Antiplasticization and phase behavior in phase-separated modified starch-sucrose blends: A positron lifetime and solid-state NMR study

Interactions, organization and dynamics within phase-separated ternary blends of hydrophobically modified starch (HMS), sucrose and water are investigated using solid-state NMR and positron annihilation lifetime spectroscopy (PALS). Antiplasticization of HMS by sucrose is demonstrated by PALS and ¹H NMR T₁ measurements. Selective solid-state ¹³C NMR experiments show that a large fraction of sucrose is in molecular contact with HMS even at high sucrose contents, indicating that the HMS-sucrose phase separation is only partial. Sucrose is observed to migrate away from the HMS-rich domains at temperatures that are above the lower T_g, but still below the upper T_g. ¹H spin diffusion experiments indicate that phase separation occurs on a nanometric scale, in line with recent theory (Van der Sman, Food Hydrocolloids 87, 360–370 (2019)). We infer that the nanoscale structure of the HMS-rich phase allows for intimate molecular contact between the HMS-rich and the sucrose-rich phases and explains the unusual dynamic behavior.