Synthesis of Polyampholyte Janus-like Microgels by Coacervation of Reactive Precursors in Precipitation Polymerization

Controlling the distribution of ionizable groups of opposite charge in microgels is an extremely challenging task, which could open new pathways to design a new generation of stimuli-responsive colloids. Herein, we report a straightforward approach for the synthesis of polyampholyte Janus-like microgels, where ionizable groups of opposite charge are located on different sides of the colloidal network. This synthesis approach is based on the controlled self-assembly of growing polyelectrolyte microgel precursors during the precipitation polymerization process. We confirmed the morphology of polyampholyte Janus-like microgels and demonstrate that they are capable of responding quickly to changes in both pH and temperature in aqueous solutions.     

Formation of Salt Bonds in Polyampholyte Chains

In this paper we study the influence of the formation of intrachain ion pairs (salt bonds) and the distribution of counterions on the behavior of single polyampholyte chains in a dilute solution. It has been shown that neutral polyampholyte chains can undergo jump‐like collapse transition from the swollen state to the globular state with the formation of ion pairs between oppositely charged ions of the chain. A polyampholyte chain with an excess charge shows the behavior of a conventional polyelectrolyte chain and counterions play an important role in the chain behavior. We distinguish three possible states of counterions: free counterions inside and outside the macromolecule, and a bound state of counterions forming ion pairs with the corresponding ions of the polymer chain. We found a non‐monotonous behavior of the chain upon increasing the excess charge on the chain: the chain swells from a compact state to elongated conformation and shrinks again to the compact state when the excess charge of the chain is increased.     

Synthesis and aqueous solution properties of sterically stabilized pH-responsive polyampholyte microgels

Emulsion copolymerization of poly(methacrylic acid) and poly(2-(diethylamino)ethyl methacrylate) (PMAA/PDEA) yielded pH-responsive polyampholyte microgels of 200-300 nm in diameter. These microgels showed enhanced hydrophilic behavior in aqueous medium at low and high pH, but formed large aggregates of approximately 2500 nm at intermediate pH. To achieve colloidal stability at intermediate pH, a second batch of microgels of identical monomer composition were synthesized, where monomethoxy-capped poly(ethylene glycol)methacrylate (PEGMA) was grafted onto the surface of these particles. Dynamic light-scattering measurements showed that the hydrodynamic radius, Rh, of sterically stabilized microgels was approximately 100 nm at intermediate pH and increased to 120 and 200 nm at pH 2 and 10, respectively. Between pH 4 and 6, these microgels possessed mobility close to zero and a negative second virial coefficient, A2, due to overall charge neutralization near the isoelectric pH. From the Rh, mobility, and A2, cross-linked MAA-DEA microgels with and without PEGMA retained their polyampholytic properties in solution. By varying the composition of MAA and DEA in the microgel, it is possible to vary the isoelectric point of the colloidal particles. These new microgels are being explored for use in the delivery of DNA and proteins.     

Synthesis strategies to incorporate acrylic acid into N‐vinylcaprolactam‐based microgels

Two different synthesis strategies were used to synthesize N‐vinylcaprolactam (VCL)‐acrylic acid (AA)‐based temperature‐ and pH‐sensitive microgels under the adequate conditions to avoid possible hydrolysis of VCL due to the presence of carboxylic groups provided by AA. Polymeric and colloidal features of the microgels were analyzed: the partial conversion evolutions of each comonomer were determined by ¹H NMR and the swelling/deswelling behavior by means of Photon Correlation Spectroscopy. Considering that microgels are porous soft nanoparticles, conductimetric titrations at the swollen state were carried out to calculate the volumetric charge density. The results indicate that the addition of AA after 30 minutes of reaction time helped to incorporate higher amounts of AA into microgels and as a result, to obtain both temperature‐ and pH‐sensitive nanoparticles. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Noble‐Metal‐Free Janus‐like Structures by Cation Exchange for Z‐Scheme Photocatalytic Water Splitting under Broadband Light Irradiation

Z‐scheme water splitting is a promising approach based on high‐performance photocatalysis by harvesting broadband solar energy. Its efficiency depends on the well‐defined interfaces between two semiconductors for the charge kinetics and their exposed surfaces for chemical reactions. Herein, we report a facile cation‐exchange approach to obtain compounds with both properties without the need for noble metals by forming Janus‐like structures consisting of γ‐MnS and Cu₇S₄ with high‐quality interfaces. The Janus‐like γ‐MnS/Cu₇S₄ structures displayed dramatically enhanced photocatalytic hydrogen production rates of up to 718 μmol g⁻¹ h⁻¹ under full‐spectrum irradiation. Upon further integration with an MnO_x oxygen‐evolution cocatalyst, overall water splitting was accomplished with the Janus structures. This work provides insight into the surface and interface design of hybrid photocatalysts, and offers a noble‐metal‐free approach to broadband photocatalytic hydrogen production.     

