Unveiling the Thermomechanical Behavior of Free-Standing Polymer Thin Films on an Ionic Liquid Surface at Sub-Ambient Temperatures Down to –10 °C

The thermomechanical behavior of free-standing polymer thin films at sub-ambient temperatures is critical to the thermomechanical reliability of wearable electronics operating in outdoor environments, as the films become more brittle with decreasing temperature. However, the behavior has not been reported due to the fragility of free-standing thin films and the absence of an adequate cooling system for them. In this study, we investigate the thermomechanical behavior of free-standing polymer thin films on an ionic liquid surface at sub-ambient temperatures down to –10 °C, using a novel direct testing method developed based on a Peltier cooling system. Specifically, this study focuses on thermoplastic polymers [polystyrene (PS) and regiorandom poly(3-hexylthiophene) (RRa P3HT)] and thermoset polymers [polydimethylsiloxane (PDMS) and polyimide (PI)], based on their internal structures. Interestingly, it is demonstrated that the crack-onset strain (COS) of PS thin films increases sharply from 13.1 ± 0.5% at –10 °C to 60.0 ± 5.3% at 25 °C due to crazing. Furthermore, the tensile and thermal expansion properties of RRa P3HT thin films change significantly across the glass transition temperature (Tg). Also, the tensile strength of PDMS thin films decreases from –10 to 25 °C, although PDMS is known to be relatively less temperature-dependent at sub-ambient temperatures. This study initiates research on the thermomechanical behavior of polymer thin films at low temperatures and underscores the necessity of characterizing this behavior, proposing a direct thermomechanical testing platform at low temperatures.