Gelation behavior of thermoplastic-modified epoxy systems during polymerization-induced phase separation

The rheological behavior and gelation characteristics of epoxy blends are of critical importance to property study and industrial application. In this work, we studied the rheological behavior and structural transition of different thermoplastics, including polyetherimide, polymethylmethacrylate, and polyethersulfone (PES), modified epoxy systems by using rheometry instrument, differential scanning calorimetry, time-resolved light scattering, and scanning electronic microscopes. At the same molecular weight level of thermoplastics, different epoxy blends show profound diversities on the rheological and gelation behavior due to the large differences in phase separation and curing process. For early phase-separation systems of PES-modified epoxy blends, two gel points are identified, which correspond to physical gelation and chemical gelation, respectively. With the variation of the PES molecular weight and curing rate, dramatic changes in gel time and critical exponent were observed. As the molecular weight of thermoplastics is increased, the gelation time becomes shorter and the gel strength gets lower, while the faster curing rate would increase the physical gel strength significantly.     

Gelation process of amylose-DMSO-water system

Amylose gel is obtained by cooling amylose aqueous solution at a certain cooling rate. In order to clarify the gelation process of amylose in the mixed solvent of water and DMSO, the characteristics of turbidity and dynamic viscoelasticity of the solution as a function of temperature have been studied by optical and rheological methods, respectively. Accordingly, cloud temperature (Tc) at which the aggregation of amylose occurs, and gelation temperature (Tgel) at which the elasticity began to appear were obtained. Tc and Tgel were strongly dependent on cooling rate, and these values shifted to higher temperatures with decreasing cooling rate. However, there was difference between Tc and Tgel, and Tc was higher than Tgel, indicating the gelation process occurs in two stages.     

Rheological behavior and structural interpretation of waxy crude oil gels

A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.     

Stability of water/crude oil emulsions based on interfacial dilatational rheology

The dilatational viscoelasticity behaviors of water/oil interfaces formed with a crude oil and its distilled fractions diluted in cyclohexane were investigated by means of an oscillating drop tensiometer. The rheological study of the w/o interfaces at different frequencies has shown that the stable w/o emulsions systematically correspond to interfaces which present the rheological characteristics of a 2D gel near its gelation point. The stability of emulsions was found to increase with both the gel strength and the glass transition temperature of the gel. As expected, the indigenous natural surfactants responsible for the formation of the interfacial critical gel have been identified as the heaviest amphiphilic components present in the crude oil; i.e., asphaltenes and resins. Nevertheless, we have shown that such a gel can also form in the absence of asphaltene in the oil phase.     

Distinct kinetic pathways generate organogel networks with contrasting fractality and thixotropic properties

The kinetics of the isothermal transformation of sols, comprised of a low molecular-mass organogelator (LMOG) and an organic liquid, to their organogel phases have been followed by circular dichroism (CD), fluorescence, small angle neutron scattering (SANS), and rheological methods. The thixotropic properties (in the sense that severe shearing followed by rest lead to reestablishment of viscoelasticity) of the gels have been examined as well by rheological measurements. The compositions of the samples were either 5alpha-cholestan-3beta-yl N-(2-naphthyl) carbamate (CNC) in an n-alkane (n-octane or n-dodecane) or 3beta-cholesteryl N-(2-naphthyl) carbamate (CeNC) in ethyl acetate. Values of Df, the mass fractal dimension of the microcrystalline self-assembled fibrillar networks (SAFINs) in the gels, have been extracted from the kinetic data using a model developed by Dickinson (J. Chem. Soc., Faraday Trans. 1997, 93, 111). The Df values, 1.1-1.3 for the CeNC gels and 1.3-1.4 or 1.6-1.8 (depending on the temperature of incubation of the sol phase) for CNC gels, are consistent with the gel network structures observed by optical microscopy. In addition, comparison of the temperature dependence of both n (the Avrami component) and K (the Avrami "rate constant") for CeNC/ethyl acetate gelation with those reported previously for gelation of CNC/n-alkane sols demonstrate that the very small change of a single bond in CNC to a double bond in CeNC causes significant differences in their gelation abilities and gel properties. The rheological measurements on CNC/n-alkane gels with spherulitic SAFIN units, formed by incubation of their sols at < or =28 degrees C, indicate that they are thixotropic. Gels with the same chemical composition but formed by incubation of their sols at > or =30 degrees C, leading to fiberlike SAFIN units, remain liquidlike after shearing regardless of the periods they are at rest. The time-dependent viscoelastic properties of the gel networks are treated according to a stretched exponential model. The observations from these studies provide detailed insights into the mechanisms of formation of molecular organogel phases and demonstrate the extreme sensitivity of the SAFINs and viscoelastic properties of such organogels to slight modifications in LMOG structure or sample history.     

