Mononuclear rearrangement of heterocycles in zwitterionic micelles of amine oxide surfactants

The pH-induced swelling of poly(2-vinylpyridine) microgel particles was studied using dynamic light scattering. The increase in particle diameter with decreasing pH was modeled using a well-established thermodynamic model for microgel swelling. The Maxwell construction was applied to extend the model and yield a prediction for a pH-responsive microgel across the entire pH range. The model predicts a first order phase transition for polymer-solvent combinations with a Flory interaction parameter, χ, greater than a critical value χ(c). The modified theory compared favorably to the dynamic light scattering data for the hydrodynamic diameter of microgel particles based on 2-vinylpyridine at different pH values. In particular, the swelling transition is both predicted theoretically and observed experimentally to occur at a pH lower than the pK(a) of the polymer.     

The diffusion coefficient of a swollen microgel particle

The drag on a permeable particle traversing through a Newtonian liquid is calculated. This is in terms of a single dimensionless group, Da, the Darcy number, which relates the particle permeability to the radius. For small values of the Darcy number the solution reverts to the well-known Stokes drag for smooth hard particles. For larger values of the Darcy number the drag is reduced. This drag allows calculation of the diffusion coefficient of such particles, again as a function of the Darcy number. The results are discussed in relation to microgel particles, which display swelling under certain conditions. The size of such particles is typically measured with dynamic light scattering, which measures the diffusion coefficient of particles and as such the analysis presented here shows the conditions under which dynamic light scattering is accurate. The Darcy number for microgel particles is estimated to be on the order of 10(-7).     

Microstructure and rheology of stimuli-responsive microgel systems--effect of cross-linked density

The effect of cross-linked density on the rheological behavior of model pH-responsive microgel systems consisting of methacrylic acid-ethyl acrylate (MAA-EA) cross-linked with di-allyl phthalate (DAP) was examined. Neutralization of acid groups increases the osmotic pressure exerted by counter-ions trapped in the polymeric network against the ions in bulk solution, which is responsible for the swelling and increase in viscosity. The viscosity exhibits a maximum at approximately 1 wt.% DAP and it decreases to a steady value at 4 wt.% DAP, which is independent of pH and particle concentrations. Static light scattering results confirmed this optimum density as the critical point where sufficient cross-link points are present to produce permanent junctions that permit optimal swelling of the microgel particles. In addition, the variation of relative swelling with cross-linked densities of our model microgel systems agrees with the theoretical scaling law, Q alpha (yalphaN(x))(3/2) for cross-linked densities beyond this optimum point (Q is the swelling ratio, y is the acidic MAA content, N(x) is the average number of monomer units between two cross-linked points, and alpha is the degree of neutralization). By combining the results from light scattering and rheological measurements, we are able to correlate the microstructural evolution of the colloidal systems with their bulk rheological behavior.     

Deformation controlled assembly of binary microgel thin films

We describe the assembly of two-component, hydrogel microparticle (microgel) monolayer films onto solid substrates via passive Coulombic adsorption from solution. By using two different microgel types with nearly identical sizes but different degrees of softness, the influence of particle deformation on film composition was determined. Determination of the microgel properties using a variety of light scattering techniques allowed for predictions of the film composition as a function of solution composition using a random sequential adsorption (RSA) model. The films were then studied via atomic force microscopy (AFM), and surface coverage and population statistics were determined from the images and compared to the model predictions. Deviations from the predicted particle adsorption behavior can be directly traced to differences in particle softness, deformation, and particle footprint following adsorption, which biases the particle coverage to the more rigid (smaller footprint) particles. Furthermore, by using a mixture of degradable and nondegradable core/shell particles, the identity of the particles can be unambiguously determined by measuring AFM height changes following erosion of the core from the microgels. These results show that, regardless of the solution diffusion properties of soft particles, their competition for surface adsorption from a binary mixture is largely dictated by their interactions with the surface and their deformation at the surface.     

