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Dynamics of linear poly(N -isopropylacrylamide) in water around the phase transition investigated by dielectric relaxation spectroscopy

: Füllbrandt, M.; Ermilova, E.; Asadujjaman, A.; Hölzel, R.; Bier, F.F.; Klitzing, R. von; Schönhals, A.


Journal of physical chemistry. B 118 (2014), No.13, pp.3750-3759
ISSN: 1089-5647
ISSN: 1520-6106
ISSN: 1520-5207
Journal Article
Fraunhofer IBMT ()

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 1011 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.