Influence of Polymer Polarity and Association Strength on the Properties of Poly(alkyl ether)-Based Supramolecular Melts

Abstract

The present work focuses on the impact of polymer polarity and hydrogen-bonding (H-bonding) end groups association strength on the structure and dynamics of poly(alkyl ether)-based supramolecular model polymers studied by means of small angle scattering (SAS), rheology, dielectric relaxation spectroscopy (DRS), and differential scanning calorimetry (DSC). These consist of poly(propylene oxide) (PPO) or poly(ethylene oxide) (PEO) backbone that self-assembly in the bulk via hydrogen bonding (H-bonding). Diaminotriazine (DAT) and thymine-1-acetic acid (THY) as well as 2-ureido-4[1H]-pyrimidinone (UPY) are the H-bonding end groups of choice. Both PPO and PEO bearing either DAT or THY as end groups associate into linear chains, but PPO-THY/DAT forms longer chains in comparison with PEO-THY/DAT. The lower polarity of the backbone results in stronger associations for the same end groups. However, the temperature dependence of the association/dissociation lifetimes for this H-bonding type is independent of the different main chain polarity as these breaking times do not exceed the rouse chain dynamics as expected within the Cates model. PPO-UPY forms a transient network characterized by the interplay between small transient bonds controlled by the characteristic times of association/dissociation between pairs of UPY groups involving probably just two ends, revealed by the similar dielectric spectra with PPO backbone, and a more stable bond type of associate that forms phase separated UPY clusters. The latter are responsible for the physical cross-links in the network and result in the rubbery-like plateau of the rheological spectra. PPO-UPY has similar structure to PEO-UPY, as validated by small angle scattering, but the UPY detachment times in the clusters are longer for PEO-UPY than for PPO-UPY, though the activation energies are very close and match the typical energy barrier specifically imposed by the hydrogen bonds. Moreover, while the segmental relaxation times and the glass transition temperature (Tg) of PPO-UPY is similar to PPO, PEO-UPY has the highest Tg value and a much slower segmental relaxation as compared to PEO and PPO based supramolecular polymers. This seems to indicate that the H-bonding association dynamics of the UPY groups is strongly influenced by the main chain polarity that play a fundamental role in defining the time scale of the association process.