Volume transition and internal structures of small poly(N‐isopropylacrylamide) microgels

Monodispersed poly(N‐isopropylacrylamide) (PNIPAM) nanoparticles, with hydrodynamic radius less than 50 nm at room temperature, have been synthesized in the presence of a large amount of emulsifiers. These microgel particles undergo a swollen–collapsed volume transition in an aqueous solution when the temperature is raised to around 34 °C. The volume transition and structure changes of the microgel particles as a function of temperature are probed using laser light scattering and small angle neutron scattering (SANS) with the objective of determining the small particle internal structure and particle–particle interactions. Apart from random fluctuations in the crosslinker density below the transition temperature, we find that, within the resolution of the experiments, these particles have a uniform radial crosslinker density on either side of the transition temperature. This result is in contrast to previous reports on the heterogeneous structures of larger PNIPAM microgel particles, but in good agreement with recent reports based on computer simulations of smaller microgels. The particle interactions change across the transition temperature. At temperatures below the transition, the interactions are described by a repulsive interaction far larger than that expected for a hard sphere contact potential. Above the volume transition temperature, the potential is best described by a small, attractive interaction. Comparison of the osmotic second virial coefficient from static laser light scattering at low concentrations with similar values determined from SANS at 250‐time greater concentration suggests a strong concentration dependence of the interaction potential. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 849–860, 2005     

Osmotic compressibility of soft colloidal systems

A turbidimetric analysis of particle interaction of model pH-responsive microgel systems consisting of methacrylic acid-ethyl acrylate cross-linked with diallyl phthalate in colloidal suspensions is described. The structure factor at zero scattering angle, S(0), can be determined with good precision for wavelengths greater than 500 nm, and it measures the dispersion's resistance to particle compression. The structure factor of microgels at various cross-linked densities and ionic strengths falls onto a master curve when plotted against the effective volume fraction, phi(eff) = kc, which clearly suggests that particle interaction potential and osmotic compressibility is a function of effective volume fraction. In addition, the deviation of the structure factor, S(0), of our microgel systems with the structure factor of hard spheres, S(PY)(0), exhibits a maximum at phi(eff) approximately 0.2. Beyond this point the osmotic de-swelling force exceeds the osmotic pressure inside the soft particles resulting in particle shrinkage. Good agreement was obtained when the structural properties of our microgel systems obtained from turbidimetric analysis and rheology measurements were compared. Therefore, a simple turbidimetric analysis of these model pH-responsive microgel systems permits a quantitative evaluation of factors governing particle osmotic compressibility.     

Structures and dynamics of thermosensitive microgel suspensions studied with three-dimensional cross-correlated light scattering

The structure factors, short- and long-time diffusion coefficients, and hydrodynamic interactions of concentrated poly(N-isopropylacryamide) microgel suspensions were measured with simultaneous static and dynamic three-dimensional cross-correlated light scattering. The data are interpreted through comparison to hard sphere theory. The structure factors are known to be described well by the hard sphere approximation. When the structure factor is fit to an effective hard sphere volume fraction and radius, the diffusion and hydrodynamic interactions are also well described by the hard sphere model. We demonstrate that one single hard sphere volume fraction is sufficient to describe the microgel structures, hydrodynamic interactions, and long- and short-time collective diffusion coefficients. This result is surprising because the particle form of the microgels at these temperatures is not rigid, but rather "fuzzy" spheres with dangling polymer chains.     

Photoresponsive surfactants in microgel dispersions

Microgel particles are cross-linked polymer particles. When dispersed in a good solvent for the polymer concerned, they are able to respond to a range of external stimuli by changing volume. Hence, microgel particles are suited to numerous applications (for example, controlled uptake and release) in the pharmaceutical, coatings, and water treatment industries. In this work, pH-sensitive, 0.5 wt % cross-linked poly(2-vinylpyridine) (PVP) microgel particles have been prepared and characterized. When the dispersion pH is decreased below 4.5, the pyridine groups become protonated and the microgel network becomes positively charged, causing the particles to expand. To investigate the possibility of using light as a trigger for effecting volume changes, the interaction of these microgel particles with a photodegradable anionic surfactant, 4-hexylphenylazosulfonate (C(6)PAS), has been investigated using dynamic light scattering and electrophoretic mobility measurements. The electrostatic attraction between the positively charged microgel network (at solution pH 3) and the negatively charged headgroups on the surfactant molecules caused a dramatic decrease in particle volume, and charge-reversal of the particles occurred with increasing surfactant concentration. The UV irradiation of phenylazosulfonate surfactants destroys the anionic headgroup of the molecules, and the microgel particles re-swell. The irradiation of PVP dispersions in the presence of C(6)PAS, along with mixed surfactant systems of sodium dodecyl sulfate plus C(6)PAS, has been investigated.     

