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Dynamics of Linear Poly(N- isopropylacrylamide) in Water around the Phase Transition Investigated by Dielectric Relaxation Spectroscopy
Zitatschlüssel fuel14
Autor Füllbrandt, M. and Ermilov, E. and Asadujjaman, A. and Hölzel, R. and Bier, F. F. and von Klitzing, R. and Schönhals, A.
Seiten 3750–3759
Jahr 2014
DOI 10.1021/jp501325x
Journal J. Phys. Chem.
Jahrgang B 118
Nummer 118
Zusammenfassung The molecular dynamics of linear poly(N-isopropylacrylamide) (pNIPAM) in aqueous media at temperatures below and above the lower critical solution temperature (LCST) are investigated using broadband dielectric relaxation spectroscopy in a frequency range from 10(-1) to 10(11) Hz. Below the LCST, two relaxation processes are observed in the megahertz and gigahertz region assigned to the reorientation of dipoles of the solvated polymer segments (p-process) and water molecules (w-process), respectively. Both relaxation processes are analyzed using the Havriliak-Negami (HN) function, taking special attention to the w-process. Above the LCST, the dielectric spectra of the pNIPAM solutions resemble that of pure water, showing only the high frequency relaxation process of the water molecules with a more or less Debye-type behavior. The non-Debye behavior of the w-process below the LCST is mainly induced by the interactions between water and pNIPAM chains via hydrogen bonding. The relaxation time and strength of the w-process is studied with dependence on the concentration, temperature, and the polymer chain length (molecular weight). The information obtained is useful for a deeper understanding of the dehydration behavior at the phase transition. The suggestion of dehydration of the pNIPAM chains at the LCST is confirmed by calculating a dehydration number.
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