Characterisation of alginate gels using quasi-static and dynamic methods

The rheological behaviour of the calcium salts of several commercial mannuronic and guluronic alginates in the time and frequency domain was assessed by quasi-static (uniaxial compression and compressive stress relaxation) and oscillatory dynamic tests. Both tests gave almost the same range (1–10%) of apparent linear viscoelasticity.Use of the Friedrich and Heymann model [Friedrich, Chr., & Heymann, L. (1988). Extension of a model for crosslinking polymer at the gel point. Journal of Rheology, 32, 235–241] allowed the transient responses of the network of alginate gels to be approximately characterised by a single relaxation function, its empirical parameters being independent of the experimental test provided that the applied strains were kept within the elastic and proportional limit. The linear interrelation between the material compressive (Sα) and shear (Sβ) parameters was roughly checked using an independent estimate of the Poisson’s ratio via the NOL test [Warfield, R. W., Cuevas, J. E., & Barnet, F. R. (1970). Single specimen determination of Young’s and bulk moduli of polymers. Rheologica Acta, 9, 439–446]Since the information collected from simple compression or shear tests in the linear viscoelasticity regime was inter-convertible, the small amplitude oscillatory dynamic tests, which are simpler, quicker, less material consuming and thus less expensive than the quasi-static ones, might be recommended to discriminate between alginates of different origin and composition.