Synthesis and Characterization of Novel pH‐Responsive Polyampholyte Microgels

Summary: We synthesized for the first time novel pH‐responsive polyampholyte microgels consisting of poly(methacrylic acid) and poly(2‐(diethylamino)ethyl methacrylate) (PMAA‐PDEA) that are sterically stabilized with poly(ethylene glycol) methyl ether methacrylate (PEGMEM). These microgels showed enhanced hydrophilic behavior in aqueous medium at low and high pH but become hydrophobic and compact between pH 4 and 6 near the isoelectric point. Dynamic‐light scattering measurements showed that the hydrodynamic radius, R_h of these microgels is approximately 100 nm between pH 4 and 6 and increases to around 140 and 170 nm at pH 2 and 10, respectively. It is evident that the cross‐linked MAA‐DEA microgel that is sterically stabilized with PEGMEM retains the polyampholyte properties in solution.

Sterically stabilized cross‐linked MAA‐DEA microgel.     

Control of colloidal interactions between microgels with stimulus-responsive properties

Microgels can switch their chemical/physical properties with external stimulus, and the colloidal behavior of microgels is strongly affected by interparticle interactions. In this article, we introduce smart microgels, focusing on Janus microgels and oscillating microgels developed by our group. Janus microgels show anisotropic shape and chemical/physical properties, and thus the structures of their flocs are also anisotropic. Oscillating microgels show autonomous swelling/deswelling and dispersing/flocculating oscillations through synchronization with chemical reactions. The interparticle interactions of these microgels are discussed. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3021–3026     

The Compressibility of pH‐Sensitive Microgels at the Oil–Water Interface: Higher Charge Leads to Less Repulsion

pH‐responsive microgels are unique stabilizers for stimuli‐sensitive emulsions that can be broken on demand by changing the pH value. However, recent experiments have indicated that electrostatic interactions play a different role to that in conventional Pickering emulsions. The influence of charges on the interactions between microgels at the oil–water interface is now described. Compression isotherms of microgels with different charge density and architecture were determined in a Langmuir trough, and counter‐intuitive results were obtained: Charged microgels can be compressed more easily than uncharged microgels. The compressibility of microgels is thus not determined by direct Coulomb repulsion. Instead, the different swelling of the microgels in the charged and the uncharged states is proposed to be the key parameter.     

Phase separations in pH-responsive polyelectrolyte multilayers: charge extrusion versus charge expulsion

Polyelectrolyte multilayers with continuously variable amounts of ionizable weak acid functionality were prepared by blending ionizable and nonionizable polyelectrolytes in the deposition solutions. Diluting ionizable groups in this way yielded multilayers that were more structurally stable, shown by thickness and atomic force microscopy measurements, as their internal polymer charge was varied by the pH of the external solution. Multilayers prepared with opposite surface charge to that appearing within the bulk (as a result of ionization) were more stable, as were thinner films, both results suggesting the extrusion of bulk charge to the surface. These multilayers were able to control the direction and magnitude of electroosmotic flow in microfluidics systems. Multilayers bearing only one, diluted layer ofionizable material were surprisingly effective in this respect.     

Self‐Consistent Theory of Structures and Transitions in Weak Polyampholytes

Weak polyampholytes are charged polymers, where the charge asymmetry can be tuned by varying the solution pH. We determine the size of a randomly charged weak polyampholyte in dilute solution as a function of charge asymmetry, Bjerrum length, salt concentration, pH, and degree of polymerization, using a self‐consistent method. It is known that in the limiting cases of low and high charge asymmetries, polyampholytes behave as neutral polyampholytes and polyelectrolytes, respectively. We explore in detail the regime of intermediate charge asymmetry where a polyampholyte show non‐monotonic change in the chain size as a function of Bjerrum length. A hierarchy of structures exists at different length scales, ranging from ideal coils at low Bjerrum length, extended rod‐like state at intermediate Bjerrum length to globular states at high Bjerrum length. The transition between ideal coil and rod‐like states is continuous, while that between rod‐like and globular states is discontinuous. The addition of salt changes the nature of the rod‐to‐globule transition from discontinuous to continuous. The effective free energy shows a double minimum at intermediate charge asymmetry, indicating the coexistence of globules and extended states. The size as a function of the solution pH shows a minimum at the isoelectric point. The size of neutral polyampholytes at the isoelectric pH increases with the increase in the salt concentration. The size of charge excess polyampholytes far away from the isoelectric pH decreases with the increase in the salt concentration.

