NMR and DSC study of temperature-induced phase transition in aqueous solutions of poly(N-isopropylmethacrylamide-co-acrylamide) copolymers
Abstrakt
Combination of H-1 NMR spectroscopy and differential scanning calorimetry (DSC) was used to investigate temperature-induced phase transition in D2O solutions of poly(N-isopropylmethacrylamide-co-acrylamide) random copolymers. Both the NMR and DSC data showed dependence on the acrylamide (AAm) content in the copolymer; with increasing AAm content, the phase transition is shifted to higher temperatures, and both phase-separated fractions determined by NMR and change of the enthalpy determined by DSC decrease faster than the content of thermosensitive N-isopropylmethacrylamide (NIPMAm) units in the copolymer.
NMR data were used to construct van't Hoff plots, and changes of the enthalpy Delta H and entropy Delta S, characterizing the phase transition, were determined. As it follows from comparison of NMR and DSC thermodynamical parameters (Delta H values), the size of the cooperative units (domains), undergoing the transition as a whole, decreases with increasing AAm content in the copolymer since the NIPMAm collapsed domains are separated by regions with hydrated AAm and surrounding NIPMAm sequences.