The queous solutions of 4 sodium alginate samples with different molecular weights

molecular weight distributions and M／G ratios were characterized with GPC

elemental anlaysis

and 1H-NMR．The sol-gel transition was determined by monitoring the dynamic moduli of these solutions with increasing their concentration within the linear viscoelasticity region at 25℃．This transition was found to obey the Winter and Chambon’s relaxation modulus equation G(t)．From the independency of tanδ on frequency ω

the concentration of the gel point cgel and the critical exponent for the sol-gel transition n were estimated．For all samples cgel is 7.6wt％ ～8.0wt％

almost independent of molecular weight Mw

and n equals 0.32～0.38 for three samples with higher Mw and 0.61 for the sample with lower Mw．This finding indicates that physical gelation in aqueous solutions of sodium alginate is mainly induced by the interaction between repeat units of the macromolecules

and that the longer chains contribute more perfect crosslinking structure at the gel point．