Microgels In Solution Research Articles

Water-swellable polyelectrolyte microgels polymerized in an inverse microemulsion using a nonionic surfactant

A series of poly(dimethylacrylamide-co-2-acrylamido-2-methyl-1-propanesulfonic acid) microgels slightly crosslinked by methylene-bis-acrylamide (MBA) were polymerized in a novel inverse microemulsion polymerization (IMEP) system. To determine a suitable composition of the IMEP system, the phase diagram of a pseudoternary system was made. The pseudoternary polymerization system consisted of n-hexane, a nonionic surfactant (polyoxyethylene oleyl ether, C 18E n ), and an aqueous monomer solution. Polymerization was performed in a single-phase reversed micelle solution. The reversed micelles were about 50 nm in diameter, as determined by FF-TEM. The viscometric characteristics of the polymers extracted from the IMEP system were studied in 3 mM sodium chloride aqueous solution. The intrinsic viscosity values for the noncrosslinked and crosslinked (0.1 mol% MBA was incorporated) samples were 25 and 7.4 dl/g, respectively. The overlap concentration ( c ∗ ) of crosslinked polymer microgel occurred at c[ η]=1 in the solvent. When the volume fraction ( φ) of the microgel was 0.7, the value of the apparent yield stress of the microgel solution was observed. These results show that the microgel has a significant thickening effect above c ∗ due to friction between the microgel particles. It is assumed that the microgels polymerized in a confined space retain the shape or size of the nanosized reactor with a diameter on the order of 50 nm.

Phase behavior of ionic microgels.

We employ effective interaction potentials between spherical polyelectrolyte microgels in order to investigate theoretically the structure, thermodynamics, and phase behavior of ionic microgel solutions. Combining a genetic algorithm with accurate free energy calculations we are able to perform an unrestricted search of candidate crystal structures. Hexagonal, body-centered orthogonal, and trigonal crystals are found to be stable at high concentrations and charges of the microgels, accompanied by reentrant melting behavior and fluid-fcc-bcc transitions below the overlap concentration.

Gelation of polymer solutions under shear flow

Abstract Gelling a polymer solution under constant shear rate was recently found to be an attractive procedure for preparing size-controlled microgels. In the theoretical approach developed in this paper, the microgels formed during gelling are modelled as non-covalent ‘star’ polymers. This approach provides scaling laws to predict the effects of the shear rate applied during gelation on: (1) the gelling kinetics in three flow regimes; (2) the microgel size dependence in weak and strong flows; and (3) the rheological and elastic properties of the microgel solutions. The reported experiments were carried out by gelling semi-dilute high molecular weight polymer solutions by a crosslinker small enough to diffuse inside macromolecules and thus able to form both intra and intermolecular cross-links. All available experimental results are in agreement with our theoretical predictions, but further experiments are needed for a complete validation of the proposed theory.

Synthesis of the flower type microgels

Polymer micelles with polystyrene (PS) shell and poly(2-vinyl pyridine) (P2VP) core were formed with P(2VP-b-S-b-2VP) triblock copolymers in a toluene/cyclohexane mixture that was good for the PS and poor for the P2VP. Then, P2VP core parts of the polymer micelles were chemically crosslinked with 1,4-diiodobutane. The change in the hydrodynamic diameters of the micelles upon crosslinking was measured by dynamic light scattering. The shape and the size of the crosslinked products (microgels) were observed by transmission electron microscopy. Viscosity of the microgel solution was measured in the semi-dilute region. Mechanical properties of the triblock copolymer films and the flower shaped microgel films were measured by dynamic mechanical spectroscopy and thermal mechanical analysis.

Phase separation in free-radical crosslinking copolymerization: formation of heterogeneous polymer networks

A model combining both the thermodynamic and kinetic aspects of free-radical crosslinking copolymerization (FCC) is presented to predict the formation conditions and the properties of heterogeneous (porous) networks. The model involves thermodynamic equations describing the phase equilibria between the network and separated phases during FCC of vinyl/divinyl monomers and kinetic equations giving the concentration of reacting species and the polymer properties as a function of the monomer conversion. Calculation results are presented for styrene/ m-divinylbenzene (S/ m-DVB) copolymerization system in the presence of inert diluents. S/ m-DVB copolymerization system at a high m-DVB concentration, or, at a low monomer concentration phase separates at the gel point and results in the formation of a microgel solution. The calculation results also show that the heterogeneity in S/ m-DVB copolymer networks increases on increasing DVB or diluent concentration, or, on decreasing the solvating power of the diluent, in accord with the experimental data published previously. The model also predicts correctly the equilibrium volume swelling ratio of heterogeneous networks in solvents.

