Temperature-Dependent, High-Resolution Magic-Angle-Spinning (HRMAS) NMR Studies of Poly(N-isopropylacrylamide-co-acrylic Acid)

Abstract

High-resolution magic-angle-spinning NMR spectroscopy is used to investigate the phase transition of poly(N-isopropylacrylamide-co-acrylic acid) hydrogels crosslinked with N,N’-methylene bisacrylamide, hereafter poly(NIPAAm-co-AAc). Van't Hoff ΔH and ΔS for polymer dehydration are derived from temperature-dependent NMR spectra, and the thermodynamic data strongly support a four-stage dehydration mechanism for pure poly(NIPAAm). Acrylic acid stabilizes the hydration sphere around the polymer chains. Reduced amounts of water released during the phase transition translates into smaller values for ΔH and ΔS. Enhanced rehydration kinetics for poly(NIPAAm-co-AAc) is attributed to water remaining in the samples at elevated temperatures, which may produce facile diffusion pathways and enable faster rehydration kinetics than poly(NIPAAm).

Keywords:

• acrylic acid
• HRMAS NMR spectroscopy
• hydrogels
• kinetics
• poly (N-isopropylacrylamide)
• thermodynamics

Acknowledgments

This work was supported by the University of Oklahoma and a CAREER Award from the National Science Foundation to CVR (CHE-0449622). CMB is indebted to Northeastern State University for partial support of the project.