Novel Polyurethane Gels: The Effect of Structure on Gelation

Novel polyurethane gels have been reported in common solvent like dimethyl formamide (DMF). Polyurethanes have been synthesized from diisocyanates, diols and rigid chain extenders. We have illustrated the influence of chemical structure of the chain extenders on gelation rate, thermal property and morphology of the gels in DMF. Gelation rate increases significantly with the rigidity of the chain extender. Introduction of more rigid chain extender molecules in polyurethane prepolymer enhanced the thermal stability of the pure polymer. On the contrary, the solvent retention power of the gels gradually decreases with increasing rigidity of chain extender presumably because of the poor dispersion/greater aggregation of the hard segments in the soft segment matrix. Morphology and formation of gelation have been discussed.     

Tunable thermoassociation of binary guanosine gels

It is well-known that aqueous solutions of individual guanosine compounds can form gels through reversible self-assembly. Typically, gelation is favored at low temperature and acidic pH. We have discovered that binary mixtures of 5'-guanosine monophosphate (GMP) and guanosine (Guo) can form stable gels at neutral pH over a temperature range that can be tuned by varying the relative proportions of the hydrophobic Guo and the hydrophilic GMP in the mixture. Gelation was studied over the temperature range of 5-40 degrees C or 60 degrees C at pH 7.2 using visual detection, circular dichroism (CD) spectroscopy, and CD thermal melt experiments. Solutions with high GMP/Guo ratios behaved similar to solutions of GMP alone while solutions with low GMP/Guo formed firm gels across the entire temperature range. Most interesting were solutions between these two extremes, which were found to exhibit thermoassociative behavior; these solutions are liquid at refrigerator temperature and undergo sharp transitions to a gel only at higher temperatures. Increasing the GMP/Guo ratio and increasing the total concentration of guanosine compounds shifted the onset of gelation to higher temperatures (ranging from 20 to 40 degrees C), narrowed the temperature range of the gel phase, and sharpened the reversible phase transitions. The combination of self-assembly, reversibility, and tunability over biologically relevant temperature ranges and pH offers exciting possibilities for these simple and inexpensive materials in medical, biological, analytical, and nanotechnological applications.     

Effect of sol-gel transition on shear-induced drop deformation in aqueous mixtures of gellan and kappa-carrageenan

In this work, we present an experimental methodology to investigate the dynamics under shear flow of a drop that is gelling as a consequence of a temperature quench. The experiments were carried out on the system water/gellan/kappa-carrageenan in the biphasic region of the phase diagram, the gellan-rich phase being used as the dispersed phase. Gelation was brought about by lowering the temperature during flow after steady state drop deformation had been reached. Simple shear flow was applied by using a parallel plate apparatus equipped with optical microscopy and image analysis, which made it possible to monitor drop shape evolution before, during, and after gelation. The onset of gelation trapped drop deformation, thus producing anisotropic particles. The fingerprint of gelation was the simultaneous tumbling of the drops, which rotated as rigid ellipsoids under the action of shear flow. Interfacial tension between the two equilibrium phases was determined at different times during the temperature quench by analyzing drop retraction upon cessation of flow. Up to gelation, no significant change was observed in the measured values.     

Protein aggregation and gel formation studied with scattering methods and computer simulations

We review recent scattering experiments on the reversible and irreversible aggregation and gelation of globular proteins. Globular proteins are compact with a well-defined structure in the native state, but are not perfect spheres and the details of the interaction between proteins may be intricate involving for instance specific binding sites. Nevertheless, it turns out that often the structure of globular protein aggregates and gels can be understood by treating them as spherical colloids with relatively simple interaction potentials. Computer simulations of interacting hard spheres give detailed structural information that can be used to understand some aspects of protein aggregation and gel formation.     

Polymer-solvent co-crystallization during thermoreversible gelation in solutions of the helical polypeptide PBLG

Macromolecular aggregation during thermoreversible gelation in solutions of the helical polypeptide poly(γ-benzyl-L-glutamate) [PBLG] in benzyl alcohol [BA] were studied by small angle neutron and small angle X-ray scattering. Gelation is apparent as a large increase in the intensity scattered at low angles, signifying formation of a microfibrillar PBLG network. The aggregated phase in isotropic gels from semidilute solutions contains about 28% solvent. A periodic structure is observed when gelation is induced by rapid cooling to a low temperature, but not by slow cooling or gelation at a higher temperature. In gels from concentrated liquid crystal solutions, two crystalline structures are observed, depending on whether the solution is rapidly quenched and then annealed or slowly gelled at an elevated temperature. A phase diagram for the PBLG/BA system is presented and the observed microstructural transitions are rationalized in terms of a gelation mechanism involving a combination of liquid-liquid phase separation and crystallization in the form of polymer-solvent co-crystals.     

