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Contact Mechanics at the Molecular Scale

Siles Brügge, Oscar (2017) Contact Mechanics at the Molecular Scale. PhD thesis, University of Sheffield.

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Abstract

A better understanding of the adhesive interactions between surfaces at the molecular scale is of growing importance as miniaturization efforts continue. To this end, Lifshitz theory of continuum mechanics was used to calculate the interaction energies between hydrocarbon surfaces in over 200 liquids, and compared to those obtained from the Hunter model of hydrogen bond solvation thermodynamics. In alkanes, amines, and primary alcohols, both theories yielded comparable results. However, in cases where the refractive index between interacting phases diverges greatly, a large disparity between the Lifshitz work of adhesion and Hunter free energy of complexation was found. In addition to some of the liquids showing differing results between the two theories, binary mixtures of benzyl alcohol and methanol were also identified for further experimental analysis. Slight modifications were applied to Lifshitz to allow for predictions of polar surfaces, and these too were compared to those provided by the Hunter model. Using force spectroscopy and friction force microscopy the tribological properties of hydrocarbon self-assembled monolayers, in the liquids identified previously, were investigated. While interactions in non-polar liquids were well described by both Lifshitz theory and the Hunter model, the former was found to consistently underestimate the work of adhesion in polar liquids, especially in water (Wad > 40 mJ/m2). In contrast, good agreement was generally obtained between the Hunter model and the experimentally obtained interaction energies. This was also true for binary mixtures of benzyl alcohol and methanol, where Lifshitz theory was completely unable to predict the form of the interaction. Friction-load plots were also obtained for the same systems of non-polar surfaces, and the form of their relation in different media was found to be dependent on the previously obtained adhesive energies. At interaction energies below 6 mJ/m2 linear friction-load relationships were observed, while yielding sublinear plots at work of adhesion values above this, corroborating the idea that friction can be considered to consist of load- and area-dependent terms. Mechanochemical removal of NPEOC photoprotecting groups from surfaces with adsorbed OEG-NPEOC-APTES monolayers using an AFM probe was also performed, with feature sizes up to 20 nm being achieved. The dependence on the width and depth of the patterned features on the applied load was investigated, with a positive relation being found for both, up to a critical load; no such change was observed with increasing write speeds. Changing the tip chemistry and environment (i.e. via immersion in different liquids) yielded no change in the size and quality of the patterns obtained, suggesting the lithographic process relies solely on the physical interaction between tip and sample surface. Modification of the surface through derivatization using TFAA and GFP indicates that only the NPEOC protecting group is being removed. Density functional theory was employed to investigate possible reaction pathways of the usual photodeprotection pathway of NPEOC-APTES, and how the mechanical interaction of the tip with the surface may promote one of these to occur without a high energy photon. It was discovered that a compression of the NPEOC leads to a shift in the UV/Vis absorbance spectrum towards higher wavelengths, and it is suggested that the mechanochemical deprotection of OEG-NPEOC-APTES SAMs occurs via this mechanism.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Science (Sheffield) > Chemistry (Sheffield)
Depositing User: Dr Oscar Siles Brügge
Date Deposited: 27 Nov 2017 09:06
Last Modified: 27 Nov 2017 09:06
URI: http://etheses.whiterose.ac.uk/id/eprint/18786

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