Tuning the morphology of cellulose acetate gels by manipulating the mechanism of phase separation

The effect of dihydric alcohol (nonsolvent) addition on the rheological and microstructural behavior of cellulose acetate (CA) in a ternary CA, N,N-dimethylacetamide (DMA), nonsolvent system was investigated. Increasing the dihydric alcohol concentration led to enhanced steady shear viscosity and dynamic viscoelastic properties that were dependent on CA concentration. Changing the dihydric alcohol from 1,2-ethanediol to 1,4-butanediol and 1,6-hexanediol increased the moduli and decreased the concentration of nonsolvent at which the sol-gel transition occurred. At 10 wt % CA concentration the modulus and gel morphology of the 1,2-ethanediol and 1,4-butanediol systems were quite similar and distinctly different from that of 1,6-hexanediol. In the former, the gel morphologies were more heterogeneous, evident of more extensive coarsening, and likely obtained via nucleation and growth and spinoidal decomposition of off-critical mixtures. The latter exhibited more uniform dense network morphology, indicative of a spinoidal decomposition of near-critical mixtures. The gels were fractal in nature and exhibited different fractal dimensions in-line with the observed differences in microstructure; D ～ 1.87 ± 0.02 (1,2-ethanediol and 1,4-butanediol) and D ～ 1.97 ± 0.02 (1,6-hexanediol). However, at 15 wt % CA content, the gels exhibited more similar viscoelastic behavior and gel microstructures; D ～ 1.97 ± 0.02 for all three dihydric alcohol systems.     

Effect of alkali pretreatment on the rheological properties of concentrated agar-agar gels

Differential scanning calorimetry and thermogravimetry in the solid state and dynamic mechanical measurements of gels have been carried out for agar-agars of Chilean and Argentinian origin in order to elucidate the rheological changes in the gel as a result of alkali pretreatment. The elastic modulus of the gel prepared from Chilean agar-agar increased with increasing sodium hydroxide concentration up to 10%, while that of Argentinian agar-agar increased with increasing sodium hydroxide concentration up to 7%, and then began to decrease at higher concentrations. The increase in elastic modulus has been attributed to the structural stabilisation induced by the formation of 3,6-anhydro-l-galactose, while the decrease in elastic modulus in Argentinian agar-agar has been ascribed to chain breakage.     

Rheology of protein gels synthesized through a combined enzymatic and heat treatment method

Whey protein gels prepared under acidic conditions (pH<4.6) remain largely unutilized because of their weak and brittle nature in contrast to the favorable elastic gels produced at neutral or basic conditions. However, such usage is important, as low pH food products are desirable due to their shelf stability and less stringent sterilization processes. In this study, we use a two-step process involving enzyme followed by heat treatment to produce whey protein gels at low pH (4.0). Dynamic rheological measurements reveal that the gel elastic modulus and yield stress increase substantially when heat treatment is supplemented with enzyme treatment. Both the elastic modulus and yield stress increase with increasing enzyme concentration or treatment time. In contrast, the dynamic yield strain decreases with enzyme concentration but increases with time of enzyme treatment. These results are explained in terms of the enzyme treatment time affecting the diffusion of the enzyme within the gel. This in turn leads to two types of gel microstructure at short and long enzyme treatment times, with the extent of enzyme diffusion modulating the structure at intermediate times.     

预热处理金针菇蛋白对猪肉肌原纤维蛋白凝胶特性的影响

本文研究预热处理（preheat treatment,PT）金针菇蛋白（Flammulina filiformis protein,FFP）对肌原纤维蛋白（myofibrillar protein,MP）凝胶特性的影响。试验将FFP在65、75、85和95℃下分别预热处理30、60、90和120min,以蛋白溶解度、总巯基含量和表面疏水性为指标,确定FFP预热处理的最佳温度与时间;将预热处理的FFP（PT-FFP）与MP以不同的比例（0:10、1:9、2:8、3:7、4:6,M/M）混合制备复合凝胶（总蛋白质量浓度均为40mg/mL）,以凝胶强度、持水性、流变学特性与微观结构为指标,考察PT-FFP对MP凝胶特性的影响。结果表明,75℃预热处理60min,FFP的溶解度与表面疏水性最高,总巯基含量最低,故确定该预热处理条件为最佳;将该条件下制备的PT-FFP以不同比例与MP混合制备凝胶,结果发现PT-FFP比FFP更有利于复合凝胶的凝胶强度和持水性的提高（P<0.05）,特别是当PT-FFP与MP混合比例为1:9时,复合凝胶的凝胶强度和持水性达到最高值121.38g和85.25%;流变学结果也表明,PT-FFP能够提高复合凝胶的弹性模量Gʹ和损耗模量G″;电镜观察可以看出PT-FFP与MP复合的凝胶具有更强的连续性,结构更为致密。总之,经过75℃预热处理60min后的FFP和MP以1:9混合时能够改善肌原纤维蛋白凝胶特性和流变学特性。     

