Microbial-derived cellulose-reinforced biocomposites

Abstract

The preparation and characterisation of novel nano-scale biodegradable biocomposite materials, consisting of bacterial cellulose (BC) in a poly(lactic acid) (PLA) matrix, are investigated. BC exhibits high purity, high mechanical strength and an ultra-fine fibrous 3D network structure, while PLA is low cost, biodegradable matrix material derived from natural resources. In this work, composites of BC reinforced PLA were prepared using a solution exchange process and compression molding. The microstructure of the raw materials and composites was characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The thermal properties and crystallinity of PLA and composites were measured using differential scanning calorimetry (DSC). The mechanical properties of pure PLA and composite materials were evaluated using static and dynamic mechanical analysis (DMA). In order to improve the interfacial adhesion between the BC and PLA matrix, BC was acetylated (ABC) or treated with 3-aminopropyltriethoxysilane (APS) coupling agent (SBC). The PLA was plasticized with glycerol (PLAG) in order to increase its ductility. As compared to the Young's modulus of neat PLA (1.9 GPa), ABC generated the highest increase in Young's modulus (4.8 GPa) of the resulting composites followed by BC (4.6 GPa) and SBC (4.5 GPa). The tensile strength of PLA (31 MPa) also was enhanced to 75 MPa with BC, 72 MPa with SBC or 38 MPa with ABC. The ductility of PLAG was degraded with the addition of glycerol. A large amount voids led to a reduction in the mechanical properties of PLAG and PLAG based composites. Every reinforcement led to an improvement in the storage modulus (E') of the neat PLA and PLAG, especially at temperatures above the glass transition temperature (Tg). The DMA results showed that the presence of BC based reinforcements significantly reduced the damping properties of PLA. The reinforcements also influenced the crystalline procedure of PLA. With the addition of BC or ABC to the PLA matrix, the melting points of the composites were increased ~ 4-7 ℃ with a slight change on crystallinity; the crystallinity of SBC-PLA composite decreased from 31.9 % to 26.9 % with only a change of ~ 1 ℃ in the melting point.