Ionic strength/temperature-induced gelation of aqueous poly(N-isopropylacrylamide-co-vinylimidazole) solution

A high molecular weight copolymer of N-isopropylacrylamide (NiPAAm) and vinyl imidazole (VI) was synthesized and its phase transition behavior in aqueous solutions (5 wt%) by simultaneous changes of ionic strength and temperature was investigated. At low ionic strengths, the copolymer solution showed two phases (clear and opaque solutions), which were freely mobile, as increasing temperatures up to 65°C due to repulsion interaction of positive charges developed by basic imidazole group on the polymer aggregates. However, at the physiological condition (I=0.15, T=37°C), four distinctive phases (clear solution, opaque solution, gel and shrunken gel) were observed because of charge shielding effect by added salts. The gel state was stable and maintained from 32°C to 55°C. In particular, the phase transition from opaque solution to gel rapidly occurred by the change in ionic strength (from ∼ 0 to 0.15) at 37°C. This characteristic can be utilized as a liquid embolic agent.     

Preparation and properties of chitosan‐poly(N‐isopropylacrylamide) semi‐IPN hydrogels

Chitosan (CS), CS‐poly(N‐isopropylacrylamide)(PNIPAM) and their dyed (pyrene) hydrogels were prepared using glutaraldehyde (Glu) as a crosslinker. The gelation rate, swelling behaviors in ethanol/water mixtures, electricity‐induced contraction and thermoresponse of the gels were investigated using fluorescence probe technique. Results showed that CS/Glu, and PNIPAM‐containing CS/Glu gels exhibited similar properties in all aspects examined, except that the transparence of the CS‐PNIPAM/Glu gel is very dependent upon the temperature. The CS‐PNIPAM/Glu gel is transparent below 30°C, whereas opaque above 32°C. It is expected that this observation may be useful for the design and preparation of new kinds of hydrogel devices. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 474–481, 2000     

Cephalexin synthesis using immobilizedXanthomonas citri cells

For continuous production of cephalexin, whole cells ofXanthomonas citri were immobilized by entrapment in polyacrylamide gel and kappa-carrageenan gel. It wasfound that cells immobilized with kappa-carrageenan showed better thermal stability compared to those immobilized by polyacrylamide gel. The cells immobilized with kappa-carrageenan were treated with glutaraldehyde and hexamethylenediamine to prevent gel destruction during prolonged operation. By immobilizing intact cells, the optimal temperature for the synthetic enzyme reaction shifted higher by 8°C and the optimal pH became broader around 6.2 In continuous operation, the immobilized cells retained better operational stability at 25 than at 37°C, and also showed maximal conversion up to 83% at 25°C.     

Fast responsive poly(<Emphasis Type="Italic">N,N</Emphasis>-diethylacrylamide) hydrogels with interconnected microspheres and bi-continuous structures

We prepared thermo-responsive polymer hydrogels by γ-ray irradiation of aqueous solutions of N, N-diethylacrylamide at different temperatures below and above its lower critical solution temperature (LCST). Poly(N, N-diethylacrylamide) gel had a transparent and homogeneous structure when the radiation-induced polymerization and crosslinking were carried out below the LCST (25 °C) of the polymer. On the other hand, cloudy and heterogeneous gels were formed at temperatures above the LCST of the polymer (>35 °C). From environmental scanning electron microscopy observations, the gels prepared at 35 and 40 °C were seen to show sponge-like bi-continuous porous structures, while those prepared at 50 °C showed a porous structure consisting of interconnected microspheres. For temperature changes between 10 and 40 °C, gels with porous structures showed rapid volume transitions on a time scale of about a minute, not only for shrinking but also for swelling processes, which is in remarkable contrast to the porous poly(N-isopropylacrylamide) hydrogels.     

