Preparation and characterization of N-isopropylacrylamide/acrylic acid copolymer core-shell microgel particles

A new method has been developed to prepare smart copolymer microgels that consist of well defined temperature sensitive cores and pH sensitive shells. The microgels were obtained from N-isopropylacrylamide (NIPAAm) and acrylic acid (AAc), containing different mole ratios of AAc. Transmission electron micrographs of the microgels show that the colloidal copolymers are nearly monodisperse spheres (core-shell structures). The lower critical solution temperatures (LCSTs) (or phase separation temperatures) of the aqueous microgel solutions were measured by cloud-point method. At slight acidic conditions, the LCST decreased with increase in AAc content, which suggests that the hydrophobic group of NIPAAm has a greater influence on the LCST than the polar COOH group at those conditions. An increase of pH value leads to a significant increase in LCST due to the formation of a more hydrophilic copolymer. The LCST were studied as a function of copolymer composition over the pH range from 4.0 to 6.5. Because the pK(a) of the polymers can be tuned to fall close to neutral pH, these polymer compositions can be dispersed to have phase transitions triggered near physiological pH or at slight acidic pH values that fall within acidic gradients found in biology. Because of their stimuli-responsive behavior, these nanoscale materials are excellent candidates for biotechnology and biomedical applications where small changes in pH or temperature are of great consequence.     

Symmetric and asymmetric adsorption of pH-responsive gold nanoparticles onto microgel particles and dispersion characterisation

The interaction between carboxylic acid-stabilised gold nanoparticles (AuNP) and pH-responsive microgels is shown. The microgel particles are a copolymer of N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA) and N-isopropylacrylamide (NIPAM). The microgel properties are presented by their hydrodynamic diameter and electrophoretic mobility in response to pH. These microgel particles are pH-responsive under neutral conditions decreasing in diameter beyond pH 7. The dispersion characteristics of AuNP adsorbed onto the microgel network are shown with respect to adsorbed amount and the pH-responsive properties of the AuNP. This data is presented between pH 3 and 6 where the microgel properties remain constant. Asymmetric adsorption of AuNP onto poly(DMAPMA-co-NIPAM) microgels is achieved by adsorption of nanoparticles, from the aqueous phase, onto microgel-stabilised oil-in-water emulsions. These asymmetrically modified microgels display very different dispersion behaviour, in response to pH, due to their dipolar nature.     

Influence of charge density on the swelling of colloidal poly(N-isopropylacrylamide-co-acrylic acid) microgels

The volume phase transition of colloidal poly(N-isopropylacrylamide-co-acrylic acid) microgels depends in a complex way on the effective charge density within the polymer network. A series of monodisperse PNIPAM/AAc microgels with different content of acrylic acid were synthesized by surfactant-free emulsion polymerization employing sonication instead of a conventional stirring technique. Subsequently, the colloids were characterized by dynamic light scattering and electron microscopy. Potentiometric titrations provided the amount of carboxyl groups incorporated into the copolymer. The effective charge density was systematically controlled by the content of acrylic acid monomers, the pH value of the suspension, and the salt concentration. The hydrodynamic dimensions of the microgels have been measured by dynamic light scattering. The swelling/deswelling behavior is determined by the delicate balance between hydrophobic attraction of NIPAM and the repulsive electrostatic interactions of the carboxylate group of the acrylic acid moieties. Compared to their macroscopic counterparts the charged microgel particles show a significantly different swelling/deswelling behavior. This manifests in the occurrence of a two-step volume phase-transition process with increasing acrylic acid content. Hydrogen bonding has to be considered to understand this two step volume phase transition uniquely observed for colloidal microgels. Another interesting phenomenon presented here is the reversible formation of well-defined aggregates at low pH and under high salt conditions.     

Functionalised Microgels for Acrylic Coatings

Summary: Aqueous acrylic dispersions of hydroxy-functionalised copolymer microgel particles crosslinked with allyl methacrylate were synthesized by emulsion polymerization. The microgels were investigated as reactive polymer fillers in mixtures with a water-borne film-forming dispersion. Properties of coatings cast from mixtures of aqueous dispersion of hard microgel particles and film-forming water-borne dispersion were investigated. The swelling behaviour of microgels in selected solvents (aliphatic ketones) as a function of microgel composition is discussed as well. It was found that the swelling ability of microgels decreased with growing degree of crosslinking. Microgels comprising copolymerised butyl methacrylate swelled less in aliphatic ketones than microgels without this comonomer. This work was focused mainly on the influence of microgels incorporated in the commercial solvent-borne acrylic binders on the properties of coatings. It was shown that the application of microgels that were redispersed in acetone did not affect the surface appearance and transparency of coatings. Moreover, the presence of microgel network precursors accelerated film curing at ambient temperature and improved the final hardness of coatings.     

