Double‐Stimuli‐Responsive Spherical Polymer Brushes with a Poly(ionic liquid) Core and a Thermoresponsive Shell

The synthesis of poly(ionic liquid) (PIL) nanoparticles grafted with a poly(N‐isopropyl acrylamide) (PNIPAM) brush shell is reported, which shows responsiveness to temperature and ionic strength in an aqueous solution. The PIL nanoparticles are first prepared via aqueous dispersion polymerization of a vinyl imidazolium‐based ionic liquid monomer, which is purposely designed to bear a distal atom transfer radical polymerization (ATRP) initiating group attached to the long alkyl chain via esterification reaction. The size of the PIL nanoparticles can be readily tuned from 25 to 120 nm by polymerization at different monomer concentrations. PNIPAM brushes are successfully grafted from the surface of the poly(ionic liquid) nanoparticles via ATRP. The stimuli‐responsive behavior of the poly(ionic liquid) nanoparticles grafted with PNIPAM brushes (NP‐g‐PNIPAM) in aqueous phase is studied in detail. Enhanced colloidal stability of the NP‐g‐PNIPAM brush particles at high ionic strength compared to pure PIL nanoparticles at room temperature is achieved. Above the lower critical solution temperature (LCST) of PNIPAM, the brush particles remain stable, but a decrease in hydrodynamic radius due to the collapse of the PNIPAM brush onto the PIL nanoparticle surface is observed.

     

A study of oil-in-water emulsions stabilized by whey proteins

Like many other emulsifiers, whey protein concentrates stabilize oil-in-water emulsions. However, the emulsifying capacity of whey proteins is affected by several factors, e. g., type of homogenizer, degree of homogenization, protein concentration, oil volume fraction, pH and ionic strength of the aqueous phase. For the present study, oilin-water emulsions were made by homogenizing known amounts of whey protein concentrate with a vegetable oil (i. e. grapeseed oil) at different pH. The emulsifying properties of whey proteins are expressed as a function of the particle size and size distribution of oil droplets as measured by light scattering, and of the surface charge density derived from the electrophoretic mobility.The whey protein concentrate was shown to have an isoelectric point at pH 4.4. Near this pH value, the oil-in-water emulsions exhibited poor stability as expected from the low surface coverage.     

Electrical Properties and Stability of Poly(N-isopropylacylamide-co-methacrylic Acid) Core-Shell Microgel

Graphical Abstract: A doubly responsive microgel with core shell shape was prepared by seed polymerization. The electrical properties were characterized. The latex stability was evaluated and discussed according to the colloid theory.

A core-shell microgel of poly(N-isopropylacylamide-co-methacrylic acid) was prepared by seeded polymerization. Dynamic light scattering measurements indicated that the microgel had a narrow size distribution. Zeta potential decreased gradually with increasing ionic strength, and when salinity reached a certain concentration the surface charge was almost screened out. Further addition of salt led to the shrinking and final flocculation. For increasing temperature, the zeta potential under higher ionic strength exhibited an abrupt change for trending to zero. Total interaction energy between particles was calculated with colloid theory. Meanwhile, thermal stability was evaluated and interpreted the experimental phenomenons.     

Steric stabilization of thermally responsive N-isopropylacrylamide particles by poly(vinyl alcohol)

Poly(vinyl alcohol) (PVA) was used as a steric stabilizer for the dispersion polymerization of cross-linked poly(N-isopropylacrylamide) (PNIPAM) in water. A series of reactions were carried out using PVA of varying molecular weight and degree of hydrolysis. Under appropriate conditions, PNIPAM particles of uniform and controllable size were produced using PVA as the stabilizer. The colloidal stability was investigated by measuring changes in particle size with temperature in aqueous suspensions of varying ionic strength. For comparison, parallel colloidal stability measurements were conducted on PNIPAM particles synthesized with low-molecular-weight ionic surfactants. PVA provides colloidal stability over a wide range of temperature and ionic strength, whereas particles produced with ionic surfactants flocculate in moderate ionic strength solutions upon collapse of the hydrogel as the temperature is increased. Experimental results and theoretical consideration indicate that sterically stabilized PNIPAM particles resulted from the grafting of PVA to the PNIPAM particle surface. The enhanced colloidal stability afforded by PVA allows the temperature-responsive PNIPAM particles to be used under physiological conditions where electrostatic stability is ineffective.     

