylated cellulose nanofibrils (CNFC) and dopamine (DOPA) have been prepared and layer-by-layer (LbL) films of these modified nanofibrils have been built up in combination with a branched polyelectrolyte, polyethyleneimine (PEI), to obtain robust, adhesive, and wet-stable nanocoatings on solid surfaces. It is shown that the chemical functionalization of CNFCs with DOPA molecules alters their conventional properties both in liquid dispersion and at the interface and also influences the LbL film formation by reducing the electrostatic interaction. Although the CNFC–DOPA conjugates show a lower colloidal stability in aqueous dispersions due to charge suppression, it was possible to prepare the LbL films through the consecutive deposition of the building blocks. Adhesive forces between multilayer films prepared using chemically functionalized CNFCs and a silica probe are much stronger in the presence of Fe<sup>3+</sup> than those between a multilayer film prepared from unmodified nanofibrils and a silica probe. The present work demonstrates a facile way to prepare chemically functionalized cellulose nanofibrils whereby more extended applications can produce novel cellulose-based materials with different functionalities
Water Drop Friction on Superhydrophobic Surfaces
To
investigate water drop friction on superhydrophobic surfaces,
the motion of water drops on three different superhydrophobic surfaces
has been studied by allowing drops to slide down an incline and capturing
their motion using high-speed video. Two surfaces were prepared using
crystallization of an alkyl ketene dimer (AKD) wax, and the third
surface was the leaf of a Lotus (Nelumbo Nucifera). The acceleration of the water droplets on these superhydrophobic
surfaces was measured as a function of droplet size and inclination
of the surface. For small capillary numbers, we propose that the energy
dissipation is dominated by intermittent pinning–depinning
transitions at microscopic pinning sites along the trailing contact
line of the drop, while at capillary numbers exceeding a critical
value, energy dissipation is dominated by circulatory flow in the
vicinity of the contacting disc between the droplet and the surface.
By combining the results of the droplet acceleration with a theoretical
model based on energy dissipation, we have introduced a material-specific
coefficient called the superhydrophobic sliding resistance, <i>b</i><sub>sh</sub>. Once determined, this parameter is sufficient
for predicting the motion of water drops on superhydrophobic surfaces
of a general macroscopic topography. This theory also infers the existence
of an equilibrium sliding angle, β<sub>eq</sub>, at which the
drop acceleration is zero. This angle is decreasing with the radius
of the drop and is in quantitative agreement with the measured tilt
angles required for a stationary drop to start sliding down an incline
Green Strategy to Reduced Nanographene Oxide through Microwave Assisted Transformation of Cellulose
A green
strategy for fabrication of biobased reduced nanographene
oxide (r-nGO) was developed. Cellulose derived nanographene oxide
(nGO) type carbon nanodots were reduced by microwave assisted hydrothermal
treatment with superheated water alone or in the presence of caffeic
acid (CA), a green reducing agent. The carbon nanodots, r-nGO and
r-nGO-CA, obtained through the two different reaction routes without
or with the added reducing agent, were characterized by multiple analytical
techniques including FTIR, XPS, Raman, XRD, TGA, TEM, AFM, UV–vis,
and DLS to confirm and evaluate the efficiency of the reduction reactions.
A significant decrease in oxygen content accompanied by increased
number of sp<sup>2</sup> hybridized functional groups was confirmed
in both cases. The synergistic effect of superheated water and reducing
agent resulted in the highest C/O ratio and thermal stability, which
also supported a more efficient reduction. Interesting optical properties
were detected by fluorescence spectroscopy where nGO, r-nGO, and r-nGO-CA
all displayed excitation dependent fluorescence behavior. r-nGO-CA
and its precursor nGO were evaluated toward osteoblastic cells MG-63
and exhibited nontoxic behavior up to 200 μg mL<sup>–1</sup>, which gives promise for utilization in biomedical applications
Understanding the Dispersive Action of Nanocellulose for Carbon Nanomaterials
This work aims at understanding the
excellent ability of nanocelluloses to disperse carbon nanomaterials
(CNs) in aqueous media to form long-term stable colloidal dispersions
without the need for chemical functionalization of the CNs or the
use of surfactant. These dispersions are useful for composites with
high CN content when seeking water-based, efficient, and green pathways
for their preparation. To establish a comprehensive understanding
of such dispersion mechanism, colloidal characterization of the dispersions
has been combined with surface adhesion measurements using colloidal
probe atomic force microscopy (AFM) in aqueous media. AFM results
based on model surfaces of graphene and nanocellulose further suggest
that there is an association between the nanocellulose and the CN.
