Role of Molecular Weight in the Mechanical Properties and Charge Transport of Conjugated Polymers Containing Siloxane Side Chains.

The molecular weight is a key factor affecting the properties of conjugated polymers. To determine the critical molecular weights of conjugated polymers modified with siloxane side chains, poly-diketo-pyrrolopyrrole-selenophene (PTDPPSe-5Si) samples with molecular weights ranging from 20 to 350 kDa are synthesized. The critical molecular weight of the polymer is determined in the range of 60-100 kDa by testing the viscosity of the solution. When the molecular weight of the 27-60 kDa polymers is below the critical molecular weight, they exhibit a high crystallinity and low ductility. When the molecular weight of the 100 kDa polymer reaches the critical molecular weight, the crystallinity decreases, and the ductility increases. As the molecular weight increases, the polymer film also gradually changes from brittle to ductile. Furthermore, when the molecular weight of the 315 kDa polymer is much higher than the critical molecular weight, the film exhibits a significant ductility, which results in the polymer films showing no pronounced cracks after high-percentage stretching. Additionally, due to the oriented alignment of the molecular chains caused by stretching, the carrier mobility in the parallel direction becomes 2.14-fold of the initial film.