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Surface nanomechanical properties of hydrogels and coatings

Time: Fri 2020-11-27 13.00

Location:, Stockholm (English)

Subject area: Chemistry

Doctoral student: Gen Li , Yt- och korrosionsvetenskap

Opponent: Professor Jan van Stam,

Supervisor: Professor Per M. Claesson, Yt- och korrosionsvetenskap

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Characterizing mechanical and wear properties at the nanometer scale emerged as a fascinating and challenging topic in the last decades due to recognition of many interesting and unusual properties of nanomaterials. Due to recent developments of the atomic force microscopy (AFM) technique, it is now possible to measure nanomechanical properties and topography simultaneously, as well as performing and evaluating wear measurements at the nanometer scale. The aim of my Ph.D work was to elucidate nanomechanical (surface stiffness, deformation, tip-sample adhesion, apparent elastic modulus) and nanotribological (friction, wear and wear mechanism, stick-slip) properties of different kinds of materials in different environments. This contributes to the general strive of understanding and counteracting material breakdown, and thus also to a more sustainable society.

The nanomechanical and nanotribological properties of macroscopic poly-HEMA (HEMA = 2-hydroxyethyl methacrylate) hydrogels with two different cross-linking densities were characterized by AFM, using different modes of operation, in aqueous environment at different loads. The stiffness of the two poly-HEMA hydrogels was found to increase with increasing load, while only for the less cross-linked poly-HEMA hydrogel the tip-sample adhesion was affected by increasing load. Further, since different cross-linking density affects the flexibility of the polymer chains, poly-HEMA hydrogels with different cross-linking density showed distinct responses when challenged by the combined action of normal load and shear.

The nanomechanical properties of adsorbed temperature responsive poly-NIPAm microgel particles were characterized in aqueous environmentacross its volume phase transition temperature (VPTT). The influence of the preparation method of the microgel and its cross-linking density were mainly explored, while the effects of some experimental parameters such as indentation speed and probe size were also elucidated. The results suggest that poly-NIPAm microgels prepared by the classical batch process presented a “hard core-soft shell” structure. In contrast, the semi-batch process greatly improved the homogeneity of the microgel particles with no obvious radial variation being observed. Further, the cross-linking density affected the extent of deswelling of the microgel particles, which rendered less cross-linked poly-NIPAm microgel particles stiffer than more highly cross-linked ones just above the VPTT.

The nanomechnical and nanowear properties of a cellulose nanocrystal (CNC) reinforced nanocomposite coating for corrosion protection were investigated both in air and in water. The results suggest that minor water uptake softens the surface of the coating. In addition, the exposure to a corrosive aqueous environment irreversibly changed the surface properties of the coating, leading to reduced robustness and thus increased wear. This occurred well before any deterioration of anti-corrosion properties were found.