Hierarchical Dynamics in Bottlebrush Polymers

Bottlebrush polymers, with densely grafted side chains along a flexible backbone, emerge as a new class of building blocks for soft materials. Their unusual architecture profoundly reshapes molecular motion, yet the origin of their complex dynamics remains elusive. Here, we reveal a hierarchy of relaxation processes in bottlebrush polymers that spans multiple length and timescales. By tuning the side chain and the backbone length, we transform their behavior from that of soft, particulate objects to flexible, polymer-like chains. Linear rheology reveals five distinct dynamic modes, while dielectric spectroscopy captures the three fastest, showing how different probes sense complementary molecular motions. We identify a modulus plateau at the intermediate time scales arising from the renormalized segmental motion of bottlebrush polymers, with an onset governed by the undulation of the bottlebrush backbone. Remarkably, the stiffness of this plateau decreases with side chain length, reminiscent of densely packed soft particulates with increasing compressibility. At long times, the viscosity (η) of unentangled bottlebrushes scales with backbone molecular weight (M) as η~M³, a behavior reminiscent of entangled linear polymers due to reptation. These results uncover how molecular architecture programs collective dynamics in macromolecules, bridging principles of polymer and colloidal physics.