In each sample, a certain amount of EC was firstly dissolved in 20mL acetone at 30◦C with magnetic stirring. After complete dissolution, plasticizer was added into the EC solution and stirred at 30◦C for 1h. The concentration of plasticizer was in a range of 5–30%. For example, ESO-15 represents 85% EC (1.7 g) and 15% ESO (0.3 g). Thereafter, the solution was cast into polypropylene mold with casting area of 13 cm × 13 cm. The mold was covered with a paper board immediately to control the evaporation rate of ace- tone. After drying in an oven at 40◦C for 6h, the film was peeled off the mold and stored at constant temperature (23±0.5◦C) and relative humidity (50 ± 5%) for at least 2 days before testing. Films made by casting were reproducible as a result of the controllable variables in operation and environment.