Rheological Consequences of Hydrogen Bonding: Linear Viscoelastic Response of Linear Polyglycerol and Its Permethylated Analogues as a General Model for Hydroxyl-Functional Polymers

Viscoelastic properties of linear, hydroxyl-functional polymers are only little understood with respect to the effect of functional group interactions. Melt rheology and thermal phase transitions of linear polyethers (polyglycerol, linPG-OH) and their methylated analogues (linPG-OMe) in a broad molecular weight range (M-n = 1-100 kg/mol) with low polydispersities (PDI) have been investigated as a general model for hydroxyl-functional polymers with respect to their functionality and hydrogen bond interactions. We provide detailed insight into the rheodynamics of nonentangled and well-entangled polyethers bearing one functional group per monomer unit. BooijPalmen plots (BBP) revealed failure of the timetemperature superposition principle (TTS) for both types of polymers in the segmental relaxation region, while TTS holds in the terminal relaxation region. The characteristic modulus of linPG-OMe derived from the BBP clearly reflects the transition from the nonentangled to the fully entangled state with increasing molecular weight. Quantitative analysis of these data allows for different estimates of the entanglement molecular weight, which is approximately 14 kg/mol. In case of linPG-OH a lower apparent entanglement molecular weight (8 kg/mol) leads to estimated 36 entanglement interactions in a cube of 10 nm edge length together with 47 association sites in the same volume. This can be determined from the molecular-weight-independent plateau modulus only, which is significantly lower than for linPG-OMe. This is explained as a consequence of the overlay of an entanglement network and an association network created by hydrogen bonding of the OH groups with themselves and with the ether linkages.