Viscoelastic properties of solutions of ovine submaxillary mucin

The linear viscoelastic and rheological properties of high molecular weight ovine submaxillary mucin (OSM) solution have been investigated in terms of the Newtonian steady-flow viscosity [eta(gamma)], the complex oscillatory viscosity [eta*(omega)], and the storage and loss shear moduli [G'(omega) and G"(omega)]. It was observed that tau(gamma), eta*(omega), and G'(omega) are always higher when OSM is dissolved in 0.1M NaCl than when at the same concentration in 6M GdnHCl. This is consistent with previous observations that submaxillary mucins self-associate in 0.1M NaCl to form large aggregates, which are disrupted in 6M GdnHCl. As the OSM concentration increases, the appearance of a plateau shear modulus indicates the formation of a gel network in both solvents. The results suggest gelation involves specific intermolecular interactions, perhaps due to hydrophobic forces between interdigitated oligosaccharide side chains. The viscoelastic behavior of OSM solution at high concentration is thus similar to that reported in the literature for porcine gastric mucin (PGM). However, the OSM gels are mechanically weaker, having moduli that are an order of magnitude lower than those for PGM gels of comparable concentration. The oligosaccharide side chains of OSM consist of only 1-2 sugar units compared to 10-15 for PGM, but it appears that this is sufficient to allow for intermolecular interaction and the formation of weak gels.     

Effect of lipids on the structure and rheology of gels formed by canine submaxillary mucin

Rheological experiments have shown that canine submaxillary mucin (CSM) forms gels in aqueous solution at low ionic strength and in 6M GdnHCl. Examination of specimens of intact CSM and also its subunits prepared by reduction and carboxymethylation showed that the presence of lipid increases the gel-forming capability, probably as a result of enhancement of the intermolecular hydrophobic interactions. The rheological evidence for gelation is that substantially larger values of the oscillatory storage modulus, G'(ω), and the dynamic complex viscosity, η^*(ω), are observed for lipid-containing CSM. TMs is backed up by electron micrographs of freeze fractured specimens, where we observe a network morphology in which the cross-links are formed as a result of non-bonded interactions between a number of CSM chains. The intermolecular interactions responsible for gelation probably involve hydrophobic association between the interdigitated oligosaccharides, and/or between the non-glycosylated regions of the protein core, and can occur even in a highly chaotropic medium (6M GdnHCl). In contrast to previous experiments with porcine submaxillary mucin and human tracheobronchial mucin, which form microphase-separated gels in aqueous solution, CSM solutions undergo macroscopic phase separation into polymerrich (gel) and polymer-poor (sol) phases. These data point to stronger hydrophobic interactions in lipid-containing CSM.     

Viscoelastic properties of human tracheobronchial mucin in aqueous solution

Human tracheobronchial mucin isolated from cystic fibrosis patients (CF HTBM) was purified using a combination of gel filtration and density gradient centrifugation. The resulting mucin was fractionated to reduce polydispersity and to facilitate studies of the molecular weight dependence of mucin viscoelasticity in concentrated solution. The viscoelastic properties of CF HTBM were examined in distilled water, 0.1M salt solutions and chaotropic solvents. In controlled strain experiments (strain ≥ 5%) with increasing mucin concentration, a crossover from sol to gel behavior is observed. The gel strength, as measured by the magnitude of the storage modulus at comparable mucin concentrations, is greatest for distilled water, intermediate for 0.1M NaCl, and lowest far 6M GdnHCl. In distilled water, high molecular weight mucin undergoes a sol-gel transition at ～ 12 mg/mL, and shows evidence of a plateau modulus at higher concentrations. The storage and loss moduli of concentrated high molecular weight fractions in 6M GdnHCl exhibit a power law dependence on frequency typical of weak gels near the sol–gel transition at 20 mg/mL. Similar rheology is observed in 0.1M NaCl and 0.091M NaCl/3 mM CaCl₂, but with evidence for additional weak associations at low frequency. The power law exponent in these systems is 0.70 ± 0.02, in good agreement with prediction for networks formed by a percolation mechanism. Low molecular weight fractions in these solvents exhibit a fluid-like viscoelastic response. However, low molecular weight mucin in distilled water shows a strain-dependent increase in elasticity at low frequency indicative of weak intermolecular associations. Comparison of the rheological behavior of CF HTBM with our earlier studies of ovine submaxillary mucin lends support to the idea that carbohydrate side-chain interactions are important in the gelation mechanism of mucins. © 1995 John Wiley & Sons, Inc.     