Synthesis of Spherical Polyelectrolyte Brushes by Thermo‐controlled Emulsion Polymerization

A novel method of thermo‐controlled emulsion polymerization has been employed to synthesize spherical polyelectrolyte brushes that consist of a solid polystyrene core and a poly (acrylic acid) (PAA) shell covalently attached on the core surface densely by one end. The growth of brushes from the core surface was monitored by dynamic light scattering (DLS). The particle size of PS core latex showed a narrow size distribution when observed by scanning electron microscopy (SEM). The brush size changed significantly upon changing pH value and ionic strength, and displayed similar behavior to brushes prepared by photo‐emulsion polymerization. The grafting density of the PAA brush, which was determined by cutting off the PAA chains using alkali hydrolysis, confirmed the formation of PAA brushes.

     

Impact of initiators in preparing magnetic polymer particles by miniemulsion polymerization

Sub-micron sized polystyrene particles containing magnetite more than 30 wt.% were prepared by miniemulsion polymerization with commercially available ferricolloid. The effects of some water-soluble initiators and/or oil-soluble initiators on the particles characteristics, such as the size, morphology, magnetic properties and colloidal stability, were studied. The size of monomer droplets/polymer particles increased from 60 to 300 nm during polymerization, keeping magnetic in core when potassium persulfate (KPS) or ammonium persulfate (APS) was used as the sole initiator. These particles were easily separated from the medium within short time scale in external magnetic field, while such characteristics were controlled by the amount of persulfate used for the polymerization. In contrast, when 2,2′-azobis isobutyronitrile (AIBN) was used as the initiator, the size of droplets/particles was retained to be 90 nm at the most and magnetite nanoparticles located at the surface of polystyrene particles, which were so colloidally stable that they were not separated in external magnetic field. The above-mentioned effect of initiators on particle size in persulfate system was likely originated from the decrease of pH value and the increase of ionic strength, which induced the fusion of droplets/particles containing magnetite. Mixed-initiators system resulted in intermediate characteristics, compared with each initiator system. The location of magnetite in the particle seems to depend on where initiation/polymerization occurred in each initiator system.     

Synthesis and characterization of polyacrylonitrile nanoparticles by dispersion/emulsion polymerization process

Polyacrylonitrile nanoparticles in sizes ranging from approximately 35 to 270 nm were prepared by dispersion/emulsion polymerization of acrylonitrile in a continuous aqueous phase in the presence of potassium persulfate as initiator and various alkyl-sulfate and sulfonate surfactants. The influence of various polymerization parameters (e.g., concentration of monomer and initiator, type and concentration of surfactant, temperature and time of polymerization, ionic strength, pH and co-solvent concentration) on the properties (e.g., size and size distribution, yield, stability, etc.) of the particles has been investigated. The polymerization of acrylonitrile may occur in two major locations: in the aqueous continuous phase (dispersion polymerization) and/or within the surfactant micelles (emulsion polymerization). A discussion concerning the role of these two mechanisms under different conditions, including comparison with previous literature, is also presented. Surface and bulk characterizations of the particles were performed by methods such as transmission and scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, zeta potential, and gravimetric measurements.     

Preparation of Polystyrene Particles by Dispersion Polymerization in an Ionic Liquid

Submicron‐sized monodisperse polystyrene (PS) particles were successfully prepared by dispersion polymerization of styrene in an ionic liquid, N,N‐diethyl‐N‐methyl‐N‐(2‐methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide ([DEME][TFSI]) at 70 °C with poly(vinyl pyrrolidone) (PVP) as a stabilizer. At the optimum PVP and styrene concentrations with regard to preparation of stable polymer particles, the number‐average diameter and coefficient of variation were 350 nm and 5.7%, respectively. The particle size increased with a decrease in the PVP concentration and an increase in the styrene concentration. Moreover, we succeeded in producing PS particles by thermal polymerization in the absence of a radical initiator at 130 °C in [DEME][TFSI] using a conventional reactor (not autoclave) utilizing the advantages of non‐volatility and thermal stability of the ionic liquid.

     

Synthesis of pH-responsive particles with shape anisotropy

Seeded emulsion polymerization is used to produce large quantities of shape anisotropic, amphoteric particles in a size range of about 1 μm. Copolymer dicolloids (CDCs) containing pyridine groups are synthesized by swelling spherical, lightly cross-linked polystyrene seeds with a mixture of styrene and pH-responsive monomer 2-vinyl pyridine followed by secondary polymerization to contrast with their analogue homopolymer dicolloids (HDCs) where the swelling step is carried out with styrene alone. After the particles are coated with a nonionic surfactant to minimize van der Waals attractions, surface potentials and aggregation properties of dilute suspensions are studied as functions of pH and ionic strength. Compared to HDCs, which remain stable at all pH values studied (3 < pH < 9) up to an ionic strength of 5 M, the CDC particles show amphoteric behavior with strong attractions under conditions where dipolar interactions are expected to dominate.     

