Showing papers in "Colloid and Polymer Science in 2011"

Microgel applications and commercial considerations

Abstract: The term microgel has been used to describe a variety of particles that differ substantially in structure, physicochemical properties, preparation and application and has been used interchangeably with terms such as nanogel, microsphere and macrogel. Many excellent and wide-ranging reviews have been published on the numerous particle types considered to fall within the broad sphere of nano-/microparticles. The aim of this review is to focus on applications of microgel particles that are synthesised by polymerisation, are of colloidal size and exhibit conformational changes in response to changes in environmental conditions. It is not the intention to attempt to cover every potential microgel application; instead, a selected range of areas will be covered and the commercial implications of scaling up the production of microgels for such purposes will be discussed. A brief description of the characteristics of microgel particles is followed by discussion of applications such as enhanced oil recovery, biomaterials and catalysis, before issues of commercialising microgel production are considered.

Drops and shells of liquid crystal

Abstract: We review the state of the art concerning drops and shells of liquid crystal. We especially focus on the defect structures observed with liquid crystals with different degrees of order under different boundary conditions and on the transitions between these structures. We conclude with an overview where we emphasize those areas that are still unexplored.

Effect of cross-linker density of P(NIPAM-co-AAc) microgels at solid surfaces on the swelling/shrinking behaviour and the Young’s modulus

Abstract: The effect of the amount of cross-linker in poly(N-isopropylacrylamide-co-acrylic acid) microgel particles on the swelling behaviour and their elasticity is studied. The distribution of the stiffness through the particle is also investigated. Therefore, the swelling ratio obtained from dynamic light scattering measurements in aqueous solutions is compared with the one after adsorption at polycation-coated silicon wafers. The studies of the swelling behaviour at the surface are carried out with scanning force microscopy (SFM) against liquid. The Young’s modulus is determined by indentation experiments with an SFM. With increasing amount of cross-linker, the ability to shrink as well as the shift in the lower critical solution temperature and in particle size (hysteresis) during the heating and cooling processes decreases. In addition, the particles at the surface preserve their height/width ratio at high amount of cross-linker, while at low amounts the shrinking and swelling mainly takes place with respect to changes in height. The particles show their highest Young’s modulus in the centre of the particles and become stiffer with increasing the amount of cross-linker and the temperature.

An effective nanocomposite of polyaniline and ZnO: preparation, characterizations, and its photocatalytic activity

Abstract: The polyaniline/zinc oxide (PANI/ZnO) nanocomposites were prepared by in situ polymerization of aniline monomer with ZnO nanomaterials and applied as a photocatalyst for the degradation of methylene blue (MB) dye. The morphological observations elicited the agglomerations of PANI sheets which occurred due to the interaction between PANI and ZnO nanomaterials in PANI/ZnO nanocomposites. As compared to pristine PANI, the UV–vis spectra exhibited that the absorption peak of π–π* transitions considerably shifted to higher wavelength at 360 nm from 325 nm in the nanocomposites. The photocatalytic activity results indicated the substantial degradation of MB dye by ~76% over the surface of PANI/ZnO nanocomposite catalyst under light illumination. The PANI/ZnO nanocomposites showed three times higher photocatalytic activity to MB dye degradation compared to pristine PANI might due to high photogenerated electron (ē)–hole (h+) pairs charge separation.

