K. C. Tam

pH-Responsive polymers: synthesis, properties and applications

pH-Responsive polymers are systems whose solubility, volume, and chain conformation can be manipulated by changes in pH, co-solvent, and electrolytes. This review summarizes recent developments covering synthesis, physicochemical properties, and applications in various disciplines. A variety of synthetic methodologies comprising of emulsion polymerization and living radical polymerization techniques are described, and some of their salient features are highlighted. Several polymeric systems, such as homopolymers, block copolymers, microgels, hydrogels and polymer brushes at interfaces are reviewed, where important characteristics that govern their behavior in solutions are described. Potential applications of these systems in controlled drug delivery, personal and home care, industrial coatings, biological and membrane science, viscosity modifiers, colloid stabilization, and water remediation, are discussed.

A Nitrogen and Sulfur Dual-Doped Carbon Derived from Polyrhodanine@Cellulose for Advanced Lithium–Sulfur Batteries

A sulfur electrode exhibiting strong polysulfide chemisorption using a porous N, S dual-doped carbon is reported. The synergistic functionalization from the N and S heteroatoms dramatically modifies the electron density distribution and leads to much stronger polysulfide binding. X-ray photoelectron spectroscopy studies combined with ab initio calculations reveal strong Li(+) -N and Sn (2-) -S interactions. The sulfur electrodes exhibit an ultralow capacity fading of 0.052% per cycle over 1100 cycles.

Thermo- and photo-responsive polymeric systems

Since the first reported thermal phase transition of poly(N-isopropylacrylamide) by Heskin in 1968, this unique polymer has continued to gain popularity. Because of their potential applications in the field of biomedical science, various responsive polymeric systems, such as those induced by pH, salt, co-solvent, thermal, light, electric and magnetic field, have been synthesized and studied. This review reports on recent developments (over the last 10 years) of thermo- and photo-responsive homopolymers, copolymers, microgels, hydrogels and polymer brushes at interfaces, where the synthesis, physicochemical properties, and potential applications are highlighted. Although homopolymers and microgels undergo phase transitions upon the application of external stimuli, block copolymers, however, self-assemble into different nanostructures. Such reversible phase transitions and self-assembly behaviors have generated many robust structures that can be applied in coating industries, personal/home care, petroleum, drug/protein/DNA delivery and separation processes.

Hydroxyapatite nanostructure material derived using cationic surfactant as a template

This article reports the successful synthesis of mesoporous hydroxyapatite, Ca10(PO4)6(OH)2 (denoted HA) using cationic surfactant as the template. The wide-angle (2θ > 10°) diffraction data revealed characteristic peaks of HA, where a hexagonal lattice structure can be deduced. The lattice structure is found in space group P63/m, the parameters of which are in excellent agreement with reference data; i.e.a = b = 0.9418 nm, c = 0.6884 nm. For small angle diffraction (2θ < 10°), the characteristic peaks occur at 2θ values of 3.30, 5.75, 7.25 and 8.30°, indicating the presence of atomic planes with a periodical spacing of 2.677 nm. Nitrogen adsorption indicated a pore size distribution of approximately 3 nm, and a corresponding pore volume of 0.0113 cm3 g−1, hence the volume ratio of the mesopores was found to be ∼0.036. SEM micrographs reveal a rod-like structure of HA, possessing a thickness of about 50–100 nm and length ranging from 500 to 1000 nm, while TEM micrographs revealed that nano-channels are formed within the rod-like structure. These nano-channels align in a lengthwise direction within the rods, consistent with the cavities being generated by the removal of “organic” CTAB templating structure during calcination. The channels have dimensions of around 3.5 nm and the spaces between the nano-channels are filled with an ordered crystalline HA structure. Comparing the pore size and the spacing between neighboring channels, the pore volume ratio of each rod was calculated to be ∼0.029, which is in agreement with the result from gas adsorption. A probable mechanism is that CTAB–PO43− mixtures form rod-like micelles, which contain many PO43− groups on the surface, and in the presence of Ca2+, Ca9(PO4)6 clusters are preferentially condensed on the rod-shaped micellar surface due to the conformation compatibility between the identical hexagonal shapes of the micelles and Ca9(PO4)6 clusters. The micelles act as nucleating points for the growth of HA crystals. During the thermal incubation stage, CTAB–HA complexes are produced and they coalesce to form a stable three-dimensional rod-like structure. The morphology of the final product shows about 10 layers of HA crystal grows on one micelle during the reaction in water.

