Facile Preparation of Starch-Based Electroconductive Films with Ionic Liquid

Here, we discovered that starch could be straightforwardly processed into optically transparent electroconductive films by compression molding at a relatively mild temperature (55 or 65 °C), much lower than those commonly used in biopolymer melt processing (typically over 150 °C). Such significantly reduced processing temperature was achieved with the use of an ionic liquid plasticizer, 1-ethyl-3-methylimidazolium acetate ([C₂mim][OAc]). A higher [C₂mim][OAc] content, lower processing temperature (55 °C), and/or higher relative humidity (RH) (75%) during the sample postprocessing conditioning suppressed the crystallinity of the processed material. The original A-type crystalline structure of starch was eliminated, although small amounts of B-type and V-type crystals were formed subsequently. The starch crystallinity could be linked to the mechanical properties of the films. Moreover, the processing destroyed the original lamellar structure of starch, and the amorphous starch processed with [C₂mim][OAc]/water could aggregate on the nanoscale. The films displayed excellent electrical conductivity (>10^-3 S/cm), which was higher with a lower processing temperature (55 °C) and a higher conditioning RH (75%). The incorporation of [C₂mim][OAc] reduced the thermal decomposition temperature of starch by 30 K, while the formulation and processing conditions did not affect the film thermal stability.