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Title: Development of grafting strategies for the polymer functionalisation of graphene
Author: Au, Yin-Nei Heather
ISNI:       0000 0004 6423 7293
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Graphene is well-known for its exceptional mechanical, electrical, and thermal properties, but its potential is yet to be fully realised in bulk applications due to difficulties in obtaining a large yield of high-quality individually-dispersed graphene sheets. In this thesis, reductive exfoliation of bulk graphite is demonstrated as a promising and versatile method which allows the isolation of single- and few-layer graphenes; the production of reduced graphene, or ‘graphenide’, solutions shows varying efficiency in different solvents. Subsequent functionalisation of graphenide dispersions with various electrophiles, including 1-bromododecane, anionic monomers such as methyl methacrylate, and bromine, results in increased solubility in organic solvents, without damage to the graphene basal plane. Exact characterisation and quantification of grafting is complicated by the presence of solvent remaining between graphene layers, a phenomenon which is not significant in other carbon nanomaterials. Reductive alkylation was carried out on five different graphitic starting materials including two types of natural flake graphite, shear-exfoliated graphite platelets, graphite nanofibres, and few-layer graphene. The study reveals pronounced differences in the obtained grafted species with respect to the degree of functionalisation and residual solvent, exfoliation efficiency and product homogeneity. These results are shown to be dependent on the size and nature of the starting material, with few-layer graphene showing the highest grafting ratios. Few-layer graphene was also functionalised with various molecular weight poly(methyl methacrylate) (PMMA) polymers by grafting-to and grafting-from approaches; the grafting ratios were higher for the grafting-from approach and the products showed a far greater dispersibility in acetone (up to 920 µg/mL). In parallel with these direct polymer-grafting strategies on few-layer graphene, the reduction method was used to dissolve and brominate few-layer graphene sheets, achieving direct covalent attachment of bromine to the graphene framework. The brominated few-layer graphenes provide a convenient, stable, liquid-phase precursor, suitable for the synthesis of a variety of directly functionalised graphenes. As an example, the brominated species was used to initiate atom transfer radical polymerisation, to obtain PMMA-grafted graphene, which was six times more dispersible in acetone than controls. In addition, brominated graphene is active for nucleophilic substitution reactions, as illustrated by the preparation of methoxypolyethylene glycol- and hydroxyl-substituted derivatives. Grafting ratios for these polymer-functionalised materials varied between 6 and 25% and all graphene derivatives showed increased solubility in organic solvents, highlighting the potential of this route for preparing large quantities of dispersed graphene with minimal damage to the carbon framework.
Supervisor: Shaffer, Milo Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral