title: Dynamics of linear poly(N-isopropylacrylamide) in water around the phase transition investigated by dielectric relaxation spectroscopy.
abstract: 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.
authors.affiliation: BAM Bundesanstalt für Materialforschung und -prüfung, Unter den Eichen 87, 12205 Berlin, Germany.
authors.forename: Marieke
authors.initials: M
authors.lastname: Füllbrandt
authors.forename: Elena
authors.initials: E
authors.lastname: Ermilova
authors.forename: Asad
authors.initials: A
authors.lastname: Asadujjaman
authors.forename: Ralph
authors.initials: R
authors.lastname: Hölzel
authors.forename: Frank F
authors.initials: FF
authors.lastname: Bier
authors.forename: Regine
authors.initials: R
authors.lastname: von Klitzing
authors.forename: Andreas
authors.initials: A
authors.lastname: Schönhals
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry*; D058266:Dielectric Spectroscopy; D056004:Molecular Dynamics Simulation; D044367:Phase Transition; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D000180:Acrylic Resins; D014867:Water; C052970:poly-N-isopropylacrylamide