Plasticizing-antiplasticizing effects of water on physical properties of tapioca starch films in the glassy state

The effects of moisture sorption on physical properties of native and cross -linked starch films in the glassy state were studied. Water played a dual role as a plasticizer or an antiplasticizer, depending on the physical prop erty measured. Plasticizing effects were clearly evident in the case of the calorimetric glass transition temperature (T-g), tensile modulus, linear e xpansion, and water vapor permeability. In contrast, antiplasticization by water resulted in maxima in tensile strength, strain-at-break, and toughnes s of films that were observed at an intermediate moisture content ranging f rom 4% to 8% (RVP 0.1 to 0.4). The seemingly contradictory effects of water on mechanical properties associated with lower and higher deformation of s tarch films were reconciled by assigning different roles to water operating primarily via opposite entropic/free volume effects. Relationships, if any , between DSC thermal events and mechanical antiplasticization were not app arent.