18 research outputs found
Ensayo aleatorizado del cierre de orejuela izquierda vs varfarina para la prevención de accidentes cerebrovasculares tromboembólicos en pacientes con fibrilación auricular no relacionada con valvulopatía. Estudio PREVAIL
The successful application of poly(<i>N</i>-vinylcaprolactam)-based
microgels requires a profound understanding of their synthesis. For
this purpose, a validated process model for the microgels synthesis
by precipitation copolymerization with the cross-linker <i>N</i>,<i>N</i>′-methylenebis(acrylamide) is formulated.
Unknown reaction rate constants, reaction enthalpies, and partition
coefficients are obtained by quantum mechanical calculations. The
remaining parameter values are estimated from reaction calorimetry
and Raman spectroscopy measurements of experiments with different
monomer/cross-linker compositions. Because of high cross-propagation
reaction rate constants, simulations predict a fast incorporation
of the cross-linker. This agrees with reaction calorimetry measurements.
Furthermore, the gel phase is predicted as the major reaction locus.
The model is utilized for a prediction of the internal particle structure
regarding its cross-link distribution. The highly cross-linked core
reported in the literature corresponds to the predictions of the model
Reversible Switching and Recycling of Adaptable Organic Microgel Catalysts (Microgelzymes) for Asymmetric Organocatalytic Desymmetrization
Adaptable enzyme-mimetic catalysts
based on temperature-responsive
polymer microgels (microgelzymes) have been developed. By a simple
change in the temperature, a microgel catalyst can be reversibly switched
into its soluble or precipitated form, thus combining the advantages
of homogeneous and heterogeneous catalysis. The responsive microgel
reactors show high reactivity and selectivity, as well as good recycling
properties in the alcoholysis of <i>cis</i>-tetrahydrophthalic
anhydride
Surfactant-Free Synthesis of Polystyrene Nanoparticles Using Oligoglycidol Macromonomers
We investigate the synthesis of functional polystyrene/oligoglycidol
particles by surfactant-free emulsion polymerization. Oligoglycidol
macromonomers with linear and branched oligoglycidol structure and
variable oligoglycidol chain lengths were synthesized. These macromonomers
were used as surfmers (surfactants and comonomers) in emulsion polymerization
of styrene. Monodisperse colloidally stable polystyrene particles
were obtained, decorated with oligoglycidol chains with diameters
between 100 and 600 nm. The increase of the macromonomer concentration
induced a decrease of the particle size and broadening of the particle
size distribution. The macromonomers with branched architecture were
more effective and produced monodisperse particles even at low concentrations.
Due to the steric stabilization provided by the hydrophilic oligoglycidol
layer on the particle surface, the emulsion of polystyrene/oligoglycidol
particles obtained exhibited very good resistance against electrolytes.
The chemical and enzyme catalyzed grafting polymerization of ε-caprolactone
from the polystyrene/oligoglycidol particle surface was demonstrated
along with formation of composite particles
Microgel-Based Adaptive Hybrid Capsules with Tunable Shell Permeability
In the present work, we demonstrate
the preparation of adaptive
hybrid capsules with microgel/SiO<sub>2</sub> composite walls. During
the first stage of the capsule synthesis process, microgel particles
stabilize the oil droplets in water and become self-assembled on the
oil/water interface. At the second stage, microgels are subsequently
glued by the sol–gel reaction of a silica precursor-hyperbranched
polyethoxysiloxane (PEOS) which occurs at the oil/water interface.
Consequently, hybrid capsules consisting of silica shell with embedded
microgels are obtained. Responsive microgels present in the capsule
wall act as transportation channels for different encapsulated materials.