Analysis of the aggregation-fragmentation population balance equation with application to coagulation

Coagulation of small particles in agitated suspensions is governed by aggregation and breakage. These two processes control the time evolution of the cluster mass distribution (CMD) which is described through a population balance equation (PBE). In this work, a PBE model that includes an aggregation rate function, which is a superposition of Brownian and flow induced aggregation, and a power law breakage rate function is investigated. Both rate functions are formulated assuming the clusters are fractals. Further, two modes of breakage are considered: in the fragmentation mode a particles splits into w2 fragments of equal size, and in the erosion mode a particle splits into two fragments of different size. The scaling theory of the aggregation-breakage PBE is revised which leads to the result that under the negligence of Brownian aggregation the steady state CMD is self-similar with respect to a non-dimensional breakage coefficient theta. The self-similarity is confirmed by solving the PBE numerically. The self-similar CMD is found to deviate significantly from a log-normal distribution, and in the case of erosion it exhibits traces of multimodality. The model is compared to experimental data for the coagulation of a polystyrene latex. It is revealed that the model is not flexible enough to describe coagulation over an extended range of operation conditions with a unique set of parameters. In particular, it cannot predict the correct behavior for both a variation in the solid volume fraction of the suspension and in the agitation rate (shear rate).     

Systematic design of profile control chemistry. Control of gelation of mixed poly(vinyl alcohol-co-vinylamine) crosslinked with dialdehydes

Quantitative chemistry and modeling allow us to predict the behavior of a new, environmentally friendly, non-heavy-metal-containing profile control agent as a function of well-bore conditions. We present an analysis of the kinetics and mechanism of gelation delay for dialdehyde crosslinking of mixed poly(vinyl alcohol-co-vinylamine) (PVA-VAM) copolymers. In the present system, gelation at 90–110°C takes place within minutes at pH below 6.5 and not at all at pH above 7.5 when the crosslinking agent is the acetal-protected form of the dialdehyde. We exploit this sharp pH transition by buffering to pH 9–12 and then allowing the pH to drop by in situ hydrolysis of triethyl phosphate. The fact that a pH much below 7 is not needed for gelation means that consumption of acid by carbonate rock will not be a problem as far as gelation is concerned. In fact, neutralization by rock carbonate after gelation can be expected to promote gel stability.

The kinetics of the gelation are determined by the hydrolysis kinetics of triethyl phosphate and the stability of the dialdehyde. Triethyl phosphate hydrolysis has both base-catalyzed and uncatalyzed kinetics so it is advantageous to avoid a high pH in order to extend the gel time. It is also important to avoid a high pH in order to minimize the attack of clay minerals. Gelation systems using NaOH, Na₄EDTA, and Na₃PO₄ at initial pH below 10 were developed along with a mathematical model which satisfactorily predicts their gelation times.

Spectroscopic analysis indicates that the crosslinks formed initially in the mixed polymer system are different from those in poly(vinyl alcohol) (PVA). In PVA the crosslinks are hemiacetal and 1,3-dioxane groups formed from adjacent OH groups but in PVA-VAM the amine function is more reactive and the crosslinks are hemiaminal and dihydro-1,3-oxazine groups. These are potentially more stable than the crosslinks in PVA. We have not determined the nature of the crosslinks formed after aging, but there is some evidence that the oxazine linkages convert to dioxane links. Gels stable at 110°C for 40 weeks have been demonstrated.     

Comparison of irreversible and reversible cluster formation

Covalent and reversible cluster molecules were synthesized by an A₃B₂ type gelation. Crosslinking of three-arm hydroxyl-terminated star polymers with 2,4-toluenediisocyanate gave branched polymers, while the reversible analogue was made by crosslinking of tertiary amine-terminated star polymers with bis(4-hydroxy-3,5-dinitrophenyl) adipate. Gelation process was followed by static and dynamic light scattering. The extent of reacted groups was measured with UV spectroscopy. Growth of the covalent clusters could be described in terms of percolation scaling laws. The experimental gel point (P_{OH, cr} = 0.70) was shifted significantly from the theoretical predicted gel point (P_{OH, cr} = 0.50), indicating extensive ring formation during the gelation. The reversible endlinking reaction gave no macroscopic gelation, though increase of the cluster dimensions was observed. Ring formation proved to be an important side reaction in both cases; however, the ring formation ability seems to change in a different manner during the course of a gelation.     

