Direct measurement of stress relaxation in polymeric thin films*

Understanding the stress relaxation in polymeric thin films is critical for the field of deformable and wearable electronics. In the past decade, numerous high-performance semiconducting polymers have been synthesized, while their fundamental relaxation behaviors under external stress are less concerned. This is mainly due to the lack of materials and limited testing methods. In this work, we report the direct measurement of stress relaxation in polymer thin films through both film-on-water and free-standing tensile tests. The effects of film thickness and experimental temperature are investigated on polymers with different glass transition temperatures (T_g). We found that thin-film relaxation follows an Arrhenius-type temperature dependence and accelerates as film thickness decreases. Significant depression of compensation temperature is observed in thin-films due to the acceleration effect of water on thin glassy film dynamics. Meanwhile, the water is found to depress the chain dynamics for films floated on water versus free-standing ones.