Role of Molecular Weight in Microstructural Transition and Its Correlation to the Mechanical and Electrical Properties of P(NDI2OD-T2) Thin Films

Semiconducting polymers with high mobility and mechanical robust properties are strongly dependent on their molecular weight. However, the relationship between molecular weights and solution chain entanglements, film microstructures, charge carrier mobility, and mechanical properties for donor-acceptor conjugated polymers remains less understood. Herein, P(NDI2OD-T2) with a weight-average molecular weight (M-w) from 34.0 to 280 kDa was investigated as a model system. The polymer chain exhibited three regions in chloroform solutions: fewer entanglements (34.0-77.7 kDa), enhanced entanglements (170 kDa), and severe entanglements (280 kDa). This chain solution behavior resulted in three distinct film microstructures: (1) 34.0-77.7 kDa, liquid-crystalline-like morphologies with highly ordered chain arrangements and large crystallite lengths (l(c)) yet relatively low tie-chain densities that increased with M-w; (2) 170 kDa, small fibril morphology with less ordered chain arrangements and a decreased l(c) of only 5.6 nm yet a high tie-chain density; and (3) 280 kDa, a seemingly amorphous film with vast well-connected local aggregates embedded in an entangled network. The structural change in films significantly affected the electrical and mechanical performances. The electron mobility increased continuously with M-w, correlating well with the tie-chain density. By contrast, the crack-onset strain was less than 3% at 34.0-77.7 kDa and then jumped to 36.4 +/- 0.9 and 60.4 +/- 2.1% for 170 and 280 kDa, showing a close correlation with the solution entanglement density, which could be inherited into films. This study contributes to structural development of rigid chains with M-w and demonstrates that the microstructure containing vast well-connected local aggregates and adequate entanglements is promising toward mechanically robust semiconducting films.

Automated summary

The study investigated the effects of molecular weight on the behavior, microstructures, charge carrier mobility, and mechanical properties of donor-acceptor conjugated polymers. It was found that different molecular weights of P(NDI2OD-T2) led to varying states in solution and distinct impacts on film microstructures and electrical performances.