The basic physical, chemical, and biological properties of materials are remarkably altered as the size of their constituent grains decreases to a nanometre scale (Wan and Bai 2006). Nanomaterials that contain nanoparticles from a biological source, like protein fibrils, will be referred to as bionanomaterials in this thesis. Nanocomposites are multiphase materials where at least one of the constituent phases has one dimension less than 100 nm (Ajayan et al. 2003, Manocha 2006). The promise of nanocomposites lies in their multifunctionality and the feasibility of obtaining unique combinations of properties unachievable with traditional materials (Koo 2006).
1.8.1 Polymer nanocomposites
Polymer nanocomposites consist of a polymeric material and a reinforcing nanoscale material and show improvements in mechanical properties of the polymer, depending on factors such as the polymer matrix, nanocomposite morphology and the type of nanoparticle (Koo 2006). There are different types of commercially available nanoparticles, of which CNTs are attracting considerable interest as reinforcing materials in polymer nanocomposites (Calvert 1999). The driving force for the development of polymer nanocomposites is to control properties of nanomaterials with the introduction of nanometre sized structures, without changing the material chemical composition (Koo 2006).
This thesis investigated the incorporation of amyloid fibrils into a film matrix based on polyvinyl alcohol (PVOH). Nanometre sized fibres can strengthen a polymer matrix and the vast majority of studies have been carried out on CNTs (Bhattacharyya et al. 2006, Bhattacharyya et al. 2005, Calvert 1999, Coleman
2006). The background literature available on CNTs, protein and polymer nanocomposites were utilised as a platform for investigations in this thesis. 1.8.2 Carbon nanotubes and their functionalisation for nanocomposites CNTs can be described as a nano-sized tube made up of a graphite sheet (Nepal and Geckeler 2006). Their stiff and robust structure is owed to the carbon-carbon bonds in their graphite-like structures, which are some of the strongest in nature (Thostensona et al. 2001). Of the wide spectrum of potential applications, CNTs are attracting considerable interest as reinforcing materials in polymer composites due to their superior structural and transport properties (high strength and flexibility, high thermal and electrical conductivity, and low density) (Calvert 1999, Koo 2006). Mechanical reinforcement of polymers by CNTs requires good interfacial bonding to ensure efficient stress transfer from the polymer matrix to the nanotube lattice. Unlike proteins, CNTs don’t intrinsically have any side chain groups on the surface of their walls, to be able interact with a matrix. Studies supporting the modification and functionalisation of CNTs with amino acids and peptides (Figure 1.9) have become important for the bottom-up design of nanotubes (Lin et al. 2004). Furthermore, solubilisation
via chemical modification and functionalisation is effective in imparting biocompatibility to CNTs, especially for the stable conjugation of CNTs with a variety of biological and bioactive molecules like proteins (Lin et al. 2004).
Figure 1.9 An example of functionalising CNTs with biological molecules like protein
Unlike CNTs, the protein side chains of amyloid are an intrinsic part of their structure and can be functionalised with additional peptide and protein groups or entities such as fluorophores and metalloporphyrins (Baldwin et al. 2006, MacPhee and Dobson 2000) allowing fibrils to interact with their environment or living cells. Preliminary studies into bilayered nanotubular structures suggested that hollow tubular arrangements of amyloid can be formed of uniform dimensions with charged curve surfaces representing scaffolds highly accessible for modification (Lu et al. 2003). This feature imparts a significant advantage over CNTs, which are limited by the lack of reactivity along their sidewalls (Lin et al. 2003).
1.8.3 Polyvinyl alcohol
Polymer nanocomposites have a polymer as the matrix, which will be PVOH in this study, and the nanoparticles, dispersed within it, in this case protein amyloid fibrils. PVOH is a nontoxic, water-soluble, biocompatible, and biodegradable synthetic polymer, which can be widely used in biochemical and biomedical materials. It has good film-forming properties, highly hydrophilic properties, and high mechanical strength, and has been studied as a membrane (Lin et al. 2006, Shang and Peng 2007). PVOH is a semicrystalline polymer with hydroxyl groups that give rise to intermolecular and intramolecular hydrogen bonds. PVOH films have a high tensile strength and oxygen barrier properties, comparable to many other polymers (Kim et al. 2007). In addition, since PVOH is biocompatible and nontoxic, and exhibits minimal cell adhesion and protein absorption, PVOH membranes have been developed for biomedical applications (Choi et al. 2007, Chuang et al. 1999, Kobayashi et al. 2005, Koyano et al. 2000, Oh et al. 2004, Qi et al. 2004). For instance, PVOH/ polyvinylpyrrolidone/chitosan hydrogels were studied for wound dressing application in rats (Huang and Yang 2008).
PVOH was selected as the model polymer to determine the potential of fibrils as nanocomposite components in this thesis, and the large amount of work done on PVOH provides support for its use as a model in a film forming system. More specifically, the selection of PVOH for investigating the impact of protein fibrils on the mechanical properties of the polymer was motivated by PVOH/protein
based film studies, such as PVOH/wheat (Dicharry et al. 2006), PVOH/collagen hydrolysate (Alexy et al. 2003), and PVOH/gelatin (Bergo et al. 2006, Darder et al. 2006, Ma et al. 2006, Mendieta-Taboada et al. 2008, Su et al. 2007, Wang et al. 2005, Yi et al. 2006)), PVOH/soluble egg shell membrane (Yi et al. 2006), PVOH/chitosan (Wang et al. 2005), and PVOH/cellulose fibres (Wang and Sain 2007), PVOH/soy protein (Su et al. 2007), and PVOH/membrane protein (Ma et al. 2006). To improve the mechanical characteristics of protein-based films, these proteins are mixed with synthetic polymers (Tharanathan 2003), such as PVOH. (Chiellini et al. 2003) have demonstrated biodegradability of protein blended PVOH films.
1.8.4 Proteins in films
Work to date reveals that films can be made from wheat protein, milk protein, zein protein and other proteins (Budi Santosa and Padua 1999). The biocompatibility, biodegradability, and structural support, plus the availability of an industrial supply of proteins (Aoi et al. 2000, Tharanathan 2003) has led to studies on blends of polymers with polypeptides (Takasu 1997) and, more recently, the development of protein-based packaging materials (Tharanathan 2003). In particular, proteins derived from various animal and plant sources have successfully been formed into films and/or coatings (Gounga et al. 2007, Lee et al. 2008, Naushad and Stading 2007).
Edible films have been produced by heating sunflower proteins to 85°C and solution casting (Meixueir et al. 2000). Film formation in such proteins is through intermolecular disulfide bridges and hydrogen bonds, accompanied by surface dehydration. Whey protein isolate and milk protein also produce edible film and coatings with different functionalities and applications (Gounga et al. 2007, Khwaldia et al. 2004, Sothornvit et al. 2007). An example of this type of natural polymers is gelatin, a polypeptide derived from the structural protein collagen, that is able to form tough films (Park et al. 2008) and transparent, elastic and thermoreversible gels (Darder et al. 2006). The various applications of proteins, both in the functional and aggregated form, discussed in the sections above further support the potential for amyloid fibrils in bionanomaterials (Gras 2007a, Gras 2007b, Gras et al. 2006).