Abstract
The majority of commodity polymers are derived from crude oil, and do not readily degrade. These issues extend to advanced applications of polymers, such as those used in healthcare and battery technologies, raising environmental concerns over the use of polymers. This work details the modification of sugar-derived polymers for the development of functional materials, with the aim to relieve the reliance of advanced polymeric materials on fossil fuels.In Chapter Two, bioconjugation of d-mannose-derived polycarbonates, poly(M-1), is achieved. First, conditions for the controlled removal of acetal protecting groups were found. Specific degrees of deprotection could be targeted, leading to water-soluble, deprotected polymers with varying thermal properties. In particular, glass transition temperatures, Tg, were found to be proportional to the extent of deprotection, reaching 164 °C when fully deprotected. Secondly, a bio-reactive moiety was installed onto polycarbonates using a maleimide-functional initiator during ring opening polymerisation (ROP). Polymerisation conditions that yield linear polymers with maleimide α-chain ends were found, as evidenced by NMR and mass spectrometry analysis. The ability of maleimide end-groups to undergo Michael addition with thiols was demonstrated, in both organic and aqueous environments, using small molecule thiols. Bovine serum albumin (BSA) was used as a model protein for bioconjugation. Attachment of oligomeric polycarbonates to native and reduced BSA was evidenced by MALDI ToF MS, leading to bioconjugates of up to 76 kDa.
Chapter Three describes the synthesis and characterisation of boron crosslinked organogels and thin films. These materials were developed for use as gel or solid polymer electrolytes in lithium ion battery technologies. First, novel organogels were synthesised by reaction of a diol-containing polyether dp-poly(D-1), derived from the sugar d-xylose, with 1,4-phenylenediboronic acid (PDBA). The crosslinked materials were analysed by thermal gravimetric analysis (TGA), scanning electron microscopy (FE-SEM), and rheology. The rheological properties of the materials were tuneable: materials showed gel or viscoelastic behaviour depending on the concentration of polymer, and mechanical stiffness increased with
the amount of PDBA crosslinker. Organogels demonstrated self-healing capabilities, and recovered their storage and loss moduli instantaneously after application and subsequent release of strain.
Lithiated organogels were also synthesised in Chapter Three, through incorporation of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) into the crosslinked matrix. When analysed by electrochemical impedance spectroscopy (EIS), lithium–borate polymer gels showed high ambient ionic conductivity values of up to 3.71 × 10−3 S cm−1 at 25 °C, high lithium transference numbers (tLi+ = 0.88 - 0.92) and electrochemical stability (4.51 V). This novel
system provided a promising platform for the production of self-healing polymer gel electrolytes (GPEs) derived from renewable feedstocks. Prototype solid polymer electrolytes (SPEs) were also developed through crosslinking under aqueous conditions. Borax was utilised as a water soluble crosslinker, and PDBA and boric acid were pre-lithiated to yield water soluble reagents. Where possible, films were characterised by EIS, but were typically too brittle or too thin for electrochemical characterisation. Films made using 0.50 equiv. of PBDA/LiOH showed ionic conductivity of 1.88 × 10−6 S cm−1 at 60 °C.
Chapter Four concerns self-healing, stretchable and re-processable boronic acid hydrogels. Namely, aqueous crosslinking of dp-poly(D-1) and PDBA in the presence of sodium hydroxide (NaOH) was achieved. Rheological analysis demonstrated the self-healing ability of the hydrogels, as well as their tuneable material properties. Self-healing was further investigated in cut-heal experiments, and demonstrated the re-shaping characteristic of the hydrogels. Tensile testing revealed extreme stretchability: hydrogels stretched to at least 12,500% times their original length without rupture. Stretching produced thin but strong fibres with Young’s moduli, E, of up to ∼10.6 MPa, and ultimate tensile strength (UTS) in the range of 230 MPa. The material properties of both hydrogels and stretched fibres were dependant on the degree of crosslinking. Samples with higher degrees of crosslinking were stronger (with UTS of up to 22.2 kPa and 227.2 MPa for hydrogels and fibres, respectively), while less crosslinked samples could be stretched further.
Steps towards the development of molecularly imprinted hydrogels, organogels, and films were made in Chapter Four. Crosslinking in the presence of cortisol and sodium lactate was performed in organic and aqueous conditions respectively to form molecularly imprinted materials (MIMs). After template removal, the materials were tested towards their ability
to rebind their templates from solution. Although small amounts of cortisol (ca. 5%) could be rebound by imprinted organogels and films, in general these materials did not produce successful MIMs. This was likely due to the dynamic nature of boronic acid crosslinking. Glyoxal and terephthaloyl chloride (TPC) are introduced as alternative crosslinkers, that would provide irreversible crosslinking, and potentially more effective MIMs.
Chapter Five presents the computational modelling of protected and deprotected poly(M1) polycarbonates. First, an appropriate model system was selected and validated against experimental characteristics of poly(M-1) and density functional theory (DFT) calculations. This included selection of the polymer structure, system size, and force fields for the polymer and solvent systems. Polymers with a degree of polymerisation (DP) of 20, with explicit α-chain ends were taken forward to molecular dynamic (MD) simulations, performed within the GROMACS suite of programmes using an OPLS force field. Four key dihedral angles within the polycarbonate backbone were investigated, along with radial distribution functions (RDFs). When modelled in vacuo, the identity of the α-chain end and the removal of acetal protecting groups was found to effect polymer conformation and chain packing. Solvated models revealed more efficient chain packing in deprotected systems: isolated dp-poly(M-1) chains adopted globular conformations, and chains were found to entangle in multi-chain simulations. RDF and hydrogen bond analysis revealed significantly more intra- and inter-chain interactions within deprotected systems compared to their protected counterparts. Hydrogen bonding at the site of acetal protection was observed, explaining the enhanced chain packing and more compact chain conformation observed in dp-poly(M-1) chains. These observations were used to explain experimental characteristics of dp-poly(M-1): increased hydrogen bonding in dp-poly(M-1) leads to increased glass transition temperatures and enhanced chain packing obscures α-chain ends, leading to reduced activity towards bioconjugation.
As a whole, this collection of work demonstrates the versatile applications of sugar-derived polymers for the first time. In particular, the development of d-mannose and d-xylose derived bioconjugates and electrolyte materials proves the ability of sustainable polymers to replace non-degradable, fossil-derived materials without loss of performance. While refinement is needed, initial investigation of renewable molecularly imprinted materials and stretchable hydrogels demonstrates the potential breadth of application for these polymer systems. As plastic pollution and depleting resources continue to be key challenges in our society, the work presented here is an important step towards the sustainable future of functional materials.
| Date of Award | 13 Sept 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Antoine Buchard (Supervisor), Steve Parker (Supervisor) & Hannah Leese (Supervisor) |