Thermo-responsive behavior and microenvironments of poly(N-isopropylacrylamide) microgel particles as studied by fluorescent label method

Temperature sensitive poly(N-isopropylacrylamide) (pNIPAM) microgels are prepared and investigated using dielectric spectroscopy in a frequency range from 10 À1 Hz to 10 6 Hz at temperatures from 15 C to 50 C. The microgels were synthesized with different crosslinker molar ratios resulting in microgels with structural differences. From the dielectric response of the pNIPAM microgels the swelling/ deswelling behaviour is monitored by both the temperature (T) and the frequency ( f ) dependence of the conductivity spectra s*(f, T). The volume phase transition (VPT) at the lower critical solution temperature (LCST) is deduced by a change in the T-dependence of the DC conductivity s 0 DC . It can be explained by a decrease in the effective charge mobility and a reduction in the effective charge number contributing to s 0 DC at T > LCST. Addressing the f-dependence of the real part of the conductivity s 0 , a pronounced frequency dependence at temperatures above the LCST can be observed whereas at temperatures below the LCST the conductivity spectra resemble that of the pure solvent (water) which is frequency independent. The f-dependence of s 0 at T > LCST is assigned to the collapse of the microgel particles. At the interfaces of the collapsed particles charge carriers are blocked and/or entrapped giving rise to Maxwell-Wagner-Sillars (MWS) polarization effects. The dependence of the MWS effect on the crosslinker amount is studied in detail and conclusions concerning the internal structure of the microgels with respect to their crosslinking density are drawn. Moreover the dielectric data are related to dynamic light scattering data. A correlation between the MWS polarization effect and the swelling/ deswelling ratio expressed by the hydrodynamic radius R h at different temperatures is established for the first time.

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“…19 Structural heterogeneity has been also investigated using dye labelling microgels 20,21 or by incorporation of nanoparticles (e.g. Au).…”

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confidence: 99%

“…The fact that high crosslinked gels offer fewer interfaces (within the particle) where ions can be entrapped explains the less pronounced MWS effect. The relationship between the swelling/deswelling ratio (DR h ¼ R h, 20 C À R h,45 C ) monitored by dynamic light scattering and the step height h is shown in the inset of Fig. 9.…”

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confidence: 99%

The dielectric signature of poly(N-isopropylacrylamide) microgels at the volume phase transition: dependence on the crosslinking density

2013

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“…1a) were synthesized as reported previously [23]. The ratio of NIPAM to DBD-AE units and NI-PAM to MBAM units in the gel particles were 2500/1 and 100/1, respectively.…”

Section: Methodsmentioning

confidence: 99%

“…blue-shifted and the fluorescence intensity was increased with the temperature--induced phase transition [23,38]. The increase in the fluorescence intensity (∆FI), which is here the intensity difference between after and before the trapping, and blue-shift of the λ max can be interpreted on the basis of the surrounding condition of DBD-AE units in the PNIPAM gel particle.…”

Section: Phase Transition By Optical Trappingmentioning

confidence: 99%

“…In addition, the hydrophilicity inside the gel particles at swollen state could decrease with the increase in crosslinker concentration [23]. However, all these results are based on ensemble measurements which yield information only on average properties of gel particles and the phase transition behavior is rarely examined at single-particle level.…”

Section: Introductionmentioning

confidence: 99%

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Photon Force-Induced Phase Transition Dynamics of Single Hydrogel Nanoparticles in Water

et al. 2007

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We investigated laser-induced phase transition dynamics of µm sized single poly(N -isopropylacrylamide) hydrogel particles in water. Single poly(N --isopropylacrylamide) gel particles labeled with a polarity-sensitive fluorescent probe monomer were optically trapped by a focused laser beam and its fluorescence dynamics was analyzed. The fluorescence intensity of trapped single gel particles was increased with the trapping laser power, while the fluorescence peak wavelength was not changed. Temperature-induced changes of fluorescence properties of the single particle were confirmed to be similar to those of the bulk solution. These behaviors are well interpreted by considering that the fluorescence intensity and fluorescence peak reflect local interactions between the fluorescent probe and attaching (bound) water molecules and effective polarity determined by (free) water content in the particle, respectively. A change in the fluorescence peak wavelength after laser trapping was followed and its blue-shift was confirmed to occur within a few hundreds seconds, indicating that a single gel particle gradually attains to a globular state on this timescale, expelling initially free water molecules and then bound ones.

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Core–Shell Particles with a Temperature‐Sensitive Shell

Kawaguchi

2012

Hydrogel Micro and Nanoparticles

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Thermo-responsive behavior and microenvironments of poly(N-isopropylacrylamide) microgel particles as studied by fluorescent label method

Cited by 27 publications

References 34 publications

The dielectric signature of poly(N-isopropylacrylamide) microgels at the volume phase transition: dependence on the crosslinking density

The dielectric signature of poly(N-isopropylacrylamide) microgels at the volume phase transition: dependence on the crosslinking density

Photon Force-Induced Phase Transition Dynamics of Single Hydrogel Nanoparticles in Water

Core–Shell Particles with a Temperature‐Sensitive Shell

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