Interaction of nonionic surfactants with copolymer microgel particles of NIPAM and acrylic acid

Binding of the nonionic surfactants Triton X-100 and Triton X-405 onto linear copolymers of N-isopropylacrylamide (NIPAM) and acrylic acid and to cross-linked microgel particles of similar composition but differing in their cross-link densities has been studied. The binding capacities vary for each of these polymeric systems, being smallest for the linear copolymer. The binding is also significantly less in all cases for the more hydrophilic surfactant, namely, Triton X-405. By comparing estimates of the pore or "cage" size within the microgel particles with the dimensions of the free micelles in solution, it is concluded that micelles of Triton X-100 form within the microgel particles more readily for the lower cross-linked microgel particles. However, micelles do not form as easily inside either microgel for Triton X-405. The swelling/deswelling behavior of each of the two microgels, in the presence of the surfactants, has been explained in terms of their relative binding behavior and how this contributes to the osmotic pressure difference inside and outside the microgel particles and also in terms of micelle "bridging" of the polymer network, causing shrinkage.     

Review on the dynamics and micro-structure of pH-responsive nano-colloidal systems

This review presents an overview on the research on pH-responsive microgel particles in the last 10 years. Microgels are cross-linked latex particles that are swollen in a good solvent. Significant quantitative studies have been conducted to investigate the swelling behavior (microscopic) and rheological (macroscopic) properties of the pH-responsive microgel particles as a function of neutralization degree, ionic strength, and cross-linked density. Mono-dispersed, alkali-swellable microgels containing carboxylic acid lattices, whose properties display extreme pH sensitivity in water is considered in detail in terms of swelling behavior and rheological properties. Their stability in solution and ability to undergo reversible volume phase transitions in response to pH makes them ideal model systems for the development of a semi-empirical as well as theoretical approach for predicting the viscosity of dilute and concentrated hard and soft sphere systems. The review concludes with a discussion of some recent applications of pH-responsive microgel particles.     

Preparation and Swelling Properties of Poly(NIPAM) "Minigel" Particles Prepared by Inverse Suspension Polymerization

The characterization of temperature- and pH-sensitive poly-N-isopropylacrylamide (poly-NIPAM) microgel particles, produced by surfactant-free emulsion polymerization, has been extensively reported. In the work described here poly(NIPAM) gel particles, cross-linked with N-N'-methylenebisacrylamide (BA), have been produced using inverse suspension polymerization. These particles have been termed "minigels" here since they are somewhat larger than conventional microgels. Results suggest that minigel particles are formed as a dilute suspension, within the aqueous dispersed (droplet) phase. The hydrodynamic diameter of the minigel particles produced in this work is 相似文献   

Microgels containing methacrylic acid: effects of composition on pH-triggered swelling and gelation behaviours

pH-responsive microgels are cross-linked polymer colloids that swell when the pH approaches the pK _a of the particles. In this work, we present a comprehensive investigation of pH-triggered particle swelling and gel formation for a range of microgels containing methacrylic acid (MAA). The microgels investigated have the general composition poly(A/MAA/X), where A and X are the primary co-monomer and cross-linking monomer, respectively. The primary co-monomers were methyl methacrylate (MMA), ethyl acrylate (EA) or butyl methacrylate. The cross-linking monomers were either butanediol diacrylate (BDDA) or ethyleneglycol dimethacrylate (EGDMA). The microgels were studied using scanning electron microscopy, photon correlation spectroscopy (PCS) and dynamic rheology measurements. Gel phase diagrams were also constructed. The particles swelled significantly at pH values greater than approximately 6.0. It was shown that poly(EA/MAA/X) microgels swelled more strongly than poly(MMA/MAA/X) microgels. Furthermore, greater swelling occurred for particles prepared using EGDMA than BDDA. Concentrated dispersions of all the microgels studied exhibited pH-triggered gel formation. It was found that the fluid-to-gel transitions for the majority of the six microgel dispersions investigated could be explained using PCS data. In those cases, gelation was attributed to a colloidal glass transition. Interestingly, the microgels that were considered to have the highest hydrophobic content gelation occurred under conditions where little particle swelling was evident from PCS. The data presented show that gelled poly(EA/MAA/BDDA) and poly(MMA/MAA/EGDMA) microgel dispersions have the strongest elasticities at pH = 7.     