Reaction kinetics and microgel particle size characterization of ultraviolet‐curing unsaturated polyester acrylates

The free‐radical reaction kinetics and microgel formation of UV‐curing unsaturated polyester acrylates were studied in terms of the effects of internal maleic and terminal acrylate unsaturations. A triacrylate‐functional monomer, trimethylolpropane triacrylate, was used as the reactive diluent. A time‐resolved Fourier transform infrared technique was used to evaluate the consumption of double bonds and showed that internal (maleic) double bonds were involved in microgel formation at a rate similar to that of the more reactive terminal (acrylic) double bonds. Dynamic light scattering was used to measure the microgel particle size. The introduction of internal unsaturations caused smaller microgels, whereas terminal acrylate unsaturations resulted in larger particle sizes. These results were attributed to the higher tendency of the internal maleic double bonds toward intramolecular cyclization reactions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6544–6557, 2006     

Small-angle neutron scattering study of structural changes in temperature sensitive microgel colloids

The structure of temperature-sensitive poly(N-isopropylacrylamide) microgels in dilute suspension was investigated by means of small-angle neutron scattering. A direct modeling expression for the scattering intensity distribution was derived which describes very well the experimental data at all temperatures over an extensive q range. The overall particle form as well as the internal structure of the microgel network is described by the model. The influence of temperature, cross-linking density, and particle size on the structure was revealed by radial density profiles and clearly showed that the segment density in the swollen state is not homogeneous, but gradually decays at the surface. The density profile reveals a box profile only when the particles are collapsed at elevated temperatures. An increase of the cross-linking density resulted in both an increase of the polymer volume fraction in the inner region of the particle and a reduction of the smearing of the surface. The polymer volume fraction inside the colloid decreased with increasing particle size. The structural changes are in good agreement with the kinetics of the emulsion copolymerization used to prepare the microgel colloids.     

聚(N-异丙基丙烯酰胺)水凝胶微球体积相变的研究

窄分散的聚（Ｎ 异丙基丙烯酰胺）水凝胶微球用乳液聚合方法制备，并用动态和静态光散射对其体积相变进行了研究．与水中聚（Ｎ 异丙基丙烯酰胺）线性单链比较，水中凝胶微球的体积相变温度较高，对温度的响应比较平缓．相变是连续的，有别于大块凝胶非连续的体积变化．在体积相变过程中，凝胶微球始终是密度均一的热力学稳定球体．从相变过程网络密度的变化可以确定，绝大部分的水在收缩过程被排了出来，但在紧缩的凝胶微球中仍含有约７０％的水．     

Polymer dynamics in responsive microgels: influence of cononsolvency and microgel architecture

The dynamics of polymers on the nm and ns scales inside responsive microgels was probed by means of Neutron Spin Echo (NSE) experiments. Four different microgels were studied: poly(N-isopropylacrylamide) (PNIPAM) and poly(N,N-diethylacrylamide) (PDEAAM) microgels, a P(NIPAM-co-DEAAM) copolymer microgel and a core-shell microgel with a PDEAAM core and a PNIPAM shell. These four different microgel systems were investigated in a D(2)O/CD(3)OD solvent mixture with a molar CD(3)OD fraction of x(MeOD) = 0.2 at 10 °C. The PNIPAM and the P(NIPAM-co-DEAAM) microgels are in the collapsed state under these conditions. They behave as solid diffusing objects with only very small additional contributions from internal motions. The PDEAAM particle is swollen under these conditions and mainly Zimm segmental dynamics can be detected in the intermediate scattering function at high momentum transfer. A cross-over to a collective diffusive motion is found for smaller q-values. The shell of the PDEAAM-core-PNIPAM-shell particle is collapsed, which leads to a static contribution to S(q,t); the core, however, is swollen and Zimm segmental dynamics are observed. However, the contributions of the Zimm segmental dynamics to the scattering function are smaller as compared to the pure PDEAAM particle. Interestingly the values of the apparent solvent viscosities inside the microgels as obtained from the NSE experiments are higher than for the bulk solvent. In addition different values were obtained for the PDEAAM microgel, and the PDEAAM-core of the PDEAAM-core-PNIPAM-shell particle, respectively. We attribute the strongly increased viscosity in the PDEAAM particle to enhanced inhomogeneities, which are induced by the swelling of the particle. The different viscosity inside the PDEAAM-core of the PDEAAM-core-PNIPAM-shell microgel could be due to a confinement effect: the collapsed PNIPAM-shell restricts the swelling of the PDEAAM-core and may modify the hydrodynamic interactions in this restricted environment inside the microgel.     