The use of smart microgels allows us to design capsules with controlled
size, morphology, and wall permeability able to operate in both water
and organic solvents
Correlated Morphological Changes in the Volume Temperature Transition of Core–Shell Microgels
PVCL and PNIPMAAm core–shell
components in microgel particles are shown to have different volume
phase temperature transition behavior than the respective homopolymer
microgel particles due to confinement effects. A combination of dynamic
light scattering (DLS) data that gives access to the temperature dependence
of hydrodynamic radius and modified Flory–Rehner theory in
the presence of networks confinement allowed obtaining information
about correlated morphological changes of components inside of core–shell
microgels. The core–shell components individual temperature
behavior is analyzed by modifying the Flory–Rehner transition
theory in order to account for core–shell morphology and the
existence of an interaction force between core and shell. Describing
the dependence on temperature of the radial scale parameter, the ratio
between the radius of the core and the hydrodynamic radius, we gain
access to the swelling behavior of the core and shell components irrespective
of the swelling behavior of the total hydrodynamic radius. Furthermore,
the theoretical description of volume phase temperature transition
permits the development of scenarios for the correlated changes in
the core and shell radial dimensions for the two microgels with reversed
morphologies. The fact that the theoretical model is appropriate for
the treatment of core–shell microgels is proved <i>a posteriori</i> by obtaining a temperature dependence of the components that is
in accordance with the expected physical behavior. Novel core–shell
microgel systems of PVCL (poly(<i>N</i>-vinylcaprolactam))-core/PNIPMAAm
(poly(<i>N</i>-isopropylmethacrylamide))-shell and PNIPMAAm-core/PVCL-shell,
with a double volume phase temperature transition due to the thermoresponsive
components, were used for this study
Electrostatic Interactions and Osmotic Pressure of Counterions Control the pH-Dependent Swelling and Collapse of Polyampholyte Microgels with Random Distribution of Ionizable Groups
In
this work, different systems of colloidally stable, ampholytic
microgels (μGs) based on poly(<i>N</i>-vinylcaprolactam)
and poly(<i>N</i>-isopropylacrylamide), wherein
the anionic and cationic groups are randomly distributed, were investigated.
Fourier transmission infrared spectroscopy and transmission electron
microscopy confirmed the quantitative incorporation and random distribution
of ionizable groups in μGs, respectively. The control of hydrodynamic
radii and mechanical properties of polyampholyte μGs at different
pH values was studied with dynamic light scattering and in situ atomic
force microscopy. We have proposed a model of pH-dependent polyampholyte
μG, which correctly describes the experimental data and explains
physical reasons for the swelling and collapse of the μG at
different pHs. In the case of a balanced μG (equal numbers of
cationic and anionic groups), the size as a function of pH has a symmetric,
V-like shape. Swelling of purely cationic μG at low pH or purely
anionic μG at high pH is due to electrostatic repulsion of similarly
charged groups, which appears as a result of partial escape of counterions.
Also, osmotically active counterions (the counterions that are trapped
within the μG) contribute to the swelling of the μG. In
contrast, electrostatic interactions are responsible for the collapse
of the μG at intermediate pH when the numbers of anionic and
cationic groups are equal (stoichiometric ratio). The multipole attraction
of the charged groups is caused by thermodynamic fluctuations, similar
to the those observed in Debye–Hückel plasma. We have
demonstrated that the higher the fraction of cationic and anionic
groups, the more pronounced the swelling and collapse of the μG
at different pHs
Gadolinium-Loaded Poly(<i>N</i>‑vinylcaprolactam) Nanogels: Synthesis, Characterization, and Application for Enhanced Tumor MR Imaging
We
report the synthesis of poly(<i>N</i>-vinylcaprolactam)
nanogels (PVCL NGs) loaded with gadolinium (Gd) for tumor MR imaging
applications. The PVCL NGs were synthesized via precipitation polymerization
using the monomer <i>N</i>-vinylcaprolactam (VCL), the comonomer
acrylic acid (AAc), and the degradable cross-linker 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5,5]-undecane
(VOU) in aqueous solution, followed by covalently binding with 2,2′,2″-(10-(4-((2-aminoethyl)amino)-1-carboxy-4-oxobutyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)
triacetic acid (NH<sub>2</sub>-DOTA-GA)/Gd complexes. We show that
the formed Gd-loaded PVCL NGs (PVCL-Gd NGs) having a size of 180.67
± 11.04 nm are water dispersible, colloidally stable, uniform
in size distribution, and noncytotoxic in a range of the studied concentrations.