How xerogel carbonization conditions affect the reactivity of highly disperse SiO<Subscript>2</Subscript>–C composites in the sol–gel synthesis of nanocrystalline silicon carbide

A transparent silicon polymer gel was prepared by sol–gel technology to serve as the base in the preparation of highly disperse SiO₂–C composites at various temperatures (400, 600, 800, and 1000°C) and various exposure times (1, 3, and 6 h) via pyrolysis under a dynamic vacuum (at residual pressures of ~1 × 10^–1 to 1 × 10^–2 mmHg). These composites were X-ray amorphous; their thermal behavior in flowing air in the range 20–1200°C was studied. The encapsulation of nascent carbon, which kept it from oxidizing in air and reduced the reactivity of the system in SiC synthesis, was enhanced as the carbonization temperature and exposure time increased. How xerogel carbonization conditions affect the micro- and mesostructure of the xerogel was studied by ultra-small-angle neutron scattering (USANS). Both the carbonization temperature and the exposure time were found to considerably influence structure formation in highly disperse SiO₂–C composites. Dynamic DSC/DTA/TG experiments in an inert gas flow showed that the increasing xerogel pyrolysis temperatures significantly reduced silicon carbide yields upon subsequent heating of SiO₂–C systems to 1500°C, from 35–39 (400°C) to 10–21% (1000°C).     

Network parameters of poly(N‐isopropylacrylamide)/montmorillonite hydrogels: effects of accelerator and clay content

The results of network parameters of poly(N‐isopropylacrylamide)/montmorillonite (PNIPAAm/MMT) hydrogels prepared with two different types and concentrations of accelerators (tetramethylethylenediamine, TEMED and quaternary TEMED containing decyl bromide, QTEMED) using three different clay contents (1.0, 3.0, and 5.0 wt% of total monomer concentration) at two different polymerization temperatures (10 and 32°C) were presented and discussed. It was observed that when the reaction temperature of PNIPAAm crosslinked with MMT was chosen as 10°C both volume swelling ratios (i.e., thermodynamic interaction) and shear moduli, G increased. In hydrogels, thermodynamic interaction is reflected by polymer–water interaction parameter (χ). The lower is the value of χ, the stronger is the interaction between polymer and water, and the lower is the interaction between hydrophobic groups or between polymer chains. In addition, increase in MMT content was slightly affected on these intermolecular interactions. Therefore, this effect that combines high‐shear modulus with lower value of χ in the same molecular structure was defined as clay content‐independent diffusion behavior. On the other hand, in the case of the hydrogels synthesized with the higher concentration of TEMED and MMT, the values of χ and G decreased. These facts were interpreted that increase in both accelerator concentration and clay content, for fixed NIPAAm concentration made the number of effective crosslinks between MMT layers smaller. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of sodium poly(styrene sulfonate) on thermoreversible gelation of gelatin

The effect of an added polyanion, sodium poly(styrene sulfonate) (NaPSS), on the thermoreversible gelation and remelting of gelatin gels has been investigated by polarimetry and rheology. The presence of NaPSS can either enhance or reduce collagenlike helix formation, depending on the polymer concentration relative to that of gelatin and the gelation temperature. At temperatures < 20°C, the helical content is reduced by increasing the amount of added NaPSS, demonstrating the disruption of helical structure of gelatin by the polyanion. Synchronous measurements of optical rotation and modulus at 25°C, in both gelation and remelting, indicate that the optical rotation at the gel point for the pure gelatin is lowered on addition of NaPSS. At low frequency, the storage modulus of gelatin is increased by the addition of a small amount of NaPSS relative to that of gelatin, but decreased with excess NaPSS. The mechanical properties of gelatin with and without NaPSS will be discussed in light of the competition between network junction formation by strands of triple helices among gelatin chains and temporary ionic crosslinking between gelatin and the polyanion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2287–2295, 1999     

The microfibrillar network in gels of poly(γ-benzyl-L-glutamate) in benzyl alcohol

The gelation and gel-melting phenomena in semidilute isotropic solutions of poly(γ-benzyl-L -glutamate) (PBLG) in benzyl alcohol were studied by small-angle neutron scattering measurements, using a deuterated solvent, and by cryotransmission electron microscopy. The reversible gels are formed when the solution is cooled below the gelation temperature, and the gels melt upon heating. Hysteresis, of about 15°C, is observed between gelation and melting temperatures. In the isotropic solution, PBLG exists as isolated helices. Gelation is apparent as a large increase in the intensity scattered at low angles, signifying the heterogeneous microstructure of the gel. Direct visualization by electron microscopy of vitrified gel samples shows the formation of a microfibrillar network. The dimension of the observed microfibrils is about 10 nm. Upon melting, microstructural changes appear in a temperature range of about 10°C. The unique feature of the gel melting is that initially only the intensity in the mid-angle range decreases. This is interpreted as thickening of the microstructure due to melting of the thinner microfibrils. The final stage marks the melting of the thicker microfibrils. © 1996 John Wiley & Sons, Inc.     

