Relationship between temperature-induced changes in internal microscopic structures of poly(N-isopropylacrylamide) microgels and organic dye uptake behavior

Temperature-induced changes in the internal structures of poly(N-isopropylacrylamide) (pNIPAm) microgels were evaluated by small-angle X-ray scattering (SAXS), and the results were used to explain organic dye uptake by the microgels. The dye uptake experiments were conducted using two organic dyes: cationic rhodamine 6G (R6G) and anionic erythrosine. In the SAXS investigation, the internal structures of the microgels were characterized in terms of the correlation length, ξ, and the distance, d*, which originated from the local packing of the isopropyl groups of two neighboring chains. With increasing temperature up to the volume phase transition temperature (VPTT) of the microgels, the correlation length, ξ, was increased and the distance, d*, was decreased. At the same time, the amounts of the dyes taken up by the pNIPAm microgels were increased, despite a decrease in the volume of the microgels. The results indicated that the pNIPAm chains were closer to each other due to the hydrophobic association of isopropyl groups, which resulted in the growth of the hydrophobic domains. Thus, the hydrophobic interactions between the dyes and pNIPAm were probably accompanied by the domain formation. With a further increase of temperature above the VPTT, the correlation length, ξ, was decreased and then not defined because the Ornstein-Zernike type contribution disappeared, and the distance, d*, was not largely changed. At the same time, the uptake amounts of the dyes per unit volume of the microgels were also not largely changed, which behaved similar to the distance, d*. It was probably due to the fact that the internal structures of the microgels were not largely changed because the isopropyl groups were in contact with each other. The view was supported by the result of the uptake study of the nonthermoresponsive microgels which did not have the hydrophobic isopropyl groups.

Photothermal control of membrane permeability of microcapsules for on-demand release

We report the use of a simple microfluidic device for producing microcapsules with reversible membrane permeability that can be remotely controlled by application of near-infrared (NIR) light. Water-in-oil-in-water (W/O/W) double-emulsion drops were prepared to serve as templates for the production of mechanically stable microcapsules with a core-shell structure and highly uniform size distribution. A biocompatible ethyl cellulose shell was formed, containing densely packed thermoresponsive poly(N-isopropylacrylamide) (pNIPAAm) particles in which gold nanorods were embedded. Irradiation with a NIR laser resulted in heating of the hydrogel particles due to the photothermal effect of the gold nanorods, which absorb at that wavelength. This localized heating resulted in shrinkage of the particles and formation of macrogaps between them and the matrix of the membrane. Large encapsulated molecules could then pass through these gaps into the surrounding fluid. As the phase transition behavior of pNIPAAm is highly reversible, this light-triggered permeability could be repeatedly switched on and off by removing the laser irradiation for sufficient time to allow the gold nanorods to cool. This reversible and remote control of permeability enabled the programmed release of encapsulants, with the time and period of the open valve state able to be controlled by adjusting the laser exposure. This system thus has the potential for spatiotemporal release of encapsulated drugs.

Supramolecular polymer gels with potential model-network structure

Supramolecular polymer networks with different strengths of transient connectivity can be formed with nanometer-scale topologies close to those of regular model networks by transition-metal complexation of monodisperse star-shaped building blocks with terpyridine endgroups.

Aggregation Kinetics of Metal Chalcogenide Nanocrystals: Generation of Transparent CdSe(ZnS) Core(shell) Gels

Transparent CdSe(ZnS) sol-gel materials have potential uses in optoelectronic applications such as light emitting diodes (LEDs) due to their strong luminescence properties and the potential for charge transport through the prewired nanocrystal (NC) network of the gel. However, typical syntheses of metal chalcogenide gels yield materials with poor transparency. In this work, the mechanism and kinetics of aggregation of two sizes of CdSe(ZnS) core(shell) NCs, initiated by removal of surface thiolate ligands using tetranitromethane (TNM) as an oxidant, were studied by means of time-resolved dynamic light scattering (TRDLS); the characteristics of the resultant gels were probed by optical absorption, transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). At low concentrations of NCs (ca. 4 × 10(-7) M), the smaller, green-emitting NCs aggregate faster than the larger, orange-emitting NCs, for a specific oxidant concentration. The kinetics of aggregation have a significant impact on the macroscopic properties (i.e. transparency) of the resultant gels, with the transparency of the gels decreasing with the increase of oxidant concentration due the formation of larger clusters at the gel point and a shift away from a reaction limited cluster aggregation (RLCA) mechanism. This is further confirmed by the analyses of the gel structures by SAXS and TEM. Likewise, the larger orange-emitting particles also produce larger aggregates at the gel point, leading to lower transparency. The ability to control the transparency of chalcogenide gels will enable their properties to be tuned in order to address application-specific needs in optoelectronics.

Impact of polymer network inhomogeneities on the volume phase transition of thermoresponsive microgels

Thermoresponsive polymer gels exhibit pronounced swelling and deswelling upon changes in temperature, rendering them attractive for various applications. This transition has been studied extensively, but only little is known about how it is affected by nano- and micrometer-scale inhomogeneities in the polymer gel network. In this work, droplet microfluidics is used to fabricate microgel particles of strongly varying inner homogeneity to study their volume phase behavior. These particles exhibit very similar equilibrium swelling and deswelling independent of their inner inhomogeneity, but the kinetics of their volume phase transition is markedly different: while gels with pronounced micrometer-scale inhomogeneity show fast and affine deswelling, homogeneous gels shrink slowly and in multiple steps.

