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.

Quantification of solvent water and hydration dynamic of thermo‐sensitive microgel by dielectric spectroscopy

“…In general, the rotation of pure water will induce a typical Debye relaxation in an external electric field, and the characteristic relaxation time at room temperature is 8.3 ps (19 GHz) . Besides, the relaxation of PNIPAM, including the rotational reorientation of side group, usually occurs at frequency below 100 MHz, as indicated by Kita and coworkers and Scho¨nhals . Thus, we believe that the relaxation near GHz is caused by the rotational motion of water molecules.…”

An Anisotropic Hydrogel Actuator Enabling Earthworm‐Like Directed Peristaltic Crawling

“…Differential scanning calorimetry (DSC) showed the TiNS/AuNP hydrogel underwent an LCST-type phase transition at 34 8 8C, analogous to apristine PNIPAh ydrogel reference ( Figure S3). [10] We then evaluated the photothermal conversion ability of the TiNS/AuNP hydrogel. TheA uNP species we employed exhibited an absorption band at 520 nm due to ap lasmonic resonance,aswell as abroad absorption band in the shorterwavelength region due to the transition of electrons from the valence band to the conduction band ( Figure 2b); both of these absorptions have been reported to permit efficient photothermal conversion.…”

“…[9a] Because TiNSs carry dense negative charges,their cofacial geometry imparts to the hydrogel interior al arge anisotropic electrostatic repulsion between TiNSs.F or thermally modulating this repulsive force,w ee mployed ah ydrogel network composed of poly(N-isopropylacrylamide) (PNIPA) [9b] that reversibly dehydrates and rehydrates upon heating and cooling,respectively,i nw ater due to its lower critical solution temperature (LCST;3 2 8 8C). [10] Because this molecular event reversibly changes the apparent electrostatic permittivity of the hydrogel interior, [10e] the anisotropic electrostatic repulsion in the hydrogel can synchronously be modulated, thereby causing reversible expansion and contraction of the hydrogel isovolumetrically along adirection orthogonal to the TiNS plane. [9b] Our previous hydrogel actuator operates thermally.H ence, we expected that this hydrogel can be made operable optically by accommodating ap hotothermal converter in its interior.F or this purpose,w ec hose gold nanoparticles (AuNPs) as the photothermal converter,b ecause they are dispersible in water, compatible with TiNSs,a nd carry an excellent photothermal conversion ability.…”

“…Meanwhile,r eference PNIPAh ydrogels containing AuNPs prepared in presence and absence of a1 0T magnetic field without TiNSs indicated that AuNPs have no contribution to the formation of the anisotropic structure of our hydrogel, as confirmed by small-angle X-ray scattering (SAXS) and optical measurements ( Figure S2). [10] We then evaluated the photothermal conversion ability of the TiNS/AuNP hydrogel. [10] We then evaluated the photothermal conversion ability of the TiNS/AuNP hydrogel.…”

Rupturing cancer cells by the expansion of functionalized stimuli-responsive hydrogels

“…However, the fundamental mechanism of how the PNIPAM hydrogel expands when the temperature is lowered is not completely understood at the molecular level. 35,36 One explanation is based on the hydrophobic groups (that is, the isopropyl groups and the polymeric backbone) and the hydrophilic groups (that is, the amide groups) of the PNIPAM hydrogel. At low temperature, the polymeric chains adopt an extended conformation stabilized by hydrogen bonding between the hydrophilic groups of the polymer and the water molecules.…”