Probing the Collapse Dynamics of Poly(N-isopropylacrylamide) Brushes by AFM: Effects of Co-nonsolvency and Grafting Densities

Authors

  • Xiaofeng Sui,

    1. Department of Materials Science and Technology of Polymers, University of Twente, MESA+ Institute for Nanotechnology, P.O. Box 217, 7500 AE Enschede, The Netherlands
    Current affiliation:
    1. [+] These authors contributed equally to this work.
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  • Qi Chen,

    1. Department of Materials Science and Technology of Polymers, University of Twente, MESA+ Institute for Nanotechnology, P.O. Box 217, 7500 AE Enschede, The Netherlands
    Current affiliation:
    1. [+] These authors contributed equally to this work.
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  • Mark A. Hempenius,

    1. Department of Materials Science and Technology of Polymers, University of Twente, MESA+ Institute for Nanotechnology, P.O. Box 217, 7500 AE Enschede, The Netherlands
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  • G. Julius Vancso

    Corresponding author
    1. Department of Materials Science and Technology of Polymers, University of Twente, MESA+ Institute for Nanotechnology, P.O. Box 217, 7500 AE Enschede, The Netherlands
    • Department of Materials Science and Technology of Polymers, University of Twente, MESA+ Institute for Nanotechnology, P.O. Box 217, 7500 AE Enschede, The Netherlands.
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Abstract

Collapse of poly(N-isopropylacrylamide) (PNIPAM) brushes in the mixed solvent system (water/methanol 50% v/v) is studied by in-situ atomic-force microscopy (AFM). PNIPAM brushes with three different grafting densities and similar chain lengths are synthesized via surface-initiated atom-transfer radical polymerization. By changing the solvent from water to a water/methanol (50% v/v) mixture, the polymer brushes switch from a swollen to collapsed state. AFM force measurements using a silica colloidal probe attached to the tip are employed to obtain the Young’s moduli of the polymer brushes in different solvation states. The collapse dynamics of the brush is followed by monitoring the pull-off force (adherence) in situ. The modulus of the swollen high-density polymer brush is four times lower than that of the same brush in the collapsed state. It is shown that in the case of the high-density polymer brush with a thickness (tin water) of 900 nm, the collapse takes place in a time scale of ~25 s, whereas the collapse occurs faster for the medium-density brush (tin water = 630 nm) and much more rapidly for the low-density brush (tin water = 80 nm). This difference in the response kinetics is primarily ascribed to the time needed for solvent exchange in the polymer brushes.

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