Effect of multi-walled carbon nanotubes on crystallization behavior of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Abstract: The effect of multi-walled carbon nanotubes on the crystallization behavior of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) has been investigated The results have shown that carbon nanotubes (CNTs) act as an effective heterogeneous nucleation agent, inducing an increase in crystallinity and crystallite sizes Comparing with the double melting peaks in pure PHBV, there is only one peak in the melting curves of nanocomposites The isothermal crystallization kinetics of PHBV and its nanocomposite containing 05% CNTs were examined based on Avrami equation, indicating that the crystallization half-time decreases while the overall crystallization rate k increases dramatically with CNTs addition The spherulitic nucleation and growth kinetics were also discussed grounded on Lauritzen–Hoffman equation It is found that there is a spherulitic growth rates (G) maximum within selected temperature range in our study Also, the temperatures corresponding to G maximum shift to a high level with addition of CNTs The parameters of the equilibrium melting temperature $$ T_m^0 $$ , the nucleation parameter K g , the lateral surface free energy σ, the fold surface free energy σ e , and the work of chain folding q of PHBV and its composite containing 05% CNTs were all calculated The reductions of K g , σ e and q values of nanocomposite are in agreement with the fact that the crystallization rate of PHBV increases greatly by addition of CNTs

Surface active and aggregation behavior of methylimidazolium-based ionic liquids of type [Cnmim] [X], n = 4, 6, 8 and [X] = Cl−, Br−, and I− in water

Abstract: The surface active and aggregation behavior of ionic liquids of type [C n mim][X] (1-alkyl-3-methylimidazolium (mim) halides), where n = 4, 6, 8 and [X] = Cl−, Br− and I− was investigated by using three techniques: surface tension, 1H nuclear magnetic resonance (NMR) spectroscopy, small-angle neutron scattering (SANS). A series of parameters including critical aggregation concentrations (CAC), surface active parameters and thermodynamic parameters of aggregation were calculated. The 1H NMR chemical shifts and SANS measurements reveal no evidence of aggregates for the short-chain 1-butylmim halides in water and however small oblate ellipsoidal shaped aggregates are formed by ionic liquids with 1-hexyl and 1-octyl chains. Analysis of SANS data analysis at higher concentrations of [C8mim][Cl] showed that the microstructures consist of cubically packed molecules probably through π–π and hydrogen bond interactions.

Effect of cooling rate on the crystal/mesophase polymorphism of polyamide 6

Abstract: The solidification of the quiescent polyamide 6 (PA 6) melt has been analyzed as a function of the cooling rate in a wide range between 1.67 × 10−2 and close to 2 × 102 K s−1, by means of differential scanning calorimetry at a low cooling rate of up to about 1 K s−1, and by the recording of continuous cooling curves and time-resolved X-ray diffraction on cooling at a higher rate. The performed experiments allowed for the first time to establish the relationship between the cooling rate, the crystallization temperature, and the X-ray structure of PA 6. The exclusive formation of monoclinic α-crystals is only detected if the crystallization temperature is higher than about 430 K or if the cooling rate is slower than about 5 K s−1, respectively. The formation of α-crystals is increasingly replaced by the development of mesophase with increasing cooling rate, accompanied with a decrease of the temperature of crystallization/ordering. Finally, completely amorphous samples were obtained on cooling faster than about 102 K s−1. The continuous decrease of the temperature of crystallization with increasing cooling rate, regardless of the specific structure formed, precludes a primary effect of the nucleation mechanism on the α-crystal/mesophase polymorphism of PA 6. A preliminary discussion of the effect of molar mass of PA 6 on the cooling rate-dependent polymorphism is also included.

Synthesis and electrorheological characteristics of sea urchin-like TiO2 hollow spheres

Abstract: TiO2 hollow microspheres with sea urchin-like hierarchical architectures were synthesized by a simple hydrothermal method. The as-synthesized hollow microspheres with hierarchical architectures consisting of many rhombic building units exhibit high specific surface area. Electrorheological (ER) properties of hierarchical hollow TiO2-based suspension were investigated under steady and oscillatory shear. The hollow TiO2-based suspensions show much higher yield stress and elasticity than pure TiO2 suspension at the same electric field strength. This phenomenon was elucidated well in view of their dielectric spectra analysis. The sea urchin-like architectures result in stronger interfacial polarization of hollow TiO2 suspension upon an electric field, showing higher ER activity. Also, hollow interiors of TiO2 particles increase the long-term stability of suspensions and further merit the ER effect.