Swelling Kinetics of Microgels Embedded in a Polyacrylamide Hydrogel Matrix

Composite hydrogels—macroscopic hydrogels with embedded microgel particles—are expected to respond to external stimuli quickly because microgels swell much faster than bulky gels. In this work, the kinetics of the pH‐induced swelling of a composite hydrogel are studied using turbidity measurements. The embedded microgel is a pH‐ and thermosensitive poly(N‐isopropylacrylamide‐co‐acrylic acid) microgel and the hydrogel matrix is polyacrylamide. A rapid pH‐induced swelling of the embedded microgel particles is observed, confirming that composite hydrogels respond faster than ordinary hydrogels. However, compared with the free microgels, the swelling of the embedded microgel is much slower. Diffusion of OH^? into the composite hydrogel film is identified as the main reason for the slow swelling of the embedded microgel particles, as the time of the pH‐induced swelling of this film is comparable to that of OH^? diffusion into the film. The composition of the hydrogel matrix does not significantly change the characteristic swelling time of the composite hydrogel film. However, the swelling pattern of the film changes with composition of the hydrogel matrix.     

Poly(N-vinylcaprolactam-co-glycidyl methacrylate) aqueous microgels labeled with fluorescent LaF3:Eu nanoparticles

We describe the synthesis and properties of functional microgel particles based on poly(N-vinylcaprolactam-co-glycidyl methacrylate) (PVCL/PGMA) copolymer. A series of colloidally stable microgel particles with a range of glycidyl methacrylate content were prepared by surfactant-free heterophase polymerization in water. The microgel particles obtained had hydrodynamic radii between 250 and 350 nm and were fairly monodisperse in size; however, a broadening of the particle size distribution was observed for samples with a low GMA content. The PVCL/PGMA microgel particles exhibit thermally responsive reversible changes in diameter in water, and the swelling degree increased with the PVCL fraction in the copolymer structure. These microgels were then modified with photoluminescent europium-doped lanthanum fluoride nanoparticles (LaF3:Eu-AEP) through reaction of the 2-aminoethyl phosphate surface ligands with epoxy groups present in the microgel. These hybrid microgels were colloidally stable and thermally responsive in aqueous solution.     

Inducing pH responsiveness via ultralow thiol content in polyacrylamide (micro)gels with labile crosslinks

Here we present the synthesis and characterization of pH responsive polyacrylamide microgels, synthesized via free radical polymerization of acrylamide and bis (acryloylcystamine) (BAC). The gels were made with ultralow amounts of thiol functional groups incorporated into the polymer. The resulting gel monoliths were mechanically chopped into microgel particles with size distributions ranging from 80 to 200 mum. The gels exhibit an interesting reversible pH-dependent rheological behavior which led to gelling of the colloidal suspension when the pH was increased, and a low-viscosity suspension was obtained when the pH was taken back to the original value. The viscosity of the colloidal system containing MBA crosslinked microgels remained insensitive to pH. This observation motivated further analysis; viscosity measurements of the highly viscous (gel-like) state of the BAC crosslinked microgel colloidal suspension were carried out to further understand the rheological behavior of the colloidal system. Electrophoretic mobility measurements as function of pH of the BAC and MBA crosslinked colloidal polyacrylamide microgel suspensions were performed. The swelling behavior of the microgels for both colloidal systems was also determined as function of pH using static light scattering. This swelling behavior was used to rationalize the observed rheological behavior. The work presented here demonstrates that free thiol groups present within a polymer gel matrix confer pH responsive behavior to the gel in solution. The viscosity of a BAC crosslinked microgel suspension was also measured under reducing conditions. The viscosity of the microgel suspension reduced with time, due to the breakage of the disulfide bonds in the crosslinkers.     

Photo-, thermally, and pH-responsive microgels

Microgels with photo-, thermally, and pH-responsive properties in aqueous suspension have been synthesized and characterized using dynamic light scattering and UV-visible spectroscopy. The new route involved first preparing poly(N-isopropylacrylamide) (PNIPAM)-allylamine copolymer microgels and a spiropyran photochrome (SP) bearing a carboxylic acid group. Then the functionalized spiropyran was coupled to the microgel via an amide bond. The dark-equilibrated gel particles feature spiropyran molecules in the polar, merocyanine form. After irradiation of visible light, the particle size becomes smaller because spiropyran changes to the relatively nonpolar, closed spiro form. The PNIPAM-SP microgels undergo a volume phase transition in water from a swollen state to a collapsed state with increasing temperature under all light conditions. However, the transition temperature range of the PNIPAM-SP is much broader than that for the PNIPAM without SP. The PNIPAM-SP microgels are monodisperse and self-assemble into a crystalline lattice while in suspension. The UV-visible spectra of an aqueous suspension of PNIPAM-SP microgel in the dark-adapted, merocyanine form showed both an absorption peak around 512 nm due to the merocyanine (giving a reddish color to the suspension) and two sharp peaks from Bragg diffraction of colloidal crystallites. Upon visible irradiation, the 512-nm band bleached significantly due to spiropyran photoisomerization. The spiropyran photoisomerization and accompanying color changes of the suspension were reversible upon alternating dark, UV, and visible light irradiation. Due to the residues of amine groups, the swelling capability of PNIPAM-SP microgels reduces as the pH value is changed from 7 to 10.     

