17 research outputs found
Structurally Controlled Dynamics in Azobenzene-Based Supramolecular Self-Assemblies in Solid State
Light-responsive
supramolecular self-assemblies exhibit interplay
between order and dynamics of the self-assembling motifs, through
which the thermal isomerization rate of azobenzene chromophores can
be tuned by orders of magnitude. By using supramolecular complexes
of 4-(4-alkylphenylazo)phenols hydrogen-bonded to poly(4-vinylpyridine)
as model systems, we demonstrate that the thermal isomerization rate
of the hydroxyazobenzene derivatives increases 5700-fold when the
material undergoes a transformation from a disordered, low-azobenzene-concentration
state to a high-concentration state exhibiting lamellar, smectic-like
self-assembly. Drastically smaller thermal isomerization rates are
observed in disordered structures. This allows us to attribute the
change to a combination of increased number density of the hydroxyazobenzenes
inducing plasticization, and cooperativity created by the chromophore–chromophore
interactions through self-assembled molecular order and alignment.
Our results pinpoint the importance of molecular self-assembly and
intermolecular interactions in modifying the dynamics in supramolecular
complexes in a controlled manner. We foresee this to be important
in light-controlled dynamic materials
Hierarchical Supramolecular Cross-Linking of Polymers for Biomimetic Fracture Energy Dissipating Sacrificial Bonds and Defect Tolerance under Mechanical Loading
Biological
structural materials offer fascinating models how to
synergistically increase the solid-state defect tolerance, toughness,
and strength using nanocomposite structures by incorporating different
levels of supramolecular sacrificial bonds to dissipate fracture energy.
Inspired thereof, we show how to turn a commodity acrylate polymer,
characteristically showing a brittle solid state fracture, to become
defect tolerant manifesting noncatastrophic crack propagation by incorporation
of different levels of fracture energy dissipating supramolecular
interactions. Therein, poly(2-hydroxyethyl methacrylate) (pHEMA) is
a feasible model polymer showing brittle solid state fracture in spite
of a high maximum strain and clear yielding, where the weak hydroxyl
group mediated hydrogen bonds do not suffice to dissipate fracture
energy. We provide the next level stronger supramolecular interactions
toward solid-state networks by postfunctionalizing a minor part of
the HEMA repeat units using 2-ureido-4[1<i>H</i>]-pyrimidinone
(UPy), capable of forming four strong parallel hydrogen bonds. Interestingly,
such a polymer, denoted here as p(HEMA-<i>co</i>-UPyMA),
shows toughening by suppressed catastrophic crack propagation, even
if the strength and stiffness are synergistically increased. At the
still higher hierarchical level, colloidal level cross-linking using
oxidized carbon nanotubes with hydrogen bonding surface decorations,
including UPy, COOH, and OH groups, leads to further increased stiffness
and ultimate strength, still leading to suppressed catastrophic crack
propagation. The findings suggest to incorporate a hierarchy of supramolecular
groups of different interactions strengths upon pursuing toward biomimetic
toughening
Preservation of Superhydrophobic and Superoleophobic Properties upon Wear Damage
Superhydrophobicity and self-cleaning require a combination
of surface topography and low-energy surfaces, where mechanical damage
of the topography or contamination with oils lead to loss of the nonwetting
properties. We show that such vulnerability can be solved by superamphiphobic
(i.e., both superhydrophobic and superoleophobic) surfactant-coated
aerogel surfaces. Using silica aerogels as model materials, the self-similar
network structure allows fresh re-entrant surface topographies even
after removal of the uppermost layer upon mechanical abrasion, and
superoleophobicity suppresses oil contamination. Given the recent
progress toward mechanically strong aerogels, we foresee that the
concept can open routes for robust self-cleaning coating technologies
Preservation of Superhydrophobic and Superoleophobic Properties upon Wear Damage
Superhydrophobicity and self-cleaning require a combination
of surface topography and low-energy surfaces, where mechanical damage
of the topography or contamination with oils lead to loss of the nonwetting
properties. We show that such vulnerability can be solved by superamphiphobic
(i.e., both superhydrophobic and superoleophobic) surfactant-coated
aerogel surfaces. Using silica aerogels as model materials, the self-similar
network structure allows fresh re-entrant surface topographies even
after removal of the uppermost layer upon mechanical abrasion, and
superoleophobicity suppresses oil contamination. Given the recent
progress toward mechanically strong aerogels, we foresee that the
concept can open routes for robust self-cleaning coating technologies
Thermal Isomerization of Hydroxyazobenzenes as a Platform for Vapor Sensing
Photoisomerization
of azobenzene derivatives is a versatile tool
for devising light-responsive materials for a broad range of applications
in photonics, robotics, microfabrication, and biomaterials science.