Dual Responsive Pickering Emulsion Stabilized by Poly[2-(dimethylamino)ethyl methacrylate] Grafted Cellulose Nanocrystals

A weak polyelectrolyte, poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA), was grafted onto the surface of cellulose nanocrystals via free radical polymerization. The resultant suspension of PDMAEMA-grafted-cellulose nanocrystals (PDMAEMA-g-CNC) possessed pH-responsive properties. The grafting was confirmed by FTIR, potentiometric titration, elementary analysis, and thermogravimetric analysis (TGA); the surface and interfacial properties of the modified particles were characterized by surface tensiometer. Compared to pristine cellulose nanocrystals, modified CNC significantly reduced the surface and interfacial tensions. Stable heptane-in-water and toluene-in-water emulsions were prepared with PDMAEMA-g-CNC. Various factors, such as polarity of solvents, concentration of particles, electrolytes, and pH, on the properties of the emulsions were investigated. Using Nile Red as a florescence probe, the stability of the emulsions as a function of pH and temperature was elucidated. It was deduced that PDMAEMA chains promoted the stability of emulsion droplets and their chain conformation varied with pH and temperature to trigger the emulsification and demulsification of oil droplets. Interestingly, for heptane system, the macroscopic colors varied depending on the pH condition, while the color of the toluene system remained the same. Reversible emulsion systems that responded to pH were observed and a thermoresponsive Pickering emulsion system was demonstrated.

Rheological properties of methacrylic acid/ethyl acrylate co-polymer: comparison between an unmodified and hydrophobically modified system

Earlier experimental studies on the emulsion polymerized methacrylic acid/ethyl acrylate co-polymer indicated that the ethyl acrylate (EA) segments are sufficiently blocky to induce hydrophobic association between the EA blocks once the methacrylic acid groups are neutralized by a base. Detailed rheological characterization on semi-dilute solutions suggests that the viscoelastic property is caused by the transient network assembled through hydrophobic associations, rather than by physical chain entanglements. In the semi-dilute solution regime, the hydrophobically modified associative polymer exhibits higher viscosities when compared to the unmodified analogue. This is due to the formation of higher proportion of intermolecular association between the polymer clusters, which enhances the hydrophobic interaction between the hydrophobic macromonomers of different polymer chains. A mechanistic model is proposed to describe the nature of associations between the blocky EA and the hydrophobic segments of the polymer.

Microgel Iron Oxide Nanoparticles for Tracking Human Fetal Mesenchymal Stem Cells Through Magnetic Resonance Imaging

Stem cell transplantation for regenerative medicine has made significant progress in various injury models, with the development of modalities to track stem cell fate and migration post‐transplantation being currently pursued rigorously. Magnetic resonance imaging (MRI) allows serial high‐resolution in vivo detection of transplanted stem cells labeled with iron oxide particles, but has been hampered by low labeling efficiencies. Here, we describe the use of microgel iron oxide (MGIO) particles of diameters spanning 100‐750 nm for labeling human fetal mesenchymal stem cells (hfMSCs) for MRI tracking. We found that MGIO particle uptake by hfMSCs was size dependent, with 600‐nm MGIO (M600) particles demonstrating three‐ to sixfold higher iron loading than the clinical particle ferucarbotran (33‐263 versus 9.6‐42.0 pg iron/hfMSC; p < .001). Cell labeling with either M600 particles or ferucarbotran did not affect either cellular proliferation or trilineage differentiation into osteoblasts, adipocytes, and chondrocytes, despite differences in gene expression on a genome‐wide microarray analysis. Cell tracking in a rat photothrombotic stroke model using a clinical 1.5‐T MRI scanner demonstrated the migration of labeled hfMSCs from the contralateral cortex to the stroke injury, with M600 particles achieving a five‐ to sevenfold higher sensitivity for MRI detection than ferucarbotran (p < .05). However, model‐related cellular necrosis and acute inflammation limited the survival of hfMSCs beyond 5‐12 days. The use of M600 particles allowed high detection sensitivity with low cellular toxicity to be achieved through a simple incubation protocol, and may thus be useful for cellular tracking using standard clinical MRI scanners. STEM CELLS 2009;27:1921–1931