Rheological and structural properties of aqueous alginate during gelation via the Ugi multicomponent condensation reaction

Turbidity, structure, and rheological features during gelation via the Ugi multicomponent condensation reaction of semidilute solutions of alginate have been investigated at different polymer and cross-linker concentrations and reaction temperatures. The gelation time of the system decreased with increasing polymer and cross-linker concentrations, and a temperature rise resulted in a faster gelation. At the gel point, a power law frequency dependence of the dynamic storage modulus (G' proportional, variant omega(n)(')) and loss modulus (G' ' proportional, variant omega(n)(' ')) was observed for all gelling systems with n' = n' ' = n. By varying the cross-linker density at a fixed polymer concentration (2.2 wt %), the power law exponent is consistent with that predicted (0.7) from the percolation model. The value of n decreases with increasing polymer concentration, whereas higher temperatures give rise to higher values of n. The elastic properties of the gels continue to grow over a long time in the postgel region, and at later stages in the gelation process, a solidlike response is observed. The turbidity of the gelling system increases as the gel evolves, and this effect is more pronounced at higher cross-linker concentration. The small-angle neutron scattering results reveal large-scale inhomogeneities of the gels, and this effect is enhanced as the cross-linker density increases. The structural, turbidity, and rheological features were found to change over an extended time after the formation of the incipient gel. It was demonstrated that temperature, polymer, and cross-linker concentrations could be utilized to tune the physical properties of the Ugi gels such as structure, transparency, and viscoelasticity.     

Effect of hydrophobic modification on rheological and swelling features during chemical gelation of aqueous polysaccharides

Rheological characteristics during chemical gelation with the cross-linker ethylene glycol diglycidyl ether (EGDE) of semidilute aqueous solutions of hydroxyethylcellulose (HEC) and of two hydrophobically modified analogues (HM-1-HEC and HM-2-HEC) are reported. In addition, rheological features of gelling samples (dextran and its hydrophobically modified analogue (HM-dextran)) of a different structure have been examined. Some swelling experiments on these gels in the postgel region are also reported. The gelation time of the hydroxyethylcellulose systems decreased with increasing cross-linker concentration, and incorporation of hydrophobic units of HEC resulted in a slower gelation. The time of gelation for the dextran system was only slightly affected by the incorporation of hydrophobic groups (HM-dextran). At the gel point, a power law frequency dependence of the dynamic storage modulus (G' proportional to omegan') and loss modulus (G' proportional to omegan') was observed for all gelling systems with n' = n' = n. The attachment of hydrophobic moieties on the dextran chains had virtually no impact on the value of n (n = 0.77), and the percolation model describes the incipient dextran gels. By increasing the number of hydrophobic groups of the HEC polymer, the value of n for the corresponding incipient gel drops significantly, and the value of the gel strength parameter increases strongly. Incorporation of hydrophobic units in the HEC chains promotes the formation of stronger incipient gels because of the contribution from the hydrophobic association effect. The frequency dependence of the complex viscosity reveals that all the investigated gels become more solidlike in the postgel domain. Far into the postgel region, the hydrophobicity of HEC plays a minor role for the strength of the gel network, whereas the values of the complex viscosity are significantly higher for HM-dextran than for the corresponding dextran gel. The swelling experiments on HEC, HM-1-HEC, and HM-2-HEC systems disclose that the degree of swelling of the postgels in water is quite different, depending on the relative distance from the gel point at which the cross-linker reaction is quenched. At a given distance from the gel point, the swelling of the HEC gel is less pronounced than for the corresponding hydrophobically modified samples. At this stage, the swelling of the HM-dextran gel is stronger than for the dextran gel.     

Temperature dependence of the elastic modulus of alginate gels

The temperature dependence of the elastic modulus for alginate gels was studied using two different gel systems: covalently crosslinked Na-alginate gels and in-situ prepared Ca-alginate gels. The modulus of physically crosslinked gels showed a complex behaviour. The temperature coefficient of the modulus of covalently crosslink gels changed from positive for the lowest crosslinked gels to negative for the highest crosslinked gels. This suggests a change from rubberlike to enthalpy-driven elasticity with an increasing degree of crosslinking for these gel networks.     

