Multi-Scale Structure And Dynamics Of Dendronized Polymers With Varying Generations

Dendronized polymers (denpols) are thick polymers comprising a linear backbone with grafted treelike structures (dendrons). The latter give rise to a molecular thickness that is a function of dendron generation. In this work, we investigate the structure and dynamics of a series of denpols with varying degrees of polymerization and generations (up to the fifth) in the melt, using X-ray scattering, dielectric spectroscopy, and shear rheometry. These polymers are well characterized and exhibit primarily topological interactions. Our findings indicate that the molar mass needed to form entanglements depends on the dendron generation. Furthermore, regardless of their molar mass, all denpols of fourth and fifth generations remain unentangled, at least in the range of the studied backbone degrees of polymerization. Stress relaxation bears signatures of different modes associated with dendron interpenetration and backbone motion and bears analogies to that of classic bottlebrushes. Interestingly, denpols of the third generation exhibit a distinct viscoelastic relaxation spectrum, which is discussed in view of the pertinent structural information. This mode is attributed to the interdigitation of dendrons, which becomes less dominant at larger generations as backfolding prevails. This interplay of dendron interdigitation and backfolding, which can be interrogated by combining rheometry and scattering, is believed to give rise to a nonmonotonicity of the terminal relaxation time with increasing generation. The fast (segmental) dynamics of the denpols is also rich. Denpols of the second generation show two glassy modes, one reflecting local liquid crystallinity, with the overall local dynamics being broader and slower compared to higher generations. A generalized representation of normalized viscosity versus degree of polymerization for different polymers of the same class (denpols, bottlebrushes, Cayley trees) shows different scaling regimes between Rouse and reptation limits. These findings bring into focus the distinct properties of these macromolecules, and at the same time, they provide ingredients for extending the current state of the art of polymer dynamics to thick polymers.