铅离子敏感N-异丙基丙烯酰胺共聚物微凝胶的制备

采用对N-异丙基丙烯酰胺-丙烯酸共聚微凝胶进行改性的方法合成了含苯并18-冠-6功能基团的PNIPAM微凝胶.红外和核磁光谱等手段证明苯并18-冠-6基团被引入到微凝胶中.改性后的微凝胶仍具有很好的温敏性,但是相转变温度由改性前的30℃提高到42℃,并且溶胀度也大大增加.在不控制离子强度的条件下微凝胶的粒径随Na+浓度增加而减小,但是随Pb2+浓度增加微凝胶粒径先减后增.在控制离子强度不变的条件下Na+浓度对微凝胶的粒径影响很小,但是随Pb2+浓度增加微凝胶粒径明显增大,显示较强的铅离子敏感性。     

Stimulus-responsive particulate emulsifiers based on lightly cross-linked poly(4-vinylpyridine)-silica nanocomposite microgels

Lightly cross-linked poly(4-vinylpyridine)-silica nanocomposite microgel particles have been recently reported to act as pH-responsive particulate emulsifiers [Fujii, S.; Read, E. S.; Armes, S. P.; Binks, B. P. Adv. Mater. 2005, 17, 1014]. In this work, the synthesis and performance of such nanocomposite microgel particles are studied in more detail. Scanning electron microscopy, dynamic light scattering, nitrogen microanalyses, thermogravimetric analysis, aqueous electrophoresis, and acid-base titration were used to characterize the nanocomposites in terms of their particle size and morphology, polymer and silica contents, surface compositions, and critical swelling pH, respectively. Depending on the polarity of the oil phase and the purity of the nanocomposite particles, either oil-in-water or water-in-oil emulsions could be prepared at pH 8-9, but not at pH 2-3. These emulsions were characterized in terms of their emulsion type, mean droplet diameter, and morphology using electrical conductivity, light diffraction, and both electron and optical microscopy. In some cases, rapid demulsification could be induced by lowering the solution pH: addition of acid led to protonation of the 4-vinylpyridine residues, which imparted cationic microgel character to the nanocomposite particles. Cross-linking of the nanocomposite microgel particles is essential for their optimum performance as a pH-responsive emulsifier, but unfortunately it is not sufficient to allow recycling.     

Molecular mass characteristics and solution behavior of some cardo polybenzimidazoles

The behavior of dilute solutions of cardo polybenzimidazoles based on 3,3′4,4′-tetraaminodiphenyl ether; 3,3′,4,4′-tetraaminodiphenyl sulfone; and 4,4′-diphenylphthalidedicarboxylic acid in solvents of various natures has been studied by the methods of dynamic light scattering, sedimentation, and viscometry. All of the polymers have been found to contain a microgel fraction. For each fraction, the diffusion coefficient and the particle size are determined. The experimental characteristics of macromolecules correspond to the conformational rigidity calculated by a computer simulation procedure.     

Photo-responsive properties of poly(NIPAM-co-AAc) microgel particles with absorbed, hydrophobically modified organic salts

The absorption of two hydrophobically modified organic salts (HMOSs), containing azobenzene units, into poly(N-isopropylacrylamide-co-acrylic acid) microgel particles has been studied at pH 8 and 20 °C. These dispersions were then irradiated with UV light (wavelength 365 nm) for 10 min to observe the effect on the microgel particle properties, such as the adsorbed amount of the HMOS, the particle size, and the electrophoretic mobility. We show that irradiation of these dispersions with UV light can lead to induced, partial desorption of the HMOS molecules, with concomitant changes in the size and electrophoretic mobility of the microgel particles. This is due to a conformational switch (trans-form to cis-form) in the HMOS molecules, which reduces the strength of the hydrophobic interaction between the HMOS molecules and the isopropyl moieties within the microgel network. Moreover, the original absorbed amounts, size, and electrophoretic mobility values can be largely restored after storage in the dark for extended periods.     

