Nanoparticle-Based Tough Polymers with Crack-Propagation Resistance.

Although thin elastomer films of polymer nanoparticles are regarded as environmentally friendly materials, the low mechanical strength of the films limits their use in various applications. In the present study, we investigated the fracture resistance of latex films composed of acrylic nanoparticles where a small quantity of a rotaxane crosslinker was introduced. In contrast to conventional nanoparticle-based elastomers, the latex films composed of the rotaxane-crosslinked nanoparticles exhibited unusual crack propagation behavior; the direction of crack propagation changed from a direction parallel to the crack to one perpendicular to the crack, resulting in an increase in tear resistance. These findings will help to broaden the scope of design of new types of tough polymers composed of environmentally friendly polymer nanoparticles.

Controlling the mechanical properties of hydrogels via modulating the side-chain length.

Even though the toughness of hydrogels is usually adjusted by changing the cross-linking density and structure, or the polymer concentration, we have discovered a new strategy to control the toughness via modulating the side-chain length. In this study, this strategy was applied to biocompatible poly(oligo(ethylene glycol) methyl ether methacrylate) with long ethylene-oxide side chains.

Nanostructure and thermoresponsiveness of poly(N-isopropyl methacrylamide)-based hydrogel microspheres prepared via aqueous free radical precipitation polymerization.

Thermoresponsive hydrogel microspheres (microgels) are smart materials that quickly respond to external stimuli, and their thermoresponsiveness can be tuned by varying the constituent chemical species. Although uniformly sized microgels can be prepared via aqueous free radical precipitation polymerization, the nanostructure of the obtained microgels is complex and remains unclear so far. In the present study, the nanostructure and thermoresponsiveness of poly(N-isopropyl methacrylamide) (pNIPMAm)-based microgels, which have a volume-transition temperature of ∼43 °C, were evaluated mainly using temperature-controllable high-speed atomic force microscopy. These observations, which are characterized by high spatio-temporal resolution, revealed that the pNIPMAm microgels have a peculiar heterogeneous structure, for example a core-shell and non-thermoresponsive nanostructure in the core region, that originates from the precipitation polymerization process. Furthermore, it was found that the adsorption concentration of the microgels on the substrate is one of the keys for controlling their thermoresponsiveness. These findings can be expected to advance the design of new materials such as thermoresponsive nanosheets and stimuli-responsive coatings.

Mechanical properties of temperature-responsive gels containing ethylene glycol in their side chains.

The mechanical properties of temperature-responsive and biocompatible poly(oligo-ethylene glycol methyl ether methacrylate)-based gels were investigated using dynamic viscoelasticity measurements so as to find applications in tissue and biomedical engineering. The gels were copolymerized using two ethylene glycol methacrylate monomers with diethylene glycol side chains: diethylene glycol methacrylate (MeO2MA), which contains two ethylene oxide units, and oligo-ethylene glycol methyl ether methacrylate (OEGMA) with either four or five ethylene oxide units. The storage (G') and loss (G'') moduli of these gels exhibit unique temperature-responsive behavior and depend on the copolymerization ratio. In MeO2MA-rich gels, phase separation occurred with increasing temperature, resulting in a significant increase in G' and the disappearance of the frequency dependence of G''. Although phase separation of OEGMA-rich gels was also observed with increasing temperature, it resulted in only a slight increase in the storage modulus due to the steric hindrance of the side chain. The mechanical properties of these gels are thus found to be strongly affected by a slight difference in the number of ethylene oxide groups in their side chains.

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.

Concentration dependence of the dynamics of microgel suspensions investigated by dynamic light scattering.

The dynamics of colloidal gel particle suspensions, i.e., microgel suspensions, has been investigated by dynamic light scattering (DLS) over a wide concentration range from the (I) dilute (φ < φcp) to the (II) intermediate (φ ≈ φcp) and (III) high concentration regions (φ ≫ φcp), where φ and φcp are the volume fraction of the gel particles in the suspension and the random close packing fraction, φcp ≈ 0.64, respectively. The time-intensity correlation function exhibited a distinct change with increasing φ, i.e., from ergodic behaviour (region I and II) to nonergodic behaviour (region III). A mode transition from translational (region I) to cooperative diffusion (the so-called gel mode) (region II) was also observed due to the soft and deformable nature of the microgels. Different from the dynamics of hard colloidal glass suspensions, the gel mode remained even at φ ≫ φcp. By using the ensemble-averaged time-correlation function, IE, we quantify the relationship between φ and their dynamics, and show that the soft microgels are deswollen in the densely packed state.

Non-Thermoresponsive Decanano-sized Domains in Thermoresponsive Hydrogel Microspheres Revealed by Temperature-Controlled High-Speed Atomic Force Microscopy.

Despite the tremendous efforts devoted to the structural analysis of hydrogel microspheres (microgels), many details of their structures remain unclear. Reported in this study is that thermoresponsive poly(N-isopropyl acrylamide) (pNIPAm)-based microgels exhibit not only the widely accepted core-shell structures, but also inhomogeneous decanano-sized non-thermoresponsive spherical domains within their dense cores, which was revealed by temperature-controlled high-speed atomic force microscopy (TC-HS-AFM). Based on a series of experiments, it is concluded that the non-thermoresponsive domains are characteristic for pNIPAm microgels synthesized by precipitation polymerization, and plausible structures for microgels prepared by other polymerization techniques are proposed.

Hydrogel Microellipsoids that Form Robust String-Like Assemblies at the Air/Water Interface.