Core–shell-structured silica-coated magnetic carbonyl iron microbead and its magnetorheology with anti-acidic characteristics

Abstract: We synthesized silica-coated soft magnetic carbonyl iron (CI) particles through a modified Stober method, in which the CI particles were pretreated with a grafting agent to enhance the affinity of a precursor of silica. Synthesized magnetic microbeads were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and anti-acid test in HCl solution. Silica-coated CI shows not only improved wettability to silicone oil with a lower off-state shear viscosity as a better magnetorheological fluid under an applied magnetic field but also enhanced anti-acidic property.

Stimuli-responsive nanocomposite gels

Abstract: A new class of polymer hydrogels, nanocomposite hydrogels (NC gels), consisting of a unique organic (polymer)/inorganic (clay) network structure, was synthesized by in situ free-radical polymerization in the presence of exfoliated clay nanoparticles in an aqueous system. The resulting NC gels overcame most of the disadvantages associated with chemically cross-linked hydrogels, such as mechanical fragility, structural heterogeneity, and slow de-swelling rate. By using thermo-sensitive poly(N-isopropylacrylamide) (PNIPA) as a constituent polymer, NC gels with remarkable mechanical, optical, and swelling properties as well as thermo-sensitivity were obtained. The various properties of NC gels, such as transparency, gel volume, cell culturing, and surface friction changed significantly in response to the temperature and surrounding conditions. All the excellent properties and new stimuli-responsive characteristics of NC gels are attributed to the unique PNIPA/clay network structure. The thermo-sensitivities and the transition temperature can largely be controlled by varying the clay content and by the addition of solutes.

Encapsulation of enzymes in microgels by polymerization/cross-linking in aqueous droplets

Abstract: In the present paper, we describe the synthesis of poly(N-isopropylacrylamide) microgel particles in water/oil inverse emulsion used as carriers for enzyme encapsulation. Since enzymes can be used in different biotechnological applications, their immobilization on polymer colloidal carriers is of great importance. In this work, laccase from Trametes versicolor and also several peroxidases were immobilized in microgels. The polymerization process occurred via radical initiation in aqueous droplets in the presence of the comonomer vinylimidazole and the cross-linker N′,N′-methylene-bis-acrylamide at room temperature. Non-ionic surfactants were used to stabilize aqueous droplets in heptane. Due to the formation of microgels in aqueous media and the low reaction temperature, this technique allows the encapsulation of enzymes without risk of their denaturation, which can be controlled by variation of reaction parameters. Enzyme-containing microgels obtained by this method were analyzed in detail concerning their particle size, swelling properties, zeta potential, as well as resulting enzyme activity. Our experimental data indicate that the presence of imidazole groups in microgel structure allows increasing enzyme loading without reduction of enzyme activity.

Effects of heat treatment and poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhedral Pt nanoparticles towards their methanol oxidation

Abstract: In this paper, the polyhedral Pt nanoparticles under control were prepared by polyol method using AgNO3 and poly(vinylpyrrolidone) (PVP) in the reduction of H2PtCl6 with ethylene glycol (EG). Transmission electron microscopy (TEM) and high resolution (HR) TEM measurements were used to investigate their characterization. In the case of the previous removal of PVP by washing and heating at 300°C, the specific morphologies of polyhedral Pt nanoparticles were still observed. However, the removal of PVP only by heat treatment at 300°C without washing causes the significant variation of their morphology. The large Pt particles were observed in the self-aggregation and assembly of the as-prepared polyhedral Pt nanoparticles. The pure Pt nanoparticles by washing and heat treatment showed the electrocatalytic property better than PVP-Pt nanoparticles by heat treatment due to the incomplete removal of PVP and by-products from AgNO3. Therefore, the removal modes of PVP without changing their characterization are required to obtain the good catalytic performance.

