Extremely Softened Polyimides with Long, Branched Alkyl Side Chains: Shape Memory Characteristics and Anticounterfeiting Application

Here, a polyimide with a reactive carboxyl side group on a relatively flexible main chain was manufactured as a parent polymer, and then a long, branched alkyl chain was introduced to the side group via Steglich esterification to obtain a significantly softened polymer. The longer the introduced alkyl chain, the more remarkable the plasticization effect was, and the glass transition was observed at approximately 103 °C, 165 °C lower than that of the parent polymer. Introducing a long, branched alkyl side chain also lowered the polymer’s elastic modulus and tensile strength to 0.79 GPa and 28.1 MPa, respectively. This polymer could easily deform at a temperature slightly higher than the glass transition temperature, and the deformed shape was stably maintained when cooled to room temperature. When the temperature was raised again to around the glass transition temperature, it instantly restored to its original shape, exhibiting typical shape memory characteristics. The paper coated with this polymer did not tear easily and was not wetted easily with water. Moreover, this coated paper was easily debossed owing to the polymer’s intrinsic softness. The debossed image served as a watermark and was readily erased by heating because of the shape memory characteristics. The soft polyimide is expected to be adopted as a coating material to prevent counterfeiting in the production of security documents or paper currency.