Polycaprolactone / Poly (lactic Acid) Blends Form And Performance

Biodegradable polymers have generated great interest in the materials research field. As typical one of such kind of materials, the thermoplastic aliphatic polyester has been extensively studied. Among them, poly(ε-caprolactone) (PCL) and polylactide (PLA) are two most important ones due to their good processability. But the rubbery PCL with high toughness and slow degradation rate shows low strength, while the glassy PLA with high intensity and rapid rate of degradation shows poor toughness. Thus to blending them is a convenient strategy to obtain new biodegradable material with high performance due to their property complementarity. However, these two biodegradable polymers are immiscible and the large-scale phase separation restrains property improvement of their blend. Thus, to design and prepare biodegradable polymer alloy with reasonable structures and controllable performance, it is necessary to deeply explore the dynamical and thermodynamic aspects influencing the phase morphology of the poly(ε-caprolactone)/polylactide (PCL/PLA) blend.In this work, therefore, the PCL/PLA blend was first prepared by melt mixing. The immiscible phase morphology and the viscoelastic behavior of the blend system were then studied by morphological characterization and structural rheology. The effects of compositions, viscosity ratio, temperature, shear flow and other dynamic aspects on the morphological evolution were deeply explored, aiming at relating phase morphology and long-range structure to structural rheology of the immiscible blend. Furthermore, a block copolymer (PCL-b-PLA) and the multi-walled carbon nanotubes (MWCNT) were used as the compatibilizer to improve the phase morphology and the performance of PCL/PLA blend. The microstructures and the mesco-structures were then studied in order to establish the relationship between the interfacial behavior and the propeties of the compatibilized blends. (1) Incompatible PCL/PLA BlendFor the incompatible PCL/PLA blend, three typical immiscible morphologies, i.e., spherical droplet, fibrous and co-continuous structures can be observed at various compositions. The phase inversion is close to the PLA weight of 60 %. The classic viscous models fail to depict inversion point because the elasticity ratio also plays an important role together with high viscosity ratio ( p≈16) on the phase inversion behavior. Therefore, the emulsion models can only be used to predict the viscoelastic properties of the blends with"sea-island"structures but not those of the co-continuous blend.(2) PCL/PLA Blend with"Sea-Island"StructureFor the PCL/PLA blend with"sea-island"structure (70/30 w/w), the results show that the steady-state shear flow can destroy the original balance between break-up and coalescence for the dispersed PLA droplets. As a result, the discrete phase domains will further evolve, and finally achieving to a new dynamical equilibrium. But the evolution level depends strongly on the viscosity ratio. For the blend with higher viscosity ratio ( p≈16), the PLA droplets can not be broken up and are only deformed and coalesce with each other; while for the blend with lower viscosity ratio ( p≈1), PLA droplets can be broken up into smaller droplets in the experimental ranges of flow rates. Such a dynamical equilibrium between break-up and coalescence also shows temperature dependence because the viscosity ratio reduces with increase of temperatures, which can promote the break-up of droplet. Thus the time-temperature superposition (TTS) is no longer applicable to the blend systems above.(3) Compatibilized PCL/PLA BlendA block copolymer was used as the compatibilizer to blend with the PCL/PLA blend. The addition of copolymer reduces the radii of the discrete droplets and the interfacial tension evidently, enhancing the interface adhesion and improving the mechanical properties of the blend as a result. Owing to the increased thickness of the interface layer, the phase morphology becomes more stable. Accordingly, TTS is applicable to the compatibilized blend. Compared with the uncompatibilized blend, the compatibilized one shows a long-time scaled terminal relaxation behavior, which is attributed to the superposition of the shape relaxation and the interface relaxation.(4) Compatibilized and Reinforced PCL/PLA BlendThe multi-walled carbon nanotubes (MWCNT) were used to improve morphology and properties of the PCL/PLA blend. The results show that the carboxylic MWCNT are selectively dispersed in the matrix PCL phase and on the interface between two phases, leading to simultaneous occurrence of thermodynamically and kinetically driven compatibility. Those interface-localized MWCNT prevent coalescence of the discrete domains and enhance the phase interfacial adhesion as well. As a result, the phase morphology of the ternary composites is improved remarkably in contrast to that of the uncompatibilized PCL/PLA blend. Owing to that unique selective interface-localization and improved phase morphology, the ternary composites present far lower rheological and conductive percolation thresholds than those of the binary composites (PLA/MWCNTs, PCL/MWCNTs), and also present extraordinary mechanical properties even at very low loading levels of the MWCNTs. Therefore, the amphiphilic MWCNT are believed to act as the reinforcements as well as the compatibilizer in the immiscible PCL/PLA blend.