Dynamic rheology studies of in situ polymerization process of polyacrylamide–cellulose nanocrystal composite hydrogels

Abstract: A series of dynamic small-amplitude oscillatory shear experiments for in situ polymerization process of polyacrylamide–cellulose nanocrystal (PAM–CNC) nanocomposite hydrogels were performed to investigate the relationship between rheological properties and synthesis parameters including chemical cross-linker concentration, polymerization temperature, initiator concentration, and CNC aspect ratios The results showed that CNCs accelerated the onset of gelation (tonset) and acted as a multifunctional cross-linker during the gelation reaction The composite hydrogels exhibited enhanced steady-state elastic modulus \( \left( {G_\infty^\prime } \right) \) and plateau loss factor (tanδ) compared to these of the pure PAM hydrogels, indicating that adding CNCs not only reinforced but also toughened PAM hydrogels \( \left( {G_\infty^\prime } \right) \) and the effective network junction density (N) increased with increased cross-linker concentration, polymerization temperature, and CNC aspect ratios, but decreased with increased initiator concentration The changes of plateau tanδ were opposite to that of \( G_\infty^\prime \) The sol–gel transition kinetics of PAM–CNC hydrogels accelerated with increased cross-linker concentration and polymerization temperature and, however, reached optimization at 025 wt% of initiator concentration CNCs with lower aspect ratios promoted tonset and the sol–gel transition of PAM–CNC hydrogels, suggesting the fact that CNCs with lower aspect ratios further facilitated the formation of network of PAM–CNC nanocomposite hydrogels

Synthesis and Physicochemical Properties of Cationic Microgels Based on Poly(N-isopropylmethacrylamide)

Abstract: Surfactant-free, radical precipitation co-polymerization of N-isopropylmethacrylamide (NIPMAm) and the cationic co-monomer N-(3-aminopropyl) methacrylamide hydrochloride (APMH) was carried out to prepare microgels functionalized with primary amines. The morphology and hydrodynamic diameter of the microgels were characterized by atomic force microscopy (AFM) and photon correlation spectroscopy (PCS), with the effect of NaCl concentration and initiator type on the microgel size and yield being investigated. When a V50-initiated reaction was carried out in pure water, relatively small microgels (~160 nm diameter) were obtained in low yield (~20%). However, both the yield and size increased if the reaction was carried out in saline or by using APS as initiator instead of V50. Stable amine-laden microgels in the range from 160 nm to 950 nm in diameter with narrow size distributions were thus produced using reaction media with controlled salinity. Microgel swelling and electrophoretic mobility values as a function of pH, ionic strength and temperature were also studied, illustrating the presence of cationic sidechains and their influence on microgel properties. Finally, the availability of the primary amine groups for post-polymerization modification was confirmed via modification with fluorescein-NHS.

Soft magnetic carbonyl iron microsphere dispersed in grease and its rheological characteristics under magnetic field

Abstract: Magnetorheological (MR) grease, comprised of a suspension of soft magnetic carbonyl iron (CI) microspherical particles dispersed in a grease medium, was fabricated by a mechanical stirring method. As potential medium oil for MR system, shear viscosity of the pure grease was measured as a function of temperature. Its MR characteristics were investigated using a rotational rheometer under an external magnetic field. Flow curve responses (shear stress and shear viscosity), yield stress, and elasticity were investigated using various magnetic field strengths ranging from 0 to 342 kA/m. It was confirmed that MR grease has a yield stress under no external magnetic field due to the inherent property of grease. In addition, CI based MR grease exhibited a characteristic of a Bingham fluid.

