‘Gel collapse’ in the saponified starch-g-polyacrylonitrile/water-alcohol system

Abstract

A sample of starch-g-polvacrylonitrile (grafting ratio 82%) has been prepared at 303 K using the Mn 3+ complex ion as initiator in aqueous acid solution with gelatinized starch as substrate. Subsequent alkaline hydrolysis converts the nitrile groups to carboxamide (∼30%) and sodium carboxylate (∼70%) groups. The swelling of the copolymer formed which retains about 700 g water per g of dry copolymer, has been studied in alcohol/water mixtures of increasing alcohol content at 294, 304 and 314 K. The liquid retention vs. alcohol concentration indicates one minor transition for ethanol/water mixtures and possibly two small transitions for methanol/water mixtures. The main transition for both alcohol/water mixtures is a collapse of the gel at 50–60 volume% ethanol and 60–70 volume% methanol, respectively. Increasing the temperature from 294 to 314 K increases the liquid retention and moves the transitions to higher alcohol concentrations by 5–10 volume%. When the starch copolymer is hydrolysed in dilute mineral acid to remove the starch moiety, the retention values increase by a factor of approximately 3 for both pure water and alcohol/water mixtures. The retention vs. methanol concentration curves show three transitions which are similar to those of the starch copolymer before acid hydrolysis. A tentative interpretation of the transitions is proposed and discussed.