Structure–properties relationship in cross-linked high amylose starch cast films

Cross-linked high amylose starch cast films were prepared to study the effect of cross-linking degree on various properties in normal environmental conditions. Mechanical tensile properties (Youngʹs modulus, elongation at break, tensile strength), water vapour transmission rate (WVTR) and oxygen permeability coefficients of cast films were determined as a function of cross-linking degree and percentage of free humidity. Cross-linking degree and degree of crystallinity are closely related and seem to have non-negligible opposite effect on the properties of interest. By using increased amounts of cross-linking agents, the effect of cross-linking degree tends to reduce the degree of crystallinity modulating thus mechanical properties, water vapour permeability and oxygen permeability coefficients. Yield strength, tensile strength at break, WVTR versus cross-linking degree showed a non-monotonous behaviour. Maximal values for these properties were reached for moderate cross-linking degree. Optimal crystalline/amorphous ratio in the films may induce interactions and balanced effects, which would be responsible for the non-linear behaviour of some of the investigated properties. By cross-linking with epichlorohydrin in the range 1–10 g crosslinker/100 g polymer, the mechanical properties of films are still related to water content and water vapour permeability remains high compared to some synthetic polymeric materials.