Study On The Modification Of Poly(l-Lactide) And Its Blends By Carbon Nanotubues

Poly (L-lactide) (PLLA) is a kind of biodegradable eco-friendly material. While poor crystallization ability and fracture toughness greatly limit the application of PLLA. Recently, improving its crystallization is still a critical problem for many researchers who are researching the modification of PLLA. On the other hand, many recent researches on PLLA toughening show that, although the toughness of PLLA can be greatly improved by elastomers and plasticizer, the tensile strength and modulus usually deteriorate to a certain degree. As a consequence, the comprehensive properties of PLLA and its blends are not very good. In the present work, functionalized multi-walled carbon nanotubes (f-MWCNTs) have been introduced into PLLA to improve the crystallization behavior. It has been proved that f-MWCNTs have a great influence on cold crystallization of PLLA due to the large length-diameter ratio and specific surface area. However, the melt-crystallization behavior of PLLA is still poor. Thus, low molecular polymer, i.e. polyethylene glycol (PEG) which usually acts as a plasticizer for other polymers, has been introduced into PLLA. As expected, PEG improves the mobility of PLLA chain segments greatly and largely decreased glass transition temperature has been reported. Consequently, there is a synergestic effect of PEG and f-MWCNTs on crystallization behavior of PLLA and such effect is greatly dependent upon the content of f-MWCNTs. In the last of the work, we attempt to imprve the toughness of PLLA through combination of elastomer and f-MWCNTs. Polar elastomer, ethylene-co-vinyl acetate (EVA), has been introduced into PLLA. Our attention is focused on the effect of monomer vinyl acetate (VA) content in EVA on phase morphologies, crystallization and mechanical properties of PLLA/EVA blends. Furthermore, f-MWCNTs has been introduced into PLLA/EVA blends, too. According to the previous reaserch, some interesting results have been achieved as shown in the following.(1) f-MWCNTs have been prepared through the acidification of strong acid and graft modification with maleic anhydride (MAH). The interfcial interaction between PLLA and f-MWCNTs is greatly improved consequently. PLLA/f-MWCNTs nanocomposites with homogeneous distribution of f-MWCNTs have been prepared successfully.(2) A few amount of f-MWCNTs exhibit great heterogeneous nucleation effect on crystallization of PLLA, improving its cold crystallization apparently in all conditions selected in this work. PLLA/f-MWCNTs nanocomposites exhibit higher glass transition temperature compared with pure PLLA due to their high degree of crystallinity.(3) f-MWCNTs and PEG exhibit synergistic effect on PLLA crystallization. Compared with binary blends, ternary bends can crystallize more easily with higher degree of crystallinity and more perfect crystal structure. Furthermore, f-MWCNTs content can also affect the crystallize behavior of PLLA. When the content of f-MWCNTs is lower than 2 wt%, they exhibit heterogeneous nucleation effect for PLLA, improving the crystallization behavior of PLLA greatly. However, if the content of f-MWCNTs is over 2 wt%, percolated f-MWCNTs physical network structure forms in the material, which restricts the mobility of PLLA chain segment and prevents the growth of spherulites.(4) VA content has influence on the miscibility and interfacial interaction between EVA and PLLA, which further affects the morphologies and crystallization behaviors of PLLA/EVA blends. When VA content is lower than 28 wt%, the blends exhibit typical sea-island morphology features; whereas when VA content is over 28 wt%, the blend exhibit cocontinuous morphology feature. Meanwhile, VA content influences the crystallization of PLLA/EVA. At lower VA content, EVA and PLLA are immiscible. Polar VA segments in EVA promote the mobility of PLLA chain segments and induce heterogeneous nucleation of PLLA at the interface, leading to the improvement of PLLA crystallization. At relatively high VA content, the miscibility of PLLA and EVA is very good. The presence of large amounts of VA segments prevents the phase separation of PLLA chain segments from EVA domains in the interface, preventing the nucleation and spherulites growth of PLLA consequently.(5) With the addition of EVA, the ductility and fracture toughness are enhanced. The deteriorated tensile modulus and strength can be compensated by annealing.(6) The crystallization behavior of PLLA in PLLA/40EVA is greatly improved by addition of functionalized MWCNTs. The tensile strength and tensile modulus of PLLA/40EVA/f-MWCNTs nanocomposites increase with the increasing of f-MWCNTs contents. The maximum elongation at break is achieved when 2 wt% f-MWCNTs are present in the nanocomposites, indicating the reinforcement and toughening effects of f-MWCNTs for immiscible PP/40EVA blend.