Poly(organosiloxan)mikrogele als Trägermaterialien für heterogene Methylalumoxancokatalysatoren

Redispersible poly(organosiloxane)microgels are excellent materials for the immobilization of methylalumoxane. The subsequent treatment of a poly(organosiloxane)microgel solution in toluene with trimethylaluminum and water at −78°C yields a completely immobilized methyl-alumoxane cocatalyst, which can be used in the metallocene catalyzed, heterogeneous polymerization of ethylene. Functional groups at the surface of these poly(organosiloxane)microgels lead to catalyst systems with different properties.

Collective and self diffusion of PS microgels in solution as observed by thermal diffusion forced Rayleigh scattering

Thermal diffusion in solutions of polystyrene micro-network spheres (microgels) in toluene has been studied by the holographic scattering technique of thermal diffusion forced Rayleigh scattering (TDFRS) and by photon correlation spectroscopy (PCS). Size distributions of microgels of different crosslink ratios are obtained from TDFRS measurements on dilute solutions at very low q-values around 4000cm−1. At low concentrations a single diffusive mode is observed and the diffusion coefficient increases with concentration. It is attributed to the collective diffusion of the microgels and the solvent. At high concentrations an additional slow mode appears whose diffusion coefficient decreases with increasing concentration. Both diffusive modes are observed with PCS and TDFRS. Contrary to PCS, heterodyne TDFRS-measurements reveal a negative amplitude of the slow mode. We attribute the slow mode to self-diffusion of the microgels, made visible by the polydispersity of their size distribution. It is discussed in terms of a fast coupled thermal diffusion with subsequent decoupling of the individual microgels and relaxation into a new Soret equilibrium by self-diffusion of the microgels.

Molecular motion in concentrated solutions of spherical polystyrene microgels studied with the pulsed field gradient n.m.r.

Results of a pulsed field gradient n.m.r. study of the motion of swollen spherical microgels in solution are presented. We have measured the echo attenuation (or the incoherent dynamic structure function) of the protons in the microgels in the dynamic range from qR⪡1 up to qR ≈ 1.8 (where q=scattering vector and R = particle radius), and in the timescale from a few milliseconds up to 100 ms. Rotational diffusion of the microgel spheres could not be detected with certainty. However, restricted diffusion of the spheres within a cage was observed, in particular for the large microgel with R =125 nm, where the short-time diffusion could be monitored. For apparent volume fractions (φ > 0.6, the diffusion is restricted within a space scale of root mean square displacement, 〈z 2〉 1 2 120 nm . With increasing volume fraction of the microgels in solution, (φ > 0.6, the diffusion becomes increasingly restricted. This crossover corresponds to the dynamic glass transition observed by Bartsch et al. for a similar system using quasielastic light scattering.

Dynamic light scattering study of concentrated microgel solutions as mesoscopic model of the glass transition in quasiatomic fluids

This paper presents a light scattering study of the dynamics of concentrated solutions of nearly monodisperse (σ≊0.16) spherical micronetwork particles consisting of highly cross-linked polystyrene dissolved in carbon disulfide, i.e., a ‘‘good’’ solvent. Above volume fractions of φ=0.50 the intermediate scattering function, measured over a time window of 10−7 to 103 s using the ALV5000 correlator, decays in two steps and shows indications of nonergodic behavior for φ≥0.64. Such behavior is typical for glass forming systems and has recently been found close to the glass transition of a hard sphere colloidal system [W. van Megen and P. N. Pusey, Phys. Rev. A 43, 5429 (1991)]. Thus the introduced system can be used for modeling the glass transition of atoms on a mesoscopic scale. The traditional analysis of structural relaxation in terms of a Kohlrausch–Williams–Watts distribution yields a mean relaxation time which follows the empirical Mooney equation as a function of concentration and thus corresponds to Vogel–Fulcher–Tammann behavior. However, the necessity to add an unspecified ‘‘intermediate’’ process between the short and long time KWW decays demonstrates the limitations of this ‘‘pragmatic’’ approach. The mode coupling theory of the glass transition interprets the intermediate scattering function consistently over nearly seven decades in time, the intermediate region corresponding to the crossover from β to α relaxation (von Schweidler law). The critical volume fraction of 0.636 derived by this analysis corresponds to a value of 0.59 for an ideal monodisperse system which is well in accord with other experimental and computer simulation studies of the glass transition of atomic systems.

