Optimization of the mechanical performance of bacterial cellulose/poly(L-lactic) acid composites

Understanding the nature of the interface between nanofibers and polymer resins in composite materials is challenging because of the complexity of interactions that may occur between fibers and between the matrix and the fibers. The ability to select the most efficient amount of reinforcement for stress transfer, making a saving on both cost and weight, is also a key part of composite design. The use of Raman spectroscopy to investigate micromechanical properties of laminated bacterial cellulose (BC)/poly(l-lactic) acid (PLLA) resin composites is reported for the first time as a means for understanding the fundamental stress-transfer processes in these composites, but also as a tool to select appropriate processing and volume fraction of the reinforcing fibers. Two forms of BC networks are investigated, namely, one cultured for 3 days and another for 6 days. The mechanical properties of the latter were found to be higher than the former in terms of Young's modulus, stress at failure, and work of fracture. However, their specific Young's moduli (divided by density) were found to be similar. Young's modulus and stress at failure of transparent predominantly amorphous PLLA films were found to increase by 100 and 315%, respectively, for an 18% volume fraction of BC fibers. BC networks cultured for 3 days were shown to exhibit enhanced interaction with PLLA because of their higher total surface area compared, as measured by nitrogen adsorption, to the material cultured for 6 days. This enhanced interaction is confirmed by using the Raman spectroscopic approach, whereby larger band shift rates, of a peak initially located at 1095 cm(-1), with respect to both strain and stress, are observed, which is a quantitative measure of enhanced stress transfer. Thermal analysis (differential scanning calorimetry) and electron microscopy imaging (scanning electron microscopy) of the samples also confirms the enhanced coupling between the resin and the BC networks cultured for 3 days, compared to those cultured for 6 days. These results are shown to have implications for the use of BC networks for composite reinforcement, whereby less material can be used for the same specific mechanical properties. The technique also gives opportunities to study the interfaces in these composite materials in detail.