NMR Study of Thermoresponsive Hyperbranched Polymer in Aqueous Solution with Implication on the Phase Transition

Abstract

High-resolution 1H NMR has been used on the thermoresponsive hyperbranched polyethylenimines (HPEIs) modified with isobutyramide (IBAm) groups (HPEI-IBAm), to study the structure and dynamics of the macromolecules in aqueous solution before and after the phase transition. It shows that the HPEI-IBAm macromolecule having a high IBAm substitution degree has a clear phase transition in aqueous solution, whereas the HPEI-IBAm macromolecule having a low IBAm substitution degree does not. The different phase transition behaviors have been attributed to the content as well as the distribution of the IBAm groups in the macromolecules. In order to deepen the understanding of the phase transition, the hydrophobic–hydrophobic interaction inside the HPEI-IBAm macromolecules was investigated by monitoring the 1H–1H NOEs between the different hydrophobic groups. An enhanced hydrophobic–hydrophobic interaction was observed in the HPEI-IBAm macromolecule having a high IBAm substitution degree after the phase transition, ...