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@PHDTHESIS{Nothdurft:825952,
      author       = {Nothdurft, Katja},
      othercontributors = {Richtering, Walter and Bardow, André},
      title        = {{C}ononsolvency of microgels : equilibrium and dynamics},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      address      = {Aachen},
      publisher    = {RWTH Aachen University},
      reportid     = {RWTH-2021-08634},
      pages        = {1 Online-Ressource : Illustrationen},
      year         = {2021},
      note         = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
                      University; Dissertation, RWTH Aachen University, 2021},
      abstract     = {Microgels are three-dimensional, cross-linked polymeric
                      networks swollen by a good solvent.
                      Poly-N-isopropylacrylamide (PNIPAM)-based materials are of
                      ongoing scientific interest due to their unique responsive
                      behavior to external parameters such as temperature and
                      solvent composition. Although PNIPAM microgels are the
                      subject of a variety of application-oriented studies,
                      fundamental aspects about the underlying processes of the
                      responsive behavior are still under discussion. The present
                      work focuses on the sensitivity of PNIPAM to the composition
                      of water-methanol mixtures, the so-called cononsolvency
                      effect. Here, the gels are swollen in either of the pure
                      solvents, whereas water-rich mixtures around 20 $mol\%$
                      methanol cause deswelling of the gels. In the first part of
                      this thesis, the internal properties of the microgels in
                      different swelling states in comparison to their
                      surroundings were studied under equilibrium conditions. Mass
                      balance experiments combined with Raman microspectroscopy
                      revealed an enrichment of methanol inside the macroscopic
                      PNIPAM gels for the cononsolvency-inducing water-methanol
                      mixtures. This preferential adsorption of methanol was
                      confirmed by measurements of the fluorescence lifetime of
                      solvatochromic microgels beads. In addition, the collapsed
                      PNIPAM microgel beads exhibit a lower polarity in
                      unfavorable mixtures than the respective binary
                      water-methanol mixtures. The possibility to control the
                      properties of the microenvironment provided by microgels
                      using external stimuli was exploited to modulate the
                      catalytic activity of PNIPAM-based microgel-catalysts.
                      Furthermore, the mechanical properties of microgel beads
                      were investigated by variation of the external osmotic
                      pressure. Upon compression, a transition from a soft,
                      deformable polymer network to a stiffer, partially collapsed
                      object was observed. In the second part, the dynamics of the
                      volume phase transition were studied. Many applications of
                      responsive microgel systems rely on the fast and reversible
                      adaptability of the polymer network to changes in the
                      environment. To further elucidate the kinetics of the
                      polymer response, fluorescently labeled PNIPAM microgel
                      beads in the micrometer range were studied. A custom-made
                      microfluidic setup allows a fast solvent exchange from pure
                      water to the unfavorable mixture of 20 $mol\%$ methanol in
                      water. The deswelling behavior is described by a two-step
                      process. The major volume change occurs in the initial,
                      rapid process where the microgels are still porous. In the
                      second, slower process, only minor changes in size are
                      observed. The dependence of the relaxation times on the
                      microgel’s diameter is discussed taking into account the
                      adhesion-induced deformation of the gels and the physical
                      processes underlying the collapse.},
      cin          = {153310 / 150000},
      ddc          = {540},
      cid          = {$I:(DE-82)153310_20140620$ / $I:(DE-82)150000_20140620$},
      pnm          = {SFB 985 B03 - Kinetik der Volumenänderung schaltbarer
                      Mikrogele (B03) (221473085) / DFG project 191948804 - SFB
                      985: Funktionelle Mikrogele und Mikrogelsysteme (191948804)},
      pid          = {G:(GEPRIS)221473085 / G:(GEPRIS)191948804},
      typ          = {PUB:(DE-HGF)11},
      doi          = {10.18154/RWTH-2021-08634},
      url          = {https://publications.rwth-aachen.de/record/825952},
}