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Nanomechanical mapping of a high curvature polymer brush grafted from a rigid nanoparticle
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0002-2288-819X
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
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2012 (English)In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 8, no 32, 8312-8320 p.Article in journal (Refereed) Published
Abstract [en]

Analysis of interaction forces when probing a silica core-polyelectrolyte brush shell nanoparticle, adsorbed on a silica substrate and bathed by aqueous electrolyte solution, with an ultrasharp atomic force microscopy (AFM) tip provides a spatially resolved map of heterogeneous mechanical properties across the nanoparticle. The deformation of the brush is mainly compressive when probed directly above the nanoparticle centre and mainly deflective when probed at a finite horizontal distance away from the centre. The brush is significantly stiffer against compression than against deflection, and ionization of the brush has a greater stiffening effect against compression than deflection. Whereas a height image of the core-shell nanoparticle was unremarkable, showing a monotonic decrease in height with increasing horizontal distance from the centre, brush deformation, energy dissipation and adhesion displayed local minima over the centre and maxima at a finite horizontal distance away from the centre, corresponding to a position near the rigid core nanoparticle edge. The different response to brush deformation depending on the angle of probing is relevant to the interactions of brush-decorated macroscopic surfaces with submicrometer roughness and to the interactions of brush-decorated nanoparticles with ultrafine structures in their environments.

Place, publisher, year, edition, pages
2012. Vol. 8, no 32, 8312-8320 p.
Keyword [en]
Transfer Radical Polymerization, Polyelectrolyte Brushes, Frictional Forces, Solid-Surfaces, Interface, Afm, Lubrication, Microscopy, Dynamics, Particle
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-101138DOI: 10.1039/c2sm26086gISI: 000306855000009Scopus ID: 2-s2.0-84864452225OAI: oai:DiVA.org:kth-101138DiVA: diva2:546515
Note

QC 20120823

Available from: 2012-08-23 Created: 2012-08-23 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Grafted Molecular Layers for Control of Surface Properties
Open this publication in new window or tab >>Grafted Molecular Layers for Control of Surface Properties
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The goal of this thesis work was to develop responsive surface grafted brushlayers for control of surface properties and to gain insights in the molecular mechanisms that control these properties. Three types of grafted layers were investigated, as outlined below. In the first system studied, poly(AAc) was synthesized by a grafting from approach, utilizing a photopolymerization reaction from a macroinitiator cast onto QCM substrates. The responsiveness in terms of frequency change, Δf, of the resulting brushes to changes in bulk pH was studied with QCM. Further, the friction properties of poly(AAc) was elucidated with colloidal probe AFM as a function of pH and counterion valency. High friction (μ=0.27) was found in presence of CaCl2 at high pH (7.5), but not under any other condition explored. It was concluded that the high friction was due to intralayer COO--Ca2+--OOC bridges. QCM-D was utilized for studying viscoelastic properties of growing poly(AAc) films during in situ photopolymerization. By Voigt modeling, the thickness, shear elasticity and shear viscosity were extracted. These parameters were observed to undergo sudden transitions at a critical thickness, and from this thickness the grafting density of the growing poly(AAc) layers was determined. In addition, the sensitivity to changes in Δf and ΔD with respect to the thickness of the poly(AAc) films was investigated, and the results showed that high sensitivity in ΔD is retained at higher film thicknesses than for Δf, and that the sensitivity with respect to noise can significantly alter the thickness that is best suited forthe study of viscoelastic changes in sensor applications. The work with QCM-D also involved the detection of structural variations within a thick brush layer of poly(AAc). Since lower overtones have higher penetration depth, these frequencies sense polymer segments further out in the brush. It was found that the apparent pKa of the poly(AAc) was higher for lower overtones, indicating therelative ease of acid dissociation in segments further out in the polyelectrolytebrush. In the second system studied, phenylethylamine (PEA) was electrografted to flatsurfaces of glassy carbon (GC). The nanomechanical properties, such as topography, deformation, adhesion and dissipation, were investigated using PeakForce quantitative nanomechanical mapping (QNM). One main finding is that globular domains of 40 to 50 nm indiameter appear in the electrografted PEA layer. They are assigned to clusters of PEA formed due to less rapid reactions between radicals and the GC surface compared to reactions with already grafted PEA. The interactions between the PEA layer and a silica sphere were further investigated by surface force measurements. A main finding is that the PEA surfaces were heavily charge regulated due to the limited net charge of the PEA layer compared to that of silica. In the third system studied, the mechanical response of polyelectrolytes as a function of applied load, probing angle and pH was investigated with PeakForce QNM. The used polyelectrolyte was poly(2-dimethylaminoethyl methacrylate)(PDMAEMA) with pKa of 6.5 to 7.5 and grafted to silica nanoparticles. While most research on polyelectrolyte brushes is conducted by employing flat and smooth surfaces, with a roughness on the nanometer scale, real surfaces are rarely ever ideally flat but rather they possess topographic irregularities on nano- and micro-scales, which locally imparts high curvatures. The spherical geometry of the core-shell nanoparticles serves as a model for real surfaces with respect to topographical irregularities of real surfaces. The nanoscale brushes were probed with an ultrasharp AFM tip, providing nanoscale resolution of topography, deformation, adhesion and dissipation. It was found that the mechanical response of the polyelectrolyte corona is dependent on the applied load and the polar angle of the tip-brush interaction. All nanomechanical data show a non-monotonic variation with horizontal position, and the peak values are shifted in magnitude and position as a function of peak force. The effect of pH on deformation was further investigated. The results showed that the brush is more resistant against compression over the centre than it is to deflection at larger horizontal positions, and this effect is amplified by charging the brush. This work provided understanding of the direction dependence of the mechanical properties and is relevant for the design of brush boundary lubricating agents for rough surfaces, where the polymer chains are both being bent and compressed under the influence of load and shear.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012
Series
Trita-CHE-Report, ISSN 1654-1081 ; TRITA-CHE Report 2012:49
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-103703 (URN)978-91-7501-502-6 (ISBN)
Public defence
2012-11-09, K2, Teknikringen 28, KTH, Stockholm, 13:00 (English)
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Note

QC 20121018

Available from: 2012-10-18 Created: 2012-10-18 Last updated: 2012-10-30Bibliographically approved

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