Supramolecular Chemistry at the Polymer-Oil Interface
The movement away from metals towards polymers for automobile bearing coatings opens a new area for the possible modification of these coatings. This project, done in collaboration with Castrol, was to understand adsorption of small molecules at the surface of these polymers and work towards engineering host-guest interactions at the surface to provide binding of specific molecules.
Initially, the commercial polyamide-imide was characterised via Nuclear Magnetic Resonance (NMR), Infrared (IR) and Ultra-Violet/Visible (UV-Vis) spectroscopy to understand the nature of the functional groups and structures present. This revealed that the imidisation reaction was incomplete so the functionality within the polymer could change and thus alter how molecules interact with it. This reaction was then investigated via IR and NMR spectroscopy and it was shown that the degree of imidisation varied with temperature.
Then the characterised polyamide-imide was used to study the adsorption of alkylphenols, a class of molecules that have structures and functionality like common additives used in engine oils. Additionally, the adsorption of water was studied as it is a common contaminant. These two systems were studied via solution depletion isotherms and neutron reflectivity measurements. The isotherms confirmed the adsorption of the molecules whilst neutron reflection was used to characterise the layers. For alkylphenol, a rather sparsely packed layer of the molecules and solvent existed at the surface with their alkyl tails extending into the solvent. For water; the molecules diffuse into the polyamide-imide. It can be partially removed by washing the surface with dry dodecane; however, some water remains in the polymer layer.
A viologen-cucurbituril binding site was chosen as the supramolecular surface interaction. The binding unit was incorporated into the polyamide-imide. The synthesis of the binding site polymer was achieved in two stages; the reproduction of the commercial polymer and the synthesis of a polymer containing 100% viologen with and without threaded cucurbituril. Whilst the synthesis appeared successful, neutron reflectivity measurements showed that, when in contact with a solution containing a second guest, no adsorption was seen on the polymer containing cucurbituril.
In order to study the supramolecular interaction in non-polar solvent, a series of rotaxanes were synthesised with viologen-cucurbituril cores and bulky stopper groups to prevent unthreading of the cucurbituril as well as enhancing the solubility of the system. The two components were linked via an amidisation reaction between an acid chloride and an amine. Due to the low solubility of the products confirmation of synthesis was only possible in one case.
As well as using a viologen-cucurbituril binding site, binding in cucurbiturils via halogen bonding was investigated as common halogen bonding species show good solubility in non-polar solvents. Initially co-crystals of 2,5-diiodo-1,3,4,6-tetrafluorobenzene with tertiary amides were studied. With N,N-dimethylformamide, a 1:2 co-crystal was seen but the structure with N-methyl-2-pyrrolidone had a stoichiometry of 1:1. In both structures the oxygen of the amide is involved in a halogen bonding; however, for N-methyl-2-pyrrolidone it is bifurcated leading to the formation of chains. A 50-50 mixture of pyridine and formic acid was found to solubilize both halogen bond donors and cucurbit[n]uril. Over the course of these experiments the structure of the co-crystals of two halogen bond donors, 2,5-diiodo-1,3,4,6-tetrafluorobenzene and 2,4,6-triiodo-1,3,5-trifluorobenzene, with pyridine were determined. Crystals of the cucurbituril-adducts weren’t of sufficient quality to determine the structure. The presence of binding solution was confirmed by 1H and 19F NMR experiments on the methyliodide-cucurbituril and Trans-diiodooctrafluoroazobenzene-cucurbituril systems.