Structure And Properties Of Poly Lactic Acid / Carbon Nanotube Composites

Biocompatible and biodegradable polylactide (PLA) is a new aliphatic thermoplastic polyester with good performance. However, the applications of PLA have been more or less limited in the engineering fields as a single-component material because of its brittleness and poor thermal stability. Filled with filler is hence an effective strategy to improve the performance of the PLA and to further extend its applications. Nowadays, to incorporate PLA with carbon nanotube (CNT) has attracted much attention because the CNT is also of good biocompatible and shows excellent mechanical properties. However, the research work on the PLA/CNT composites is still in the preliminary stage and, the relations between the structure and the properties of such system have not yet been established till now.In this work, therefore, the PLA/CNT composites were prepared via melt mixing. Then, the micro- and mesco-structures, viscoelasticity, crystallization, mechanical properties, electrical conductivity and degradation behavior of the composites were studied by the approaches of morphology characterization, rheology, and thermal and mechanical analysis. The effects of dispersion state and mesoscopic structure of the CNT on macroscopic properties of the composites were described using linear viscoelastic theory, nonlinear percolation theory, suspension model and crystallization kinetic models, aiming at exploring the relations between hierarchical structure of the CNT a and properties of the PLA/CNT composites.(I) The linear rheological behavior of the composites show strong dependence on the content of CNT. With addition of CNT, the dynamic modulus of composites increases significantly, showing evident solid-like response in the low-frequency region due to formation of the transient percolation network. Moreover, such a percolated network structure is very sensitive to the steady shear and, is readily destroyed in the shear flow even with small rates, resulting in a sharp decrease of low-frequency modulus. However, the network structure can be reorganized in the annealing process, leading to a transient stress overshoot behavior of the composites. The structural rheology can be used as a powerful tool to describe the short-range structure (nano-dispersion) and the long-range structure (mesoscopic network structure) of the CNT, therefore.(II) The composites with various urface modificatd CNTs show large difference in their rheological responses. The carboxylic CNT presents far smaller percolation threshold in the PLA matrix than that of the hydroxyl one. At the identical contents, the former presents the effective filling volume fraction 3 times higher than the latter. As a result, the composite with carboxylic CNT shows far higher modulus than the composite hydroxyl CNT. This is attributed to better affinity between PLA chains and surface carboxyl of the CNT. The rheological responses of composites also depend on the aspect ratio of CNT strongly. Despite the fracture of CNT during melt processing, the CNT with higher aspect ratio still has larger hydrodynamic volume than the one with lower aspect ratio, leading to an evident increase of probability in the collision and the friction among the CNTs themselves. Thus the percolation threshold is far lower than the CNT with lower aspect ratio.(III) The presence of CNT has an evident heterogeneous nucleating effect, promoting formation of the nuclei of PLA matrix both under isothermal and non-isothermal conditions. The crystallization rate, however, is also dependent on the crystallization history. In the isothermal annealing process, the nucleation rate is very high and the average separation of nuclei approaches the width of molecular stems. In this case, the crystallization from molten state is controlled by nucleation, while the crystallization in solid state is governed by growth. As a result, the presence of CNT accelerates the melt crystallization, while retards the overall kinetics of the cold crystallization. In the non-isothermal annealing process, however, the presence of CNT accelerates the cold crystallization because the nucleation is the dominant role on the overall kinetics. (IV) The addition of CNT improves both the tensile strength and impact strength of the PLA. As the content of CNT achieving up to 1.5 wt% and above, however, the mechanical strength of the composites decreases due to local aggregation of the CNT. Compared with that of the pure PLA, the conductivity of the composite increases by about 14 orders and, the conductive percolation threshold is about 1 wt%, which is a little higher than the rheological percolation threshold (0.77 wt%). This is due to the structural difference in these two percolation network. Moreover, the conductivity of the composite also depends on the thermal histories. The crystallinity of the matrix PLA increases in the annealing process, leading to an increase of effective filling fraction of the CNT in the amorphous zone, and finally further enhances the conductivity of the composite.(V) The degradation behavior of the composites also shows strong dependence on the surface modification of the CNT and the thermal histories. To the thermal degradation, the presence of carboxylic CNT improves the thermal stability of PLA, while the presence of hydroxyl CNT is just the opposite. To the biodegradation, the presence of carboxylic CNT slows down the degradation rate due to its barrier effect. Moreover, the increased crystallinity of matrix PLA can also decrease the degradation rate. In the same period, a lower degradation level is observed both on the surface and inside the sample with melt crystallization history in contrast to the one with cold crystallization history and, the amorphous sample always presents highest degradation levels.