Physicochemical properties and mechanical characters of methanol-modified melamine-formaldehyde (MMF) shell microPCMs containing paraffin

Abstract: Microcapsules containing phase change materials (microPCMs) with melamine-formaldehyde (MF) shells have been applied in many thermo-regulation or thermo-saving fields. However, it is still essential to decrease the residual formaldehyde and enhance the mechanical properties of MF shells. The objective of this work was to fabricate a series of microPCMs containing paraffin by an in situ polymerization method using methanol-modified melamine-formaldehyde (MMF) prepolymer as shell material and investigate the physicochemical properties and mechanical characters of these microPCMs. FT-IR analysis indicates that the methanol-modified method can reduce the free formaldehyde in shell material through increasing the cross-linking structure. Optical microphotographs and SEM morphologies show that the microPCMs have regular globe shape with smooth surface. With the increasing of emulsion stirring rates from 1,000 to 5,000 rpm, the average diameters decreased sharply from 27 to 2.5 μm. The phase change temperature (T m) of microPCMs samples with the core/shell ratios of 3/1, 2/1, 1/1, and 2/1 are 22.6, 23.0, 23.4, and 23.9 °C, which are nearly equaled to the T m of pure paraffin (22.5 °C). Mechanical properties test data show that the MMF shells have larger yield point value than that of MF shell for microPCMs with the same core/shell ratio, which means that the methanol-modified method shell can greatly enhanced the resistance of deformation for MF shells. Moreover, MMF shells can resist the interface extrusion force in epoxy resin owing to their higher yield point of enhanced MMF shell.

Well defined hybrid PNIPAM core-shell microgels: size variation of the silica nanoparticle core

Abstract: Thermoresponsive core-shell hybrid microgels with different core sizes were prepared by radical precipitation polymerization of the monomer N-isopropylacrylamide (NIPAM) in the presence of functionalized silica cores. The size of the cores was varied in a range of 70–170 nm in diameter. Characterization of the hybrid microgels was done by means of imaging techniques such as transmission electron microscopy (TEM) and atomic force microscopy (AFM). In addition, scattering techniques were used to study the swelling behavior and network structure of the responsive polymer shells. While dynamic light scattering (DLS) was employed to investigate the overall particle dimensions, SANS allowed to determine the correlation length ξ of the polymer network. Additionally, SANS also provides the average core size and the polydispersity of the cores in-situ using the method of contrast variation.

Polyelectrolyte complexes of chitosan/heparin and N,N,N-trimethyl chitosan/heparin obtained at different pH: I. Preparation, characterization, and controlled release of heparin

Abstract: Chitosan and N,N,N-trimethyl chitosan (TMC) were treated with heparin (HP) to produce polyelectrolyte complexes (PECs). The structures of PECs were characterized by Fourier transform infrared and CP-MAS 13C NMR spectroscopies. A thermal stability of TMC/HP complex was evaluated by thermogravimetric analyses, and morphology was investigated by SEM images. The ratio between the carbons bonded to noncharged nitrogen atoms (CΦ) and the carbons bonded to positively charged nitrogen atoms (Cω) in TMC was calculated through of the degree quaternization (DQ), being the DQ obtained through 1H NMR data. Experiments of controlled release of HP were performed in distilled water at pH ∼7, using methylene blue/heparin system as solute. The release profiles of HP from TMC/HP complex are quite different each other. The value of CΦ/Cω ratio was used for explaining such an effect. The CΦ/Cω ratio tends to be constantly (∼0.69) as high as the pH of medium in which the PEC of TMC/HP was obtained. The change of CΦ/Cω ratio on the pH was also considered to explain the improvement of thermal stability of PEC2 with increase of pH. Data show that PEC of TMC/HP formed at pH 8 is a promising material for uses in oral site-specific HP release systems.