Submaxillary mucins. Intermolecular interactions and gel-forming potential of concentrated solutions.

The intermolecular interactions in concentrated solutions of pig submaxillary mucin (PSM) and sheep submaxillary mucin (SSM) were studied by mechanical spectroscopy. PSM and SSM were purified from detectable protein and nucleic acid by equilibrium centrifugation in a CsCl density gradient. PSM and SSM isolated in the presence of proteinase inhibitors showed distinct differences from preparations isolated in the presence of 0.2 M-NaCl alone, the latter having a carbohydrate and amino acid analysis similar to other preparations isolated by precipitation or ion-exchange techniques. Gel-filtration studies showed that preparations isolated in the presence of 0.2 M-NaCl alone were dissociated into smaller-sized glycoprotein units by 3.5 M-CsCl or 2.0 M-NaCl (SSM), pH 2.0 (PSM) or heating at 100 degrees C for 10 min (PSM and SSM). Preparations isolated in the presence of proteinase inhibitors were not dissociated by these treatments. Proteolysis fragmented all submaxillary mucin preparations into small glycopeptides of Mr 13,700 for PSM and of Mr 14,000 and 15,000 for SSM. PSM preparations when concentrated formed viscoelastic gels, as determined by mechanical spectroscopy. In contrast, SSM showed characteristics of a weak viscoelastic liquid under comparable conditions (coil overlap). PSM glycoprotein isolated in proteinase inhibitors formed weak viscoelastic gels at concentrations between 5 and 15 mg/ml. Preparations of PSM glycoprotein isolated in the presence of 0.2 M-NaCl (concentration 10-97 mg/ml) had the same overall mechanical gel structure as those preparations extracted in the presence of proteinase inhibitors. This gel structure was seen to collapse following proteolysis of both preparations or after acid treatment of the glycoprotein isolated in the presence of 0.2 M-NaCl, consistent with the breakdown in size of the polymeric glycoprotein. Treatment of PSM gel with 0.2 M-2-mercaptoethanol caused a surprising increase in gel strength, which was further markedly increased on removal of the reducing agent by dialysis. An association of reduced subunits of PSM was observed by gel filtration after removal of 0.2 M-2-mercaptoethanol. These results point to intermolecular disulphide exchange occurring on reduction of these PSM glycoprotein preparations. These results demonstrate that gel formation in PSM glycoprotein is similar to that for other gastrointestinal mucus glycoproteins from stomach to colon. Gel formation in PSM, as in other mucins, depends on polymerization of subunits.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dynamic light scattering and circular dichroism studies on heat-induced gelation of hard-keratin protein aqueous solutions

Animal hairs consist of aggregates of dead cells filled with keratin protein gel. We succeeded in preparing water-soluble hard-keratin proteins and reconstructing the keratin gels by heat-induced disulfide linkages in vitro. Here, the roles of intermolecular hydrophobic interaction and disulfide bonding between the proteins in the gel were discussed. Water-soluble keratin proteins consisting of mixtures of type I ( approximately 48 kDa) and type II ( approximately 61 kDa) were prepared from wool fibers as S-carboxymethyl alanyl disulfide keratin (CMADK). The gelation was achieved by heating an aqueous solution containing at least 0.8 wt % CMADK at 100 degrees C. CMADK solutions with different urea or N-ethylmaleimide concentrations or pH were exposed to dynamic light scattering (DLS) and circular dichroism (CD). DLS clarified the gelation point of CMADK solutions and provided information on the changes in keratin cluster size. DLS suggested two types of gelation mechanism. One was the regenerated chemical disulfide bonding between keratins from CMAD parts of chains. After the gel formed, this bond became important to maintain the gel structure. The other was the physical assembly due to hydrophobic interaction between alpha-helix parts of keratin chains. This hydrophobic assembly also played an important role during gelation. CD confirmed a conformational change in the keratin protein, resulting heat-induced gelation. CD clarified the relationship between keratin protein conformation and gelation, i.e., a rodlike conformation with many alpha-helix structures was necessary to associate keratin chains and form a gel network.     

