Microgels are solvent-swollen nano- and microparticles that show prevalent colloidal-like behavior despite their polymeric nature. Here we study ultra-low crosslinked poly(N-isopropylacrylamide) ...microgels (ULC), which can behave like colloids or flexible polymers depending on dimensionality, compression or other external stimuli. Small-angle neutron scattering shows that the structure of the ULC microgels in bulk aqueous solution is characterized by a density profile that decays smoothly from the center to a fuzzy surface. Their phase behavior and rheological properties are those of soft colloids. However, when these microgels are confined at an oil-water interface, their behavior resembles that of flexible macromolecules. Once monolayers of ultra-low crosslinked microgels are compressed, deposited on solid substrate and studied with atomic-force microscopy, a concentration-dependent topography is observed. Depending on the compression, these microgels can behave as flexible polymers, covering the substrate with a uniform film, or as colloidal microgels leading to a monolayer of particles.
Stimuli responsive magnetic emulsions are produced by using “smart” microgels as stabilizers. The oil droplets in the emulsions are magnetic, can be separated with a static magnetic field and are ...destabilized by an ac magnetic field. This allows for a remote control on separation and stability of the emulsion.
Recent theoretical phase diagrams for loosely cross-linked ionic microgels with a low monomer volume fraction ( Gottwald ; et al. Phys. Rev. Lett. 2004, 92, 068301 ) have predicted a re-entrant ...order−disorder transition (i.e., fluid−FCC−BCC−fluid) as a function of concentration and so far there has been no experimental verifications of these theoretical predictions. Here, we present experimental results on phase behavior of loosely cross-linked charged poly(N-isopropylacrylamide co acrylic acid) (PNIPAm-co-AAc) microgesls with a low monomer volume fraction (∼0.003) for a wide range of concentrations (0.02−0.6 wt %) using static and dynamic light scattering methods. These microgel dispersions exhibit a short-range liquid order at low concentration (<0.03 wt %), a FCC crystalline order at intermediate concentrations (0.03− 0.3 wt %). In addition, we suggested a possible coexistence of BCC and FCC phases at higher concentration crystalline suspension (∼0.34 wt %). These results clearly demonstrate the experimental verification of above theoretical prediction below the overlap concentration and also reveal that the interaction potential between the microgel particles is of screened Coulomb repulsive type within these concentration ranges. At further higher concentration (∼0.57 wt %), we once again observed a disordered state and this disordered state from dynamic light scattering was confirmed to be a glass. These initial results are discussed in the light of previously reported results on the phase behavior of ionic microgel colloidal dispersions.
A novel non-stirred precipitation polymerization for rapid small-scale synthesis of monodisperse temperature-sensitive poly(
N
-isopropylacrylamide) microgels is introduced. A practical framework for ...the final particle size control is established, and low-temperature synthesis is highlighted as an easy alternative for producing large particles in contrast to the temperature ramp method. Furthermore, in situ 3D-DLS method is used to determine the kinetic rate law of the precipitation polymerization of
N
-isopropylacrylamide. The power law exponents for the reaction are determined to be 0.97 ± 0.12 and 0.46 ± 0.01 for the monomer and the initiator concentration, respectively. In conjunction with other evidence, it is suggested that the reaction follows conventional radical polymerization kinetics and takes place in the continuous phase. Number concentration of particles in the batch is recognized to be the determining factor for the final particle volume of the microgels.
In this comparative study we investigate the influence of the side chain substitution pattern on the volume phase transition temperature (VPTT) of thermo-sensitive polymer microgels. We especially ...focus on cononsolvency of water and methanol. We compare literature data for poly(N-isopropylacrylamide) (PNIPAM) that shows cononsolvency to experimental data for secondary amide poly(N-n-propylacrylamide) (PNNPAM) as well as tertiary poly(N,N-diethylacrylamide) (PDEAAM) and poly(N-vinyl-caprolactam) (PVCL). As methods we apply size determination by dynamic light scattering (DLS) as well as differential scanning calorimetry(DSC) and theoretical calculations using COSMO-RS (conductor-like screening model for real solvents). Clear differences are found for PDEAAM and PVCL in comparison to PNIPAM and PNNPAM. We ascribe the significant differences between PNIPAM and PNNPAM on the one hand and PDEAAM and PVCL on the other hand to the amide proton, which is present in PNIPAM and PNNPAM but not in the other two polymers.
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•Polymers from secondary and tertiary amides are compared.•Thermoresponsive polyacrylamides with amide proton show cononsolvency.•Thermoresponsive polyacrylamides without amide proton do not show cononsolvency.•Tertiary poly(N-vinylcaprolactam) behaves like tertiary polyacrylamides.•Calculations reproduce the experimental results.
