No abstract

Failure stresses and strains were measured in compressive, tensile and torsional modes on gellan gels at four polymer (O&1.8% w/v) and seven Ca++ (1.5-60 mM) concentrations. Shear stresses at failure were equal in all three testing modes and proportional to gellan content. Low calcium gels increased linearly in strength with Ca+ + concentration until it reached a level of about 0.5 calcium ions per repeat tetrasaccharide unit of gellan gum polymer. Gel strength decreased linearly with Ca+ + at higher concentrations. Low calcium gels were extensible with failure strains decreasing as the logarithm of Ca+ +; whereas high calcium gels were brittle and failed at a constant strain, the value of which was twice as high in compression and torsion as in tension.

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Mechanical behavior of gels formed with gellan polymer crosslinked by calcium and magnesium ions was studied to determine the inthtence of divalent ion type and polymer concentration. Failure strength and deformation were measured in compression and related to concentrations of gellan and bound cations in gel matrices. Insufficient cations formed weak, extensible gels. Maximum gel strength was achieved at 0.5 divalent cations/repeat tetrasaccharide unit, assumed to be the condition for maximal numbers of complete junction zones. At optimum cation levels gels with Ca++ were about 1.2 times stronger than gels with Mg" at the same polymer concentration. Excessive cations weakened the gels. Twice as much reduction in gel strength resulted from additional Ca++ as compared to the same additional amount of Mg". Differences between strengths of the gels may be attributable to polymer configurations at junction zones in relation to cation size.