     

微凝胶增强两性复合水凝胶的制备与性能

将核壳微凝胶包埋在两性基质中,制备了复合水凝胶(CAH)。 研究发现,利用微凝胶与聚合物链之间的物理缠结作用,可以使复合凝胶具有致密的网络结构,力学性能显著提高;复合凝胶对pH和离子强度敏感,呈现出典型的两性聚电解质凝胶的溶胀行为。 同时微凝胶的存在和特殊的复合结构,可赋予CAH两性凝胶基质所不具有的响应性,并实现在高温下快速响应。     

Directional Shuttling of a Stimuli‐Responsive Cone‐Like Macrocycle on a Single‐State Symmetric Dumbbell Axle

Rotaxane‐based molecular shuttles are often operated using low‐symmetry axles and changing the states of the binding stations. A molecular shuttle capable of directional shuttling of an acid‐responsive cone‐like macrocycle on a single‐state symmetric dumbbell axle is now presented. The axle contains three binding stations: one symmetric di(quaternary ammonium) station and two nonsymmetric phenyl triazole stations arranged in opposite orientations. Upon addition of an acid, the protonated macrocycle shuttles from the di(quaternary ammonium) station to the phenyl triazole binding station closer to its butyl groups. This directional shuttling presumably originates from charge repulsion and an orientational binding preference between the cone‐like cavity and the nonsymmetric phenyl triazole station. This mechanism for achieving directional shuttling by manipulating only the wheels instead of the tracks is new for artificial molecular machines.     

Preparation of Poly(N‐isopropylacrylamide) Microgels using Different Initiators Under Various pH Values

Ammonium persulfate (APS), 2,2′‐azobis(amidinopropane) dihydrochloride (V50) and 4,4′‐azobis(4‐cyanovaleric acid) (ACVA) were utilized to prepare temperature‐sensitive poly(N‐isopropylacrylamide) (PNIPAM) microgels by precipitation polymerization under various reaction pH conditions. Their particle sizes and swelling ratios depended on the reaction pH due to the pH dependence on the ionization degree of the decomposed fragments originating from the initiators and their hydrophilicity‐hydrophobicity. The more hydrophobic initiator, under the reaction pH conditions used, could be partitioned to a greater extent into the microgel particles due to the hydrophobicity of PNIPAM chains at the reaction temperature, which led to a more cross‐linked structure inside the microgels resulting in their smaller swelling ratio. pH dependence of surface charge density of the microgels with amidino groups or carboxylic acid groups on their surfaces was evidenced by the variation of their zeta potentials as a function of pH.     

Interacting nanoparticles with functional surface groups

Functionalized nanoparticles with ionizable groups have generated a large variety of structures with important potential applications in technology. The nature of their interactions is crucial to determining their solubility and to exploring assemblies with diverse symmetries. Here, we use a molecular theory to describe the interactions between two nanoparticles coated with short polymer chains that contain ionizable (functional) end‐groups immersed in aqueous salt solution. It is shown here that the fraction of ionized functional groups in the system depends on factors such as the ionic strength and pH of solution, grafting density of polymer chains, the chain length, as well as the separation distance between the nanoparticles. The interactions between two neighboring nanoparticles influence the charge regulation of the end‐groups, which consequently induces an asymmetric distribution of these charged end‐groups on the nanoparticles, and thus confers a preferred directionality in nanoparticle–nanoparticle interactions. We show that the charge regulating system is less repulsive than an equivalent system with a fixed charge distribution. This is due to a decrease in the charge density of the weak acid end‐groups, to avoid a local increase in counterion confinement (condensation) in the region between neighboring nanoparticles, when their separation decreases. The anisotropic degree of ionization found in our results can be used to design aggregates of nanoparticles with reduced symmetries. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

PEG-POSS assisted facile preparation of amphiphilic gold nanoparticles and interface formation of Janus nanoparticles

A general approach to transfer water-soluble nanoparticles with different shapes, sizes, and surface charges into organic solvents, retaining their surface charge properties was developed, and its application in fabrication of hybrid Janus particles with opposite charges in solution was also demonstrated.     

