Study On The Mechanical Properties And Toughening Mechanisms Of PLLA/EVA Immiscible Blends By Adding Carbon Nanotubes

As is known to us, polymer blending is the important way to develop new materials nowadays. Unfortunately, it is very hard to satisfiy the industrial demand through blending two or more polymers together because most of the polymers are immiscible. To solve the problem, nano-particles are incorporated into immisible blends in recent years. It is very interesting to find that the nano-particles can not only selectively distribute in the two phases but also induce peculiar morphologies which have a great influence on the mechanical properties of the materials. There are many commonly used nano-particles such as silicone dioxide (SiO2), carbon black (CB), montmorillonite (MMT), carbon nanotubes (CNTs) and so on. Compared with other nano-particles, CNTs have the characteristics of large aspect ratio, high migration rate and strongly "self-network" capacity. It has been reported that the impact toughness of some immiscible blends can be enhanced by adding CNTs. Actually, CNTs cannot improve the impact strength for most of immiscible blends, which restricts the application of CNTs to some extent.In order to extend the application of CNTs in modifying immiscible blends, the toughening mechanism of CNTs on immiscible blends was studied in this work. Firstly, CNTs were introduced into poly(L-lactide) (PLLA)/poly(ethylene-co-vinyl acetate) (EVA) immiscible blend. The relationships between the mechanical properties of the materials, the microstructure and the interaction between the two phases were investigated systematically. Secondly, the interaction between PLLA and EVA was regulated by changing the VA content and the prerequisite of toughening immiscible blends was comparatively investigated. Besides, reactive auxiliary was further incorporated to construct chemical-physical double networks in the materials and super toughened composites were achieved.The main results through the systematic research are listed as follows:(1) CNTs were introduced into PLLA/EVA immiscible blends with sea-island structure through melt-blending. It concludes that CNTs not only induce the larger EVA particles but also significatanly enhance the interaction between PLLA and EVA through the selevtive localization of CNTs at the interface between PLLA and EVA and the construction of CNTs network structure, which lead to a great improvement in the notched Izod impact strength of the materials. Besides, the evolution of the morphologies induced by CNTs is not effective for improving the impact strength according to Wu’s theory, which verifies the toughening mechanism of CNTs on immiscible blends further.(2) Different PLLA/EVA/CNTs composites with sea-island structure were prepared via changing VA content. It concludes that CNTs have negative effect on toughening the immiscible blends with lower VA content. On the contrary, CNTs have positive effect on toughening the immiscible blends with higher VA content. Consequently, the morphological change, selective distribution of CNTs at the interface between PLLA and EVA and the construction of CNTs network structure are not the main factors which influence the toughening efficiency of CNTs. Specifically, the crystallization behavior of EVA phase where CNTs are located, the modulus ratio of PLLA to EVA and the interaction between the two phases are the main factors which influence the toughening efficiency of CNTs, and also are the prerequisite of toughening immiscible blends by CNTs.(3) Dicumyl peroxide (DCP) and CNTs were simultaneously introduced into PLLA/EVA immiscible blends through melt-blending. It concludes that DCP and CNTs have synergistic toughening efficiency on PLLA/EVA blends. The interaction between PLLA and EVA could be enhanced and the EVA particles become smaller and more uniform because of chemical-physical double networks constructed by DCP and CNTs. Consequently, the matrix ligament thickness is greatly decreased and the stress field around EVA particles could superimpose easily under external force so that the shear yielding pervades the whole matrix. At last, super toughened composites were successfully achieved.