Factors affecting the stability of O/W emulsion in BSA solution: stabilization by electrically neutral protein at high ionic strength

Bovine serum albumin (BSA) was used as an emulsifier to disperse corn oil in aqueous media with various protein concentration, pH, and ionic strength. Quantitative estimation was made on the homogenizing activity of BSA and dispersion stability of oil particles by measuring particle size, turbidity, and creaming rate. Dispersion stability strongly depended on pH and became a minimum around pH 5.0 which was the isoelectric point of BSA. The interfacial tension between BSA solution and corn oil was minimized at pH 5.0. Interesting results were obtained concerning the ionic-strength dependence of stability. When the ionic strength was set below 30 mM, the emulsions became more stable with the increase of BSA concentration at pH 6.7 but the opposite behavior (enhanced destabilization) was confirmed at pH 5.0 with the BSA content. In high ionic strength conditions (ca. > or = 80 mM NaCl), however, BSA-stabilized emulsions became fairly stable even at pH 5.0. These results suggested that BSA molecules having no net charge induced some attractive interactions (e.g., bridging or depletion) in low ionic strength but steric stabilization in high ionic strength, respectively.     

Cationic Salt‐Responsive Bottle‐Brush Polymers

A class of cationic bottle‐brush polymers that show ionic strength‐dependent stimuli responsiveness is prepared. Brush polymers with norbornene as backbone and quaternary ammonium (QA)‐containing polycaprolactone copolymers as side chains are synthesized by a combination of ring‐opening metathesis polymerization, ring‐opening polymerization, and click reaction. In water with low ionic strength, brush polymers are soluble due to the strong electrostatic repulsion between cationic QA groups. As the addition of salt to increase ionic strength, single brush polymers undergo a transition from extended conformation to collapsed state and finally become insoluble in solution due to the screening effect of salts that yield the once‐dominant electrostatic interactions among QA species to hydrophobic–hydrophobic interactions.

     

Visualizing the interaction between poly-L-lysine and poly(acrylic acid) microgels using microscopy techniques: effect of electrostatics and peptide size

The interaction between lightly cross-linked poly(acrylic acid) (pAA) microgels (50-150 microm in diameter) and poly-L-lysine (pLys) was studied as a function of pH, ionic strength, peptide size, and concentration. The swelling response and distribution of polypeptides within microgel particles was monitored by micromanipulator-assisted light microscopy and confocal laser scanning microscopy, while binding isotherms of pLys in the microgels were determined spectrophotometrically. Conformational changes of pLys were investigated by circular dichroism. The molecular weight of pLys was found to influence the degree of peptide-induced microgel deswelling, largely due to limitation of peptides larger than the effective network mesh size to penetrate the entire gel. Large peptides were concentrated within a surface layer of the gel particles, and at low ionic strength this dense surface layer was shown to act as a largely steric barrier for further penetration of compounds into the gel core. Small peptides, however, distributed evenly throughout the microgel particles and were able to create large microgel volume reductions. The deswelling of microgels increased with decreasing pH, while the uptake of pLys was significantly reduced at low pH. The effect of ionic strength on the interactions of pLys and oppositely charged pAA microgels was moderate and only pronounced for deswelling of gels at high pH. A significant increase in the alpha-helix content of pLys interacting with the oppositely charged microgels was observed for high molecular weight peptides, and the extent of alpha-helix formation was as expected more pronounced at high pH, i.e., at high charge density of the microgels but reduced charge density of the peptides.     

Synthesis of Composite Polystyrene/Silica Nanoparticles via Precipitation and Emulsion Polymerization Methods

Polystyrene/silica composite nanoparticles were synthesized via precipitation and emulsion polymerization methods, in the presence of a basic co‐monomer (e.g., 4‐VP and 1‐VID), and a colloidal aqueous silica solution. The effects of key process parameters, that is, solvent type, monomer/co‐monomer volume ratio and total monomers concentration for precipitation polymerization, and reaction temperature, pH value, initial silica‐sol concentration and initial monomer/co‐monomer molar ratio for emulsifier‐free emulsion polymerization on the particle morphology, silica content, and particle size distribution of the composite nanoparticles were experimentally investigated. Stable, spherical, and uniform in size composite nanoparticles were synthesized by both techniques. The average particle diameter varied from 108 to 182 nm for the emulsifier‐free emulsion polymerization and from 400 to 800 nm for the precipitation polymerization, while the silica content was as high as 38.3 wt.‐% for the former method and up to 15.5 wt.‐% for the later. The synthesized composite polymer/silica particles were then electrolytically co‐deposited with zinc on steel plates to improve the corrosion resistance of the metal's surface.