Identification of the testing parameters in high frequency dynamic shear measurement on agarose gels

Dynamic mechanical analysis (DMA) on agarose gels can be used to validate magnetic resonance elastography (MRE) measurements as well as to provide better understanding for the biological responses of cells to the dynamic loadings in cell culture studies. Various parameters potentially affecting the repeatability and accuracy of the DMA shear modulus measurements were investigated systematically in the present study, including sample thickness, shear strain, testing frequency, and compressive clamping strain. The study showed that the thickness of the agarose gel sample must be sufficiently small (1 mm) to prevent the erroneous fluctuation in the measured modulus. The appropriate levels of shear strain (< or = 0.5%) and compressive clamping strain (5-10%) must be applied to overcome the slippage at the gel-clamp interface without causing significant boundary and stress non-uniformity or micro-cracks in the agarose gel sample.     

Effects of extraction parameters on gel properties of carrageenan from <Emphasis Type="Italic">Kappaphycus alvarezii</Emphasis> (Rhodophyta)

Carrageenan was isolated under different extraction conditions from Kappaphycus alvarezii collected in North Sulawesi, Indonesia. Its gel properties include very strong elasticity even at low concentrations. Molecular weight and rheological properties were obtained by gel permeation chromatography and dynamic viscoelasticity measurements in order to clarify the average molecular weight at various extraction temperatures (50, 70, 90°C) and times (1, 3, 5 h), as well as gel formation ability. The results showed that both the weight-average and the number-average molecular weight decreased with increasing extraction temperature. However, the gelation rate of the carrageenan was found to be constant at around 40°C, whereas the storage modulus, G′, and loss modulus, G″, of the gels differed from each other.     

Stereocontrolled Synthesis of an Octasaccharide Part of I-Active Glycolipids

The effects of glycerin and ethylene glycol on the elastic modulus and DSC thermograms of agarose and kappa-carrageenan gels were examined to clarify the relation between structure and properties. The elastic modulus of these gels as a function of the concentration of polyols increased up to a certain concentration and then decreased with increasing concentration of polyols. These polyols shifted the melting temperature of the gel to higher temperatures in kappa-carrageenan gels but to lower temperatures in agarose gels. The temperature dependence of elastic modulus was changed in opposite directions in agarose and kappa-carrageenan gels by the addition of polyols, and this is discussed on the basis of model consisting of junction zones which are connected by Langevin chains. It was suggested that the mean distance between junction zones became shorter in the presence of a small amount of polyols.     

The gelling of κ-carrageenan in light and heavy water

Gels of κ-carrageenan were prepared in H₂O and D₂O. Rheological measurements revealed that in heavy water the elastic modulus is substantially higher. From the rheological view point, this result indicates the formation of a higher number of physical-crosslinking points in the structure of the gel. These crosslinking points are mainly associated with the aggregation of the double-helix of the κ-carrageenan, which is enhanced in D₂O. The thermal stability of the gels in both solvents (relative to the sol-gel transition) was also investigated by using polarimetric measurements (to measure the relative fraction of helix-coil) and micro-differential scanning calorimetry (micro-DSC). The results obtained using these techniques are very consistent and allow us to propose a model to explain the differences for the gels formed in light and heavy water based on the degree of double-helix aggregation.     

Shear creep of gelatin gels from mammalian and piscine collagens.

This paper describes new measurements on the creep rheological behaviour of gelatin gels from both traditional mammalian and piscine sources. Measurements on a series of concentrations of gels were obtained using a high-precision controlled stress rheometer. Results for the concentration dependence of compliance are close to those expected from dynamic oscillatory measurements of gel modulus, assuming ideal elasticity. The concentration dependence of viscosity approximates power law behaviour, with eta~C( approximately 2-3), lower than the exponent expected for semi-dilute solutions. The apparent contradiction implied by this is discussed and a novel gel viscosity versus concentration state diagram presented.     

