Viscoelasticity and phase separation of aqueous Na-type gellan solution.

Viscoelastic and phase behaviors of Na-type gellan were investigated. The complex shear modulus of aqueous gellan solutions was monitored as a function of concentration, temperature, and molar mass. At relatively low concentrations, the loss shear modulus, G'', increased steeply at a certain temperature upon cooling, which was attributed to helix formation. Above a certain concentration, a steep increase and then gradual decrease of G'' was observed. The decrease of G'' was significant and is considered to be due to anisotropic phase formation, judging from the macroscopic phase separation into upper isotropic and lower anisotropic phases of a gellan solution kept quiescently in a vial. Polarized light microscopy produced an image showing a nematic liquid crystalline phase. Although gel formation was not observed when gellan alone was used in the present study, the gellan formed a gel in the presence of an appropriate amount of salt.

Evaluation of a condensation nucleation light scattering detector for the analysis of synthetic polymer by supercritical fluid chromatography.

A condensation nucleation light scattering detector (CNLSD) was adapted for use as a detector for supercritical fluid chromatography. The performance of the CNLSD was evaluated and compared to evaporative light-scattering detector (ELSD) using a well-defined equimass mixture of uniform poly(ethylene glycol) oligomers and a certified reference material of poly(ethylene glycol) 1000. The CNLSD was able to detect a 10 times less concentrated solution of uniform oligomers compared to the ELSD. The quantitativeness of CNLSD was high enough to obtain the molecular mass distribution of poly(ethylene glycol) 1000 without any calibrations; on the other hand, the original data measured by ELSD was about 4% smaller than the certified value of poly(ethylene glycol) 1000. The CNLSD was suitable for supercritical fluid chromatography as a mass concentration detector.

Molecular structures of gellan gum imaged with atomic force microscopy in relation to the rheological behavior in aqueous systems in the presence or absence of various cations.

Aqueous solutions of gellan gum with comparable molecular masses but with different acyl contents were investigated by atomic force microscopy and rheological measurements in the presence or absence of various cations. For a high-acyl sample, no continuous network structures were identified microscopically, except in the presence of Ca (2+), where structural inhomogeneity was the highest in terms of the height distribution of molecular assemblies. Rheological thermal hysteresis between sol-gel transitions was detected in the presence of K (+) and Ca (2+), particularly Ca (2+). The storage modulus at 20 degrees C was larger in the order Na (+) < Ca (2+) < K (+). For a low-acyl sample, continuous network structures were identified in the presence of each cation, involving greater thermal hysteresis than the corresponding data for the high-acyl sample. Structural homogeneity was the highest in the presence of K (+). Thermal hysteresis and elasticity of the system were discussed in terms of continuousness and homogeneity of network structures.

Protein/polysaccharide cogel formation based on gelatin and chemically modified schizophyllan.

In the work, aldehyde groups were quantitatively introduced into schizophyllan (SPG) side chains through periodate oxidation. The periodate-oxidized SPG (POSPG) forms an elastic gel with gelatin. The cogel formation is based on the Schiff-base reaction between the amino groups of gelatin chains and the aldehyde groups of POSPG chains. The POSPG/gelatin cogel has an elastomeric character with a very small value of loss tangent. The gelation kinetics and gel properties were discussed as a function of POSPG concentration, gelatin concentration, oxidation degree, temperature, and pH. This method can be used to design a large variety of cogels between SPG and proteins.

Thermally induced coil-to-helix transition of sodium gellan gum with different molar masses in aqueous salt solutions.