Some applications rely on fast isomerization kinetics, while for others,
bistable azobenzenes are preferred. However, solid-state materials
where the isomerization kinetics depends on the environmental conditions
have been largely overlooked. Herein, an approach to utilize the environmental
sensitivity of isomerization kinetics is developed. It is demonstrated
that thin polymer films containing hydroxyazobenzenes offer a conceptually
novel platform for sensing hydrogen-bonding vapors in the environment.
The concept is based on accelerating the thermal <i>cis</i>–<i>trans</i> isomerization rate through hydrogen-bond-catalyzed
changes in the thermal isomerization pathway, which allows for devising
a relative humidity sensor with high sensitivity and quick response
to relative humidity changes. The approach is also applicable for
detecting other hydrogen-bonding vapors such as methanol and ethanol.
Employing isomerization kinetics of azobenzenes for vapor sensing
opens new intriguing possibilities for using azobenzene molecules
in the future
Deoxyguanosine Phosphate Mediated Sacrificial Bonds Promote Synergistic Mechanical Properties in Nacre-Mimetic Nanocomposites
We
show that functionalizing polymer-coated colloidal nanoplatelets
with guanosine groups allows synergistic increase of mechanical properties
in nacre-mimetic lamellar self-assemblies. Anionic montmorillonite
(MTM) was first coated using cationic poly(diallyldimethylammonium
chloride) (PDADMAC) to prepare core–shell colloidal platelets,
and subsequently the remaining chloride counterions allowed exchange
to functional anionic 2′-deoxyguanosine 5′-monophosphate
(dGMP) counterions, containing hydrogen bonding donors and acceptors.
The compositions were studied using elemental analysis, scanning and
transmission electron microscopy, wide-angle X-ray scattering, and
tensile testing. The lamellar spacing between the clays increases
from 1.85 to 2.14 nm upon addition of the dGMP. Adding dGMP increases
the elastic modulus, tensile strength, and strain 33.0%, 40.9%, and
5.6%, respectively, to 13.5 GPa, 67 MPa, and 1.24%, at 50% relative
humidity. This leads to an improved toughness seen as a ca. 50% increase
of the work-to-failure. This is noteworthy, as previously it has been
observed that connecting the core–shell nanoclay platelets
covalently or ionically leads to increase of the stiffness but to
reduced strain. We suggest that the dynamic supramolecular bonds allow
slippage and sacrificial bonds between the self-assembling nanoplatelets,
thus promoting toughness, still providing dynamic interactions between
the platelets
Controlling Multicompartment Morphologies Using Solvent Conditions and Chemical Modification
The
solution self-assembly of amphiphilic diblock copolymers into
spheres, cylinders, and vesicles (polymersomes) has been intensely
studied over the past two decades, and their morphological behavior
is well understood. Linear ABC triblock terpolymers with two insoluble
blocks A/B, on the other hand, display a richer and more complex morphological
spectrum that has been recently explored by synthetic block length
variations. Here, we describe facile postpolymerization routes to
tailor ABC triblock terpolymer solution morphologies by altering block
solubility (solvent mixtures), blending with homopolymers, and block-selective
chemical reactions. The feasibility of these processes is demonstrated
on polystyrene-<i>block</i>-polybutadiene-<i>block</i>-poly(methyl methacrylate) (SBM) that assembles to patchy spherical
micelles, which can be modified to evolve into double and triple helices
or patchy and striped vesicles. These results demonstrate that postpolymerization
treatments give access to a broad range of morphologies from single
triblock terpolymers without the need for multiple polymer syntheses
Bacterial Nanocellulose Aerogel Membranes: Novel High-Porosity Materials for Membrane Distillation
We
fabricated, characterized, and tested novel fibrous aerogel
membranes in direct contact membrane distillation (MD) to elucidate
the effects of a model high-porosity membrane material on MD performance.