Preparation and characterization of pH‐responsive and thermoresponsive hybrid microgel particles with gold nanorods

We report on pH‐responsive and thermoresponsive hybrid materials based on the assembly of gold nanorods, Au NRs, into multiresponsive, crosslinked copolymer microgel particles. These microgel particles were prepared by the surfactant‐free emulsion polymerization of N‐isopropylacrylamide and acrylic acid using N, N′‐methylene bis‐acrylamide as a crosslinker, which produces particles measuring approximately 160 nm that are interconnected to one other. Cetyltrimethyl ammonium bromide‐stabilized Au NRs were also prepared independently using a seed‐mediated growth method and then loaded into swollen, deprotonated, acrylic acid‐containing microgel particles using the electrostatic interactions between the oppositely charged particles. Transmission electron micrographs of the as‐prepared hybrid Au NR–microgel particles confirmed that the Au NRs were attached to the surface of the microgel particles. The size‐dependent temperature‐responsive characteristics of the hybrid microgel particles were studied by dynamic light scattering, and it was found that as the temperature increased across the phase transition temperature, the particle size decreased to 56% of the original volume. The thermoresponsive and pH‐responsive optical properties of the hybrid microgel particles were also systematically investigated. The thermo‐ and pH‐induced shrinkage of the microgel led to an increase in the UV–vis absorption intensity and caused a significant blue shift in the longitudinal surface plasmon bands of the Au NRs. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Gold nanoparticle localization at the core surface by using thermosensitive core-shell particles as a template

We report novel thermosensitive hybrid core-shell particles via in situ gold nanoparticle formation using thermosensitive core-shell particles as a template. This method for the in situ synthesis of gold nanoparticles with microgel interiors offers the advantage of eliminating or significantly reducing particle aggregation. In addition, by using thermosensitive microgel structures in which the shell has thermosensitive and gel properties in water--whereas the core itself is a water-insoluble polymer--we were able to synthesize the gold nanoparticles only at the surface of the core, which had reactive sites to bind metal ions. After the gold nanoparticles were synthesized, electroless gold plating was carried out to control the thickness of the gold nanoshells. The dispersions of the obtained hybrid particles were characterized by dynamic light scattering and UV-vis absorption spectroscopy, and the dried particles were also observed by electron microscopy. Adaptation of the technique shown here will create a number of applications as optical, electronic, and biomedical functional materials.     

Spectral time moment analysis of microgel structure and dynamics

The structure and dynamics of crosslinked nanoparticles (microgels) made out of hydroxypropylcellulose (HPC) polymer chains were studied using dynamic light scattering spectroscopy. The microgel light scattering spectra were found to be highly nonexponential requiring a spectral time moment analysis in which the spectra were fit to a sum of stretched exponentials. Each term offers three parameters for analysis and represents a single spectral mode. At room temperature microgel spectra reveal three modes. Two faster modes are almost diffusive and correspond to apparent sizes of 25 and 450–650 nm. The slowest mode is independent of scattering angle and is reminiscent of the slow polymer mode observed in identical non‐crosslinked polymer solutions. When solution temperature is varied from 23 to 45°C and back, the microgel undergoes a reversible volume phase transition between 40 and 45°C. According to the time‐moment analysis, above the transition temperature two faster modes collapse into one with apparent hydrodynamic radius of 100–150 nm, while the slow mode remains largely unchanged. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 771–781, 2008     

Synthesis and characterization of thermoreversible biopolymer microgels based on hydrogen bonded nucleobase pairing

We describe the synthesis and characterization of a thermoreversibly cross-linked biopolymer microgel based on protein, DNA, and peptide nucleic acid (PNA) components. The DNA assembles into a trifunctional three-way junction (TWJ) with single-stranded overhangs. PNA oligomers complementary to these overhangs and bearing terminal biotin groups hybridize to the DNA TWJ and simultaneously bind to the tetrafunctional protein avidin, leading to a cross-linked system. Dynamic light scattering experiments reveal that micron-sized particles are formed. Static light scattering was used to characterize the internal structure of these microgels, which were found to have a fractal dimension of 1.85, indicative of a loose network structure. Heating disrupts the weakest component in the system, namely the PNA-DNA hybrid, resulting in dissolution of the microgel, while cooling restores the hydrogen bonding leading to reassembly of the microgel. Variation of the nucleotide sequence permits tuning of the gelation temperature with fine control.