Synthesis of Sustainable Antibacterial Materials by Surface Functionalisation of NZ Flax (Phormium tenax) Fibres and Hemp (Cannabis sativa) Hurd

Abstract

Development of sustainable antibacterial fibres for use in a range of applications has received great research interest in recent years due to increasing environmental concerns. This thesis summarises the investigation of functionalisation of Phormium tenax (NZ flax) fibre (PTF) and hemp hurd core of Cannabis sativa (HH) through surface grafting of chitosan 1, a potent novel chitosan derivative 132 (selected for the excellent bioactivity from a set of O-amine-modified chitosans synthesised in this study) and cationic polymer brushes of 2-trimethylammonioethyl methacrylate chloride (TMAEMC), so as to introduce antibacterial activity to these natural materials. Phormium tenax fibre (PTF) is the leaf fibre from NZ flax- a plant indigenous to New Zealand, abundant but currently under-utilized. Hemp hurd (HH) is a sustainable by-product from hemp (Cannabis sativa) industry. Chitosan 1 is a known antimicrobial compound, which is prepared from chitin, the second-most abundant and renewable biopolymer in existence, and a by- product of the shell-fish industry. Chitosan 1 has been shown to have significant scientific and industrial applications in fields demanding reduced environmental impact. This is due to its versatile properties including biocompatibility, biodegradability and non-toxicity, along with its potent antimicrobial properties. Preparation of O-amine-functionalised chitosan analogues was performed with an aim of improving the antibacterial activity of chitosan 1, by introducing additional amino groups, for further use in fibre surface functionalisation. A N-protection-O-grafting-N-deprotection strategy proceeded through a SN2 O-epoxide ring opening, adding twenty various amines. Benzaldehyde and phthalic anhydride were used as protecting agents (producing two series: BDs 81-87 and PDs 114-133, respectively). In total, 27 chitosan derived compounds were synthesised and assessed for their bioactivity against Staphylococcus aureus Gram-positive bacteria. Compound 132 (the (N,N-dimethylethane-1,2-diamine)-modified analogue from the PD series) was found to be the most promising, exhibiting an eight-fold activity compared to unmodified chitosan 1, and as such was further utilized in surface grafting. Grafting of chitosan 1 onto the fibre surface of both PTF and HH through a reactive chemical linker attached onto the cellulosic residue was investigated using succinic anhydride bridging, epichlorohydrin cross-linking and diazonium-based surface chemistry. X-ray photoelectron spectroscopy (XPS), water contact angle measurements and bioactivity tests of the surface functionalised PTF and HH confirmed that the diazonium method of surface functionalisation was the most efficient and promising method of chitosan-grafting onto PTF and HH surfaces. Therefore, derivative 132 was also attached onto PTF and HH using this diazonium-based strategy which produced fibres PTF-152 and HH-153. An alternate way of antibacterial surface functionalisation of PTF and HH through cationic polymer brush grafting was also performed. This was achieved through the modification of surface hydroxyl groups by 2-bromopropionyl bromide, followed by grafting of poly(2-trimethylammonioethyl methacrylate chloride) (PTMAEMC), polymer of a biocompatible monomer TMAEMC under atom transfer radical polymerization (ATRP) conditions. Water contact angle testing, X-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy, scanning electron microscopy (SEM) and biological activity assays against S. aureus confirmed the successful grafting. The ATRP grafting converted both PTF and HH into antibacterial fibres PTF-165 and HH-166, possessing excellent inhibition. In this thesis, a range of achievements are presented. This research developed a synthetic strategy to make a range of novel chitosan analogues, used this route to synthesise derivatives more active than chitosan, developed various methods to attach chitosan (and derivatives) to fibres, characterised the produced functionalised fibres, explored the antibacterial activities and assessed their mode of action. This research will be able to inform future advancement in the development and application of antibacterial, chitosan (derivative)-functionalised natural fibres.