Is the Microgel Collapse a Two-Step Process? Exploiting Cononsolvency to Probe the Collapse Dynamics of Poly- N -isopropylacrylamide (pNIPAM)

The Journal of Physical Chemistry B, vol. 125, pp. 1503–1512

Abstract

Many applications of responsive microgels rely on the fast adaptation of the polymer network. However, the underlying dynamics of the de-/swelling process of the gels have not been fully understood. In the present work, we focus on the collapse kinetics of poly- N -isopropylacrylamide (pNIPAM) microgels due to cononsolvency. Cononsolvency means that either of the pure solvents, e.g., pure water or pure methanol, act as a so-called good solvent, leading to a swollen state of the polymer network. However, in mixtures of water and methanol, the previously swollen network undergoes a drastic volume loss. To further elucidate the cononsolvency transition, pNIPAM microgels with diameters between 20 and 110 μm were synthesized by microfluidics. To follow the dynamics, pure water was suddenly exchanged with an unfavorable mixture of 20 mol% methanol (solvent-jump) within a microfluidic channel. The dynamic response of the microgels was investigated by optical and fluorescence microscopy and Raman microspectroscopy. The experimental data provide unique and detailed insight into the size-dependent kinetics of the volume phase transition due to cononsolvency. The change in the microgel’s diameter over time points to a two-step process of the microgel collapse with a biexponential behavior. Furthermore, the dependence between the two time constants from this biexponential behavior and the microgel’s diameter in the collapsed state deviates from the square-power law proposed by Tanaka and Fillmore [ J. Chem. Phys. 1979, 70, 1214−1218]. The deviation is discussed considering the adhesion-induced deformation of the gels and the physical processes underlying the collapse.

Authors 11

1. Katja Nothdurft corresponding Aachen Institute of Physical Chemistry

   RWTH Aachen University

   Affiliation as printed

   Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany

2. David Heinrich Müller Aachen Institute of Technical Thermodynamics

   RWTH Aachen University

   Affiliation as printed

   Institute of Technical Thermodynamics, RWTH Aachen University, Schinkelstr. 8, 52062 Aachen, Germany

3. Sonja D. Mürtz Aachen Institute of Physical Chemistry

   RWTH Aachen University

   Affiliation as printed

   Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany

4. Anna A. Meyer Aachen Institute of Physical Chemistry

   RWTH Aachen University

   Affiliation as printed

   Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany

5. Luis P. B. Guerzoni Aachen DWI - Leibniz Institute for Interactive Materials

   DWI – Leibniz Institute for Interactive Materials

   Affiliation as printed

   DWI-Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52074 Aachen, Germany

6. Alexander Jans Aachen DWI - Leibniz Institute for Interactive Materials

   DWI – Leibniz Institute for Interactive Materials

   Affiliation as printed

   DWI-Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52074 Aachen, Germany

7. Alexander J. C. Kühne Aachen DWI - Leibniz Institute for Interactive Materials

   DWI – Leibniz Institute for Interactive Materials

   Affiliation as printed

   DWI-Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52074 Aachen, Germany

8. Laura De Laporte Aachen DWI - Leibniz Institute for Interactive Materials Institute of Technical and Macromolecular Chemistry

   DWI – Leibniz Institute for Interactive Materials · RWTH Aachen University

   Affiliation as printed

   DWI-Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52074 Aachen, Germany

   Institute of Technical and Macromolecular Chemistry, RWTH Aachen, Worringerweg 1-2, 52074 Aachen, Germany

9. Thorsten Brands Aachen Institute of Technical Thermodynamics

   RWTH Aachen University

   Affiliation as printed

   Institute of Technical Thermodynamics, RWTH Aachen University, Schinkelstr. 8, 52062 Aachen, Germany

10. André Bardow corresponding Aachen Institute of Technical Thermodynamics

    ETH Zurich · RWTH Aachen University

    Affiliation as printed

    Department of Mechanical and Process Engineering, ETH Zürich, Tannenstr. 3, 8092 Zürich, Switzerland

    Institute of Technical Thermodynamics, RWTH Aachen University, Schinkelstr. 8, 52062 Aachen, Germany

11. Walter H. Richtering corresponding Aachen DWI - Leibniz Institute for Interactive Materials Institute of Physical Chemistry

    DWI – Leibniz Institute for Interactive Materials · RWTH Aachen University

    Affiliation as printed

    DWI-Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52074 Aachen, Germany

    Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany