Rheology of Conjugated Polymers with Bulky and Flexible Side Chains

We report on the linear viscoelasticity of dialkylfluorene- and dialkoxybenzene-based viscoelastic conjugated polymers at a wide temperature range. Three dynamic transition temperatures are probed on the rheological spectrum, which are ascribed to the backbone (Tg,bb), the inner side chain (Tsc,inner), and the outer side chain (Tsc,outer), respectively. For our polymers that contain long branched side chains, their Tg,bb values deviate from the prediction based on atomic mobility. Tsc,outer is independent of chemical structures, while Tsc,inner is in between Tg,bb and Tsc,outer. A universal William-Landel-Ferry fitting could describe the dynamic temperature dependence of backbones for different polymers at an iso-free-volume state, while the dynamics of side chains exhibit two steps of Arrhenius behavior, corresponding to motions of inner and outer side chains, respectively. The activation energy of inner side chains monotonically decreases with the increasing side chain fraction, while that of outer side chains is independent of chemical structures.