The viscoelastic properties of actin solutions

The viscoelastic properties in actin solutions were investigated by measuring their elastic modulus and viscous modulus using a rheometer. The polymerization/gelation process of actin solutions was accompanied by an increase of both parameters, indicating the formation of a protein network. High shear rotational motion destroyed this network which, however, would reanneal if left undisturbed. At 25 °C under low ionic strength conditions, the viscoelastic moduli of a Spudich-Watt globular (G) actin preparation increased with time, while G-actin, purified by gel filtration maintained low viscoelastic moduli. The rigidity of the filamentous (F) actin network in a solution of Spudich-Watt actin, measured by the elastic modulus, was somewhat lower than that of gel-filtration-purified actin at the same protein concentration. The crosslink density of these F-actin networks was estimated, using models from rubber elasticity theory. The calculated density was 1 crosslink/50 actin monomers for the purified actin and 1 crosslink/120 actin monomers for Spudich-Watt actin. The results are consistent with the idea that a small amount of regulatory factor(s), which could be removed by the gel filtration step, modulates the structure of an actin network.     

Unique gelation behavior of cellulose in NaOH/urea aqueous solution

A transparent cellulose solution was prepared by mixing 7 wt % NaOH with 12 wt % urea aqueous solution which was precooled to below -10 degrees C and which was able to rapidly dissolve cellulose at ambient temperature. The rheological properties and behavior of the gel-formed cellulose solution were investigated by using dynamic viscoelastic measurement. The effects of temperature, time, cellulose molecular weight, and concentrations on both the shear storage modulus (G') and the loss modulus (G") were analyzed. The cellulose solution having a viscosity-average molecular weight (M(eta)) of 11.4 x 10(4) had its sol-gel transition temperature decreased from 60.3 to 30.5 degrees C with an increase of its concentration from 3 to 5 wt %. The gelation temperature of a 4 wt % cellulose solution dropped from 59.4 to 30.5 degrees C as the M(eta) value was increased from 4.5 x 10(4) to 11.4 x 10(4). Interestingly, at either higher temperature (above 30 degrees C), or lower temperature (below -3 degrees C), or for longer gelation time, gels could form in the cellulose solutions. However, the cellulose solution remains a liquid state for a long time at the temperature range from 0 to 5 degrees C. For the first time, we revealed an irreversible gelation in the cellulose solution system. The gel having been formed did not dissolve even when cooled to the temperature of -10 degrees C, at which it was dissolved previously. Therefore, this indicates that either heating or cooling treatment could not break such stable gels. A high apparent activation energy (E(a)) of the cellulose solution below 0 degrees C was obtained and was used to explain the gel formation under the cooling process.     