Temperature Response of PNIPAM Derivatives at Planar Surfaces: Comparison between Polyelectrolyte Multilayers and Adsorbed Microgels

Two strategies for the design of thermosensitive coatings based on poly‐N‐isopropyl acrylamide (PNIPAM) derivatives are presented: 1) polyelectrolyte multilayers containing a diblock copolymer with a large PNIPAM block and 2) adsorption of PNIPAM microgels. The multilayers show only a small but irreversible response to the increase of outer temperature due to the strong interdigitation between the charged part and the temperature‐sensitive block, while the adsorbed microgels show a pronounced and reversible response. It will be shown that the microgel number density can be easily controlled at the substrate. The swelling and shrinking of two extremes in density are characterized: densely packed microgels, which are considered as a film, and individual microgels, which are able to swell and shrink also lateral to the surface.     

Uptake and release of surfactants from polyampholyte microgel particles

Polyampholyte microgel particles, containing both methacrylic acid and 2-(dimethylamino) ethyl methacrylate (a weak base), in a mainly N-isopropyl acrylamide network, have been prepared by free-radical dispersion polymerisation. The swelling properties of the particles have been shown to be pH and temperature dependent and to exhibit a minimum in size at the iso-electric point. The uptake and release of cetylpyridinium chloride and Triton X-100, into and from, the polyampholyte microgel particles have been investigated as a function of pH. The absorbed amounts at different pH values have been related to various specific interactions between the surfactant and the microgel network.     

负载纳米银复合微球制备及其催化性能

以具有温度和pH双重敏感性能的N-异丙基丙烯酰胺(NIPAM)共聚丙烯酸(AA) P(NIPAM-co-AA)高分子微凝胶为模板, 以乙醇为还原剂, 原位还原得到负载纳米银的微米尺度Ag/P(NIPAM-co-AA)复合微凝胶材料. 通过扫描电子显微镜(SEM)、X射线衍射(XRD)仪和紫外-可见(UV-Vis)分光光度计等对复合材料的形貌、组成和催化性能进行表征. 研究结果表明, Ag/P(NIPAM-co-AA)复合微球具有均一的表面结构, 微凝胶的限域作用显著提高了纳米银的分散性和稳定性. 另外, Ag/P(NIPAM-co-AA)复合微球对对硝基苯酚(4-NP)的还原具有较好的催化活性, 且其催化活性与微凝胶网络结构的溶胀、收缩行为有一定关系, 即模板微凝胶的温敏特性可以实现对对硝基苯酚催化反应活性的调控作用.     

Crystallization behavior of soft, attractive microgels

The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spherical, colloidal particles with attractive pair potentials have been studied by digital video microscopy. The particles were synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and N,N'-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels) with temperature- and pH-tunable swelling properties. It is observed that in a pH = 3.0 buffer with an ionic strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels, in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization for these microgels.     

Interfacial layers of stimuli-responsive poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) microgels characterized by interfacial rheology and compression isotherms

Stimuli-sensitive emulsions stabilized by microgel particles consisting of poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) and being responsive to both pH and temperature have been investigated with respect to the visco-elastic properties of the interfacial layer. Properties of the interfacial layer were probed by means of shear and dilatational rheology as well as by compression isotherms and are related to the microgel packing at the interface as visualized by cryogenic scanning electron microscopy. The corresponding pH dependent emulsion stability is strongly correlated with the visco-elastic properties of the microgel covered oil-water interface. At high pH when the microgels are charged, a structure of partially interconnected microgels is found that provides elastic, soft gel-like interfaces. At low pH, however, the uncharged microgels are densely packed and the interface is rather brittle. Obviously, these pH dependent visco-elastic properties of the microgel layer at the oil-water interface play a determining role in the stability of emulsion droplets and allow us to prepare very stable emulsions when the microgels are charged and to break the emulsion by changing the pH.     

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.     

Poly(vinylpyridine) core/poly(N-isopropylacrylamide) shell microgel particles: their characterization and the uptake and release of an anionic surfactant

The use of microgel particles for controlled uptake and release of active species has great potential. The compatibility of microgel particles with their environment and the functionality of the particles can be achieved by modification of the core microgel through the addition of a shell. In this work, core-shell microgel particles, with a pH-responsive core (polyvinylpyridine) and a temperature-responsive shell (poly-N-isopropylacrylamide), have been prepared and characterized. The uptake and release of an anionic surfactant from the microgels has been investigated as a function of solution pH and temperature. The results indicate that electrostatic attraction between the anionic surfactant and the cationically charged core of the microgel particles is the dominant mechanism for absorption of the surfactant into the core-shell microgel particles.