General approach to low-molecular-weight metallogelators via the coordination-induced gelation of an L-glutamate-based lipid

A twin-tailed glutamate-based lipid with a pyridine headgroup was prepared in good yield using standard amide coupling and protection/deprotection chemistry. The resulting Lewis basic lipid gels a wide array of hydrocarbon solvents at a critical gelation concentration (C(g)) of 0.3 wt %. The gelation of more polar solvents, such as ethanol, THF, dichloromethane, and chloroform, occurs with a C(g) of between 2 and 5 wt %, demonstrating the importance of hydrogen bonding interactions in gel formation. The importance of hydrogen bonding in this system was also demonstrated by IR observation of the amide bands, which show a substantial shift upon gelation. Solutions of this new organogelator with concentrations below C(g) rapidly form gels upon the introduction of a wide variety of metal salts or complexes, providing a convenient general method for the preparation of metallogelators. Spectroscopic evidence suggests that the enhanced gelation seen in the metal-containing systems can be explained by a cross-linking of gel fibril aggregates similar to those formed by the unmetalated gelator.     

Architecture of intracellular networks in plant matrices

Pectin is an integral component of plant cell walls. It is believed to form an interconnected network structure independent of the cellulose–xyloglucan network structure. Pectin gels are often used as a model for the pectin network structure within the plant cell wall. The middle lamella pectin can be extracted with chelating agents and is believed to be associated through cooperative binding of calcium ions in the so-called egg-box junction zones. Although a great deal is known about the nature of the junction zones in pectin gels, less is known about the long-range structure within calcium-set gels. Two plausible alternative models for long-range order in these gels are a pseudo rubber-like structure and a fibrous network structure. Atomic force microscopy studies of calcium-induced gel precursors, and fragments released from gels, suggest that association leads to a branched fibrous structure within the gels. Enzymatic de-esterification of high methoxy pectin in the presence of calcium ions can induce gelation of the pectin. Thus pectin gel networks may provide a model for a self-assembled network structure within the middle lamella region of the plant cell wall.     

Effect of deacetylation rate on gelation kinetics of konjac glucomannan

Effect of deacetylation rate on the gelation behaviors on addition of sodium carbonate for native and acetylated konjac glucomannan (KGM) samples with a degree of acetylation (DA) range of 1.38-10.1 wt.% synthesized using acetic anhydride in the presence of pyridine as catalyst was studied by dynamic viscoelastic measurements. At a fixed alkaline concentration (C(Na)), both the critical gelation times (t(cr)) and the plateau values of storage moduli (G'(sat)) of the KGM gels increased with increasing DA. While at a fixed ratio of alkaline concentrations to values of DA (C(Na)/DA), the similar t(cr) and (G'(sat)) values independent of DA were observed. On the whole, increasing KGM concentration or temperature shortened the gelation time and enhanced the elastic modulus for KGM gel. The effect of deacetylation rate related to the C(Na)/DA on the gelation kinetics of the KGM samples were discussed.     

Morphology control of liquid crystalline composite gels based on molecular self-assembling kinetics

Liquid crystalline composite gels consisting of a low molecular mass gelator and a low molecular mass liquid crystal were prepared by two types of gelation method (continuous cooling and isothermal gelating), which provide different molecular self-assembling kinetics of the low molecular mass gelator as gelation proceeds. Optical microscopy and atomic force microscopy revealed that numerous fine strands of the one-dimensionally assembled low molecular mass gelators were formed in the composite gels for both the continuous cooling method and the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the isothermal gelation product at an appropriate temperature, than in the continuous cooling process. This difference in dispersion state of the strands was shown (by polarizing optical microscopy) to have a significant influence on the molecular alignment of the low molecular mass liquid crystal in the liquid crystalline composite gel. The electro-optical response and light scattering-transmitting switching, of the liquid crystalline composite gel in an applied electric field was extremely dependent on the morphology of the gelators. High contrast light switching was achieved for the composite prepared by isothermal gelation. The response time of electro-optical switching was less than 100 µs under 30 V_rms.     

