Theory of sorption and electronic transport in amorphous polymers and single molecules

Jay, Michael and Lambert, Colin (2020) Theory of sorption and electronic transport in amorphous polymers and single molecules. PhD thesis, Lancaster University.

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Abstract

This thesis describes a series of studies of the energetics, structure, sorption and electronic transport of polymers and single molecules at the nanoscale. The first of these relates to hypercrosslinked porous polymers, which are of technological and commercial interest for their ability to efficiently absorb molecules in a wide range of contexts from pharmaceuticals to gas storage. I model a family of polymers synthesised by collaborators at the University of Strathclyde, which show very high uptake of polyaromatic hydrocarbon (PAH) molecules from solution and could potentially be used as a filtration material to remove these molecules from vehicle lubrication oils, where they are responsible for the build up of soot and the long term degradation of both oil and engine. Density functional theory calculations identified the structural units of the polymers contributing most to sorption, and the relative binding strengths of several PAHs which correspond to the experimental trends. Molecular dynamics simulations of the polymer pores with PAH molecules in a heptane solution highlighted the significance of polymer flexibility and pore size in the sorption process. In the second I analyse the conductance of the first organically synthesized sp-sp3 hybridized porous carbon, OSPC-1. This new carbon shows electron conductivity, high porosity, the highest uptake of lithium ions of any carbon material to-date, and the ability to inhibit dangerous lithium dendrite formation. It therefore has potential as an anode material for lithium-ion batteries with high capacity, excellent rate capability, long cycle life, and potential for improved safety performance. Detailed simulation of the variation of the conductivity of this amorphous material versus length using a tight binding methodology showed that the measured conductance is consistent with an inelastic scattering length of the size of an OPSC-1 fragment. The third project is in the field of molecular electronics, where a crucial area of research aims to identify molecular anchor groups to bind molecules to electrodes. This study presents a series of oligo(phenylene-ethynylene) wires with one tetrapodal anchor and a phenyl or pyridyl head group. The new anchors are designed to bind strongly to gold surfaces without disrupting the conductance pathway of the wires. Density functional theory was used to simulate the structures of the molecules and the nature of their binding to the Au surface. Quantum transport calculations provided insight into the conductance pathway through the molecules and confirmed the decoupling between surface binding and electronic coupling. This feature may enable the inclusion in junctions of a wider range of functional groups, in particular those with strong electronic coupling, but only weak physical binding.