Rapid stress relaxation of high-Tg conjugated polymeric thin films

Conjugated polymers consist of complex backbone structures and side-chain moieties to meet various optoelectronic and processing requirements. Recent work on conjugated polymers has been devoted to studying the mechanical properties and developing new conjugated polymers with low modulus and high-crack onset strain, while the thin film mechanical stability under long-term external tensile strain is less investigated. Here we performed direct mechanical stress relaxation tests for both free-standing and thin film floated on water surface on both high-T-g and low-T-g conjugated polymers, as well as a reference nonconjugated sample, polystyrene. We measured thin films with a range of film thickness from 38 to 179 nm to study the temperature and thickness effect on thin film relaxation, where an apparent enthalpy-entropy compensation effect for glassy polymer PS and PM6 thin films was observed. We also compared relaxation times across three different conjugated polymers and showed that both crystalline morphology and higher modulus reduce the relaxation rate besides higher glass transition temperature. Our work provides insights into the mechanical creep behavior of conjugated polymers, which will have an impact on the future design of stable functional organic electronics.