Morphology control of liquid crystalline composite gels based on molecular self-assembling kinetics

Liquid crystalline composite gels consisting of a low molecular mass gelator and a low molecular mass liquid crystal were prepared by two types of gelation method (continuous cooling and isothermal gelating), which provide different molecular self-assembling kinetics of the low molecular mass gelator as gelation proceeds. Optical microscopy and atomic force microscopy revealed that numerous fine strands of the one-dimensionally assembled low molecular mass gelators were formed in the composite gels for both the continuous cooling method and the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the isothermal gelation product at an appropriate temperature, than in the continuous cooling process. This difference in dispersion state of the strands was shown (by polarizing optical microscopy) to have a significant influence on the molecular alignment of the low molecular mass liquid crystal in the liquid crystalline composite gel. The electro-optical response and light scattering–transmitting switching, of the liquid crystalline composite gel in an applied electric field was extremely dependent on the morphology of the gelators. High contrast light switching was achieved for the composite prepared by isothermal gelation. The response time of electro-optical switching was less than 100?µs under 30?V_rms.     

Thermal gelation of cellulose in a NaOH/thiourea aqueous solution

Utilizing a novel solvent of cellulose, 6 wt % NaOH/5 wt % thiourea aqueous solution, for the first time, we prepared the thermally induced cellulose gel. We investigated the thermal gelation of cellulose solutions with rheometry and the structure of the gel with 13C NMR, wide-angle X-ray diffraction, environmental scanning electron microscopy, and atomic force microscopy. The cellulose solutions revealed an increase in both the storage modulus (G') and the loss modulus (G") with an increase in the temperature during gelation. The temperature at the turning point, where G' overrides G" because of the onset of gelation, decreased from 38.6 to 20.1 degrees C with an increase of cellulose concentration from 4 to 6 wt %. Given enough time, G' of all solutions can exceed G" at a certain temperature slightly lower than the gelation temperature, indicating that the occurrence of the gelation is also a function of time. Each of the assigned peaks of NMR of the cellulose gel is similar to that of the cellulose solution, suggesting that the gelation resulted from a physical cross-linking. The gels were composed of relatively stable network units with an average diameter of about 47 nm. At either a higher temperature (at 60 degrees C for 30 s) or a longer gelation time (at 30 degrees C for 157 s), the gel in the 5 wt % cellulose solution could form. A schematic gelation process was proposed to illustrate the sol-gel transition: the random self-association of the cellulose chains having the exposed hydroxyl in the aqueous solution promotes the physical cross-linking networks.     

EFFECTS OF GELATION RETARDATION ON THERMAL RESPONSE BEHAVIOR OF PNIAPM GELS： A METHOD TO IMPROVE THEIR DESWELLING KINETICS

P(N-isopropylacrylamide)(PNIPAM)prepared by reversible addition fragmentation chain transfer(RAFT) polymerization exhibited gelation retardation.The intermediate before gelation was characterized and indicated the presence of branched or hyperbranched chains.The swelling behavior was investigated,and the gel by RAFT polymerization(RAFT gel)showed accelerated shrinking kinetics and higher swelling ratio comparing with conventional gels(CG).The study was extended to gels prepared by using 2-hydroxy-1-ethanethiol as chain transfer agent and by using low concentration solutions. The two systems also exhibited retardation effects and improved deswelling kinetics.The different swelling behaviors of these gels and CG could be attributed to the presence of dangling chains caused by gelation retardation.     

Gelation of PEO-PLGA-PEO triblock copolymers induced by macroscopic phase separation

The gelation behavior of aqueous solutions of poly(ethylene oxide-b-(DL-lactic acid-co-glycolic acid)-b-ethylene oxide) (PEO-PLGA-PEO) triblock copolymer containing short hydrophilic PEO end blocks is investigated using dynamic light scattering, rheology, small-angle neutron scattering (SANS), and differential scanning calorimetry (DSC). For polymer concentrations between 5 and 35 wt %, four distinct regions of the turbidity change depending on temperature were observed. Interestingly, in the turbid solution region, gel phase is formed for polymer concentrations above 14 wt % and an extremely slow relaxation was detected. In fact, a power law, which takes into account the dynamics of percolation clusters, dominates the correlation function. In rheological measurements, the local maximum in G' is observed at around the temperature of maximum turbidity. We further found that G" > G' and G' is highly dependent on frequency at the gel state implying viscoelastic characteristics, which is quite different from general concepts of gels, typically formed by the micellar packing. SANS profiles showing multiple peaks in the sol state rather than in the gel state as well as a DSC exotherm at the temperature of gels can also serve as the evidence of different gel states. Based upon the experimental data obtained in the present study, a new gelation mechanism induced by the macroscopic phase separation of triblock copolymers containing short hydrophilic PEO end blocks such as PEO-PLGA-PEO is proposed. The effect of the type ofhydrophobic middle blocks on the gelation is also discussed.     

