Plasticisation and mobility in starch-sorbitol films

The purpose of this study was to elucidate the molecular mechanism involved in the behavioural changes observed at macroscopic level in a starch-sorbitol-water system. At a low sorbitol content (<27%), maximum stress decreased and the yield at break was quite constant at around 5%. This behaviour was compared with antiplasticisation, a well-known phenomenon occurring with synthetic polymers. The study of storage loss modulus by dynamic thermal mechanical analysis (DTMA) confirmed antiplasticisation. Sorbitol at high content (>27%) acts as a plasticiser. The two relaxations observed with DTMA, alpha associated with glass transition (occurring at high temperature, >30 degrees C) and two occurring at low temperatures (<10 degrees C), were involved in behavioural changes. System mobility as a function of sorbitol content was determined by time domain nuclear magnetic resonance. Much of this study was performed by replacing H2O with deuterium oxide to mask the signal from water and therefore analyse only the starch and sorbitol signals. The results show the existence of two competitive effects: one associated with a decrease in mobility and the other with the enhanced mobility of the system. The relative importance of these two effects seems to be reversed, depending on sorbitol content, which could account for the changes observed at the macroscopic level. (C) 1999 Academic Press.