Probing the Relationship between Molecular Structures, Thermal Transitions, and Morphology in Polymer Semiconductors Using a Woven Glass-Mesh-Based DMTA Technique

The glass transition temperature (Tg) of polymers is an important parameter that determines the kinetics of molecular organization of polymeric chains. Understanding the Tg of conjugated polymers is critical in achieving a thermally stable and optimum morphology in polymer:polymer or polymer:small molecule blends in organic electronics. In this study, we have used the woven glass-mesh-based method of dynamic mechanical thermal analysis (DMTA) to evaluate the Tg of polymer semiconductors, which is generally not easy to detect using conventional techniques such as differential scanning calorimetry (DSC). More importantly, we establish the relationship between the thermal transitions and the molecular structure of polymer semiconductors. For conjugated polymers with rigid conjugated backbones and large alkyl side chains, we report the presence of separate thermal transitions corresponding to the polymer backbone as well as transitions related to side chains, with the latter being the most prominent. By systematically comparing polymer side chains, molecular weight, and backbone structure, the origin of the Tg and a sub-Tg transitions have been successfully correlated to the polymer structures. The antiplastization effect of additives has also been used to further prove the origin of the different transitions. Thermal transitions of a range of high performing polymers applied in organic photovoltaics, including TQ1, PTNT, PTB7, PTB7-Th, and N2200, have been systematically studied in this work. According to the measurements, some of these polymers have a very small amorphous part, changing the way the morphology should be described for these materials. We infer that the main phase in these polymers consists of hairy aggregates, with a few π-stacked rigid polymer chains forming the aggregates.