Unique organogel formation with macroporous materials constructed by the freeze-drying of aqueous cyclodextrin solutions

Macroporous cyclodextrin materials (MP-alpha-, beta-, and gamma-CDs) were easily fabricated by the freeze-drying of aqueous solutions of alpha-, beta-, and gamma-CDs. These MP-CDs showed the absorption ability toward various organic solvents and oils to give organogels at ambient temperature. The morphological changes of the MP-CD microstructures were observed through the absorption of organic solvents. In particular, the absorption of polar organic solvents with hydrogen-bond forming ability, including 1,4-dioxane and ethanol, by the MP-CDs caused remarkable morphological changes in the microstructures. The absorption of these polar solvents by MP-alpha- and gamma-CDs resulted in the formation of channel-type assemblies of alpha- and gamma-CDs, respectively.

Mesoporous hydrogels: revealing reversible porosity by cryoporometry, X-ray scattering, and gas adsorption

Mesoporous poly(2-hydroxyethyl methacrylate-co-ethylene glycol dimethacrylate) networks, with cross-linker contents ranging from 100 to 5 mol %, were prepared using a hard-templating approach. The imbibition of the silica pellets with a monomer/cross-linker mixture resulted in mesoporous gels with a pore size of approximately 10 nm, which corresponds well with the average size of the fumed silica particles (10-11 nm). The highly cross-linked materials showed permanent surface areas of up to 230 m(2) g(-1) and porosities up to approximately 33 vol %. The porosity of the hydrogels was investigated in both dry and water-saturated state by nitrogen sorption, cryoporometry, and small-angle X-ray scattering (SAXS). It is only polymeric materials that contain 50 mol % or more of the cross-linker that showed a significant porosity after evaporative drying. Freeze-drying is able to preserve the porosity also for hydrogels of intermediate cross-linker content, but the pores of the materials of low cross-linker content collapses completely upon solvent removal. The observed critical cross-linker ratio for pore stability compared favorably with a simple estimate of the critical cross-linker density needed to make the material sufficiently stiff to withstand the Laplace pressure during solvent removal. Analysis of the hydrogels in the water swollen state revealed that gels having cross-linker contents down to 5 mol % still possessed mesoporosity. The pores got less defined at very low cross-linker contents, while their size was rather constant at intermediate to high cross-linking densities. Closed pores could be reopened upon swelling, which suggests that the observed pore collapse upon drying may be at least partly reversible.

Gelation mechanism of poly(N-isopropylacrylamide)-clay nanocomposite hydrogels synthesized by photopolymerization

The gelation process of poly-(N-isopropylacrylamide)-clay nanocomposite hydrogels (PNIPAAm-clay NC gels) was investigated by dynamic and static light scattering (DLS and SLS), as well as by fluorescence correlation spectroscopy (FCS). The photopolymerization method chosen for the radical polymerizing system ensured that, when the irradiation is removed, the reaction stopped immediately. Experiments showed that shortly before the gelation threshold is reached, no changes in the DLS autocorrelation functions appear, while the monomer conversion can be observed by 1H NMR spectroscopy. These results correspond to the formation of microparticles, in which the PNIPAAm chains are closely attached to the clay platelets. During the further polymerization process, clay clusters are developed before the sol-gel threshold is reached. FCS measurements were performed to obtain information on the motion of the clay platelets inside the NC gel. The DLS method gives only an average of the motions in the gel. In a time window between 10 micros and 1 s, the clay sheets labeled with Rhodamine B show no characteristic motions.

Gel point determination of gelatin hydrogels by dynamic light scattering and rheological measurements

The sol-gel transition of gelatin aqueous solutions has been investigated in terms of time-resolved dynamic light scattering (DLS) and rheological measurements during the cooling process. A drastic increase in the scattering intensity, ergodic-nonergodic transition, and a power-law behavior in the scattering intensity-time correlation function were observed at the gelation temperature Tgel. Thus obtained "microscopic" Tgel was confirmed to be in good agreement with a "macroscopic" Tgel obtained by rheological measurements irrespective of gelatin concentration C . The fractal exponent Dp evaluated by DLS was found to be q and C independent and was also in good agreement with that obtained by rheology (n), i.e., Dp congruent with n congruent with 0.73, where q is the magnitude of the scattering vector.

Surface Charge Modification of Nano-Sized Silica Colloid

The surface of commercial 30-nm colloidal silica particles were modified by reaction with functional silanes. The high specific surface area and reactivity of the particles, due to their small size, makes the process susceptible to irreversible aggregation not found previously with larger particles. The present study compares surface charge results from amino silanes with one or three alkoxy groups. Measurements of the zeta potential as a function of pH, and gelation kinetics shed light on the mechanism of surface charge changes from the modification. Instability in suspensions before and after the surface modification is also studied using a new data analysis technique from simple light-scattering equipment. Experimental results show very stable particles are obtained by amino silane surface modification. Factors affecting susceptibility of small particles to irreversible aggregation caused by a non-aqueous solvent or high concentration of a trialkoxy silane, including the large number of reactive silanol groups in the surface gel layer of the particles, are discussed.