Physical properties of starch nanocrystal-reinforced pullulan films

Nanocomposite materials were prepared using sorbitol-plasticized pullulan as the amorphous matrix and an aqueous suspension of starch nanocrystals (prepared by submitting native granules from waxy maize starch to acid hydrolysis at 35 °C) as the reinforcing phase. Wide-angle X-ray diffraction analysis showed an increase of the crystallinity of the composite biopolymer films with increasing of starch nanocrystal content. The water absorption isotherms and kinetics as well as the water barrier properties of nanocomposite films filled with 0–40% (w/w) starch nanocrystals (starch nanocrystals/pullulan + sorbitol) were investigated. The water uptake of pullulan–starch nanocomposites decreased with increasing filler content whereas water vapor permeability (measured at 25 °C and 53/100 relative humidity (RH) gradient) remained constant up to 20% (w/w) and, then decreased significantly with further addition of nanocrystals. The thermo-mechanical behaviour of nanocomposite films was also investigated by means of dynamic mechanical thermal analysis (DMTA) and large deformation mechanical tests (tensile mode). The glass transition temperature (Tg) shifted towards higher temperatures with increasing amount of nanocrystals, which can be attributed to a restriction of the mobility of pullulan chains due to the establishment of strong interactions not only between starch nanocrystals but also between the filler and the matrix. Moreover, the addition of nanocrystals caused strong enhancement of the Young modulus and the tensile strength, but led to a drastic decrease of the strain at break in samples conditioned at different environments (from 43% to 75% RH).