Ion concentration of external solution as a characteristic of micro- and nanogel ionic reservoirs

Ion-sensitive hydrogel is regarded as an ionic reservoir, i.e., a system capable of changing the external pH or ionic strength by accumulating or releasing ions. The concept of a hydrogel ionic reservoir was demonstrated for hydrogel particles of three different size ranges: macrogel (1000-6000 microm), microgel (approximately 20-200 microm), and nanogel (approximately 0.2 microm). Ion sensitivity of poly(N-isopropylacrylamide-co-1-vinylimidazole) (PNIPA-VI) microgels with imidazolyl (ionizable) groups was confirmed by the pH dependence of their volume, while nanogels were characterized by dynamic light scattering. On the contrary, the volume of poly(N-isopropylacrylamide) (PNIPA) microgels without ionizable groups was pH independent in the whole range of pH from 10 to 2. Four distinct regions of pH-behavior were observed for PNIPA-VI hydrogel micro- and nanoparticles using potentiometric titration of their suspensions. Time-resolved measurements of ion concentrations in the suspension of hydrogel particles revealed a substantial difference in kinetics of pH equilibration for (i) ion-sensitive hydrogels (PNIPA-VI) vs hydrogels without ionizable groups (PNIPA) and (ii) PNIPA-VI hydrogels of different sizes. On the basis of the experimental observations, a two-step mechanism affecting the kinetics of proton uptake into the hydrogel particles with ionizable groups was proposed: (1) fast binding of ions to the immediate surface of each particle and (2) a slower successive diffusion of bound sites into the next inner layer of polymer network. In accord with the mechanism proposed, a quasi-chemical kinetic model of pH relaxation to equilibrium was developed to fit the experimental data for the time course of proton uptake by macro-, micro-, and nanogels into two exponentials with the characteristic times of tau(1) and tau(2). We believe the same kinetic model will be pertinent to describe phenomenological and molecular mechanisms controlling proton transport in/out bacteria, cells, organelles, drug delivery vehicles, and other natural or artificial multifunctional ionic containers. The approach can be easily extended for the other ions (e.g., Na(+), K(+), and Ca(2+)).     

Kinetically controlled growth of gold nanotriangles in a vesicular template phase by adding a strongly alternating polyampholyte

This paper is focused on the temperature-dependent synthesis of gold nanotriangles in a vesicular template phase, containing phosphatidylcholine and AOT, by adding the strongly alternating polyampholyte PalPhBisCarb.

UV-vis absorption spectra in combination with TEM micrographs show that flat gold nanoplatelets are formed predominantly in the presence of the polyampholyte at 45°C. The formation of triangular and hexagonal nanoplatelets can be directly influenced by the kinetic approach, i.e., by varying the polyampholyte dosage rate at 45°C. Corresponding zeta potential measurements indicate that a temperature-dependent adsorption of the polyampholyte on the {111} faces will induce the symmetry breaking effect, which is responsible for the kinetically controlled hindered vertical and preferred lateral growth of the nanoplatelets.     

From polyelectrolyte to polyampholyte microgels: comparison of swelling properties

We report the results of comparative studies of the swelling behavior of polyelectrolyte (PE) and polyampholyte (PA) microgels in response to pH, ionic strength, temperature, and solvent composition. Polyelectrolyte microgels used were cross-linked binary copolymers of poly(N-isopropylacrylamide) (polyNIPAm) and acrylic acid (AA) or vinyl imidazole (VI). The PA microgels with an excess of either cationic or anionic groups swelled at low or high pH values, respectively, analogous to PE microgels. The PA microgels with similar amounts of AA and VI groups exhibited marked swelling at both high and low pH values. All PA microgels shrank in the intermediate range of pH due to electrostatic attraction between charged AA and VI moieties. In moderately concentrated salt solutions, PA microgels underwent swelling showing antipolyelectrolyte behavior. The extent of swelling of PA microgels increased with rise in AA content. The temperature-dependent contraction of both PE and PA microgels occurred at higher temperatures when AA and VI groups were charged and hydrophilic. Ion pairing between the AA and VI groups increased the extent of the temperature-induced deswelling in PA microgels. The solvent-dependent swelling of PE and PA microgels in ethanol–water mixtures was governed by competing electrostatic and cosolvency effects.

耐电解质高吸水性树脂

在简述高吸水性树脂吸水机理的基础上,综述了改善高吸水性树脂耐电解质性能的四种途径,结合聚合方法重点讨论了提高亲水性、引入长链疏不性单体以及合成两性聚合物三种改性途径。     

铆接于固体表面的两性聚电解质链构象的分子动力学模拟

用分子动力学模拟研究了铆接于带电固体壁面上的两性无规共聚电解质链的构型及其随溶剂的介电常数、系统温度和带电壁面电场的变化。结果表明,聚离子链的构型取决于外电场对链节的作用、链内各链节间的静电作用和链节的热运动(温度)三者的综合作用结果。电场强度较低时,铆接聚离子链与非铆接聚离子链的构型变化类似。电场强度较高时,两性聚离子链构型更为舒展。溶剂介电常数减小,链节间静电作用增强,净电荷比较小的中性或非中性两性聚电解质链倾向于收缩,净电荷比较多的链则膨胀。温度的升高一般总是抵消电场和分子内静电作用的影响,当它占主导地位时将使聚离子链膨胀。