Gelation behavior of native and acetylated konjac glucomannan

Gelation kinetics of native and acetylated konjac glucomannan (KGM) samples in the presence of alkali (sodium carbonate) was studied by dynamic viscoelastic measurements. Molecular weight and other molecular parameters of KGM were determined by static light scattering and viscosity measurements. It was found that KGM molecules were degraded during acetylation treatment, but the molecular weights of acetylated samples were almost independent of the degree of acetylation (DA) and were about a half of that of a native sample. At a fixed alkaline concentration, increasing concentration of KGM or temperature shortened the gelation time, but increasing DA delayed it. The deacetylation reaction and subsequent aggregation process of acetylated samples needed longer time than that of native sample, and acetylated samples formed finally more elastic gels. It implied that the presence of acetyl groups exerts a strong influence on gelation behavior of KGM. It was suggested that the gelation rate of acetylated KGM and native KGM, which depends on the alkaline concentration and temperature, is an important factor that determines the elastic modulus of gels. This was supported by the experimental finding that the saturated elastic modulus tends to the same value when the ratio of alkali concentration to acetylated groups was kept constant. In slower gelation processes, junction zones are more homogeneously distributed and more numerous, leading to the more elastic gels.     

Rheological characterization of photochemical changes of ethyl(hydroxyethyl)cellulose dissolved in water in the presence of an ionic surfactant and a photosensitizer

The effects of addition of the photosensitizer riboflavin (RF) to semidilute solutions of the systems ethyl(hydroxyethyl)cellulose (EHEC)/water, EHEC/sodium dodecyl sulfate (SDS), and EHEC/cetyltrimethylammonium bromide (CTAB) on the turbidity and the linear viscoelasticity are studied. The turbidity behavior and the cloud point (CP) are influenced by the addition of RF to the EHEC/SDS system, whereas no discernible change is observed for the other systems. The rheological features of all systems are affected by the presence of RF at lower temperatures, whereas at temperatures close to the CP, only a slight effect is detected. Both the EHEC/SDS and EHEC/CTAB systems evolve thermoreversible gels at the same temperature (37.5 degrees C), but in the presence of RF, the EHEC/CTAB system does not form a gel, whereas the gel temperature for the EHEC/SDS system is depressed (32.5 degrees C). Light irradiation of RF in the EHEC/SDS/RF system causes fragmentation of the network and a higher temperature is required to re-form the incipient gel network. The photochemical degradation of EHEC gives rise to a decrease in the dynamic moduli and the complex viscosity for all of the three systems. The effect is strengthened at higher temperatures and it is most pronounced for the EHEC/SDS system.     

Gelation of konjac glucomannan crosslinked by organic borate

A series of thermoreversible konjac glucomannan gels crosslinked by organic borate were prepared. The gel network was formed through the crosslinking reaction between borate ions dissociated from organic borate and the cis-diol hydroxyl groups on the mannose units of polysaccharide chains. The rheological properties of the complex gels were studied by dynamic viscoelastic measurement. The gelation kinetics of the gels were studied and the critical gelation points of the gels were exactly determined by the Winter–Chambon criterion. The effects of temperature and composite ratio on the shear storage modulus (G^′), the loss modulus (G^″), and the sol-gel transition points were investigated. The critical gel-sol temperatures of the complex gels were successfully elucidated by Winter–Chambon criterion. The effect of crosslinking density on the critical gelation temperature and the elasticity of the gels were discussed.     

Rheological properties and mechanical stability of new gel-entrapment systems applied in bioreactors

The mechanical stability of gels applied for entrapment and retention of biocatalysts in bioreactors is of crucial importance for successful scale-up applications. Gel abrasion in agitated reactors will depend on liquid shear, bubble shear, and wall shear, as well as collisions between the gel particles. As a simplified standardized model system, abrasion of gel beads was studied in 1-m-high bubble columns with controlled aeration, and quantified by measuring the loss of gel material into solution. Gel beads were also taken out to measure stress-strain response during controlled compression. More general rheological properties of different gels were studied by applying a variety of regimes of controlled compression of standardized gel cylinders: Gel strength was measured by recording the fracture properties and the Young's modulus. Viscoelastic properties were revealed by recording creep during compression as well as recovery after compression. Oscillation tests up to 1000 cyclic compressions were applied to compare the fatigue of different gels. Results obtained for Ca-alginate gels, gels of chemically modified polyvinyl alcohol with stilbazolium groups (PVA-SbQ) as well as mixed gels of Ca-alginate and PVA-SbQ are compared with previously published data for kappa-carrageenan, agar, and polyethylene glycol (PEG) gels. It is concluded that material fatigue rather than mechanical properties such as stiffness or fracture stress should be considered when selecting a suitable gel material on the basis of abrasion resistance. The very soft and superelastic PVA-SbQ gel showed no significant fatigue in mechanical tests and no abrasion was detected in the standardized model system used. Ca-alginate gels, however, showed severe irreversible changes due to fatigue at oscillating loads and creep at constant load. Due to their similarities with kappa-carrageenan gels in mechanical tests, it is likely that Ca-alginate would also be sensitive to abrasion. Mixed gels of Ca-alginate and PVA-SbQ represent a complex system with intermediate properties, showing significant fatigue and creep, but elastic properties from the PVA-SbQ gel make it less sensitive than the pure Ca-alginate gel.     