Preparation of sol-gel hybrid materials from γ-methacryloxypropyltrimethoxysilane and tetramethyl orthosilicate: study of the hydrolysis and condensation reactions

Abstract: Organic–inorganic hybrid materials suitable for the development of sol–gel coatings for metallic surfaces were prepared by hydrolysis and condensation of γ-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethyl orthosilicate (TMOS). The hydrolysis of MAPTMS/TMOS was carried out in an ethanol/water solution. The prehydrolysis stage of MAPTMS/TMOS system was monitored by Fourier transform infrared spectroscopy (FTIR) and liquid-state 29Si and 13C nuclear magnetic resonance (29Si and 13C NMR). FTIR analysis indicated that the hydrolysis of MAPTMS/TMOS was accomplished as far as the (SiOMe) band corresponding to unhydrolyzed silane disappeared. The concentration of the alkoxy groups and the extent of self-condensation of mono-, di-, and trisubstituted siloxanes (T species) in the sol were estimated by using liquid-state 29Si NMR spectroscopy. The hydrolysis of the prepared sol was also evaluated by liquid-state 13C NMR spectroscopy. The results indicated that under the adopted synthesis strategy conditions, the hydrolysis process requires 4 h to be completed.

Hyperbranched polymers meet colloid nanocrystals: a promising avenue to multifunctional, robust nanohybrids

Abstract: Colloid nanocrystals (NCs) mainly include metal nanocrystals, semiconductor nanocrystals, and insulator nanocrystals, exhibiting interesting size-dependent electrical, optical, magnetic, and chemical properties that cannot be achieved by their bulk counterparts. However, there’s a critical problem that NCs tend to aggregate, which induces degradation of their performance. Hyperbranched polymers (HPs) possess excellent attributes of three-dimensional topology, low viscosity, good solubility, and plenty of modifiable terminal groups. The combination of NCs and HPs to form nanohybrids cannot only endow NCs with multifunctionality, uniform dispersibility, and splendid solubility but also can impart extra properties to HPs. This article reviews the recent progress and state-of-the-art of the synthesis and applications of NCs-HPs nanohybrids (NHBs). NHBs can be obtained by three approaches: HPs first (i.e., NCs are formed with the stabilizer of HPs), NCs first (i.e., HPs are grafted on the surface of as-prepared NCs), and ligand exchange (the original ligand of NCs is replaced with HPs). Various HPs including hyperbranched poly(amidoamine), polyethylenimine, polyglycerol, polyester, polyamide, polyurethane, and poly(3-ethyl-3-hydroxymethyloxetane), as well as sorts of NCs such as metals (e.g., Ag, Au, Pd, Pt, and Rh), quantum dots (e.g., ZnO, CdS, CdTe, CdSe, and SnO2), magnetic oxides (e.g., Fe3O4), rare earth compounds, and so forth, have been used to obtain NHBs. The NHBs can be applied in nanocatalysis, antimicrobia, biosensor, biological labeling, and other fields promising their bright future.

Fabrication and interfacial morphologies of methanol–melamine–formaldehyde (MMF) shell microPCMs/epoxy composites

Abstract: Microcapsules containing phase change materials (microPCMs) have many potential applications because of their thermoregulation or thermosaving abilities. At the same time, it is still essential to understand the interface stability of microPCMs/polymer composites during a thermal transmission. The aim of this work was to fabricate novel microPCMs containing dodecanol by an in situ polymerization using methanol-modified melamine–formaldehyde (MMF) prepolymer as shell material and investigate the interface morphologies of microPCMs/epoxy composites treated by a simulant thermal process with a ten times repeated temperature variation. A series of microPCMs were fabricated by 1,000–3,000 r·min−1 emulsion speed with the PCM contents of 40–70%. The average diameter, melting temperature, and encapsulation efficiency of microPCMs were 1–16 μm, 19.5 °C, and 97.4%, respectively. Tests results indicated that the properties of the microPCMs were greatly affected by core/shell ratios and emulsification stirring rates of preparation conditions. With the increasing of stirring rates, the average diameters of microPCMs were sharply decreased. The encapsulation efficiency (E e) values of microPCMs increased with the increasing of stirring rates. The contents of PCM in microcapsules (C t) and the average diameter of microPCMs both affected the interface morphologies of microPCMs/epoxy composites after the repeated thermal treatments. Microcracks and gaps occurred after a thermal treatment in the interface of microPCMs and epoxy matrix obviously. The internal stress generated by the expansion or shrinking of the microPCMs was the main factor leading to the interface morphology changes and damaged of composites.