In situ organically cross‐linked polymer gel for high‐temperature reservoir conformance control: A review

Polymer gel has been established as water‐blocking agents in oil recovery application. In this practice, a mixture known as gelant is injected into target area and set into a semisolid gel after a certain adequate time. Besides profile modification and water shutoff, the role of the polymer gel in conformance control is to block high permeability regions, before diverting injected water from high permeability to low permeability zones of the reservoir. It is to boost the oil displacement and sweep efficiency. This is the key to improve oil recovery in the heterogeneous oil reservoirs. However, very limited gels are applicable for harsh conditions, especially in high‐temperature reservoirs. Organically cross‐linked polymer is 1 of the materials for conformance control at high‐temperature reservoirs. Many experimental works and field applications have exhibited the potential of this technology. This paper presents a concise review on this polymer gel for conformance control at high‐temperature wells. Firstly, in situ organically cross‐linked polymer gel has been introduced, and the reason of the use over other types of polymer gels is summarized. The early studies of organically cross‐linked gel systems are also discussed, followed by the chemistry and the gelation mechanisms. An extensive review on factors that affect gelation kinetics and field applications is also discussed in some detail.     

Amylose gelation in low-concentration conditions

Amylose gels of low concentration (1–2 wt.-%) were studied by means of a rheological method and turbidity measurements. The effects of temperature and concentration were investigated. The kinetics of turbidity evolution and of the storage modulus G' variations were strongly dependent of both parameters. The overall gelation process could be described in three steps: first, an induction period, followed by a sharp variation of G' and turbidity, and finally a slow variation of G'. These observations confirm that amylose gelation proceeds through a phase separation process.     

Biocatalytic Gelation of Silica in the Presence of a Lipase

The effect of the lipase from Burkholderia cepacia (previously known as Pseudomonas cepacia) on the gelation kinetics and gel structure was examined on a type of silica aerogel made from a mixture of methyltrimethoxysilane and tetramethoxysilane. For this purpose, gels were made with increasing concentrations of lipase in otherwise constant other conditions (pH, water and Si precursors concentrations). It was found that the enzyme accelerated the gelation kinetics, hence was participating in some way to the hydrolysis of the silica precursor. The structure of the gel was simultaneously modified to produce an increasing proportion of Q⁴ silicon sites.     

酶凝干酪素物理凝胶化过程的有限元模拟

以酶凝干酪素的凝胶化过程为对象,利用有限元方法数值分析了在凝胶化过程中温度场的空间分布和时间演变规律.在此基础上,基于一阶的凝胶化动力学方程,数值模拟了凝胶体系的复剪切模量场,进而分析了材料配方、体系尺寸与冷却方案对复剪切模量场的影响规律.模拟结果表明,由于热阻的差异,体系表面的冷却速率大于内部,表面首先发生凝胶化;而由于预凝胶化阶段的平均冷却速率决定了无穷复剪切模量的值,最终体系内部的复剪切模量超过表面的.     

Kinetics of rennet casein gelation at different cooling rates

A mathematical model was developed to quantitatively analyze the rheological data of rennet casein gelation at different cooling rates. Kinetic parameters were estimated and correlated with the microstructure development of the protein network. The kinetic model identified structure development upon cooling to be first order, and the network forming energies were estimated for four protein concentrations cooled at four rates. A lower energy for network formation was observed for a slower cooling rate and a higher protein concentration. This observation resulted from the availability of more flocs at a slower cooling rate and a higher casein concentration, simplifying floc cross-linking. By analyzing the kinetics during the aging process of casein gels, no difference in the reaction mechanism was observed. This study illustrated that structure formation resulted from the addition of flocs into the protein network: not all flocs were part of the network at a defined gel point. The incubation period following cooling integrated idle flocs into the network, thereby strengthening the gel. By understanding the gelation mechanism during cooling of rennet casein gels, the structure and thus quality of dairy products, such as processed cheese, may be better controlled.     

