The author would like to emphasise on the fact that each individual chapter contains its re-spective introductory part and literature review followed by the experimental, results/discussion and conclusion parts. The atypical layout (i.e., for British PhD-theses) of the chapters pre-sented in this thesis was adopted for the sake of clarity and coherence since the research described in the following chapters is proposing to make use of bacterial cellulose nanofibrils in different fields of materials science ranging from hierarchical composites, porous media, functional bioinorganic nanohybrids and bacterial-based nanocomposites. Therefore, it would have been a very convoluted narrative to draw a unifying introduction encompassing the var-ious aspects of the author’s research in a single chapter.
The present thesis is divided into 8 chapters, as follows:
• Chapter 1 introduces the context of the research, motivation and the roadmap to this thesis.
• Chapter 2 provides a background and introduction to the analytical methods used and the experimental methodology followed.
• In Chapter 3, the biosynthesis of bacterial cellulose by the Gluconobacter xylinus strain in static and agitated fermentation conditions is characterised. An increase of the pro-ductivity of bacterial cellulose in an instrumented stirred-tank bioreactor was achieved by
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the addition of a stainless-steel mesh support inside the bioreactor vessel. The physical, chemical and thermal characteristics of the bacterial cellulose nanofibrils produced in static and agitation fermentation conditions have been characterised and compared.
• Chapter 4 describes the characterisation of all-cellulose nanocomposites produced by disso-lution and subsequent regeneration of microcrystalline cellulose. The surface modification procedure of sisal fibre, which consists of coating of bacterial cellulose onto the surfaces of sisal fibres in situ during fermentation of Gluconobacter xylinus is discussed. The mechan-ical properties of bacterial cellulose coated sisal fibres are characterised. The effect of both surface modification of sisal fibres and degree of crystallinity of the regenerated cellulose matrix on the mechanical properties of hierarchical all-cellulose composites is evaluated and discussed.
• Chapter 5 describes the preparation and characterisation of porous cryogel microspheres composed of regenerated bacterial cellulose nanofibrils. The physical and structural proper-ties of the regenerated bacterial cellulose cryogel microspheres are reported as a function of the dissolution time and concentration of bacterial cellulose nanofibrils in the DMAc/LiCl cosolvent. The evolution of the morphology of the porous network and BET surface area of the regenerated bacterial cellulose cryogel microspheres are discussed.
• Chapter 6 describes the synthesis and characterisation of thioether functionalised bacterial cellulose nanofibrils by free radical grafting polymerisation reaction in an aqueous medium.
The preparation and characterisation of bioinorganic nanohybrids composed of either gold nanoparticles or cadmium telluride quantum dots chemisorbed onto the thioether function-alised bacterial cellulose nanofibrils are reported.
• The synthesis and characterisation of hydrophobised bacterial cellulose nanofibrils by free radical grafting polymerisation in aqueous medium are reported in Chapter 7. BC was hydrophobised by the grafting of polycaprolactone chains from its surface. The thermal and mechanical properties of entirely bacterial-based nanocomposites composed of poly(3-hydroxybutyrate) (PHB) biopolymer matrix reinforced with hydrophobised bacterial cel-lulose nanofibrils are characterised and discussed.
• The general conclusions are then drawn in Chapter 8.
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Methods
The aim of this chapter is to summarise the various analytical methods employed to char-acterise the different specimen presented subsequently in the present thesis. Each analytical method is introduced succinctly with its relevant background theory. The methodology and apparatus used for the characterisation of the specimen are also described. The list of ana-lytical methods used is as follows:
• The optical properties of bacterial suspension of Gluconobacter xylinus strain was char-acterised by optical density at wavelength of 600 nm (OD600).
• The cellulose polymorphs, allomorphs and functional groups grafted from bacterial cellulose (BC) nanofibrils were characterised by ATR-FTIR spectroscopy.
• The surface plasmon resonance of colloidal suspension of gold nanoparticles (Au NPs) and cadmium telluride quantum dots (CdTe QDs) as well as suspension of functional bioinorganic nanohybrids were characterised by UV-Vis spectroscopy.
• The diameter of Au NPs was determined by dynamic light scattering technique.
• The crystal structure of cellulose specimen was analysed by X-ray diffraction (XRD).
• The surface area of dried BC nanofibrils and macroporous cryogel (i.e., freeze-dried) microspheres composed of regenerated BC nanofibrils was quantified by nitrogen BET adsorption.
• The chemical composition of functionalised BC nanofibrils was quantified using elemen-tal analysis (EA).
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• The thermal and thermomechanical properties of various type of specimen were char-acterised using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA).
• The mechanical properties of various (nano-) composites were characterised by tensile testing.
• The morphology of various specimens was observed using scanning electron microscopy (SEM).
• The moisture sorption behaviour of regenerated cellulose was assessed by dynamic vapour sorption (DVS).
2.1 Optical density
The growth of a bacterial population of Gluconobacter xylinus strain was monitored using optical density measurements at a wavelength of 600 nm (OD600) of a broth containing bacterial cells, as discussed later in Sections 3.2.2 & 3.3.2, respectively. The change in OD600
provides information about the rate of growth of the bacterial population during cultivation.
The OD600 analysis (also referred as turbidity) is based on the Lambert-Beer law, where there is a direct and linear relationship between the amount of light adsorbed and the con-centration of constituents in the sample, as long as it adsorbs or scatters the incident light.
The Lambert-Beer law is expressed as follows:
Aλ = log 1
T = − log T (2.1)
where Aλ is the sample absorbance at a specific wavelength, T is the transmittance expressed as the ratio of intensity of transmitted light It over intensity of light beam I0.
T = It
I0 (2.2)
In the case of a dilute substance, as for culture broth, the Lambert-Beer law is rewritten as follows:
Aλ = λcd (2.3)
where λ represents the coefficient of adsorption at a specific wavelength (L·mol−1), c is the concentration of the sample (mol·L−1) and d is the path length of light through the