A Study on Intrinsic Mechanical Properties of n-type Conjugated Polymer via Controlling the Molecular Weight: The Importance of Critical Molecular Weight for Stretchable Organic Electronics

The understanding on the mechanical properties of semicrystalline n-type conjugated polymers is very important for developing the stretchable electronics. In this work, we investigated the intrinsic mechanical properties of naphthalene diimide (NDI) based n-type conjugated polymer, P(NDI2OD-T2) via controlling the number-average molcular weight (M_n), varing from low to very high M_n = 15, 20, 48, 103 and 163 kg mol^-1. While we observed the general increasing trend of tensile properties as a function of M_n, a sharp transition in the strain at fracture and toughness values was observed between 48 and 103 kg mol^-1 with an increase by a factor of 26 and 160, indicating the presence of the critical molecular weight. This distinct transition from brittle to ductile is mainly attributed to large fraction of amorphous regions including tie molecules and interchain entanglements, which can effectively dissipate a substantial strain energy. The molecular weight dependence of mechanical behavior coincide well with thermal, viscoelastic and microstructural and thin film morphological properties. Therefore, our work suggests design rule for n-type conjugated polymer having good compromise between mechanical reliability and electrical performance for producing stretchable electronics.