Unsupported bacterial nanocellulose aerogels exhibit higher porosity,
thinner fibers, and lower bulk thermal conductivity than any previously
reported MD materials. Modeling and experiments demonstrate that
these material properties confer significantly higher intrinsic membrane
permeability and thermal efficiency than symmetric PVDF phase inversion
membranes with lower porosity. Development of macroporous fibrous
membranes with aerogel-like porosity and thermal conductivity (>98%
and <0.03 W m<sup>–1</sup> K<sup>–1</sup>, respectively)
in thinner-film formats may further improve MD flux
Thermoresponsive Nanocellulose Hydrogels with Tunable Mechanical Properties
Cellulose microfibrils physically
bound together by soft hemicellulose
chains form the scaffolding that makes plant cell walls strong. Inspired
by this architecture, we designed biomimetic thermoreversible hydrogel
networks based on reinforcing cellulose nanocrystals (CNC) and thermoresponsive
methylcellulose (MC). Upon dissolving MC powder in CNC aqueous dispersions,
viscoelastic dispersions were formed at 20 °C, where the storage
modulus (<i>G</i>′) is tunable from 1.0 to 75 Pa
upon increasing the CNC concentration from 0 to 3.5 wt % with 1.0
wt % MC. By contrast, at 60 °C a distinct gel state is obtained
with <i>G</i>′ ≫ <i>G</i>″, <i>G</i>′ ∼ ω<sup>0</sup>, with an order of
magnitude larger <i>G</i>′ values from 110 to 900
Pa upon increasing the CNC concentration from 0 to 3.5 wt % with constant
1.0 wt % MC, due to the physical cross-links between MC and CNCs.
Therefore, simply mixing two sustainable components leads to the first
all-cellulose thermoreversible and tunable nanocellulose-based hydrogels
Nacre-Mimetic Clay/Xyloglucan Bionanocomposites: A Chemical Modification Route for Hygromechanical Performance at High Humidity
Nacre-mimetic bionanocomposites of
high montmorillonite (MTM) clay
content, prepared from hydrocolloidal suspensions, suffer from reduced
strength and stiffness at high relative humidity. We address this
problem by chemical modification of xyloglucan in (XG)/MTM nacre-mimetic
nanocomposites, by subjecting the XG to regioselective periodate oxidation
of side chains to enable it to form covalent cross-links to hydroxyl
groups in neighboring XG chains or to the MTM surface. The resulting
materials are analyzed by FTIR spectroscopy, thermogravimetric analysis,
carbohydrate analysis, calorimetry, X-ray diffraction, scanning electron
microscopy, tensile tests, and oxygen barrier properties. We compare
the resulting mechanical properties at low and high relative humidity.
The periodate oxidation leads to a strong increase in modulus and
strength of the materials. A modulus of 30 GPa for cross-linked composite
at 50% relative humidity compared with 13.7 GPa for neat XG/MTM demonstrates
that periodate oxidation of the XG side chains leads to crucially
improved stress transfer at the XG/MTM interface, possibly through
covalent bond formation. This enhanced interfacial adhesion and internal
cross-linking of the matrix moreover preserves the mechanical properties
at high humidity condition and leads to a Young’s modulus of
21 GPa at 90%RH