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 (A
0). The A
0 values for the here
studied and literature available/calculated values for all types of
branched macromolecular topologies were found to assume an average
of A
0 = (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
× 10–3
M
0.67, [S], D = 251 × 10–3
M
–0.33, [10–7 cm2 s–1], [η] = 5.9M
0, [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.