Topology evolution and gelation mechanism of agarose gel

Kinetics as well as the evolution of the agarose gel topology is discussed, and the agarose gelation mechanism is identified. Aqueous high melting (HM) agarose solution (0.5% w/v) is used as the model system. It is found that the gelation process can be clearly divided into three stages: induction stage, gelation stage, and pseudoequilibrium stage. The induction stage of the gelation mechanism is identified using an advanced rheological expansion system (ARES, Rheometric Scientific). When a quench rate as large as 30 deg C/min is applied, gelation seems to occur through a nucleation and growth mechanism with a well-defined induction time (time required for the formation of the critical nuclei which enable further growth). The relationship between the induction time and the driving force which is determined by the final setting temperature follows the 3D nucleation model. A schematic representation of the three stages of the gelation mechanism is given based on turbidity and rheological measurements. Aggregation of agarose chains is promoted in the polymer-rich phase and this effect is evident from the increasing mass/length ratio of the fiber bundles upon gelation. Continuously increasing pore size during gelation may be attributed to the coagulation of the local polymer-rich phase in order to achieve the global minimum of the free energy of the gelling system. The gel pore size determined using turbidity measurements has been verified by electrophoretic mobility measurements.     

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.     

Studies on the cure parameters of cyanate ester–epoxy blend system through rheological property measurements

Thermosetting blend system of co-cured cyanate ester with epoxy resin is receiving importance for high technological applications because of its wide range of thermo-mechanical, rheological, and electrical properties. However, processing of these system warrant proper knowledge of the rheological behavior of the blend during the curing process. This article discusses the rheological behavior of the blend systems with respect to the pot life, gel time, gelation temperature and also evaluated fitting parameters for the prediction of gelation time and viscosity during the entire curing process from the isothermal rheological measurements.     

Rheological properties of protein-surfactant based gels

Water-based protein-surfactant gels, formed by mixing bovine serum albumin (BSA) and sodium dodecyl sulfate in water, were investigated by rheological methods. The measurements were performed for many different protein-to-surfactant ratios as a function of the applied frequency, stress, or strain, as well as by changing the temperature, in the range between 15 and 65 degrees C. The rheological behavior of the gels as a function of applied frequency is interpreted in terms of the overlapping of at least two viscoelastic relaxation processes. The rheological results indicate the presence of thermal transitions from essentially viscous to mainly elastic regimes, in analogy with the thermal gelation processes observed in polymer solutions. The thermal gelation threshold in the present system is modulated by the protein/surfactant ratio. Differential scanning calorimetry measurements were also performed to determine whether thermal gelation is somehow concomitant to protein denaturation. The results indicate that the thermal denaturation of BSA in protein-surfactant based gels occurs at slightly higher temperatures than in the bulk. Scanning electron microscopy indicates the occurrence in the gel structure of globules formed by the arrangement of fibrils.     

Rheological behavior of PMN gels for solid freeform fabrication

Lead magnesium niobate (PMN) is a relaxor ferroelectric material and have widespread applications in the manufacture of multilayer electronic devices such as ceramic capacitors, actuators and transducers. The dielectric constant of this electrostrictive material is much higher than the well known ferroelectric barium titanate. However, aqueous processing of PMN is not investigated yet especially for the novel wet shaping fabrication. In this study, concentrated aqueous colloidal PMN gels have been designed to use in the robocasting process. Concentrated PMN suspensions were stabilized by polyacrylic acid and then gelation induced by changing the pH or ionic strength of the suspension or by addition of a cationic polyelectrolyte to the system. Through this procedure it was essential to understand the solid–liquid transition under shear to explore the feasibility of forming without excessive use of polymers. Therefore, rheological response of the samples having a gel network was investigated. Results showed that gelation induced by cationic polyethylene imine or by multivalent salts were successful methods in preparation of PMN gels. However, gelation induced by changing the pH of the suspension was challenging due to ion dissolution from PMN surface.     