Dynamic mechanical properties of agarose gels modeled by a fractional derivative model

The complex modulus (E*) and elastic modulus (E') of agarose gels (2% to 4%) are measured with a dynamic mechanical analyzer in frequency sweep shear sandwich mode between 0.1 and 20 Hz. The data showed that E* and E' increase with frequency according to a power law which can be described by a fractional derivative model to characterize the dynamic viscoelasticity of the gel. The functions between the model parameters including storage modulus coefficient (H) and the power law exponent (beta) and the agarose concentration are established. A molecular basis for the application of the fractional derivative model to gel polymers is also discussed. Such an approach can be useful in tissue culture studies employing dynamic pressurization or for validation of magnetic resonance elastography.     

A study of some factors influencing amylose gelation

Amylose fractions were prepared by aqueous leaching from pea, maize and patato starch granules. The fractions were characterised by iodine binding, β-amylolysis and viscometry. Amylose starts to form a gel rather than a precipitate on cooling aqueous solutions to room temperature at concentrations above the coil overlap concentration C¹. Amylose gels are almost purely elastic, with negligible viscous flow at room temperature. The rigidity modulus is strongly dependent on concentration, c, in that above 1·5% w/w the modulus increases as a function of c⁷. The modulus of a matured gel falls only slightly with increasing temperature; at temperatures below 100°C the gel could not be melted. The non-equilibrium nature of the system is shown by the dependence of rigidity on thermal history. The shear modulus is also dependent on amylose type; higher molecular weight amylose fractions produced less rigid gels at a given concentration.     

Rheology and gelation of water-insoluble dextran from Leuconostoc mesenteroides NRRL B-523

Dynamic oscillatory testing has been used to study the rheology of water-insoluble dextran. The rheological properties (storage and loss moduli) of dextran gel were measured and dextran was found to be neither a strong gel nor a weak gel, but an entanglement network at a concentration of 250 mg/ml. The extent of gelation, illustrated by the gel elastic modulus G′, is found to decrease with increasing concentration of calcium ions. This was confirmed by shift of crossover frequencies towards higher values on the dynamic spectra and lower yield stress τ values obtained from stress ramp experiments. Finally, a comparison between gelation of dextran and alginate (a similar biopolymer) was made for clear understanding of effect of calcium ions on the dextran gelation.     

Fibrillar beta-lactoglobulin gels: Part 3. Dynamic mechanical characterization of solvent-induced systems

Oscillatory shear rheometry has been used to study the gelation of beta-lactoglobulin at ambient in 50% v/v trifluoroethanol (TFE)/pH 7 aqueous buffer and in 50% v/v ethanol (EtOH)/water at pH 2. In contrast to what was found on heating aqueous solutions at pH 2 (Part 2 of this series), a more expected "chemical gelation"-like profile was found with modulus components G' and G' ' crossing over as the gels formed and then with G' ' passing through a maximum. In addition, for the EtOH system, there was a significant modulus increase at long time, suggestive of a more complex two-step aggregation scheme. Modulus-concentration relationships were obtained for both systems by extrapolating cure data to infinite time. For the TFE gels, this data was accurately described by classical branching theory, although it could also be approximated by a constant power--law relationship. Only the latter described the modulus--concentration data for the gels in ethanol, but there were problems here of greater frequency dependence of the modulus values and much less certain extrapolation. Gel times for the TFE systems showed higher power laws in the concentration than could be explained by the branching theory in its simplest form being similar, in this respect, to the heat-set systems at pH 2. Such power laws were harder to establish for the EtOH gels as for these there was evidence of gel time divergence close to a critical concentration. Reduced G'/G'inf versus t/tgel data were difficult to interpret for the gels in ethanol, but for the TFE system they were consistent with previous results for the heat-set gels and approximated master curve superposition. The frequency and temperature dependences of the final gel moduli were also studied. In general, the networks induced by alcohols appeared more flexible than those obtained by heating.