The influence of the cross-link density on rheological properties of thermosensitive microgels was investigated. The temperature-sensitive hydrogel particles consisted of poly (N-isopropylacrylamide) ...(PNiPAM) chemically cross-linked with several different molar ratios of N,N'-methylenebisacrylamide. The variation of cross-link density leads to soft spheres that possess a different particle interaction potential and a different swelling ratio. With increasing temperature the microgel particles decrease in size and with it the effective volume fraction, which leads to strong changes in rheological properties. The relative zero-shear viscosity and the plateau modulus at different temperatures superpose to mastercurves when plotted versus the effective volume fraction. Up to an effective volume fraction of 0.5 the microgels behaved like hard spheres and the maximum volume fraction, as determined from the divergence of the zero-shear viscosity, was mainly dominated by the polydispersity of the spheres and not by the cross-link density. The plateau modulus, on the other hand, revealed soft-sphere behavior and the interaction potential became softer with decreasing cross-linker content.PUBLICATION ABSTRACT
Solutions of microgels have been widely used as model systems to gain insight into atomic condensed matter and complex fluids. We explore the thermodynamic phase behavior of hollow microgels, which ...are distinguished from conventional colloids by a central cavity. Small-angle neutron and x-ray scattering are used to probe hollow microgels in crowded environments. These measurements reveal an interplay among deswelling, interpenetration, and faceting and an unusual absence of crystals. Monte Carlo simulations of model systems confirm that, due to the cavity, solutions of hollow microgels more readily form a supercooled liquid than for microgels with a cross-linked core.
We propose a strategy to adjust the size of multiliposomal complexes made of small unilamellar anionic liposomes (lipid bilayer vesicles prepared from zwitterionic lipid ...1,2-dioleoyl-sn-glycero-3-phosphocholine DOPC and ionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine POPS with a typical diameter of 50 nm) with cationic polymers due to electrostatic attraction. Hereby, the use of quaternized poly(dimethylaminoethyl methacrylate), qPDMAEMA, allows the construction of multicompartmental carriers. Variation of the architecture and composition of various qPDMAEMA samples demonstrates the effect of introducing poly(ethylene oxide) PEO moieties and/or branching on the size of the hierarchical assemblies: both the PEO block (as seen for block copolymers and miktoarm stars) and the branching (as seen for star-shaped homopolymers and miktoarm stars) lead to smaller complexes compared to complexes with a linear qPDMAEMA homopolymer. Besides the use of cryogenic transmission electron microscopy (cryo-TEM), the formation of multicompartmental carriers composed of differently loaded liposomes is verified by two-color fluorescence cross-correlation spectroscopy (FCCS).
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•Size of complexes among anionic liposomes and cationic polymers can be adjusted.•Use of copolymers with soluble/non-ionic chains decreases the size of complexes.•Use of branched polymers further decreases the size of complexes.•Joint transport of differently loaded liposomes is possible, as seen by FCCS.
The phase behavior of supersoft spheres is explored using solutions of ultralow cross-linked poly(N-isopropylacrylamide)-based microgels as a model system. For these microgels, the effects of the ...electric charges on their surfaces can be neglected and therefore only the role of softness on the phase behavior is investigated. The samples show a liquid-to-crystal transition at higher volume fraction with respect to both hard spheres and stiffer microgels. Furthermore, stable body centered cubic (bcc) crystals are observed in addition to the expected face centered cubic (fcc) crystals. Small-angle x-ray and neutron scattering with contrast variation allow the characterization of both the microgel-to-microgel distance and the architecture of single microgels in crowded solutions. The measurements reveal that the stable bcc crystals depend on the interplay between the collapse and the interpenetration of the external shell of the ultralow cross-linked microgels.
The influence of a triblock copolymer, poly(ethylene oxide)20-b-poly(propylene oxide)70-b-poly(ethylene oxide)20 (Pluronic P123) on the phase behavior and on the shear-induced multilamellar vesicle ...(MLV, also called Onion) formation in the lyotropic lamellar phase of the nonionic surfactant C10E3 was investigated by means of rheology, small-angle neutron scattering (SANS), and microscopy. Added triblock copolymer shifted the Lα−L3 phase transition to lower temperatures. In the presence of triblock copolymer, MLV structure was not stable and easily transformed back into the lamellar phase with increasing polymer concentration and temperature. In the study of the shear-induced MLV formation, we found an increase of the critical shear rate for the onset of the shear-thickening, which also indicates the instability of MLV in the presence of the triblock copolymer. No MLV formation was observed at high polymer concentration. Suppression of the shear-induced MLV formation might be attributed to the enhancement of the effective surface tension originating from the excluded volume effect between polymers adsorbed onto the membranes.
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