Rheological characterization, crystallization, and gelation behavior of monoglyceride gels

Saturated monoglycerides can form firm gels in water. These gels are networks of stiff plate-like beta-crystals of monoglycerides (a "cardhouse"), grown from a space-filling lamellar liquid-crystalline phase. The molecular mechanism of crystallization is discussed in the light of network formation. The concentration dependence of gel development of (shear-cooled) monoglyceride gels has been studied by rheology. A gelation mechanism has been proposed, consisting of two steps: (i) After formation of a nucleus, rapid crystallization in a lateral direction occurs (probably within one bilayer) by which the first space-filling network is formed. (ii) This is followed by reinforcement of the network by which stacks of crystalline bilayers are formed. The plate-like crystals are linked in connective domains or junction zones, probably containing all the material (cosurfactants, diglycerides, etc.) that does not fit in the crystalline array. Small deformation rheology shows that above about 2 wt% monoglyceride a percolating network is formed. The large deformation rheology is typical for a particle gel with a relatively small strain at failure (both in shear deformation and compression). The connective domains or junction zones already fail when relatively small deformations are put on the system.     

Cationic temperature-responsive poly(N-isopropyl acrylamide) graft copolymers: from triggered association to gelation

In this work temperature-triggered association and gel formation within aqueous solutions of a new family of cationic poly( N-isopropyl acrylamide) (PNIPAm) graft copolymers have been investigated. Five copolymers were synthesized using aqueous atom transfer radical polymerization (ATRP) involving a macroinitiator based on quaternarized N, N-dimethylaminoethyl methacrylate units (DMA+). The PDMA+) x - g-(PNIPAmn)y copolymers have x and y values that originate from the macroinitiator; values for n correspond to the PNIPAm arm length. The copolymer solutions exhibited temperature-triggered formation of nanometer-sized aggregates at the cloud point temperature, which was 33-34 degrees C. The aggregates were investigated using variable-temperature turbidity, hydrodynamic diameter, and electrophoretic mobility measurements. The aggregates were clearly evident using SEM and flowerlike or spherical morphologies were observed. Variable-temperature electrophoretic mobility measurements revealed that the zeta potentials of the aggregates increased with DMA+ content. A study of the effect of added NaNO3 showed that electrostatic interactions controlled the size of the aggregates. The concentrated graft copolymer solutions showed temperature-triggered gelation when the copolymer concentrations exceeded 5 wt %, Fluid-to-gel phase diagrams were constructed. It was found that electrostatic interactions also controlled the gelation temperature. A correlation was found between aggregate size and the minimum copolymer concentration needed to form a gel. A mechanism for the temperature-triggered structural changes leading to the formation of aggregates (in dilute solution) or gels (in concentrated solutions) is proposed.     

Effect of thermal history on kappa-carrageenan hydrogelation by differential scanning calorimetry

The effect of thermal history on gel-sol transition was investigated by highly sensitive differential scanning calorimetry (DSC) in order to clarify the non-equilibrium state of κ-carrageenan hydrogels. κ-Carrageenan with a concentration from 0.5% to 5.0% was used. When concentration of solution was lower than 2.0%, homogeneous κ-carrageenan gel was formed when aqueous solution was fully equilibrated. When concentration exceeded 3.0%, a sub-peak could be observed at the low temperature side of the main peak. It was indicated that helices having various sizes and different kinds of defects are present in the junction zone. Thermal histories, such as cooling rate from the sol state or annealing at around gel-sol transition temperature, markedly affect the junction zone formation. A large junction zone is formed at a slow gelation rate, and also that many small junction zones form at a fast gelation speed.     