Thermally reversible polymer gels for biohybrid artificial pancreas

As an approach to replacing islets of Langerhans in an implanted biohybrid artificial pancreas, thermally reversible polymers based on N-isopropylacrylamide were synthesized and then evaluated as an extracellular matrix for islets in an immunoprotecting membrane pouch. A high molecular weight poly(N-isopropylacrylamide-co-acrylic acid (2 mole % in feed)) demonstrated gelation at 37°C and became a solution below 30°C. This polymer exhibited minimum syneresis (water separation) upon gelation from a solution state when the temperature was raised from room temperature to 37°C, while poly(N-isopropylacrylamide) exhibited considerable syneresis under the same conditions. These properties influence the efficiency of islet entrapment. The copolymer was able to entrap rat islets almost 100%, but the homopolymer entrapped less than 50%. The static insulin secretion of the islets in the copolymer matrix at high glucose concentration (16.5 mM) was comparable to that of control islets, however, the entrapped islets showed prolonged viability in vitro. These results indicate the potential of developing a rechargeable biohybrid pancreas using thermally reversible polymer gels.     

Mullite crystallization using fully hydrolyzed silica sol: the gelation temperature influence

Mullite is an aluminosilicate widely used as a structural material for high temperature applications. This paper studies the effect of the gelation temperature on the synthesis of two mullite precursors: polymeric and colloidal silica, using both in fully-hydrolyzed silica sol, derived from sodium silicate. The gels were synthesized using aqueous silicic acid and aluminum nitrate. Ethylene glycol was added into polymeric gels. Two gelation temperatures were used: 80 and 100 °C. In the polymeric precursor, the increasing of the gelation temperature caused an increase in the silica incorporation inside the mullite crystalline lattice at 1,000 °C, and it also generated an increase in the reaction extent at all calcination temperatures. In the colloidal precursors, these effects were more intense than in the polymeric precursors in terms of yield. Colloidal samples calcined at 1,250 °C crystallized cristobalite and alpha alumina in addition to mullite when they were previously gelled at 80 °C. On the other hand, the same sample gelled at 100 °C led to only crystallized mullite. The reaction extent increased by more than 20 % for colloidal samples gelled at 100 °C compared to colloidal samples gelled at 80 °C (calcined at 1,250 °C). This increase was due to the almost total incorporation of alumina and silica in the crystalline lattice of mullite.     

Morphology and mechanical properties of ultrahigh molecular weight polypropylene prepared by gelation/crystallization at various temperatures

Films of ultrahigh molecular weight (5.4×10⁶) polypropylene were produced by gelation/crystallization at various temperatures from dilute decalin solutions according to the method of Smith and Lemstra. The temperatures chosen were 20°, 30°, 50°, and 60°C. With increasing the temperature, the long period and crystallinity of the resultant gel film increased. By contrast, when the films were stretched up to 50 } 60 times, the increases in Young's modulus and crystallinity become more significant, as the temperature of the gelation/crystallization became lower. This interesting phenomenon is thought to be due to the dependence of the number of entanglements on the temperatures concerning gelation/crystallization and evaporation of solvent from the gel to form a film.     

Gelation kinetics of PAM/PEI system

Excess water production in oil and gas wells causes serious productivity and environmental problems in the oilfield. A mixture composed of a polymer, cross-linker, and water is usually injected into the reservoir to block unwanted water by forming a three-dimensional structure. This transition process from solution to gel is a function of temperature, time, salinity of mixing water, and concentration of the various components. The gelling solution was prepared by mixing polyacrylamide (PAM) with distilled water, and then polyethylenimine (PEI) was added as a cross-linker. The injection process was simulated and investigated by differential scanning calorimeter (DSC) over the temperature range of 80–120 °C. The DSC dynamic scan showed two consecutive peaks. An endothermic peak was observed at low temperature due to PAM alkaline hydrolysis which ends at around 60 °C. Another exotherm was observed at ~70 °C which corresponds to the onset of cross-linking of PAM and PEI. It was found that high temperatures lead to high release of heat due to gelation. The effect of salts on the cross-linking was also examined. More delay in cross-linking was observed in the case of NH₄Cl compared to NaCl. The gelation kinetics was modeled using a rate process model that relates fractional gelation with time. Further, Avrami model, usually used to study crystallization kinetics, was also used to model the gelation process. Kinetic parameters were obtained from the two different models, and the results showed good agreement with experimental data. The presence of salts in seawater leads to a drop of 60–80 % in the rate constant without influencing the order of the gelation reaction.     