Structural changes during heat-induced gelation of globular protein dispersions

Macroscopic and molecular structural changes during heat-induced gelation of beta-lactoglobulin, bovine serum albumin, ovalbumin, and alpha-lactalbumin aqueous dispersions were probed by the mechanical and CD spectroscopy, respectively. Aqueous solutions of the native globular proteins, except for alpha-lactalbumin, exhibited solid-like mechanical spectra-namely, the predominant storage modulus G' over the loss modulus G" in the entire frequency range examined (0.1-100 rad/s), suggesting that these protein solutions were highly structured even before gelation, possibly due to strong repulsions among protein molecules. Such solid-like structures were susceptible to nonlinearly large shear but recovered almost immediately at rest. During gelation by isothermal heating, major changes in the secondary structure of the globular proteins completed within a few minutes, while values of the modulus continued to develop for hours with maintaining values of tandelta (= G"/G') less than unity. As a result, a conventional criterion for mechanically defining the gelation point, such as a crossover between G' and G", was inapplicable to these globular protein systems. beta-Lactoglobulin gels that had passed the gelation point satisfied power laws (G' approximately G" approximately omega(n)) believed to be valid only at the gelation point, suggesting that fractal gel networks, similar to those of critical gels (i.e., gels at the gelation point), were formed.     

Mechanical characterization of network formation during heat-induced gelation of whey protein dispersions

The formation of gel network structures during isothermal heating of whey protein aqueous dispersions was probed by mechanical spectroscopy. It was anticipated that the pathway of the sol-to-gel transition of whey protein dispersions is quite different from that of ordinary cross-linking polymers (e.g., percolation-type transition), since aqueous solutions of native whey proteins have been shown to be highly structured even before gelation, in our previous study. At 20 degrees C, aqueous dispersions of beta-lactoglobulin, the major whey protein, and those of whey protein isolate (WPI), a mixture of whey proteins, exhibited solid-like mechanical spectra, i.e., the predominant storage modulus G' over the loss modulus G", in a certain range of the frequency omega (1-100 rad/s), regardless of the presence or absence of added NaCl. The existence of the added salt was, however, a critical factor for determining transitions in mechanical spectra during gelation at 70 degrees C. beta-Lactoglobulin dispersions in 0.1 mol/dm(3) NaCl maintained the solid-like nature during the entire gelation process and, after passing through the gelation point, satisfied parallel power laws (G' approximately G" approximately omega(n)) that have been proposed for a critical gel (i.e., the gel at the gelation point) that possesses a self-similar or fractal network structure. In contrast, beta-lactoglobulin dispersions without added salt exhibited a transition from solid-like [G'(omega) > G"(omega)] to liquid-like [G'(omega) < G"(omega)] mechanical spectra before gelation, but no parallel power law behavior was recognized at the gelation point. During extended heating time (aging), beta-lactoglobulin gels with 0.1 mol/dm(3) NaCl showed deviations from the parallel power laws, while spectra of gels without added NaCl approached the parallel power laws, suggesting that post-gelation reactions also significantly affect gel network structures. A percolation-type sol-to-gel transition was found only for WPI dispersions without added salt.     

Dynamic light scattering studies of porcine submaxillary mucin fractions in dilute solution at intermediate scattering vectors

We report dynamic light scattering measurements over a wide range of scattering vectors for fractionated samples of porcine submaxillary mucin (PSM) glycoproteins in two different solvents: 0.1M NaCl, and 6M GdnHCl. The relaxation spectrum has been successfully resolved into a slow mode corresponding to pure translational diffusion and a fast mode containing information on the relaxation times for intramolecular motion. Analysis of the slow mode permits a light scattering evaluation of the polydispersity of these high molecular weight mucin glycoprotein fractions. Determination of the longest intramolecular relaxation times tau 1 shows that these are much longer for the PSM fractions in 0.1M NaCl compared to 6M GdnHCl. These data are consistent with earlier studies showing that the chain conformation is the same in both solvents, but that in 0.1M NaCl, the PSM glycoprotein undergoes a self-association process that is end-to-end in nature. Since the tau 1 value is intimately related to the viscoelastic behavior of PSM solutions and gels, it is interesting to speculate that the end-to-end association process may be physiologically important.     