Steady and Dynamic Shear Properties of Aqueous Polymer Solutions

The steady and dynamic shear properties of dilute aqueous solutions of Separan AP30, xanthan gum, carboxymethyl cellulose, and Polyox WSR301 at different concentrations and temperatures were investigated. These polymer solutions exhibit both shear thinning (“power‐law”) and elastic characteristics under steady and oscillatory measurements even at polymer concentrations as low as 50 ppm. The temperature and concentration superposition principles using reduced variables are applicable to both steady and dynamic shear data. The steady‐shear viscosity data are fitted with the Carreau viscosity equation. The dynamic results agree qualitatively with the trend predicted by the Zimm molecular model. However, the longest relaxation time evaluated from the dynamic flow curve is an order of magnitude longer than the predicted value from the Zimm theory. The disagreement could be attributed to the differences in the molecular weight distribution or polydispersity, ionic interaction, and solvent chemistry. The relatio...

Cyclodextrin-assisted assembly of stimuli-responsive polymers in aqueous media

This review explores the advances in stimuli-responsive polymeric inclusion complexes, comprising of cyclodextrins, in aqueous media. Cyclodextrin complexation allows these polymeric systems to possess tailored physical properties that permit them to form novel supramolecular structures. By including and combining stimuli-responsive characteristics, these special systems can transform into various morphologies when exposed to different kinds of external stimuli. A wide variety of these stimuli-responsive polyrotaxanes and functionalized cyclodextrin polymers that possess interesting supramolecular characteristics are discussed, with specific focus on pH-, temperature-, photo- and redox- sensitive systems. These unusual polymer/cyclodextrin systems provide a basis for many useful applications, such as drug delivery, environmentally friendly coatings, personal home care products, separation processes, food processing, and microelectronics.

Rheological properties of hydrophobically modified alkali‐soluble polymers—effects of ethylene–oxide chain length

The rheological properties of hydrophobic alkali-soluble associative polymers (HASE) were studied using controlled rate (Mettler LS40) and controlled stress (TA CSL 500) rheometers. The effects of pH and polymer concentrations on the rheological properties of three HASE model polymer systems (i.e., HASE 5141, 5134, and 5142, with a degree of ethoxylation of 2.5, 10, and 40 mol, respectively) and a reference polymer without associative hydrophobes (MAAEA) were examined. As the pH is increased by addition of ammonia to greater than 5–6, the carboxyl groups ionize to carboxylate ions and the polymers become water soluble. The HASE polymers thicken mainly by hydrophobic association. Viscosity can increase by two to three orders of magnitude as pH is raised to 9. The degree of ethoxylation in the macromonomer controls the nature of the hydrophobic association junctions by altering the flexibility and hydrophobicity of the macromonomer. Optimum thickening efficiency is observed in the system with approximately 10 mol of an ethylene–oxide spacer between the polymer backbone and the macromonomer. Viscoelastic study shows that the maximum thickening efficiency also corresponds to the dominant elastic property observed in the system with 10 mol of EO. All the model systems except the control system without hydrophobe exhibit strain thickening of the viscous and elastic components.