     

On the adsorption of IgG onto polystyrene particles: electrophoretic mobility and critical coagulation concentration

An experimental investigation on the adsorption of immuno -globulin molecules on polystyrene microspheres is described. Three different IgG samples were adsorbed on latex particles. One was of polyclonal nature with a broad range of isoelectric points (6.1–8.7), whereas the other samples were of monoclonal nature, Mab 1 and Mab 2 with i.e.p. of (5.65±0.15) and (7.7±0.1), respectively. Adsorption isotherms at different ionic strengths and pH were performed. Most of the adsorption isotherms showed well-defined plateaus. Because of instability in solution of Mab 2 in the pH values of 7 and 8, no plateau values were found in the adsorption isotherms at both pH-values. Maximum protein adsorption was found around the i.e.p. of the protein. According to the findings, the IgG adsorption on polystyrene surface is strongly irreversible with respect to pH changes. The ionic-strength changes, however, exert a pronounced effect on the adsorption-desorption processes of IgG on negatively charged polystyrene surface. Also, electrophoresis experiments were performed to gain information on the electrostatic interaction between the IgG molecules and the PS latex. With increasing the adsorbed amount of IgG the absolute value of mobility decreases to reach a plateau value. The isoelectric pH of the IgG-PS complex is always smaller than the i.e.p. of the dissolved IgG, indicating that the PS surface charge must partly compensate the positive charge on the protein. Finally, the colloidal stability of the rabbit IgG/PS complex is always very low, whereas the Mab/PS complexes are very stable when the charge electrokinetically mobilized by these systems is very large.     

双敏性微凝胶的絮凝及聚沉行为

用沉降聚合法制备了聚(N-异丙基丙烯酰胺-co-甲基丙烯酸)微凝胶, 并用NMR, DLS分析测定了微凝胶结构及凝胶颗粒在不同离子强度下粒径和表面电势的变化. 25 ℃时在pH＝7的溶液中Zeta电位为－18 mV, 随离子强度增加, 逐渐减小. 当NaCl浓度达0.2 mol/L时基本不变, 表明微凝胶表面电荷受到屏蔽, 浓度继续增加主要使凝胶颗粒收缩. 加热引起微凝胶收缩, 颗粒表面电荷密度增大, Zeta电位增大. 在0.2 mol/L NaCl溶液中, 41 ℃时微凝胶的Zeta电位可达－12.4 mV, 使微凝胶稳定. 较高离子强度时, Zeta电位随温度升高发生突变, 微凝胶表面几乎为中性, 其突变温度与临界絮凝温度(CFT)相当. CFT随离子强度增加向低温迁移, 微凝胶聚集速率在高温时比低温时快.     

Amphoteric character of polyelectrolyte complex particles as revealed by isotachophoresis and viscometry

Isotachophoresis and viscometric measurements were performed on aqueous dispersions of non-stoichiometric polyelectrolyte complexes in order to elucidate the surface charge situation of the complex particles in dependence on component charge density, ratio of cationic to anionic groups in the complex, and pH and ionic strengths of the ambient medium. Components for complex formation were acryl-based anionic and cationic polyelectrolytes of the pendent type. From our results, an amphoteric character of the polyelectrolyte complex particles can be concluded, with an isoelectric point characterized by zero mobility and a minimum in reduced viscosity _spec/c of the particle dispersion, and with the sign of net surface charge depending on ambient pH and component charge density. The influence of ionic strength on the _spec/c vs pH plots can be interpreted by assuming a competition between salting-out and electrostatic shielding effects. No correlation could be established between the overall molar ratio of cationic to anionic groups and the isoelectric point of the complex particles, which obviously indicates a different composition of surface and bulk of the polyelectrolyte complex particles.     