Gelation behaviour of konjac glucomannan with different molecular weights

The deacetylation and gelation of konjac glucomannan (KGM) following alkali addition was investigated by Fourier transform infrared, while the rheological properties of KGM with different molecular weights were studied by dynamic viscoelastic measurements in shear mode and penetration force tests. It was found that gelation occurred after significant deacetylation had taken place. Rheometrical studies revealed that KGM with different molecular weights exhibited different gelation characteristics in small amplitude oscillatory shear flow. For the samples of fractionated KGM with lower molecular weights, a decrease in both the storage shear modulus (G') and loss shear modulus (G") was observed during gelation at temperatures above 75 degrees C. It is suggested that the decrease results from the wall slip between sample and measuring geometry owing to a rapid gelation process with syneresis and/or disentanglement of molecular chains adsorbed on the surface of parallel plates from those located in the bulk. Penetration force tests were employed to confirm the occurrence of slippage and thereby no decreases in rigidity of samples were observed during gelation. For the native KGM samples decreases in G' and G" during gelation were not observed, and it is suggested that this is due to the effect of the higher molecular weight and increased solution viscosity of these samples on the gelation kinetics.     

The gelation and crystallisation of amylopectin

A range of physical and chemical techniques, including viscometry, rheological measurements, dilatometry, turbidity measurements, X-ray diffraction, and differential scanning calorimetry, has been used to study the gelation of amylopectin. Gels form on cooling concentrated aqueous solutions to 1°. The development of gel stiffness is closely related to the association of amylopectin chains, as monitored by dilatometry and differential scanning calorimetry. X-Ray diffraction studies suggest that intermolecular association involves a crystallisation process. The association of amylopectin chains in the gel is substantial and is thermo-reversible at temperatures below 100°. Heterogenous acid hydrolysis of the gel followed by examination of the residue by gel-permeation chromatography showed that the associated regions contained branched fragments, the individual chains of which had a d.p. of 15. The combined data suggest that the amylopectin molecules associate by crystallisation of the branches with d.p. 15 to form a network.     

Gelation of fractionated canine submaxillary mucin in a chaotropic solvent

Rheological measurements have been performed on three molecular weight fractions of purified canine submaxillary mucin (CSM) dissolved in the chaotropic solvent 6 M guanidine hydrochloride (GdnHCI). Solutions of the lower molecular weight fractions are viscoelastic sols, and their dynamic moduli can be scaled with respect to molecular weight and concentration according to linear viscoelasticity theory. In contrast, preparations of the highest molecular weight fraction form viscoelastic gels that exhibit an equilibrium shear modulus, G_e^′, which scales with mucin concentration as G_e^′ c³. Amino acid and carbohydrate analyses of all three fractions are similar; thus, the differences in rheological behavior are attributed to molecular weight differences, which affect the degree of coil overlap in solutions of a given concentration. These observations demonstrate conclusively that mucin glycoproteins of high molecular weight form gels under conditions in which the mucin chains physically interpenetrate, even when non-covalent intermolecular interactions are extensively disrupted. A comparison of these results with previous studies of purified submaxillary and tracheobronchial mucins indicates that the carbohydrate side-chain length, in addition to molecular weight, is an important determinant of the observed elastic response and the ability to form physical gels     

Dynamic viscoelastic properties of cellulose carbamate dissolved in NaOH aqueous solution

Dynamic viscoelastic properties of cellulose carbamate (CC) dissolved in NaOH aqueous solution were systematically studied for the first time. CC was microwave-assisted synthesized from the mixture of cellulose and urea and then dissolved in 7 wt % NaOH aqueous solution precooled to -7 °C. The obtained CC solution is transparent and has good liquidity. To clarify the rheological behavior of the solution, the CC solutions were investigated by dynamic viscoelastic measurements. The shear storage modulus (G') and loss modulus (G') as a function of the angular frequency (ω), concentration (c), nitrogen content (N %), viscosity-average molecular weight (M(η)), temperature (T), and time (t) were analyzed and discussed in detail. The sol-gel transition temperature of CC (M(η) = 7.78 × 10(4)) solution decreased from 36.5 to 31.3 °C with an increase of the concentration from 3.0 to 4.3 wt % and decreased from 35.7 to 27.5 °C with an increase of the nitrogen content from 1.718 to 5.878%. The gelation temperature of a 3.8 wt % CC solution dropped from 38.2 to 34.4 °C with the M(η) of CC increased from 6.35 × 10(4) to 9.56 × 10(4). The gelation time of the CC solution was relatively short at 30 °C, but the solution was stable for a long time at about 15 °C. Moreover, the gels already formed at elevated temperature were irreversible; that is, after cooling to a lower temperature including the dissolution temperature (-7 °C), they could not be dissolved to become liquid.     