部分水解聚丙烯酰胺弱凝胶交联动力学的流变学分析

用流变学方法对部分水解聚丙烯酰胺(HPAM)苯酚甲醛间苯二酚交联体系的弱凝胶化过程进行了研究,通过对基团转化率和高分子交联转化率的分析,发现凝胶化过程在接近凝胶点时,处于反应动力学的初期,这使得交联点增加的动力学是比较简单的.通过在不同聚合物浓度和交联剂浓度并在地层温度和矿化度条件下线性粘弹性行为的研究,得到了交联体系凝胶化动力学过程的完整数据,发现聚合物浓度与交联剂浓度对凝胶点与凝胶强度的影响比较类似,反映出交联点增加的共同动力学特征.复数粘度在一个诱导期后,是以指数上升的,类似一个一级反应的特征.产生交联的临界聚合物浓度约为250mg L左右.并提出了剪切粘度数学模型,可以描述凝胶化过程中流变性质的变化.     

Oil in water emulsions stabilized by maillard conjugates of sodium caseinate-locust bean gum

Conjugates composed by sodium caseinate and locust bean gum (LBG) were produced through Maillard reaction in order to enhance their emulsifying properties. The reactions were performed (80?°C/76% relative humidity) at three protein/polysaccharide ratios (0.3, 0.6, 1.2), and the conjugates were added in emulsions at two pH values (3.5 and 7). The formation of protein-polysaccharide conjugates was confirmed by SDS-polyacrylamide gel electrophoresis and by FTIR spectra. At both pH values, the conjugates subjected to longer reaction times presented less emulsifying capability, probably due to insoluble compounds formation. Furthermore, the conjugates provided high stability in O/W emulsions at pH 7 due to the electrostatic repulsion of protein charged groups. However, even though the conjugates were not charged at pH 3.5, the stabilization process occurred at this condition probably due to the formation of a thicker interfacial layer, which provided an increase in steric repulsion between the droplets. Thus, the conjugates formed by sodium caseinate and locust bean gum can be characterized as a good emulsifying agent to be applied in different pH conditions.     

Rheology and gelation of cellulose/ammonia/ammonium thiocyanate solutions

Liquid crystalline solutions of cellulose in an ammonia/ammonium thiocyanate solvent will form thermoreversible gels at temperatures below 30°C. These gels are of interest both for processing the cellulose/ammonia/ammonium thiocyanate system and because they have an unusual structure, containing neither crystalline nor covalently bonded crosslinks. Although these gels contain neither crystalline nor covalently bonded crosslinks, the dynamic rheological behavior of the system at the gel point was found to be the same as for gels with covalent or crystalline crosslinks with a loss tangent, tan δ, independent of frequency. The kinetics of the gelation process was monitored via dynamic elastic modulus, G′. All samples revealed an exponential increase in G′ with time during gelation, very different from that observed in covalently bonded or crystalline crosslinked systems. Measurements of the loss tangent enabled precise determination of the gelation time for these systems as a function of cellulose concentration and temperature. We found the gel time to be inversely related to cellulose concentration and directly related to temperature. The strong dependence of gel time on these parameters offers a windows of spinnability that can be tailored for processing high modulus cellulose fibers. © 1996 John Wiley & Sons, Inc.     

Gelation of perfluorinated liquids by N-alkyl perfluoroalkanamides

Gels comprised of low-molecular-mass organic gelators (LMOGs), N-alkyl perfluoroalkanamides [F(CF2)(m)CONH(CH2)(n)H; FmNHn], and several perfluorinated liquids are described. The gelation ability of the amides has been compared to that of two analogous alkyl perfluoroalkanoates. The properties of these gels have been correlated with the N-alkyl and (to a lesser extent) perfluoroalkyl chain lengths in the FmNHn by X-ray diffraction, polarizing optical microscopy, infrared spectroscopy, and small-angle neutron scattering. The gels are thermally reversible and require generally very low concentrations (<2 wt %) of LMOG. Several of the gels have been stable at room temperature for >1 year, thus far. The incompatibility of the fluorocarbon and hydrocarbon segments causes the LMOGs to aggregate into lamellae within the fibrils that constitute the basic unit of the gel networks. IR spectroscopic studies of these gels indicate that additional ordering within the aggregate units is enforced by intermolecular H-bonding among amide groups.