Rheology of concentrated microgel solutions

Viscosity, modulus, and yield stress for 0–6 wt% aqueous solutions of Carbopol 941 were investigated using constant shear rate, constant shear stress, and dynamic oscillatory experiments. The microgel character of the polymer was evident from the solid-like behavior of the solutions above 1 wt%. Yield stress increased with concentration, but yield occurred at a critical shear strain of 40%, independent of concentration. The static stress-strain relationship became non-linear at ~ 25% strain, in fair agreement with the onset of non-linear response in the storage modulus at ~ 10% strain. Small strain moduli from static and low frequency measurements agreed rather well; modulus values obtained from the recoverable strain after yielding were 30–40% smaller. Solutions flowed at near-constant stress in the low shear rate regime; at higher rates the stress increases with shear rate more rapidly. The viscosity did not obey the Cox-Merz rule. Steady-state viscosity scaled with polymer concentration to the 3/4 power. Results were interpreted using a cellular, deformable sphere model for the polymer, in analogy to emulsions and foams.

Microgels—Functional Globular Macromolecules

Reactive microgels are crosslinked macromolecules of globular shape. They may be prepared by polymerization of multifunctional monomers in emulsion or solution. Microgels are soluble in water or organic solvents. Their molecular mass is usually much higher than that of linear macromolecules of comparable diameters. By emulsion copolymerization (ECP) of unsaturated polyesters (UP) and styrene (S) microgels may serve as self-emulsifying component in ECP. Results on influence of molecular mass of UP and UP/S ratio on microgel yield, on the relation between structure and viscosity of microgel solution and on the ageing mechanism in dry state are reported.

Effect of syndiotacticity on the reaction of poly(vinyl alcohol)s with iodine

AbstractPoly(vinyl alcohol)s derived from poly(vinyl trifluoroacetate), obtained by free radical polymerization of vinyl trifluoroacetate in various kinds of solvents at temperatures between 60 and −40°C, are reacted with iodine in aqueous solution, and UV‐spectra of the solutions were measured after the reaction. The absorption maximum in the UV‐spectrum appears at 600 nm for the polymers obtained in ketones, alkanes, and esters or at 590 nm in aldehydes. The absorbance of the maximum increases with increasing syndiotacticity up to a content of syndiotactic diads of Xs ≈︁ 55,5–58,5% or of triads of Xss ≈︁ 34,2–39,3%, then it passes through a maximum value and decreases with increasing Xs or Xss. It is considered that the increases is due to the stereoregularity of the PVA molecules and that the decrease is due to the formation of microgels in solution.

Microgel: An idealized polymer molecule

AbstractPure microgels (i.e., those free from any linear or branched admixed polymers) were prepared by an emulsion polymerization process. The products were characterized by ultracentrifuge measurements. The best crosslinking agent for acrylonitrile microgels was found to be N,N′‐methylene bisacrylamide, while divinylbenzene gave the best styrene and methyl acrylate microgels. A study of the viscosity of microgel solutions vs. concentration showed that there was a sudden transition from a low viscosity to a high viscosity at a characteristic gel point, G, dependent upon the degree of crosslinking. For monodisperse systems, this coincides with the point where hexagonal close packing of the swollen spherical microgel particles occurs. The following equations relating the gel point and intrinsic viscosity of a microgel were found to apply: [η] = (0.025/ρ)S and (G/ρ)S = π/3√2, where S is a swelling factor which is equal to the ratio of swollen to dry volume and ρ is the density of the dry microgels. The above relationships were developed by considering microgels as swollen Einstein spheres. Evidence is presented to support the equations by some light scattering and viscosity measurements. The Flory equation relating swelling factor, polymer‐solvent interaction parameter, and degree of crosslinking was successfully applied to microgel solutions. A simple empirical equation, found to correlate the intrinsic viscosity [η] with the mole ratio of crosslinking agent, x for the microgels, was [η] = K(1/x) + 0.025/ρ where K is a constant.