Composite aqueous microgels: an overview of recent advances in synthesis,characterization and application

In the last decade there has been a steady increase in the number of publications describing the synthesis, characterization and applications of hybrid particles. Colloidal aqueous microgels form an important subdivision of polymer colloids used for the deposition of different functional materials. Due to their numerous attractive properties, microgel particles have been used as templates for the synthesis, storage and transportation of nanostructured materials. This paper reviews the important developments in the area of composite microgels over the last decade, and is limited to microgel particles operating in aqueous media. The patent literature is not included in the review. Different synthetic approaches as well as some properties and applications of composite microgels are discussed. Copyright © 2006 Society of Chemical Industry     

Thermo-sensitive poly(N-vinylcaprolactam-co-acetoacetoxyethyl methacrylate) microgels: 2. Incorporation of polypyrrole

In a previous paper [Polymer (2003) in press DOI: 10.1016/j.polymer.2003.09.037] polymeric temperature-sensitive microgels were prepared by surfactant-free emulsion co-polymerization of N-vinylcaprolactam (VCL) and acetoacetoxyethyl methacrylate (AAEM). In the present work, VCL/AAEM microgels were used as a template for oxidative polymerization of pyrrole (Py). It was found that pyrrole polymerization takes place directly in microgel structure leading to composite particles. Influence of microgel structure and amount of incorporated polypyrrole (PPy) on particle properties is discussed. Obtained stable composite microgels show similar thermal sensitivity as VCL/AAEM particles with fully reversible collapse-swelling properties.     

The use of poly (N-isopropylacrylamide) microgels as a multi-functional processing aid for aqueous alumina suspensions

Two temperature sensitive microgels of poly (N-isopropylacrylamide) were prepared, one anionic in nature and the other cationic. The microgels were concentrated by centrifugation and the rheological properties of the microgels measured as a function of temperature. The anionic microgel showed a transition from an elastic gel like structure to a liquid like structure at 32°C, whilst the cationic microgel demonstrated the same transition, but at a slightly higher temperature of 34°C. Both systems were completely reversible. A sub micron sized alumina powder was then mixed into the microgels using an anionic dispersant, (Darvan 821A ) to stabilise the powder. The powder was added until the system was just reversible. The aim was to see which microgel behaved as the best processing aid for the alumina particles, in terms of dispersion, gelation and adhesion after drying. The resulting mixtures were freeze dried, sintered and investigated by electron microscopy. The best microgel / alumina system was the one with similarly charged particles, i.e. the anionic microgel, whereas the system containing the oppositely charged cationic microgel particles flocculated, as would be expected from simple electrostatic theory. The presence of the inorganic particles caused the sharp transition from elastic gel to viscous fluid to broaden and the systems to behave viscoelastically over the whole temperature range. Sintered products made from the two systems were remarkably different. With the cationic microgel the resulting structures crumbled, but with the anionic microgel robust structures were obtained. Hence it is feasible to use similarly charged microgels as processing aids for ceramics as long as both particles are not oppositely charged.     

Microgel/SiO2 hybrid colloids prepared using a water soluble silica precursor

In the present work we demonstrate that functional polymer microgels may act as smart self-catalyzing system inducing controlled formation of silica nanoparticles inside the polymer network and formation of hybrid colloids. We synthesized a water soluble silica precursor PEG-PEOS via post-modification of hyperbranched poly(ethoxysiloxane) (PEOS) with poly(ethylene glycol) monomethyl ether. We used poly(N-vinylcaprolactam)-based microgel functionalized with imidazole and β-diketone groups as a matrix for biomimetic deposition of silica. Composite microgel particles containing silica nanoparticles (up to 20 wt.-%) have been prepared by simultaneous PEG-PEOS conversion and silica deposition in the microgels. TEM studies indicate the infiltration of silica nanoparticles (～10 nm) inside the corona region of the microgels due to the strong acid–base interaction between the acidic silica and basic imidazole groups. The resulting composite particles were found to be colloidally stable and no aggregation was observed even after months of storage. The incorporation of silica nanoparticles increased the rigidity of the microgel particles and reduced their thermal sensitivity.     

Preparation and characterization of surfactant‐free stimuli‐sensitive microgel dispersions

A surfactant‐free method to produce responsive polymer microgels is introduced. As an example, poly(methacrylic acid) hydrogels with varying crosslinking density have been synthesized in bulk and then chopped using a high shear mechanical cutter to form microgel particles dispersed in water. The mechanical cutting technique enables the concentration and particle size distribution of the microgel suspensions to be easily controlled, therefore making the rheology of the suspensions tuneable. The particle size distribution of the dispersions, characterized using light scattering, was dependent on the speed and duration of mechanical cutting. The particle size distribution also depended on the degree of crosslinking of the hydrogel. The higher the crosslinking density, the lower the average mean diameter of the resulting microgel particles. The lower the crosslinking density of the hydrogel, the larger the difference between the maximum and minimum particle size. The time to complete swelling of the particles upon change in pH was measured to be up to 45 s, depending on the particle size. The rheology of the resulting suspensions as a function of pH was investigated. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 104: 1912–1919, 2007     

