“Hard” Sphere Behavior of “Soft”, Globular-like, Hyperbranched Polyglycerols – Extensive Molecular Hydrodynamic and Light Scattering Studies

Whether or not hyperbranched polymers behave like quasi “hard spheres” in solution is the subject of numerous fundamental discussions, also motivated by research on the perfectly branched dendrimer structures. Experimentally targeting this question, a homologous series of hyperbranched polyglycerols (HPGs) was prepared in a wide range of molar masses from ca. 3000 to 700000 g mol–1 and an overall degree of branching (DB) between 0.55 and 0.59. HPG samples have been investigated by a comprehensive set of experimental hydrodynamic and light scattering approaches, i.e., sedimentation velocity studies in analytical ultracentrifugation, dynamic and static light scattering experiments, isothermal diffusion experiments, intrinsic viscosities, and size exclusion chromatography coupled with multiangle laser light scattering. The physical soundness of the obtained average molar masses, evaluated by the different, arguably, absolute approaches to molar mass estimations was verified via the concept of the hydrodynamic invariant (A0). The A0 values for the here studied and literature available/calculated values for all types of branched macromolecular topologies were found to assume an average of A0 = (2.6 ± 0.4) × 10–10 g cm2 s–2 K–1 mol–1/3. The hyperbranched polyglycerols adopt a very compact, globular-like conformation in aqueous solution, which is accompanied by a very high level of hydration, on average 1.7 g of water per 1 g of HPG macromolecules. The correspondingly determined classical scaling relationships return values that are characteristic for a classical hard sphere conformation: s = 2.16 × 10–3M0.67, [S], D = 251 × 10–3M–0.33, [10–7 cm2 s–1], [η] = 5.9M0, [cm3 g–1]. An experimentally high level of molecular compactness is then also reflected by the corresponding contraction factors, which show up to 50 times less molecular volume of HPGs at high molar mass values than their linear analogues.