Using 5 samples of well-purified Na-gellans (Na-gellans G1-G5, weight-average molar mass M(w) = 120 x 10(3)-32 x 10(3) at 40 degrees C), the effects of molar mass on the coil-to-double-helix transition in aqueous solutions with 25 mM NaCl were studied by light scattering and circular dichroism (CD) measurements, viscometry, and differential scanning calorimetry (DSC). From the temperature dependence of M(w), molar ellipticity at 201 nm [theta]201, intrinsic viscosity [eta], and DSC exothermic curves, it was found that the coil-to-double-helix transitions for G1-G5 samples took place at almost the same temperature. The [eta] and M(w) obtained in the temperature range from 40 to 25 degrees C can be explained by a simple coil/double-helix equilibrium model using the double-helix contents determined from CD data. The van't Hoff's transition enthalpy deltaH(vH) of Na-gellans depended on M(w). It is concluded that the coil-to-double-helix transitions of Na-gellans are all-or-none type transitions, and are accelerated with increasing M(w).

Rheological characterization of schizophyllan aqueous solutions after denaturation-renaturation treatment.

Schizophyllan (SPG) with a molecular weight of 2.6x10(6), designated SPG-1, is denatured and then renatured at a concentration of 1.8 wt % by alkalization-neutralization. The prepared denatured-renatured samples (DRSPG-1) are diluted to various concentrations and equilibrated for 10 days before rheological and intrinsic viscosity measurements. When concentration (C(p)) is above 0.75 wt %, DRSPG-1 aqueous systems have weak gel-type rheological properties. However, for 0.28 wt %

Effects of citric acid on the viscoelasticity of cornstarch pastes.

The effects of citric acid on the rheological properties of cornstarch pastes were studied by steady shear and dynamic oscillatory viscoelasticity, intrinsic viscosity measurements and microscopic observation. The pH of cornstarch dispersion was adjusted between 6.0 and 3.0. The viscosity of the pastes was increased by lowering the pH (between 5.5 and 3.6), while the viscosity of samples with pH below 3.5 decreased further than that of the control (pH = 6.3). Citric acid promoted the collapse of starch granules; however, adding excessive citric acid led to the hydrolysis of glucose chains. No decrease in the viscoelasticity was observed for cornstarch pastes by adding acid at 25 degrees C after gelatinization.

Solution properties of gellan gum: change in chain stiffness between single- and double-stranded chains.

Nine samples of gellan gum in the sodium form, ranging in weight-average molar mass from 3.47 x 10(4) to 1.15 x 10(5) at 40 degrees C, were investigated by static and dynamic light scattering and viscometry in 25 mM aqueous NaCl both at 40 and at 25 degrees C. The ratios of the molar mass at 25 degrees C (in the ordered state) to that at 40 degrees C (in the disordered state) were in the range of 1.99 to 2.07, supporting the scheme of the conformational transition of gellan gum between a disassociated single chain and an associated chain composed of two molecules. Focusing on the effects of polydispersity, the intrinsic viscosities, radii of gyration, and hydrodynamic radii were analyzed on the basis of unperturbed wormlike chain models. The persistence lengths were evaluated as 9.4 nm at 40 degrees C and 98 nm at 25 degrees C.

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%.

Asymmetrical-flow field-flow fractionation with on-line multiangle light scattering detection. 1. Application to wormlike chain analysis of weakly stiff polymer chains.

Four samples of hyaluronan in the sodium form, ranging in weight-average molecular weight, M(w), from 6.67 x 10(5) to 4.23 x 10(6) were investigated by asymmetrical-flow field-flow fractionation coupled to multiangle light scattering (FlFFF-MALS) in 0.2 M aqueous NaCl at 25 degrees C. M(w) and z-average radii of gyration, R(G)(z)(), obtained via FlFFF-MALS showed a good agreement with the results obtained by conventional static light scattering. Furthermore, the molecular weight dependence of the radius of gyration for sodium hyaluronan obtained via FlFFF-MALS was analyzed on the basis of the Kratky-Porod model for unperturbed wormlike chains combined with the Yamakawa theory for radius expansion factor, and a sufficiently good agreement was observed between the theoretical prediction and experimental data. These results show the potential usage of FlFFF-MALS regarding size separation and molecular characterization even for weakly stiff chains.

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.