(14a) Surface Functionalisation of Bacterial Cellulose Nanofibrils as the Route to Produce Green Polylactide Nanocomposites with Improved Properties
Recent interest in environmental friendly products and ?the public's growing demand for greener materials has sparked the use of cellulosic materials as the reinforcement in composites. This is not surprising as cellulosic materials such as natural fibres have distinct advantages over conventional glass fibres; low cost, low density, high toughness and most importantly, biodegradability. In addition to natural fibres, bacterial cellulose is also one of the extensively studied reinforcement. Bacterial cellulose possesses a highly crystalline structure (~90%) and a high Young's modulus of about 114 GPa. This value is comparable to that of high performance glass fibres considering that bacterial cellulose has much lower density (1.25 g cm-3) than glass fibres (2.5 g cm-3). It also exists as a nano-sized material naturally and possesses a high surface area of about 150 m2 g-1. All these properties are highly favourable for the use of composite production. However, bacterial cellulose is extremely hydrophilic in nature and it has strong affinity to itself due to the presence of large amount of hydroxyl groups (-OH) on the surface. This will often result in poor interfacial adhesion between bacterial cellulose and hydrophobic polymer matrices, which limits the application of bacterial cellulose in the composites field.
Therefore in this work, we investigate the effect of surface functionalised bacterial cellulose with long chain fatty acid (acetic acid, hexanoic acid and lauric acid) for the use in polylactide based nanocomposites. By limiting the reaction only to the surface of bacterial cellulose, the highly crystalline structure will be retained whilst the cellulose surface will become hydrophobic. This will lead to an improvement in the interfacial adhesion between bacterial cellulose and polylactide. We have explored a method based on thermally induced phase separation to ensure the processability of dried bacterial cellulose in an extrusion process. A method was also developed to determine the contact angle of polylactide droplets on (modified) bacterial cellulose nanofibrils. We could show a significant improvement of the wettability of bacterial cellulose nanofibrils by polylactide with increasing hydrophobicity of the nanocelluloe fibrils, i.e. the longer the chain length of the grafted fatty acid. This indicates a much improved adhesion between the modified cellulose and polylactide. The tensile modulus and tensile strength of the resulting polylactide nanocomposites were found to increase by as much as 50% and 15%, respectively with bacterial cellulose loading fractions of only 5% compared to neat polymer. The thermal degradation and the viscoleastic behaviour of the polylactide nanocomposites were also found to improve as well. This implied that surface functionalisation of bacterial cellulose nanofibrils can be used as the route to produce truly green nanocomposites with improved properties.