Suppression of protein interactions by arginine: a proposed mechanism of the arginine effects

Arginine has been used to suppress protein aggregation and protein-protein or protein-surface interactions during protein refolding and purification. While its biotechnology applications are gradually expanding, the mechanism of these effects of arginine has not been fully elucidated. Arginine is more effective at higher concentrations, an indication of weak interactions with the proteins. The effects of weakly interacting additives, such as arginine, on protein solubility, stability and aggregation have been explained from three different approaches: i.e., (1) the effects of additives on the structure of water, (2) the interactions of additives with the amino acid side chains and peptide bonds and (3) the preferential interactions of additives with the proteins. Here we have examined these properties of arginine and compared with those of other additives, e.g., guanidine hydrochloride (GdnHCl) and certain amino acids and amines. GdnHCl is a strong salting-in agent and denatures proteins, while betaine is a protein stabilizer. Several amino acids and amine compounds, including betaine, which stabilize the proteins, are strongly excluded; i.e., the proteins are preferentially hydrated in these solutions. On the other hand, GdnHCl preferentially binds to the proteins. Arginine is intermediate between these two extreme cases and shows a more complicated pattern of interactions with the proteins. The effects of additives on water structure, e.g., the surface tension of aqueous solution of the additives and the solubility of amino acids in the presence of additives also shed light on the mechanism of the effects of the additives on protein aggregation. While arginine increases the surface tension of water, it favorably interacts with most amino acid side chains and the peptide bonds, a property shared with GdnHCl. Thus, we propose that while arginine is similar to GdnHCl in the amino acid level, arginine interacts with the proteins differently from GdnHCl.     

Light scattering studies of the effect of Ca2+ on the structure of porcine submaxillary mucin

The effects of calcium ions on the solution properties of porcine submaxillary mucin (PSM) have been investigated by static and dynamic light scattering. The weight average molecular weights of PSM fractions are unaffected by the addition of up to 0.5M CaCl2: these data are within experimental error of those for solutions in 0.1M NaCl. The distribution of relaxation frequencies derived from the dynamic data shows the existence of two distinct relaxation modes. The average relaxation times have been interpreted to yield the z-average translational diffusion coefficient and the longest intramolecular relaxation time tau1. A plot of tau1 vs the mean value of 1/Rh-3z is linear, and consistent with plots of such data recorded for PSM in 0.1m NaCl and 6M GdnHCl solutions. However, the tau values and the associated results for the mean value of R-1h-1z in 0.5M CaCl2 are smaller than those determined in 0.1M NaCl. This suggests that the conformation of PSM in CaCl2 solution is more contracted than those in the other two solvents. These results are consistent with the compact packaging of mucin in the secretary granules that have elevated Ca2+ levels.     

Rheological study into the ageing process of high methoxyl pectin/sucrose aqueous gels

The ageing process of high methoxyl pectin (HMP)/sucrose gels was followed at different ageing temperatures by small amplitude oscillatory experiments. Dynamic mechanical measurements allowed the characterisation of the point at which the system undergoes the sol/gel transition. The HMP/sucrose system is extremely sensitive to temperature variation during ageing, especially in the lower temperature range. The viscoelastic behaviour through the gel point changes with the ageing temperature, probably due to variations in mobility of the pectin chains, and consequently, in the lifetime of junction zones. Weaker pectin networks are formed under thermal conditions unfavourable to the development of hydrophobic interactions. Gel time and elastic modulus have a complex dependence on temperature, which could be attributed to the different thermal behaviour of the intermolecular interactions that stabilise the nonpermanent cross links of these physical networks.     

