Impact of Spatial Distribution of Charged Groups in Core Poly(N-isopropylacrylamide)-Based Microgels on the Resultant Composite Structures Prepared by Seeded Emulsion Polymerization of Styrene

Interfacial Electrokinetic Phenomena of Thermoresponsive Microgels with a Spatial Gradient of Charged Groups

When functional microgels are synthesized via radical copolymerization, spatial gradients of functional groups often form due to a difference in reactivity ratios between monomer and comonomer. In this study, we systematically investigated the effect of a decreasing gradient of charged groups from the core to the shell of microgels on their surface properties, which are crucial for colloidal particles, through the analysis of interfacial electrokinetic phenomena using Ohshima's equation. A series of electrophoretic analyses combined with dynamic light scattering revealed that the surface of the microgels undergoes a multistep collapse during the particle-size reduction due to dehydration upon increasing the temperature. Furthermore, the more complicated hierarchical gradient of charged groups within the microgels was elucidated by quantitatively evaluating changes in surface properties during precipitation polymerization based on interfacial electrokinetic phenomena.

Multi-layer core/shell microgels with internal complexity and their nanocomposites

In this study, we show that core/shell (CS) microgels with multiple layers can be created via a one-pot precipitation polymerization, in which monomers are added to the reaction flask multiple times once most of the previous monomer has been consumed. The complex internal structures of the microgels were examined using a combination of scattering and microscopy techniques.

Clarification of Surface Deswelling of Thermoresponsive Microgels by Electrophoresis

Although many investigations of thermoresponsive microgels have been reported, their surface properties, which are crucial in colloid science, are still not fully understood. In this study, microgels with surface-localized charged groups were synthesized by precipitation polymerization, and their electrophoretic behaviors were analyzed using a modified version of Ohshima's equation to obtain two surface properties of the soft particles: the softness parameter and the surface charge density. This systematic evaluation allows us to discuss the thermoresponsiveness of the overall microgels and their surfaces separately. Furthermore, the validity of the surface properties obtained from electrophoresis was verified by comparing them with the results of seeded emulsion polymerization in the presence of the microgels and the force-indentation curves obtained via high-speed atomic force microscopy (HS-AFM).

Methacrylic acid based microgels and hybrid microgels

Methacrylic acid based microgels have got much consideration in the last two decades because of their potential uses in different fields owing to their responsive behaviour towards external stimuli. Synthesis, properties and uses of methacrylic acid based microgels and their hybrids have been critically reviewed in this article. With minute change in external stimuli such as pH and ionic strength of medium, these microgels show quick swelling/deswelling reversibly. The methacrylic acid based microgels have been widely reported for applications in the area of nanotechnology, drug delivery, sensing and catalysis due to their responsive behaviour. A critical review of current research development in this field along with upcoming perception is presented here. This discussion is concluded with proposed probable future studies for additional growth in this field of research.

Durable gelfoams stabilized by compressible nanocomposite microgels

Compressible nanocomposite microgels can stabilize the air/water interfaces of gas bubbles for several months, which affords durable gelfoams.

Hydrophobic Monomers Recognize Microenvironments in Hydrogel Microspheres during Free-Radical-Seeded Emulsion Polymerization

The three-dimensional structure of nanocomposite microgels was precisely determined by cryo-electron micrography. Several nanocomposite microgels that differ with respect to their nanocomposite structure, which were obtained from seeded emulsion polymerization in the presence of microgels, were used as model nanocomposite materials for cryo-electron micrography. The obtained three-dimensional segmentation images of these nanocomposite microgels provide important insights into the interactions between the hydrophobic monomers and the microgels, that is, hydrophobic styrene monomers recognize molecular-scale differences in polarity within the microgels during the emulsion polymerization. This result led to the formation of unprecedented multi-layered nanocomposite microgels, which promise substantial potential in colloidal applications.

Synthetic strategies for raspberry-like polymer composite particles

The strategies used for the preparation of raspberry-like polymer composite particles are summarized comprehensively.

