Can Hyperbranched Polymers Entangle? Effect of Hydrogen Bonding on Entanglement Transition and Thermorheological Properties of Hyperbranched Polyglycerol Melts

Melt rheology and thermal phase transition of a series of hyperbranched polyglycerol samples (hbPG) (DB approximate to 60%) in a broad molecular weight range (M-n = 600-440 000 g/mol) were investigated and correlated to both molecular weight and nature of the end group (hydroxyl vs permethylated and trimethylsilylated). The well-characterized and defined flexible polyethers are particularly suitable to shed light on the linear viscoelastic behavior with respect to (i) hyperbranched topology and (ii) hydrogen bond interactions, particularly in comparison to the perfectly linear polyglycerol counterparts studied recently [Osterwinter, C.; et al.Macromolecules 2015, 48, 119-130]. We present a detailed examination of differences found in the characteristic moduli as a consequence of functionality and topology leading to an estimation of both a stickiness parameter H and a connectivity parameter B of hyperbranched molecules. The appearance of a plateau region of the dynamic moduli indicates entanglement behavior, although the determined apparent entanglement molecular weight Me 6000 g/mol is significantly higher than the molecular weight between two branching points (M-x << M-e). Zero shear viscosities and terminal relaxation times show a unique scaling behavior with respect to the nature of the multiple end groups of the hyperbranched topology, suggesting entanglement transitions with a critical molecular weight around 55 000 g/mol. Most striking and in pronounced contrast to linear PG as the perfect linear analogue, the derived structure-property relationships depend on the functionality of the repeating unit of the polymer.