对硅酸聚合机制反应级数的验证

用胶凝法研究酸化了的不同摩尔比 (Si O2 /Na2 O)的硅酸钠溶液的胶凝速度 ,发现在恒盐条件下 ,硅酸胶凝速度常数是 2 ,这个结果和我们过去所提出的硅酸聚合机制符合一致。     

Gelation dynamics and gel structure of fibrinogen

Gelation dynamics and gel structure of fibrinogen induced by serine protease, thrombin, was investigated by light scattering, real space observation using confocal laser scanning microscopy (CLSM), and turbidity. Effects of additives, such as (linear) saccharides, glucose to dextran, and cyclodextrin, were studied focusing on the interaction with fibrin(ogen) and thrombin. Light scattering measurement was ascertained to be able to characterize the gelation process and growth kinetics. Stepwise (two-step) gelation process, formation of fibrin monomers and protofibrils followed by the lateral aggregation to form fibrin fibers and gel network, was clearly ascertained. Gelation point could be characterized quantitatively. At the gelation point, dynamic light scattering exhibited a self-similar nature of the fibrin gel network, and the fractal dimension was evaluated in good accordance with the reconstructed 3D image of gel network by CLSM. The interaction between the additives and fibrin(ogen) and thrombin were studied by the inhibition test using synthesized substrate. Temporal variation of microstructure of fibrin gel network (lateral fiber growth) was investigated by turbidity in detail. Addition of saccharides affects significantly the network formation as revealed by turbidity. The interaction of dextran with fibrin fibers was examined by fluorescence microscopy, too, and the characteristic spatial distribution was observed.     

Gelation of covalently edge-modified laponites in aqueous media. 1. rheology and nuclear magnetic resonance

We describe the covalent modification of the edges of laponite with organic groups and the influence of this modification on gelation behavior. We compare three materials: an unmodified laponite, a laponite edge modified with a trimethyl moiety (MLap), and an octyldimethyl moiety (OLap). Gelation is investigated using rheology and NMR T1 relaxation measurements and nuclear Overhauser enhancement spectroscopy (NOESY). MLap and OLap show qualitatively different gelation. Gelation of MLap is very similar to laponite: MLap gels over the same time scale as laponite and has about the same solid modulus, and the MLap gel is almost as transparent as laponite. In contrast, OLap gels rapidly relative to laponite and forms a weak, turbid gel. We believe that gelation in laponite and MLap results from the formation of a network of well-dispersed platelets (or a few platelets), while in OLap, gelation results from a network of stacks of several platelets. NMR relaxation measurements indicate that gelation does not affect the average relaxation of water protons. However, T1 increases marginally for the protons in the organic moieties in MLap and decreases for protons in the organic moieties in OLap. Relaxation measurements, analyses of line width, and NOESY taken together suggest that, in OLap, gelation is a consequence of association of the organic moieties on the laponite edges, and that this association strengthens with time. Thus, the time-dependent changes in NMR suggest a structural origin for the time-dependent changes in the rheological behavior.     

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.     

二维材料的凝胶化及电化学储能应用

近年来, 二维材料由于其独特的结构以及高电化学活性在储能领域中备受关注. 然而在实际应用中, 二维材料的“面-面”堆叠极大地限制了其性能的发挥. 凝胶化作为实现纳米材料液相三维组装的重要手段, 不仅可以有效减少二维材料的团聚, 保留更多活性位点, 同时形成的三维网络结构可以提供畅通的离子电子传输通道, 提升材料在储能应用中的实用性. 不仅如此, 二维材料的凝胶化在电极材料孔结构设计以及活性物质缓冲空间定制方面具有先天优势. 本文以氧化石墨烯凝胶化过程的方法和原理为基础, 综合评述了石墨烯及其它几类较典型的二维材料的凝胶化机制及方法, 梳理了其孔结构调控策略, 并对凝胶化二维储能材料的设计以及应用进行了归纳、 总结及展望.     