Effect of Charge Groups Immobilized in Hydrogel Microspheres during the Evaporation of Aqueous Sessile Droplets

In contrast to conventional dispersions of solid microspheres, dilute dispersions containing soft hydrogel microspheres (microgels) exhibit unique drying behavior due to their selective adsorption at the air/water interface of sessile droplets. So far, the impact of the size, chemical composition, and softness (degree of cross-linking) of microgels has been investigated. In the present study, we present the impact of charged groups introduced in the microgels on the adsorption and assembly behavior of these microgels at the air/water interface using a series of microgels with different amounts and distribution of charged groups. A series of experiments under different conditions (pH value and ionic strength) afforded information that clarified the adsorption, interpenetration, and deformation behavior of such charged microgels at the air/water interface. The results indicate that the adsorption and the deformation of charged microgels at the air/water interface are suppressed by the presence of charged groups. Moreover, charged microgels adsorbed at the interface are more dynamic and not highly entangled with each other; i.e., even though the more dynamic charged microgels are arranged at the interface, these arranged structures are disrupted upon transferring onto the solid substrates. Our findings of this study can be expected to promote the further development of applications, e.g., foams and emulsions stabilized by microgels, that crucially requires an in-depth understanding of the adsorption behavior of charged microgels at the air/water interface such as coatings.

Hydrophobized nanocomposite hydrogel microspheres as particulate stabilizers for water-in-oil emulsions

Hydrophobized nanocomposite microgels can serve as particulate emulsifiers for water-in-oil (W/O) emulsions with different types of oils, including non-polar oils, which usually do not form W/O emulsions with conventional microgels.

Seeded Emulsion Polymerization of Styrene in the Presence of Water-Swollen Hydrogel Microspheres

In a previous study, we have ascertained that the charge distribution in hydrogel microspheres (microgels) plays a crucial role in controlling the nanocomposite structure of the polystyrene obtained from the seeded emulsion polymerization (SEP) of styrene in the presence of microgels. However, all these polymerizations were conducted at high temperature, where most of these microgels were dehydrated and deswollen. In the present study, we initially verified that the nanocomposite microgels can be synthesized even when the seed microgels are swollen and hydrated during the SEP of styrene. These highly swollen microgels were used as the nucleation sites for the polystyrene, and subsequently the propagation of the hydrophobic polystyrenes proceeded within water-swollen microgels.

Self-Organization of Soft Hydrogel Microspheres during the Evaporation of Aqueous Droplets

The unique drying behavior of aqueous droplets that contain soft hydrogel microspheres (microgels) upon evaporation was systematically investigated. Compared to the ring-shaped deposits that are obtained from drying solid microsphere dispersions, we have previously reported that uniformly ordered thin films are obtained from drying ∼1.2 μm-sized poly( N-isopropyl acrylamide) microgel dispersions. In the present study, we thoroughly investigated several hitherto unexplored aspects of this self-organization, such as the effect of the size, chemical structure, and "softness" of the microgels (or rigid microspheres). For the macro- and microscopic observation of the drying behavior of various microsphere dispersions, an optical microscope and a digital camera were employed. The results suggested that the convection in the aqueous droplets plays an important role for the transportation of the microgels to the air/water interface, where the softness and surface activity of the microgels strongly affects the adsorption of the microgels. On the basis of these discoveries, a design concept for the rapid formation of uniform thin films of soft microgels was proposed.

Role Transformation of Poly( N-isopropylacrylamide) Microgels from Stabilizer to Seed in Dispersion Polymerization by Controlling the Water Content in Methanol-Water Mixture

Poly( N-isopropylacrylamide) (PNIPAM)-based ionic microgels with different diameters were first prepared and then used as particulate stabilizer or seed in dispersion polymerization of styrene. The role of PNIPAM-based ionic microgels could be transformed from particulate stabilizer to seed by controlling the water content in methanol-water mixture. Generally, PNIPAM-based ionic microgels served as particulate stabilizer in methanol in the absence of water, leading to the formation of spherical polystyrene nanoparticles. However, they turned into seeds when water was added into the methanol solution, with the formation of octopus-like nanoparticles. Further study demonstrated that the mechanism for this role transition was related to the special thermosensitivity of PNIPAM microgels in methanol-water mixture. They lost their thermosensitivity in pure methanol solution but restored their thermosensitivity when increasing the water content in methanol-water mixture.

Nanocomposite Microgels for the Selective Separation of Halogen Compounds from Aqueous Solution

Nanocomposite microgels that selectively adsorb and release halogen compounds were developed. These nanocomposite microgels consist of poly(2-methoxyethyl acrylate) (pMEA) and a poly(oligo ethylene glycol methacrylate) hydrogel matrix. Therefore, the methoxy groups of the former are crucial for the halogen bonding, while the presence of the latter adds colloidal stability and allows controlled uptake/release of the halogen compounds. Such nanocomposite microgels may not only be used as dispersed carriers, but also in films and columnar formations. Thus, these unprecedented polymer/polymer nanocomposite microgels resolve a variety of problems associated with, e.g., the removal of halogen compounds from wastewater, or with the delivery of halogen-containing drugs.