Thermosensitive hyperbranched polyethylenimine partially substituted with N-isopropylacrylamide monomer: thermodynamics and use in developing a thermosensitive graphene composite

Abstract

A thermosensitive hyperbranched polyethylenimine (HPEI) partially substituted with N-isopropylacrylamide (NIPAM) monomer (HPEI-pNIPAM) was obtained and its thermodynamic properties were compared with the previously reported fully substituted hyperbranched HPEI-NIPAM polymer using calorimetric and turbidity measurements and one- and two-dimensional correlation infrared spectroscopy. With fewer NIPAM units embedded in the interior of the HPEI backbone, more hydrogen bonds between CO and D₂O in the HPEI-pNIPAM polymer could be transformed into CO/D–N bonds and fewer CO related intermediates were formed during heating. During both the heating and cooling of HPEI-pNIPAM, collapse and restoration of the branched backbone occurred first, followed by the formation and debonding of self-associated CO/D–N hydrogen bonds in the same process; all of these processes might be due to a lower conformational confinement effect in the interior backbone leading to the formation of weaker hydrophobic–hydrophobic interactions. The residual NH groups in the hyperbranched thermosensitive HPEI-pNIPAM polymer could be used to bind to graphene oxide by the reaction of NH with epoxyl and carboxyl groups; it could also serve as a blocking reagent to prevent the aggregation of graphene during the preparation process. The new thermosensitive graphene composite obtained has a good potential for use in various biomedical or biosensor applications and provides opportunities for other similar hyperbranched polymers to be developed with responsive effects to multi-stimuli.