Gelation of Covalently Cross-Linked PEG-Heparin Hydrogels

We study PEG-heparin hydrogels to identify compositions that lead to gel formation and measure the corresponding gelation kinetics. The material consists of a maleimide-functionalized high molecular weight heparin (HMWH) backbone covalently cross-linked with bis-thiol poly(ethylene glycol) (PEG). Using multiple particle tracking microrheology, we investigate a broad composition space, defined by the number of maleimide functional sites per HMWH (f = 3.9-11.8), the molecular weight of the PEG cross-linker (M(n) = 2000, 5000, and 10 000), and the concentrations of the heparin and PEG polymers. Gelation kinetics are characterized by time-cure superposition, yielding the gel time, t(c), and the critical relaxation exponent, n. Gelation times range from 5 < t(c) ≤ 45 min, with the fastest kinetics occurring for the highest HMWH maleimide functionalities. t(c) depends nonmonotonically on the PEG cross-linker molecular weight, suggesting that gelation is affected by the length of the cross-linker relative to intermolecular interactions between heparin molecules. The critical relaxation exponent decreases from n = 0.52 for PEG 2000 to n = 0.39 for PEG 10 000. Finally, 219 equilibrated samples taken over the entire composition space are identified as liquid or solid, defining the "gelation envelope". The boundaries of this empirical gelation envelope are in good agreement with Flory-Stockmayer theory. In all, microrheological measurements enable characterization over a large parameter space and provide crucial insight into the gelation of complex, multifunctional hydrogelators used in therapeutic applications.     

Microstructure and Viscosity of Aggregating Colloids under Strong Shearing Force

Disaggregation under strong shearing force is simulated for an aggregating colloid based on a sticky particle model which can describe the disaggregating and aggregating kinetics, the deformation, and the rupture of clusters with a minimum number of parameters. For a 2-dimensional system, the viscosity and coordination number of the model colloid are calculated at each time step, and the changes of microstructure with shear flow are observed directly by displaying the configuration of particles onto a monitor. The viscosity depends on both area fraction and shear rate, but coordination number depends only on shear rate. Furthermore, the viscosity and coordination number at steady state are independent of the initial state of particles, which indicates that the disaggregation and aggregation are mutually reversible. Copyright 1999 Academic Press.     

Asymptotic Particle Size Distributions Attained during Coagulation Processes

The effects of the collision kernel on the self-preserving size distribution and on the gelation phenomenon of aerosol coagulation were investigated. An analytical solution for the asymptotic width of log-normally preserving size distribution during coagulation was obtained as a function of the degree of homogeneity using arbitrary shape of homogeneous collision kernels. From the solution obtained, it was shown that when the degree of homogeneity is larger than 1, self-preserving size distribution does not exist, and gelation occurs. A very accurate numerical coagulation simulation method, the sectional method, was also used for calculating the self-preserving particle size distribution for some specific classes of coagulation kernels and the results were compared with the analytical solution obtained by the log-normal method. Copyright 2001 Academic Press.     

The Effect of the Side Chain on Gelation Properties of Bile Acid Alkyl Amides

Six bile acid alkyl amide derivatives were studied with respect to their gelation properties. The derivatives were composed of three different bile acids with hexyl or cyclohexyl side chains. The gelation behaviour of all six compounds were studied for 36 solvents with varying polarities. Gelation was observed mainly in aromatic solvents, which is characteristic for bile-acid-based low molecular weight gelators. Out of 108 bile acid-solvent combinations, a total of 44 gel systems were formed, 28 of which from lithocholic acid derivatives, only two from deoxycholic acid derivatives, and 14 from cholic acid derivatives. The majority of the gel systems were formed from bile acids with hexyl side chains, contrary to the cyclohexyl group, which seems to be a poor gelation moiety. These results indicate that the spatial demand of the side chain is the key feature for the gelation properties of the bile acid amides.     

Scaling of the kinetics of slow aggregation and gel formation for a fluorinated polymer colloid

The aggregation and gelation kinetics in moderately concentrated (0.004 相似文献   

Thermoreversible Gelation of Polybenzimidazole in Phosphoric Acid

Summary: We report a study of thermoreversible gelation of polybenzimidazole (PBI) in phosphoric acid (PA). The PBI gel in PA exhibits fibrillar network morphology and reversible first order phase transition. The gelation rate is measured by the tube tilting method and found to depend both upon gelation concentration and gelation temperature. The UV‐vis study demonstrates that the gelation process is a two‐step process: conformational transformation and aggregation which produces crystallites for gel formation. The WAXS study supports the presence of crystallites in the gel. The PA doping level of the membrane increases significantly because of gelation.

Thermoreversible gelation of polybenzimidazole in phosphoric acid and the membrane produced from the gel.     

Formation of Physical Thermally Reversible Gels in Solutions of Methyl Cellulose in Water and Dimethylacetamide and Properties of Films Thereof

Gelation conditions in solutions of methyl cellulose in water and dimethylacetamide were studied. The gelation mechanisms were studied by X-ray diffraction and photocolorimetry, and their differences in the two systems were revealed. The physicomechanical properties of methyl cellulose films prepared from solutions with preliminary gelation were determined.