Showing a rheological effect of gelation on epoxy-amine systems

The effect of gelation was studied on the rheological curves, during hardening of a resin subjected to sinusoidal shearing between parallel plates. The epoxy-amine system was chosen for the moderate value of its maximum glass transition temperature T_{g, ∞} which means that it is possible to avoid vitrification by operating at T > T_{g, ∞}. The shape of the variation curve of the storage modulus G' does not depend on temperature; G' obeys a single percolation law as a function of the reduced variable t/t_gel. On the other hand, gelation leads to a fall in the growth rate of the loss modulus G” after the gel point. This effect increases with temperature and can become considerable at very high temperatures: in this case a minimum of G” between the gel point and the rubbery plateau can be observed. We have verified that this effect is indeed a rheological effect of gelation as it begins practically at the gel point determined on the storage modulus curves. Several qualitative conclusions are drawn, and the nature of this effect is discussed.     

Rheological studies of the gelation kinetics of Cr+3-polyacrylamide solutions

A new rheological approach for the study of the gelation kinetics of aqueous Cr⁺³ polyacrylamide solutions has been investigated which relies on the assumption that at the gel point, GP, a constant amount of Cr⁺³ will have reacted, independent of the initial Cr⁺³ concentration. From this, a kinetic expression relating GP to the initial concentrations of Cr⁺³ and reactive groups on the polymer has been derived for the case of a crosslinking reaction first order in Cr⁺³ and polymer. Experimental evidence that the amount of Cr⁺³ reacted at the GP is constant for a given polymer and solution pH has been obtained. The good fit of the kinetic expression to the rheological data provides new evidence that the gelation reaction is first order in Cr⁺³ rather than second order as claimed previously.     

Interfacial routes to colloidal gelation

Colloidal gelation is a rheological transition from fluid-like to solid-like viscoelasticity in a particulate suspension and is often instigated by causing the net interparticle interaction to be attractive. In this article, three routes to colloidal gelation that have been discovered recently and involve interfacial phenomena at a fluid interface are reviewed. As in conventional systems, gelation is due to a percolating particle network that imparts elasticity to the mixture, but the network formation involves interfacial particle jamming or bridging, or capillary interactions along or across interfaces, in a mixture of immiscible fluids. Gelation imparts mechanical stability to these multiphase mixtures and paves the way for their use as templates for the synthesis of functional, microstructured materials and composites. The gel mechanical properties are mediated by the interfacial forces and the mixture's microstructure, and therefore show different dependencies on particle volume fraction across the three systems.     

凝胶形成动态过程在线监测方法的比较

以醋酸介质中戊二醛（Ｇｌｕ）交联壳聚糖（ＣＳ）水凝胶的形成为例,研究比较了粘度法,电导法,光度法和荧光探针法用于监测凝胶形成动态过程的可行性性和特点,结果表明：粘度法具有简单、直观等优点,但粘度法不能够跟踪凝胶点（ｇｅｌｌｉｎｇｐｏｉｎｔｓ）后体系的进一步变化,旋转粘度法在测定时对体系的扰动不容忽视。电导法和光度法跟踪的是引起凝胶化的交联反应过程,但溶液到凝胶的转化往往发生在交联反应完成之前,因此所得结果未必与宏观观察结果相一致,荧光探针法具有适用面宽,可供选用的探针种类多、方法灵活等特点,对于研究凝胶形成动态过程具有普适性,该法还可用于凝胶性质的研究。     

β-Casein aids in the formation of a sodium caprate-induced β-lactoglobulin B gel

The effects of sodium caprate on the gelation of β-lactoglobulin B and a β-lactoglobulin B/β-casein mixture at ambient temperature were investigated using ultrasonic spectroscopy and rheology. A 12% β-lactoglobulin B solution gelled in the presence of 3.6% sodium caprate. Conversely, sodium caprate did not induce the formation of a gel when β-casein was in isolation, regardless of the protein concentration. Although a 6% β-lactoglobulin B/1.8% sodium caprate solution did not form a gel, a gel was formed when 6% β-casein was added to a mixture containing 6% β-lactoglobulin B and 3.6% sodium caprate. This gel showed comparable rheological properties to that of a gel containing 12% β-lactoglobulin B. The results clearly indicated that β-casein aids in the gelation of a β-lactoglobulin B/sodium caprate mixture, when the concentration of β-lactoglobulin B is insufficient to allow for gelation. It appears that β-casein self-aggregation is also inhibited. Therefore, it could be concluded that β-casein can be used as a texture modifier for β-lactoglobulin gelation induced by sodium caprate.     