Modeling and simulation of pH-sensitive hydrogels

Abstract: Polyelectrolyte gels are ductile elastic electroactive materials. They consist of a polymer network with bound charged groups and a liquid phase with mobile ions. In water-based solutions, these gels show enormous swelling capabilities under the influence of different types of stimulation, such as chemical, electrical, or thermal. In the present work, a coupled multi-field formulation for polyelectrolyte gels using the finite element method is applied. Additionally to the three given fields—mechanical, electrical, and chemical fields—the dissociation reactions of the bound charges in the gel phase are considered. In this study, chemical stimulation (change of pH or salt concentration) is investigated for gels placed in solution baths. By changing the ambient conditions, we are able to simulate both pH stimulation and change of salt concentration, and to give the change of the mobile and bound ion concentrations, the electric potential, and the mechanical displacement.

Stimuli-responsive gel emulsions stabilized by microgel particles

Abstract: Using pH- and thermo-responsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgels as an solely emulsifier, we have prepared a series of oil-in-water (o/w) highly concentrated emulsions with an internal phase ranging from 65% up to 80%. The resultant emulsions are gel-like and exhibit elastic properties due to the formation of a viscoelastic three-dimensional network of interconnected microgels and emulsion droplets. The dynamical rheology measurements show that elastic modulus (G′) of the emulsion increases as the internal phase volume increases. For a fixed internal phase volume, microgels are shown to stabilize emulsion with a high elasticity at room temperature above a solution pH of 6, but below this threshold, G′ decreases gradually with pH. This indicates that a stronger transient gel network is formed between the interconnected microgels and emulsions droplets at the high pH solutions. On the other hand, for emulsions prepared at pH 4.2, gel-to-fluid transition occurs when the temperature is raised to 37 °C, whereas the emulsions produced above pH 6 are stable even when heat at 60 °C for 12 h. Therefore, the formulated gel emulsion stabilized by the microgels shows their twofold responsiveness to pH and temperature.

Colloidal crystallization of thermo-sensitive gel spheres of poly ( N -isopropyl acrylamide)

Abstract: Morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermo-sensitive gel spheres, poly (N-isopropylacrylamide) (224 nm in the hydrodynamic diameter at 25 °C) were studied. Giant colloidal single crystals formed at very low gel concentrations. Critical concentration of melting of gel spheres (0.8 wt.% without ion-exchange resins) decreased sharply to 0.01 wt.% as the gel suspension was deionized exhaustively with coexistence of the mixtures of cation- and anion-exchange resins and increased substantially as concentration of sodium chloride increased. These studies demonstrated that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded-volume effect of the gels. Most of the researchers including the authors have believed that the crystallization of the gel spheres takes place by the excluded-volume effect, in other words, by the hard-sphere model, exclusively. However, the present work clarified that the colloidal interfaces, which are inevitable for the formation of the electrical double layers, are formed firmly between the water phase and gel spheres, though the gel spheres contain a lot of water molecules in the inner the sphere region.

Cyclodextrin-responsive nanogel as an artificial chaperone for horseradish peroxidase

Abstract: The thermal stabilization and refolding of horseradish peroxidase (HRP) upon heating were investigated using an artificial molecular chaperone consisting of cholesterol-bearing pullulan (CHP) nanogels. The CHP nanogels inhibited the aggregation of HRP under heating by complexation with the denatured HRP. The enzyme activity of HRP complexed with CHP nanogels was not detected. However, the enzyme activity recovered up to 80% of native HRP after the addition of cyclodextrin (CD) to the complex. The dissociation of CHP nanogels was induced by the formation of an inclusion complex of cholesterol groups of CHP with CD. The enzyme activity of HRP was only significantly recovered by the addition of β-CD or its derivatives. Natural molecular chaperones, such as GroEL/ES, trap, fold, and release the nonnative proteins by changing the hydrophobicity of the specific sites of the molecular chaperone that interact with the nonnative protein. The functional mechanism of the nanogel chaperon system is similar to that of natural molecular chaperones. The nanogel chaperone system is a useful tool to aid the refolding and thermal stabilization of unstable proteins for post-genome research, and in medical and biological applications.