The PVCL-Gd NGs also display a r<sub>1</sub> relaxivity (6.38–7.10
mM<sup>–1</sup> s<sup>–1</sup>), which is much higher
than the clinically used Gd chelates. These properties afforded the
use of the PVCL-Gd NGs as an effective positive contrast agent for
enhanced MR imaging of cancer cells in vitro as well as a subcutaneous
tumor model in vivo. Our study suggests that the developed PVCL-Gd
NGs could be applied as a promising contrast agent for <i>T</i><sub>1</sub>-weighted MR imaging of diverse biosystems
Das Susac Syndrom - Mikroangiopathie als seltene Ursache bilateraler Taubheit
We
report on the behavior of two immiscible liquids within polymer
microgel adsorbed at their interface. By means of dissipative particle
dynamics (DPD) simulations and theoretical analysis in the framework
of the Flory–Huggins (FH) lattice theory, we demonstrate that
the microgel acts as a “compatibilizer” of these liquids:
their miscibility within the microgel increases considerably. If the
incompatibility of the liquids is moderate, although strong enough
to induce phase separation in their 1:1 composition, they form homogeneous
mixture in the microgel interior. The mixture of highly incompatible
liquids undergoes separation into two (micro)phases within the microgel
likewise out of it; however, the segregation regime is weaker and
the concentration profiles are characterized by a weaker decay (gradient)
in comparison with those of two pure liquids. The enhanced miscibility
is a result of the screening of unfavorable interactions between unlike
liquid molecules by polymer subchains. We have shown that better miscibility
of the liquids is achieved with densely cross-linked microgels. Our
findings are very perspective for many applications where immiscible
species have to be mixed at interfaces (like in heterogeneous catalysis)
Monitoring the Internal Structure of Poly(<i>N</i>‑vinylcaprolactam) Microgels with Variable Cross-Link Concentration
The combination of a set of complementary
techniques allows us
to construct an unprecedented and comprehensive picture of the internal
structure, temperature dependent swelling behavior, and the dependence
of these properties on the cross-linker concentration of microgel
particles based on <i>N</i>-vinylcaprolactam (VCL). The
microgels were synthesized by precipitation polymerization using different
amounts of cross-linking agent. Characterization was performed by
small-angle neutron scattering (SANS) using two complementary neutron
instruments to cover a uniquely broad Q-range with one probe. Additionally
we used dynamic light scattering (DLS), atomic force microscopy (AFM),
and differential scanning calorimetry (DSC). Previously obtained nuclear
magnetic resonance spectroscopy (NMR) results on the same PVCL particles
are utilized to round the picture off. Our study shows that both the
particle radius and the cross-link density and therefore also the
stiffness of the microgels rises with increasing cross-linker content.
Hence, more cross-linker reduces the swelling capability distinctly.
These findings are supported by SANS and AFM measurements. Independent
DLS experiments also found the increase in particle size but suggest
an unchanged cross-link density. The reason for the apparent contradiction
is the indirect extraction of the parameters via a model in the evaluation
of DLS measurements. The more direct approach in AFM by evaluating
the cross section profiles of observed microgel particles gives evidence
of significantly softer and more deformable particles at lower cross-linker
concentrations and therefore verifies the change in cross-link density.
DSC data indicate a minor but unexpected shift of the volume phase
transition temperature (VPTT) to higher temperatures and exposes a
more heterogeneous internal structure of the microgels with increasing
cross-link density. Moreover, a change in the total energy transfer
during the VPT gives evidence that the strength of hydrogen bonds
is significantly affected by the cross-link density. A strong and
reproducible deviation of the material density of the cross-linked
microgel polymer chains toward a higher value compared to the respective
linear chains has yet to be explained
Swelling of a Responsive Network within Different Constraints in Multi-Thermosensitive Microgels
We report on the swelling of a polymeric
network in doubly thermoresponsive
microgels. Silica-core double-shell and hollow double-shell microgels
made of an inner poly(<i>N</i>-isopropylmethacrylamide)
and an outer poly(<i>N</i>-isopropylacrylamide) shell
are studied by exploiting the distinct temperature sensitivities of
the polymers. The swelling states of the two shells can be tuned by
temperature changes enabling three different swelling states: above,
below, and between the distinct volume phase transition temperatures
of the two polymers. This enables to investigate the effect of different
constraints on the swelling of the inner network. Small-angle neutron
scattering with contrast variation in combination with computer simulation
discloses how the expansion of the inner shell depends on the material
and swelling state of its constraints. In the presence of the stiff
core, the microgels show a considerable interpenetration of the polymeric
shells: the inner network expands into the outer deswollen shell.
This interpenetration vanishes when the outer network is swollen.
Furthermore, as predicted by our computer simulations, an appropriate
choice of cross-linking density enables the generation of hollow double-shell
nanocapsules. Here, the inner shell undergoes a <i>push–pull
effect</i>. At high temperature, the collapsed outer shell pushes
the swollen inner network into the cavity. At lower temperature, the
swelling of the outer network contrary pulls the inner shell back
toward the external periphery