Volume phase transition of “smart” microgels in bulk solution and adsorbed at an interface: A combined AFM,dynamic light,and small angle neutron scattering study

In the present article the swelling behavior of copolymer microgel particles made of poly(N-isopropylacrylamide)-co-vinylacetic acid using dynamic light scattering (DLS), neutron scattering, and in situ atomic force microscopy (AFM) for various copolymerized amounts of vinylacetic acid (VA) (up to 2.5 mol%) under slightly acidic conditions is studied. The transition temperature of these microgel particles is found to be ≈32.5 ± 1 °C, independent of the VA content. Microgel particles adsorbed onto a solid substrate display a similar volume phase transition as their dissolved counterparts. However, their swelling capacity is reduced by approximately one order of magnitude compared to the bulk value. Nevertheless, the observed effect still is sufficiently large to be exploited for the use of these particles in sensors or as nanoactuators. In addition it can be concluded that the continuous character of the transition observed in solution does not arise from the polydispersity of the particles but can be attributed to the heterogeneity inside each individual microgel particle. Finally, AFM images reveal a pattern on the surface of the collapsed particles, which we attribute to globules formed by collapsed dangling polymer chains. In solution these dangling ends form a brush contributing to the hydrodynamic dimensions of the microgels.     

Thermo-sensitive poly(N-vinylcaprolactam-co-acetoacetoxyethyl methacrylate) microgels. 3. Incorporation of polypyrrole by selective microgel swelling in ethanol-water mixtures

Behaviour of temperature-sensitive core-shell VCL/AAEM microgels has been studied in binary alcohol/water mixtures. Amount of alcohol in binary mixture influences strongly the swelling and thermo-sensitive properties of microgels. Alcohol induces swelling of VCL-rich microgel shell leading to larger dimensions of microgel particles and larger surface area. Under these conditions pyrrole polymerization was carried out, and the influence of pyrrole concentration, oxidant nature and temperature on morphology and properties of composite particles was investigated. Contrary to the polymerization in water medium, this selective swelling method gives possibility to increase three times loaded polypyrrole amount and maintain the stability of the colloidal system. It was found that in case when persulfate was applied as oxidant it is possible to vary effectively the particle size of composite microgels by changing the ethanol concentration in water. Contrary, when FeCl₃ was used as oxidant formation of secondary particles was detected leading to dispersions with bimodal particle size distribution. The conductivity of the composite particles was much higher if polypyrrole synthesis was carried out in pure water.     

A study of the binding of the biologically important hematin molecule to a novel imidazole containing poly(N-isopropylacrylamide) microgel

The influence of two different comonomers incorporated separately at 1 %w/w into a poly(N-isopropylacrylamide) microgel is investigated by photon correlation spectroscopy, transmission electron microscopy and high sensitivity differential scanning calorimetry. The synthesis, characterisation and subsequent binding affinity of iron protoporphyrin IX to the microgels was also investigated. Mössbauer and electronic absorbance spectroscopies were used to characterise the resulting protoporphyrin IX iron(III) polymer complexes. Co-polymer microgels containing either sulfide or imidazole residues acted as a matrix for the binding of haems to form soluble molecular structures containing multiple iron centres.     

Effect of functionalised core–shell microgels prepared by emulsion polymerisation on acrylic coatings properties

The influence of hydroxyl-functionalised acrylic core–shell microgels incorporated into a solvent-borne acrylic binder system on the properties of coatings is described in this paper. Our approach has been to show the usefulness of prepared functionalised microgels as coating modifiers. This subject was shown to be connected with the selection of an appropriate solvent with good affinity for microgels and the film-forming polymer. Therefore the swelling behaviour of microgels in selected solvents as a function of microgel composition is discussed as well. The structured microgels were prepared by first making an aqueous emulsion via the semi-batch emulsion copolymerisation, then dehydrating the system by air drying followed by grinding in a mill. The resulting agglomerates of spherical microgel particles were dispersed in convenient organic solvent media and after that added to the thermosetting solvent-borne acrylic binder system. It was shown that the application of core–shell microgels did not affect the surface appearance and transparency of coatings. Moreover, the presence of microgel network precursors improved corrosion resistance of coatings.     

On the structure of biocompatible,thermoresponsive poly(ethylene glycol) microgels

The aim of the present study is the preparation and characterization of microgel particles which are, contrary to other microgels, thermoresponsive as well as biocompatible. Hence, monodisperse p-MeO₂MA-co-OEGMA microgel particles were synthesized by precipitation polymerization. Swelling/deswelling behavior and the structure of poly(ethylene glycol) (PEG) based microgel particles were investigated. A combination of dynamic light scattering (DLS) and small angle neutron scattering (SANS) was used. Particle size and the volume phase transition temperature (VPTT) are adjustable by changing the amount of comonomer. SANS measurements indicate an inhomogeneous structure of the PEG microgels in the swollen state. At temperatures above the VPTT a compact structure was observed. An increase of the comonomer content leads to a densely packed core and a fuzzy shell in the swollen state. Additionally, nanodomains inside the polymer network were observed in the temperature range around the volume phase transition (VPT). Due to this heterogeneous structure in the swollen state two correlation lengths of the network fluctuations were observed.