Influence of alkaline concentration on molecular association structure and viscoelastic properties of curdlan aqueous systems

Curdlan is an extracellular polysaccharide produced from soil microorganism Alcaligens faecalis var. 10C3K, and the linear structure consists of β-1,3-glycoside linkages. Curdlan is not soluble in water but it is soluble in alkaline aqueous solution, and we can obtain the gel when curdlan alkaline solution is heated above 60°C or neutralized by acids. In the present study, the gelation mechanism and dispersing structure of curdlan in the alkaline solutions are studied in terms of correlation between the molecular association structure and viscoelastic properties, using static light scattering and rheological measurements. The degree of association for the curdlan molecules in dilute solution increases with decreasing alkaline concentration. The viscoelastic properties also depend strongly on the alkaline concentration. The concentrated curdlan solution shows almost Newtonian flow at high alkaline concentrations and shows a gel-like behavior at low alkaline concentrations. It was elucidated that the molecular association in the dilute solution reflects on the viscoelastic properties of the concentrated solution and that the gelation mechanism is related to the association structure of curdlan molecules. In the case of lower NaOH concentration systems, the molecular association is likely to consist of a hydrophobic core and hydrophilic surface. The gelation mechanism above 60°C is considered to include the dissociation process of the molecular association and reformation of the network structure. © 1997 John Wiley & Sons, Inc. Biopoly 42: 479–487, 1997     

A gel network constituted by rigid schizophyllan chains and nonpermanent cross-links

This work reports a gel network formed by rigid schizophyllan (SPG) chains with Borax as a cross-linking agent. The formed cross-links are non-permanent and somewhat dynamic in nature because the cross-linking reaction is governed by a complexation equilibrium. Gelation processes are traced by dynamic viscoelastic measurements to examine the effects of Borax content, SPG concentration, temperature, salt concentration, salt type, and strain. The first-order kinetic model containing three parameters, t(0) (induction time), 1/tau(c) (gelation rate), and (saturated storage modulus), is successfully applied to describe the gelation of the SPG-Borax system. Gelation occurs faster at higher Borax content, higher SPG concentration, higher salt concentration, or lower temperature. Moreover the gelation is cation-type-specific. Storage modulus is a linear function of both Borax content and SPG concentration. The linear relationship between storage modulus and Borax content can be explained by a modified ideal rubber elasticity theory with a front factor alpha to take into account the presence of ineffective cross-links and the effect of SPG chain rigidity. On the other hand, the linear dependence of storage modulus on SPG concentration could be explained on the basis of chain-chain contacting behavior of extended SPG chains. Apparent activation energy and cross-linking enthalpy are calculated to be -74.5 and -32.4 kJ/mol for the present system. Strain sweep measurements manifest that the elasticity behavior of this gel starts to deviate from Gaussian-chain network at a small strain of 10%.     

Disentangling alpha from beta mechanical relaxations in the rubber-to-glass transition of high-sugar-chitosan mixtures

The occurrence of molecular motions in addition to those of the glass-transition region (alpha mechanism) were investigated in chitosan and a branched derivative substituted with alkyl chains having eight carbon atoms. Once hydrophobic interactions of the alkyl groups in aqueous solution were demonstrated, polymers were mixed with glucose syrup at high levels of solids. The real (G') and imaginary (G") components of the complex dynamic modulus in high-solid mixtures were measured between 0.1 and 100 rad s(-1) in the temperature range from -55 to 50 degrees C. The method of reduced variables gave superposed curves of G' and G", which unveiled an anomaly in the dispersion of the alkylated derivative both in terms of higher modulus values and dominant elastic component of the polymeric network, as compared with the glass-transition region of chitosan. It was proposed that the new mechanical feature was due to beta mechanism, and master curves of viscoelastic functions and relaxation processes were constructed to rationalize it.