Exploring the effect of humidity on thermoplastic starch films using the quartz crystal microbalance

The quartz crystal microbalance (QCM) is used as a non-destructive and efficient characterization tool for thin thermoplastic starch (TPS) films. Thin TPS films (1-2 μm) were prepared with 30% (w/w starch) plasticizers using either glycerol or an ionic liquid, 1-ethyl-3-methylimidiazolium acetate ([emim⁺][Ac^-]), as the plasticizer. The differences in the mechanical properties and environmental effects on the plasticized TPS films were explored. The modulus of starch-glycerol films was higher than starch-[emim⁺][Ac^-], consistent with literature data and bulk AFM measurements, likely due to superior plasticization by the ionic liquid. The starch-[emim⁺][Ac^-] films were shown to have relative stable properties at low humidity that may be due to some antiplasticization effects at low water content despite absorbing more water than starch-glycerol films at higher humidity.