| Poly(lactic acid) (PLA) is the most studied biodegradable polymer material in recent years. However,its strength is not high enough, and its brittleness,thermal stability and crystallization is poor. To improve its performance, we need to modify PLA. Bacterial cellulose (BC) is one of nano biomaterials, which has fine 3D mesh structure. Carbon nanotubes (CNTs) have excellent mechanical properties and thermal stability, which are the ideal nano fiber reinforced materials. BC and CNTs grafted together to modify the PLA,and it is expected to enhance the performance of PLA.In this paper, multi-walled carbon nanotubes (MWCNTs) firstly were oxidized(carboxyl-functionalized) by concentrated nitric acid and concentrated sulfuric acid to obtain MWCNTs-COOH. After that, MWCNTs grafted with KH560 (MWCNTs-KH560)were prepared by the reaction between MWCNTs-COOH and KH560. And then the graft composites of MWCNTs-KH560-BC were the reaction between MWCNTs-KH560 and BC.The PLA/MWCNTs-KH560-BC masterbatch were prepared by the method of solution-blend. Finally the PLA/MWCNTs-KH560-BC masterbatch were added to the PLA matrix in different proportions to prepared the PLA/BC/MWCNTs composites by the method of melting-blend. The structures of graft-modified MWCNTs and BC were characterized by using fourier infrared spectrum (FT-IR) and thermogravimetric analysis(TGA) and the relative grafting rate of MWCNTs to BC was estimated. The mechanical property, morphology of fracture surface, thermal stability, spherulite morphology,crystallization and melting performance etc of PLA/BC/MWCNTs composites were characterized by using electronic universal testing machine, scanning electron microscopy(SEM), TGA, polarizing microscope (POM), X ray diffractometer (XRD) and differential scanning calorimetry (DSC) respectively. The results showed that:(1 )The surface of MWCNTs treated by acid had carboxyl groups. KH560 could react with MWCNTs-COOH and it also could grafting react with BC. The relative grafting rate of MWCNTs-KH560 to the BC was 42.86wt%.(2) The mechanical property and thermal stability of PLA/BC/MWCNTs composites showed a trend of increase before decrease with the increase of the content of MWCNTs-KH560-BC. When the content of MWCNTs-KH560-BC was 0.6wt%,PLA/BC/MWCNTs composites had best mechanical properties. And its tensile strength,notched impact strength and flexural strength were increased by 9.9%, 21.7% and 9.4%compared to pure PLA respectively. MWCNTs-KH560-BC formed the network structure,which gave rise to the toughening and strengthening effects.(3 ) The thermal decomposition temperature of PLA/BC/MWCNTs composites reached its maximum when the content of MWCNTs-KH560-BC was 0.6wt%. The initial degradation temperature and maximum degradation temperature of pure PLA was 254.01 ℃and 342.27 ℃ respectively. When the content of MWCNTs-KH560-BC was 0.6wt%, The initial degradation temperature and maximum degradation temperature of PLA/BC/MWCNTs composites was 267.50℃ and 349.38 ℃ respectively. At this time, the dispersion of MWCNTs-KH560-BC in PLA matrix was the best, and it formed the largest network area, which effectively improved the decomposition temperature and VST of PLA.(4 ) The SEM images of the fracture surface of PLA/BC/MWCNTs composites indicated that MWCNTs-KH560-BC with a proper amount could be dispersed in PLA matrix uniformly and formed good interface bonding. The impact section surface of PLA/BC/MWCNTs composites was rough, and there were obvious pull-out traces, which showed typical characteristics of ductile fracture.(5) MWCNTs-KH560-BC with a proper amount could reduce the spherulite size of PLA/BC/MWCNTs composites, especially at its content of 0.6wt% the spherulite size was minimum, which was close to about 15μm. MWCNTs-KH560-BC had heterogeneous nucleation effect in the matrix of PLA, and could promote the crystallization of PLA/BC/MWCNTs composites, and also made the crystallization more perfect. With the increase of the content of MWCNTs-KH560-BC , the crystallization temperature (Tc) and crystallinity of PLA/BC/MWCNTs composites increased gradually and it reached the maximum at 0.6wt%. |
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