Preparation, Characterisation, and Rheological Behaviour of Starch‐Sodium Trimetaphosphate Hydrogels

Ionic biopolymer hydrogels were prepared by the cross‐linking of starches with sodium trimetaphosphate in alkaline medium at 40°C for 2 hours. The swelling capacity is relatively high — up to 310 g H20/g polymer. Salt solutions have a marked influence, and result in shrinkage but not in a total collapse. The effect of both the cross‐linker and substrate concentrations on the swelling and rheological properties was investigated. The influence of temperature and NaOH concentration on the rheological behaviour suggests that they are both significant in determining the gel properties because of the readiness of the diester phosphate bonds to undergo hydrolysis. The molecular weight between two entanglement points (Me) and the effectiveness of cross‐linking [ne(r)/ne(t)] were estimated from the observed Gp′ values, and those calculated from complete conversion of the cross‐linker. The effectiveness of cross‐linking lay between 0.2 and 2.74% for 10% wlw gels, and reached a maximum of 48.1% at the higher substrate concentration of 20% wlw. 13C‐NMR signals from the anhydroglucose units became broader and decreased in intensity with rising cross‐linker concentration due to the restricted motion arising from the additional bonding.