Aggregation phenomena in the suspension polymerization of VCM

In the suspension polymerization of VCM, insoluble polymer particles are formed inside the monomer droplets. The growth and aggregation of these particles are responsible for important polymer properties, such as porosity. It is well established that the most characteristic polymer particles, the primary particles, are of a narrow distribution with a size (diameter) ranging from 0.10–0.20 m. This work studied the formation of primary particles based on the aggregation phenomena that take place inside a monomer droplet. This was done by formulating a population balance equation, which was based on the following considerations: a) polymerization occurs in both the monomer and the polymer phases; b) there is continuous formation of the basic particles in the monomer phase; c) the growth of the polymer particles occurs as a result of both polymerization in the polymer phase and aggregation of the particles; d) the colloidal properties of the particles that are responsible for the aggregation phenomena were considered to be the net result of attraction and repulsion energies.It was shown that for particles carrying a constant charge it was not possible to predict the formation of primary particles of size 0.10–0.20 m. The particle size distribution had a mode diameter equal to the diameter of the basic particles. Consequently, the particle charge was allowed to vary in a way proportional to the particle radius raised to a power coefficient. For values of the coefficient greater than zero, i. e., when the particle charge increased during polymerization, the aggregation of the basic particles was efficient enough to result in the formation of large primary particles.     

Spherical Polyolefin Particles from Olefin Polymerization in the Confined Geometry of Porous Hollow Silica Particles

Porous hollow silica particles (HSPs) are presented as new templates to control the product morphology in metallocene‐catalyzed olefin polymerization. By selectively immobilizing catalysts inside the micrometer‐sized porous hollow silica particles, the high hydraulic forces resulting from polymer growth within the confined geometries of the HSPs cause its supporting shell to break up from the inside. As the shape of the support is replicated during olefin polymerization, perfectly spherical product particles with very narrow size distribution can be achieved by using HSPs exhibiting a monomodal size distribution. Furthermore, the size of the obtained product particles can be controlled not only by the polymerization time but also by the size of the support material.

     

Effect of poly(ethylene oxide)-silane graft molecular weight on the colloidal properties of iron oxide nanoparticles for biomedical applications

The size, charge, and stability of colloidal suspensions of magnetic nanoparticles with narrow size distribution and grafted with poly(ethylene glycol)-silane of different molecular weights were studied in water, biological buffers, and cell culture media. X-ray photoelectron spectroscopy provided information on the chemical nature of the nanoparticle surface, indicating the particle surfaces consisted of a mixture of amine groups and grafted polymer. The results indicate that the exposure of the amine groups on the surface decreased as the molecular weight of the polymer increased. The hydrodynamic diameters correlated with PEG graft molecular weight and were in agreement with a distributed density model for the thickness of a polymer shell end-grafted to a particle core. This indicates that the particles obtained consist of single iron oxide cores coated with a polymer brush. Particle surface charge and hydrodynamic diameter were measured as a function of pH, ionic strength, and in biological buffers and cell culture media. DLVO theory was used to analyze the particle stability considering electrostatic, magnetic, steric, and van der Waals interactions. Experimental results and colloidal stability theory indicated that stability changes from electrostatically mediated for a graft molecular weight of 750 g/mol to sterically mediated at molecular weights of 1000 g/mol and above. These results indicate that a graft molecular weight above 1000 g/mol is needed to produce particles that are stable in a wide range of pH and ionic strength, and in cell culture media.     

Grafting of amino functional monomer onto initiator-modified polystyrene particles

Polystyrene nanoparticles with grafted chains of an amino functionalized polymer were prepared by a two-step polymerization process. In the first step, the polystyrene seed particles were synthesized by the conventional batch emulsion polymerization using terpolymer HAS (hydroperoxide monomer, acrylic acid, and styrene) as a surface-active initiator. The surface of the obtained particles contains carboxyl groups, which are responsible for the latex stability, and residual undecomposed hydroperoxide groups. Therefore, in the second step, an amino functional monomer was grafted onto the hydroperoxide modified polystyrene particles by a "grafting from" approach. X-ray photoelectron spectroscopy, NMR, and scanning electron microscopy were used to examine the surface of the amino functionalized particles. The amount of incorporated amino groups onto the particles was determined by fluorescenometric titration. In general, the number of amino groups on the particle surface increased with the increase of the functional monomer content in the reaction mixture. The incorporation of the functional monomer was also confirmed by electrophoretic measurements. Final particles possess amphoteric character due to the presence of amino and carboxyl groups on the surface. Adsorption of human immunoglobulins G onto the amino functionalized particles was studied as a function of pH and ionic strength. The covalent binding of human IgG was performed using the glutaraldehyde preactivation method. The immunoreactivity of the latex-IgG complex was examined by the latex agglutination test.