Importance of Critical Molecular Weight of Semicrystalline n-Type Polymers for Producing Highly-Stretchable, Conducting Thin Films
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Mechanical properties of conducting polymers are an import issue in the field of flexible and stretchable electronics, but fundamental understanding of the polymer structure-mechanical property relationship is still very limited. Here we report systematic investigation on the dependence of mechanical and fracture properties on molecular weight for a well-known n-type conjugated polymer (P(NDI2OD-T2)). By synthetic control, the number average molecular weight (Mn) precisely tuned from 15 to 163 kg mol-1 were prepared. Importantly, the high-Mn thin films (＞ 100 kg mol-1 above the critical molecular weight) exhibited superior mechanical compliance and robustness with the 114-fold enhanced strain at fracture and 2820-fold enhanced toughness, as compared to those based on low-Mn polymers (15 kg mol-1). More importantly, a noticeable increment in mechanical properties were seen between Mn = 48 and 103 kg mol-1, yielding a 26-fold enhanced strain at fracture and 160-fold enhanced toughness.