The temperature effect on electrokinetic properties of the silica-polyvinyl alcohol (PVA) system

Abstract: The influence of polyvinyl alcohol (PVA) adsorption on the structure of the diffuse layer of silica (SiO2) in the temperature range 15–35 °C was examined. The microelectrophoresis method was used in the experiments to determine the zeta potential of the solid particles in the absence and presence of the polymer. The adsorption of PVA macromolecules causes the zeta potential decrease in all investigated SiO2 systems. Moreover this, decrease is the most pronounced at the highest examined temperature. Obtained results indicate that the conformational changes of adsorbed polymer chains are responsible for changes in electrokinetic properties of silica particles. Moreover, the structure of diffuse layer on the solid surface with adsorbed polymer results from the following effects: the presence of acetate groups in PVA chains, the blockade of silica surface groups by adsorbed polymer and the shift of slipping plane due to macromolecules adsorption.

Preparations and properties of waterborne polyurethane/allyl isocyanated-modified graphene oxide nanocomposites

Abstract: We synthesized waterborne polyurethane (WPU)/allyl isocyanate modified graphene oxide (iGO) nanocomposites by UV curing, and the effects of iGO on the mechanical, dynamic mechanical, and thermal properties of the nanocomposites were systematically investigated. It was shown that the iGO chemically incorporated into the WPU chains by covalent bonding acts as a multifunctional cross-linking agent as well as reinforcing filler. Consequently, the tensile strength, glassy and rubbery state moduli, glass transition temperature, and thermal stability of the WPU were significantly increased up to an iGO content of 1%, beyond which most of the above properties showed a decrease, due probably to the auto-inhibition of the allyl compounds.

Thermo-dependent characteristics of polyimide–graphene composites

Abstract: Polyimide–graphene composites (PIG) were prepared with variable amounts of graphene, and their thermal properties were analyzed in films on substrates or sheet states. The thermal conductivities of PIG composite sheets gradually moved upwards with increase of graphene loading. Coefficient of thermal expansion of composite sheet was higher in out-of-plane mode than in-plane mode. The residual stress of a composite film was monotonously changed in accordance with the variation of temperature and lowered with increase of graphene. In addition, the residual stress of a composite film reached to the initial stress value during cooling process after heating. The stress profiles on further heating and cooling runs closely followed the stress profile during the first cooing run.

Determination of the bulk modulus of microgel particles

Abstract: In this paper, we present an in-depth study of a non-invasive method to determine the bulk modulus of microgel particles. The method relies on measuring the particle size as a function of external osmotic pressure using dynamic light scattering to measure the particle size and dextran solutions to change the osmotic pressure. An independent characterization of the dynamic response of the dextran solutions is required in order to obtain the microgel size in the microgel/dextran mixtures. The method works fine in most cases except for slightly crosslinked microgels, where the scattering from the particles is comparable to the scattering from the dextrans.

Desiccating colloidal sessile drop: dynamics of shape and concentration

Abstract: Using lubrication theory, drying processes of sessile colloidal droplets on a solid substrate are studied. A simple model is proposed to describe temporal dynamics of both the shape of the drop and the volume fraction of the colloidal particles inside the drop. The concentration dependence of the viscosity is taken into account. It is shown that the final shapes of the drops depend on both the initial volume fraction of the colloidal particles and the capillary number. The results of our simulations are in a reasonable agreement with the published experimental data. Computations for the drops of aqueous solution of human serum albumin are presented.