Study on formation of gels formed by polymer and zirconium acetate

The gel system used in the preparation of dispersed particle gel for water shutoff treatments, which is composed of polyacrylamides and zirconium acetate, was investigated. The gelation process, the effects of various parameters on the gelation properties, the thermal stability, and the microstructure were addressed. The cross-linking reaction process is divided into three successive steps: induction, rapid cross-linking, and stabilization. High polymer and crosslinker concentrations reduce gelation time and increase gel strength. In addition, adding salts to the brine or increasing the temperature also decrease gelation time and increase gel strength. The optimum pH for the gel system is 7.49. In field applications, this gel system is recommended to be used within 130 °C using differential scanning calorimetry. The gel formed in a three-dimensional network structure was confirmed through environmental scanning electron microscopy.     

Improving the scratch resistance of sol–gel metal oxide coatings cured at 250 °C through use of thermogenerated amines

Scratch resistant sol–gel metal oxide coatings typically require a thermal post-treatment step (curing process) at temperatures between 400 and 700 °C. In this report, we demonstrate that the in situ generation of amines within sol–gel films facilitates the preparation of scratch resistant metal oxide coatings at a curing temperature of 250 °C. A selected series of carbamates was added to sol–gel formulations and included in the resulting xerogel films. During the curing process, the carbamates decomposed to liberate primary amines. These catalysed the curing process, finally leading to scratch resistant metal oxide coatings at low cure temperature.     

Evolution of water vapor from indium-tin-oxide transparent conducting films fabricated by dip coating process

Tin-doped indium oxide In₂O₃ (indium-tin-oxide) transparent conducting films were fabricated on silicon substrates by a dip coating process. The thermal analysis of the ITO films was executed by temperature-programmed desorption (TPD) or thermal desorption spectroscopy (TDS) in high vacuum. Gas evolution from the ITO film mainly consisted of water vapor. The total amount of evolved water vapor increased on increasing the film thickness from approx. 25 to 250 nm and decreased by increasing the preparation temperature from 365 to 600°C and by annealing at the same temperature for extra 10 h. The evolution occurred via two steps; the peak temperatures for 250 nm thick films were approx. 100-120 and 205-215°C. The 25 nm thick films evolved water vapor at much higher temperatures; a shoulder at approx. 150-165°C and a peak at approx. 242°C were observed. The evolution temperatures increased by increasing the preparation and the annealing temperatures except in case of the second peak of the 25 nm thick films. The evolution of water vapor at high temperature was tentatively attributed to thermal decomposition of indium hydroxide, In(OH)₃, formed on the surface of the nm-sized ITO particles. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stimuli‐responsive ABC triblock copolymers by sequential living cationic copolymerization: Multistage self‐assemblies through micellization to open association

Stimuli‐responsive ABC triblock copolymers with three segments with different phase‐separation temperatures were synthesized via sequential living cationic copolymerization. The triblock copolymers exhibited sensitive thermally induced physical gelation (open association) through the formation of micelles. For example, an aqueous solution of EOVE₂₀₀‐b‐MOVE₂₀₀‐b‐EOEOVE₂₀₀ [where EOVE is 2‐ethoxyethyl vinyl ether, MOVE is 2‐methoxethyl vinyl ether and EOEOVE is 2‐(2‐ethoxy)ethoxyethyl vinyl ether; the order of the phase‐separation temperatures was poly(EOVE) (20 °C) < poly(EOEOVE) (41 °C) < poly(MOVE) (70 °C)] underwent multiple reversible transitions from sol (<20 °C) to micellization (20–41 °C) to physical gelation (physical crosslinking, 41–64 °C) and, finally, to precipitation (>64 °C). At 41–64 °C, the physical gel became stiffer than similar diblock or ABA triblock copolymers of the same molecular weight. Furthermore, the ABC triblock copolymers exhibited Weissenberg effects in semidilute aqueous solutions. In sharp contrast, another ABC triblock copolymer with a different arrangement, EOVE₂₀₀‐b‐EOEOVE₂₀₀‐b‐MOVE₂₀₀, scarcely exhibited any increase in viscosity above 41 °C. The temperatures of micelle formation and physical gelation corresponded to the phase‐separation temperatures of the segment types in the ABC triblock copolymer. No second‐stage association was observed for AB and ABA block copolymers with the same thermosensitive segments found in their ABC counterparts. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2601–2611, 2004     