Soft colloidal particles such as hydrogel microspheres assemble at air/water or oil/water interfaces, where the soft colloids are highly deformed and their surface polymer chains are highly entangled with each other. Herein, we report the formation of robust one-dimensional, string-like colloidal assemblies through self-organization of hydrogel microspheres with shape anisotropy at the air/water interface of sessile droplets. Shape-anisotropic hydrogel microspheres were synthesized via two-step polymerization, whereby a hydrogel shell was formed onto preformed rigid microellipsoids. The shape anisotropy of the hydrogel microspheres was confirmed by transmission electron microscopy and high-speed atomic force microscopy as well as by light-scattering measurements. The present findings are crucial for the understanding of natural self-organization phenomena, where "softness" influences microscopic assembled structures such as those of Nostoc bacteria.

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.

The deformation of hydrogel microspheres at the air/water interface.

The deformation of soft hydrogel microspheres (microgels) adsorbed at the air/water interface was investigated for the first time using large poly(N-isopropyl acrylamide)-based microgels synthesized by a modified aqueous precipitation polymerization method. The deformation of the micron-sized soft microspheres could be visualized clearly and analyzed quantitatively at the air/water interface.

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.

High Reusability of Catalytically Active Gold Nanoparticles Immobilized in Core-Shell Hydrogel Microspheres.

The reusability of hybrid core-shell microgels, whose core surfaces were decorated with gold nanoparticles, was investigated in terms of catalysis activity. Hybrid core-shell microgels composed of a rigid core and water-swollen gel shell endowed the immobilized gold nanoparticles with a high dispersion stability, which resulted in excellent catalytic activity. In contrast to free Au nanoparticles and conventional hybrid microgels, where the Au nanoparticles are randomly distributed over the entire microgel templates, the hydrogel shell part of the hybrid core-shell microgels suppressed the aggregation between the microgels and Au nanoparticles in individual microgels, which improved the reusability for the catalysis reaction. The results of this study should help to develop advanced catalyst systems that require high reusability even when the chemical reactions occur in aqueous solution and external stimuli are applied.

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.

Controlled Separation and Release of Organoiodine Compounds Using Poly(2-methoxyethyl acrylate)-Analogue Microspheres.

A selective adsorption/desorption of organoiodine compounds was achieved on poly(2-methoxyethyl acrylate)-analogue microspheres, wherein the side chains in the polymers act as halogen-bonding sites. These results demonstrate that the halogen-bonding sites in the side chains exhibit adequate specific affinity for organoiodine compounds. In addition, the water-swollen pMEA-analogue microspheres (microgels) showed a thermoresponsive swelling/deswelling behavior that permitted a controlled release of the organoiodine compounds upon changing the temperature. Thus, it seems plausible that a variety of problems associated with, e.g., the recovery of rare iodine-containing compounds, such as the marine-derived iodine compounds, the delivery of iodine-containing drugs, or the removal of halogen compounds from wastewater, could be resolved by polymer microspheres that exhibit controlled halogen bonding.

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.

A series of raspberry-shaped composite microgels were synthesized by seeded emulsion polymerization of styrene in the presence of hydrogel particles with different distributions of charged groups. Unlike microgels whose charged groups are localized in their center,29 polystyrene nanoparticles were formed inside the core microgels when the microgels whose charged groups were localized on their surface were used as cores for seeded emulsion polymerization. The effects of the surface charge densities of the core microgels and the concentration of styrene monomer during the polymerization on the resultant structures of composite microgels were investigated. The surface structures of obtained composite microgels were mainly evaluated by electron microscopy, and their stimuli responsiveness was evaluated by dynamic light scattering and laser Doppler velocimetry. The internal structures of the composite microgels were visualized from ultrathin cross sections observed by transmission electron microscopy (TEM). Cryo-TEM was used to clarify the microscopic structures of composite microgels when they were in hydrated states. Through a series of characterizations, we summarize the effects of structures of core microgels on the resultant composite structures.

Investigation of Changes in the Microscopic Structure of Anionic Poly(N-isopropylacrylamide-co-Acrylic acid) Microgels in the Presence of Cationic Organic Dyes toward Precisely Controlled Uptake/Release of Low-Molecular-Weight Chemical Compound.

Changes in a microscopic structure of an anionic poly(N-isopropylacrylamide-co-acrylic acid) microgel were investigated using small- and wide-angle X-ray scattering (SWAXS). The scattering profiles of the microgels were analyzed in a wide scattering vector (q) range of 0.07 ≤ q/nm(-1) ≤ 20. In particular, the microscopic structure of the microgel in the presence of a cationic dye rhodamine 6G (R6G) was characterized in terms of its correlation length (ξ), which represents the length scale of the spatial correlation of the network density fluctuations, and characteristic distance (d*), which originated from the local packing of isopropyl groups of two neighboring chains. In the presence of cationic R6G, ξ exhibited a divergent-like behavior, which was not seen in the absence of R6G, and d* was decreased with decreasing the volume of the microgel upon increasing temperature. At the same time, the amount of R6G adsorbed per unit mass of the microgel increased upon heating. These results suggested that a coil-to-globule transition of the poly(N-isopropylacrylamide) chains in the present anionic microgel occurred because of efficiently screened, thus, short ranged electrostatic repulsion between the charged groups, and hydrophobic interaction between the isopropyl groups in the presence of cationic R6G. The combination of hydrophobic and electrostatic interaction between the cationic dye and the microgel affected the separation and volume transition behavior of the microgel.