2.4. Mechanical properties
Standardized dumbbell-shaped specimens (ISO 527-2) with a 13 mm long and 3.3 mm wide rectangular working section were cut from the films with a cutting die. These were then tested for tensile resistance on a TA Instrument DMA with a force ramp of 1 N min−1. This gave ultimate tensile strength (UTS), elastic modulus (E) and the elongation at break (EL); the film toughness was obtained from the area of the stress–strain curves, and the tensile strength–elastic modulus ratio UTS/E also obtained [29]. Five to seven measurements were made for each type of film. Dynamic mechanical analyses (DMA) of the films were performed over the temperature range −120 to 20 °C with a scanning rate of 3 °C min−1. The static force, amplitude and frequency were maintained constant at 0.2 N, 2 μm and 1 Hz, respectively. To avoid water condensation, all dumbbell specimens were covered with a layer of silicone grease, which does not affect the dynamic mechanical properties of starch-based materials [5].