Thermosensitive phase behavior and drug release of in situ gelable poly(N-isopropylacrylamide-co-acrylamide) microgels

In situ gelable poly(N-isopropylacrylamide-co-acrylamide) microgels were prepared by precipitation polymerization in the presence of various amounts of N,N′-methlenebisacrylamide as a crosslinker. The diameters of microgels were in the range of 200–300 nm with narrow distributions as determined by photo correlation spectroscopy. The equilibrium swelling ratio and thermosensitive properties of the microgels increased with decreasing crosslinker content. The volume phase transition of microgels dispersions at high concentrations were investigated by phase diagrams. The microgels dispersions experienced four phases when the temperature was increased: semitranslucent swollen gel, clear flowable suspension, cloud flowable suspension, and white shrunken gel. The related phase transition temperatures were influenced by crosslinker content and the concentration of the microgel dispersions. Herein, the gelation temperature was changed by more than 20 °C, shrinking temperatures were slightly changed by about 3 °C, and cloud point temperatures showed almost no change. The three phase transition temperatures of microgels dispersed in phosphate-buffered saline solutions were lower than that in water. As drug carriers, the release rates of bleomycin from bleomycin-loaded microgel dispersions exhibited diffusion control at human body temperature.     

Preparation and properties of N-phenylated aromatic polyamides from N,N′-diphenyl-p-phenylenediamine and aromatic diacid chlorides

N-Phenylated aromatic polyamides and copolyamides derived from N,N′-diphenyl-p-phenylenediamine, isophthaloyl, and terephthaloyl chloride were prepared by high-temperature solution polycondensation in anisole at 155°C. Factors that influenced the reaction, such as monomer concentration, solvent, temperature, and time, were studied to determine the optimum conditions for the preparation of high molecular weight polymers. Compared with analogous unsubstituted aromatic polyamides, the N-phenylated polymers exhibited better solubility in chlorinated and amide solvents, reduced crystallinity, and lower glass transition temperatures (above 200°C). All polymers except the polyterephthalamide could be solvent-cast, as well as hot-pressed, into transparent flexible films.     

Influence of TMAO as co-solvent on the gelation of silica-PNIPAm core-shell nanogels at intermediate volume fractions

We study the structure and dynamics of poly(N-isopropylacrylamide) (PNIPAm) core-shell nanogels dispersed in aqueous trimethylamine N-oxide (TMAO) solutions by means of small-angle X-ray scattering and X-ray photon correlation spectroscopy (XPCS). Upon increasing the temperature above the lower critical solution temperature of PNIPAm at 33 °C, a colloidal gel is formed as identified by an increase of I(q) at small q as well as a slowing down of sample dynamics by various orders of magnitude. With increasing TMAO concentration the gelation transition shifts linearly to lower temperatures. Above a TMAO concentration of approximately 0.40 mol/L corresponding to a 1 : 1 ratio of TMAO and NIPAm groups, collapsed PNIPAm states are found for all temperatures without any gelation transition. This suggests that reduction of PNIPAm-water hydrogen bonds due to the presence of TMAO results in a stabilisation of the collapsed PNIPAm state and suppresses gelation of the nanogel.     

Improving functional properties of ZnO nanostructures by transition-metal doping: role of aspect ratio

Localized surface plasmon resonance (LSPR) of gold nanoparticles (AuNPs) has been used for biosensing and chemical sensing applications because the LSPR peak wavelength depends on the dispersion state and local refractive index of the surrounding medium. In this study, AuNP-loaded silica gels were prepared as sensing chips with high transparency and solution holding capability. The silica gels were prepared at various sintering temperatures from 500 to 900 °C, and the AuNPs precipitated in the gels by using a subsequent thermal reduction process. At sintering temperatures of 700, 800, and 900 °C, transparent and crack-free AuNP-loaded silica gels were obtained. Transmission electron microscopy observation revealed the AuNP size to be approximately 20 nm, and they were highly dispersed in all the silica gel samples. However, the sintering temperature of the silica gels strongly affected the LSPR property of the AuNPs and the porous property of the silica gel. The samples sintered at higher temperature exhibited a lower LSPR sensing ability against the refractive index of immersing solvents. The low sensing ability was considered as a result of a decrease in the contact area between the AuNPs and immersing solvent caused by an increase in the silica gel density with sintering temperature.