Use of ultrasound spectroscopy to examine the effect of cysteine on β-lactoglobulin interactions

We investigated the effects of cysteine on β-lactoglobulin interactions using ultrasound spectroscopy, rheological measurements, and differential scanning calorimetry. Changes in ultrasonic velocity and attenuation were monitored using ultrasound spectroscopy, and we discuss the effects of cysteine on gel formation together with the results obtained using other methods. A decrease in ultrasonic velocity occurred around 54 °C, suggesting that the compressibility of the system increases at approximately this temperature. An increase in ultrasonic attenuation was observed at approximately 54 °C, which is much lower than the commonly observed denaturation temperature of 75–80 °C. The temperature coincided with the onset of phase transition by differential scanning calorimetry and the initial rise in temperature of dynamic modulus for rheological measurements under heat treatment. We conclude that cysteine promotes the polymerization processes of denatured proteins during the initial stage of gelation. The ultrasonic spectroscopic analysis is a useful tool to monitor protein molecule interactions prior to gelation.     

聚苯胺溶液的凝胶化行为

通过流变学手段对聚苯胺溶液的凝胶化行为研究表明，在室温下，聚苯胺（ＰＡｎ）／Ｎ－甲基－２－吡咯烷酮（ＮＭＰ）溶液有一转折浓度Ｃｇ（０．０５ｇ／ｍＬ），浓度高于此值时，浴液凝胶化速度急剧增大，凝胶速度还与溶剂，ＰＡｎ分子量及溶液的温度有很大关系。溶液中不溶的ＰＡｎ的存在会加速凝胶化过程。ＰＡｎ／ＮＭＰ凝胶的假塑性和触变性明显地反映出ＰＡＶＮＭＰ溶液的凝胶化是溶剂参与的ＰＡｎ分子链间的物理交联过程。     

Study of the Gelation and Rheological Behavior of Carboxymethyl Cellulose-Polyacrylamide Graft Copolymer Hydrogel

This research article deals with the synthesis of carboxymethyl cellulose-g-polyacrylamide copolymer and its suitability for the development of hydrogel system using cross-linker. The hydrogels were prepared using different concentrations of synthesized graft copolymer and chromium(III) acetate cross-linker. The gelation and rheological behaviors of this gel system were thoroughly studied. It was experimentally observed that the prepared hydrogels exhibit an acceptable gel strength and gelation time. The graft polymeric gel using 10,000 ppm copolymer and 2000 ppm chromium(III) acetate cross-linker could be deformed up to 7690% without tearing of the gel network in the range of 0.1–10,000% strain. These observations show the potential of the developed hydrogel system for its application as sealant during water shutoff jobs in oil reservoir.     

Cooling effects on a model rennet casein gel system: part I. Rheological characterization

The gelation of a model rennet casein system was studied during cooling at different rates. During cooling, casein network structure development was proposed to evolve over a few steps at different length scales: molecules, particles, flocs, or network. Rennet casein flocs are fractal in nature, and fractal dimension and floc size are two variables affecting the rheology and microstructure of a rennet casein gel. Casein structure formation during cooling from 80 to 5 degrees C at four different rates (0.5, 0.1, 0.05, and 0.025 degrees C/min) was monitored by dynamic rheological tests, and a stronger gel developed at a slower cooling rate. During different cooling schedules, similar fractal dimensions were observed due to a lack of difference in the colloidal interactions. Differences among rheological data were possibly caused by variability in floc size, as observed in the second part of this paper. A larger number of smaller-sized flocs enabled gelation at a higher temperature and created a stronger network at a slower cooling rate. Controlling cooling schemes thus provides an approach for manipulating casein gelation and the microstructure for a system of fixed chemical compositions.     

Oligonucleotide Analogues with Integrated Bases and Backbones. Part 19.

The A*[s]U⁽*⁾ dinucleosides 1 and 2 form thermoreversible gels in organic solvents. The basis of the gelation is the formation of linear aggregates by base pairing following desolvation of the nucleobases. This is evidenced by the absence of gel formation by the C(6)‐deaminated analogue 3 of 1 , the correlation of gelation with the anti‐conformation, as preferred for 1 , and the temperature‐, concentration‐, and time‐dependent CD spectra. The gels were also characterized by the minimum gelation concentration, the gel–sol transition (melting) temperature, and rheological properties.