Non-close-packed arrangement of soft elastomer microspheres on solid substrates.

Unlike rigid microparticles, soft and deformable elastomer (rubber) microspheres were found to exhibit a non-close-packed arrangement on solid substrates after the evaporation of water from their dispersions. The microscopic observation revealed that individual microspheres are ordered in regular intervals at the air/water interface of a sessile droplet and remain fixed on the substrate without being affected by the capillary forces during evaporation due to their deformability. Moreover, using the Langmuir-Blodgett method, thin films of non-close-packed structures could be successfully generated over large areas. Our findings may potentially help to control the arranged structures of elastomer microspheres, which can be expected to improve the nano-science and technology for the precise control for e.g. surface patterning.

Quantification for the Mixing of Polymers on Microspheres in Waterborne Latex Films.

Although tremendous efforts have been devoted to the structural analysis and understanding of the toughness of latex films, in which soft elastomer microspheres are interpenetrated, a method to quantitatively analyze the mixing of polymer chains at the microsphere surface, i.e., delocalization of hydrophilic charged group on the polymer chains by aging, has not yet been established. In this study, small-angle X-ray scattering was applied to characterize latex films by assuming a pseudo-two-phase system, which consists of an average-electron density microsphere core and a high-electron density interphase between the microsphere interfaces due to localized charged groups. The thus obtained parameter, i.e., the characteristic interfacial thickness (tinter), quantitatively reflects the degree of mixing of polymer chains on the microsphere surface. We found that tinter is strongly correlated to the fracture energy of the latex films. The proposed analysis method for the microscopic mixing of polymers on the microsphere surface in the film can thus be expected to shed light on design guidelines for industrial latex films and on the understanding of the mechanical properties of such films.

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.

Decoupled Thermo- and pH-Responsive Hydrogel Microspheres Cross-Linked by Rotaxane Networks.

Rotaxane cross-linked (RC) microgels that exhibit a decoupled thermo- and pH-responsive volume transition were developed. The pH-induced changes of the aggregation/disaggregation states of cyclodextrin in the RC networks were used to control the swelling capacity of the entire microgels. Different from conventional thermo- and pH-responsive microgels, which are usually obtained from copolymerizations involving charged monomers, the RC microgels respond to temperature as intended, even in the presence of charged functional molecules such as dyes in the microgel dispersion. The results of this study should lead to new applications, including drug delivery systems that require a retention of their smart functions even in environments that may contain foreign ions, for example, in in vivo experiments.

Fast Adsorption of Soft Hydrogel Microspheres on Solid Surfaces in Aqueous Solution.

The real-time adsorption behavior of polymeric colloidal microspheres onto solid surfaces in aqueous solution was visualized for the first time using high-speed atomic force microscopy (HS-AFM) to reveal how the softness of the microspheres affects their dynamic adsorption. Studies that focus on the deformability of microspheres upon dynamic adsorption have not yet been reported, most likely on account of a lack of techniques that appropriately depict the dynamic adsorption and deformation behavior of individual microspheres at the nanoscale in real time. In this study, the deformability of microspheres plays a crucial role on the adsorption kinetics, that is, soft hydrogel microspheres adsorb faster than harder elastomeric or rigid microspheres. These results should provide insight towards development of new colloidal nanomaterials that exhibit effective adsorption on specific sites in aqueous solution.

Formation of Tough Films by Evaporation of Water from Dispersions of Elastomer Microspheres Crosslinked with Rotaxane Supramolecules.

Compared to rigid microspheres that consist, for example, of polystyrene or silica, soft and deformable elastomer microspheres can be used to generate colorless transparent films upon evaporating the solvent from microsphere-containing dispersions. To obtain tough films, a post-polymerization reaction to crosslink the microspheres is usually necessary, which requires extra additives during the drying process. This restriction renders this film-formation technology complex and rather unsuitable for applications in which impurities are undesirable. In the present study, it is demonstrated that tough elastomer microspheres that are crosslinked with rotaxanes can form tough bulk films upon evaporation of water from microsphere dispersions, so that post-polymerization reactions are not required. The results of this study should thus lead to new applications including coatings for biomaterials that need complete removal of all impurities from the materials prior to use.

Water-immiscible bioinert coatings and film formation from aqueous dispersions of poly(2-methoxyethyl acrylate) microspheres.

Poly(2-methoxyethyl acrylate) (pMEA) microspheres are prepared through facile free-radical polymerization in water without additives and impurities, such as surfactants, other polymers, and organic solvents, which are usually used to synthesize pMEA chains. Clean and pure (non-factionalized and non-cross-linking) pMEA microspheres exhibit plasma-protein adsorption resistances on their surface regardless of their charged state. They are characterized in terms of the adsorbed amounts of proteins at pH 7. In addition, these soft and deformable pMEA microspheres are suitable for forming substrates coated with pMEA microspheres and free-standing films by injecting pMEA dispersion and evaporating the aqueous medium through fusion between the pMEA chains at the surface without the precoating agent and cross-linker. These pMEA coatings have been used till now in artificial heart/lung fabrication and metal products manufacturing by casting organic solvents such as 1,4-dioxane, toluene, and methanol and dissolving pMEA chains prepared using conventional solution polymerization. In this study, bioinert coatings and adhesive and transferable films are easily obtained due to the rubber-like properties of the pMEA microspheres and stable in mild and biocompatible conditions even when these impurities are not used completely, allowing us to provide a guideline for potential pMEA bioapplications such as coatings, films, barriers, and implant devices.