This association is caused by fluctuations of the counterions on the
surface of the nanocellulose inducing dipoles in the sp<sup>2</sup> carbon lattice surface of the CNs. Furthermore, the charges on the
nanocellulose will induce an electrostatic stabilization of the nanocellulose–CN
complexes that prevents aggregation. On the basis of this understanding,
nanocelluloses with high surface charge density were used to disperse
and stabilize carbon nanotubes (CNTs) and reduced graphene oxide
particles in water, so that further increases in the dispersion limit
of CNTs could be obtained. The dispersion limit reached the value
of 75 wt % CNTs and resulted in high electrical conductivity (515
S/cm) and high modulus (14 GPa) of the CNT composite nanopapers
Force Interactions of Nonagglomerating Polylactide Particles Obtained through Covalent Surface Grafting with Hydrophilic Polymers
Nonagglomerating polylactide (PLA)
particles with various interaction
forces were designed by covalent photografting. PLA particles were
surface grafted with hydrophilic polyÂ(acrylic acid) (PAA) or polyÂ(acrylamide)
(PAAm), and force interactions were determined using colloidal probe
atomic force microscopy. Long-range repulsive interactions were detected
in the hydrophilic/hydrophilic systems and in the hydrophobic/hydrophilic
PLA/PLA-<i>g</i>-PAAm system. In contrast, attractive interactions
were observed in the hydrophobic PLA/PLA and in the hydrophobic/hydrophilic
PLA/PLA-<i>g</i>-PAA systems. AFM was also used in the tapping
mode to determine the surface roughness of both neat and surface-grafted
PLA film substrates. The imaging was performed in the dry state as
well as in salt solutions of different concentrations. Differences
in surface roughness were identified as conformational changes induced
by the altered Debye screening length. To understand the origin of
the repulsive force, the AFM force profiles were compared to the Derjaguin,
Landau, Verwey, and Overbeek (DLVO) theory and the Alexander de Gennes
(AdG) model. The steric repulsion provided by the different grafted
hydrophilic polymers is a useful tool to inhibit agglomeration of
polymeric particles. This is a key aspect in many applications of
polymer particles, for example in drug delivery
Zero-Dimensional and Highly Oxygenated Graphene Oxide for Multifunctional Poly(lactic acid) Bionanocomposites
The unique strengths of 2D graphene
oxide nanosheets (GONSs) in
polymer composites are thwarted by nanosheet agglomeration due to
strong intersheet attractions. Here, we reveal that shrinking the
planar size to 0D graphene oxide quantum dots (GOQDs), together with
the intercalation of rich oxygen functional groups, reduces filler
aggregation and enhances interfacial interactions with the host polymer.
With polyÂ(lactic acid) (PLA) as a model matrix, atomic force microscopy
colloidal probe measurements illustrated that a triple increase in
adhesion force to PLA was achieved for GOQDs (234.8 nN) compared to
GONSs (80.4 nN), accounting for the excellent exfoliation and dispersion
of GOQDs in PLA, in contrast to the notable agglomeration of GONSs.
Although present at trace amount (0.05 wt %), GOQDs made a significant
contribution to nucleation activity, mechanical strength and ductility,
and gas barrier properties of PLA, which contrasted the inferior efficacy
of GONSs, accompanied by clear distinction in film transparency (91%
and 50%, respectively). Moreover, the GOQDs with higher hydrophilicity
accelerated the degradation of PLA by enhancing water erosion, while
the GONSs with large sheet surfaces gave a higher hydrolytic resistance.
Our findings provide conceptual insights into the importance of the
dimensionality and surface chemistry of GO nanostructures in the promising
field of bionanocomposites integrating high strength and multifunction
(e.g., enhanced transparency, degradation and gas barrier)
Robust and Tailored Wet Adhesion in Biopolymer Thin Films
Model layer-by-layer (LbL) assemblies
of polyÂ(allylamine hydrochloride)
(PAH) and hyaluronic acid (HA) were fabricated in order to study their
wet adhesive behavior. The film characteristics were investigated
to understand the inherent structures during the assembly process.
Subsequently, the adhesion of these systems was evaluated to understand
the correlation between the structure of the film and the energy required
to separate these LbL assemblies. We describe how the conditions of
the LbL fabrication can be utilized to control the adhesion between
films. The characteristics of the film formation are examined in the
absence and presence of salt during the film formation. The dependence
on contact time and LbL film thickness on the critical pull-off force
and work of adhesion are discussed. Specifically, by introducing sodium
chloride (NaCl) in the assembly process, the pull-off forces can be
increased by a factor of 10 and the work of adhesion by 2 orders of
magnitude. Adjusting both the contact time and the film thickness
enables control of the adhesive properties within these limits. Based
on these results, we discuss how the fabrication procedure can create
tailored adhesive interfaces with properties surpassing analogous
systems found in nature