Fabrication And Microstructure And Properties Of Carbon Nanotubes/Poly(l-lactide) Composites

The surface of MWCNTs are modified by way of mixing acids refluxing, surfactant modification and polymer grafting. The composites combined with poly(L-lacitde)(PLLA) and reinforcing fillers, pristine MWCNTs and modified MWCNTs, are fabricated by the solution casting method. The morphologies and structures of modified MWCNTs are studied by FT-IR, Raman, SEM, TEM and TGA. The chain structure, aggregation structure of the composites are analized by FT-IR, XRD and PM. The mechanical properties, thermal properties and biodegradable behavior of the composites are investigated by tensile testing, DSC, FT-IR and SEM. The effects and mechanism of modified MWCNTs to the structure and properties of PLLA based composites are revealed.It is shown that the morphology and structure of MWCNTs are affected remarkably by the different modification methods. Carboxylated groups and some defects are formed on the surface of MWCNTs after refluxing in the mixing acids(HNO3/H2SO4, HNO3/H2O2, HNO3 and HNO3/HCl). Compared to MWCNTs treated by other acids, the surface of MWCNTs treated by HNO3/H2O2 acids have less defects and MWCNTs treated by HNO3/H2O2 acids suspend in deionized water for longer time. Homogeneous sodium lauryl benzenesulfate(SDBS) layers are formed on the surface of MWCNTs by mixing method throughπ-πnoncovalend bond, which make MWCNTs suspend stably in the chloroform. Homogeneous PLLA grafted layers are formed on the surface of Fe3O4 coated MWCNTs(mMWCNTs) by means of in situ ring-opening polymerization of lactide, which are bonded to mMWCNTs firmly. The amount of grafted PLLA are controllable. The mechanism of PLLA grafted to the surface of mMWCNTs is that firstly, a compound is formed on the surface of mMWCNTs after the initiator reacts with mMWCNTs, and then the"initiator-ligand"intermediate products are obtained when the compound reacts with lactide by way of nucleophilic attack, finally, the procedure aboved repeats, leading to polymer chain propagation. PLLA grafted mMWCNTs(mMWCNTs-g-PLLA) exhibit superparamagnetic behavior and suspend stably in the chloroform, which also are aligned under an extra magnetic field.Study on the structure of composites comprising of PLLA and MWCNTs shows that the effect of pristine MWCNTs and modified MWCNTs to the chain structure and crystalline structure of PLLA is not obvious. But, compared to pure PLLA, the size of spherulite become smaller and the mixed spherulite are formed for pristine MWCNTs/PLLA, MWCNTs-COOH/PLLA and mMWCNTs-g-PLLA/PLLA composites. The formation of spherulite is prevented by SDBS/MWCNTs for SDBS/MWCNTs/PLLA composites.The tensile properties of MWCNTs/PLLA composite are investigated. It is found that pristine MWCNTs and MWCNTs treated by acids(MWCNTs-COOH) cause the tensile strength of composites decreased because aggregation of MWCNTs and MWCNTs-COOH happens and there are weak interfacial adhesion between MWCNTs, MWCNTs-COOH and PLLA matrix. The SDBS layers and grafting PLLA improve the dispersion of MWCNTs in PLLA matrix and strengthen the interfacial adhesion between MWCNTs and PLLA, resulting the tensile strength of SDBS/MWCNTs/PLLA and mMWCNTs-g-PLLA/PLLA composites increased. In contrast to SDBS/MWCNTs/PLLA composites, the enhancement ratio of tensile strength of mMWCNTs-g-PLLA/PLLA composites is much higher, which reaches 67% at 3wt% mMWCNTs-g-PLLA content.The biodegradable behavior of MWCNTs/PLLA composites is investigated. The results show that different modified surface of MWCNTs affect the biodegradable behavior of PLLA distinctly after the composites degradate in PBS solution for 56 days. The mass loss ratio of composites is as followed in high to low sequence: pristine MWCNT/PLLA, MWCNTs-COOH/PLLA, SDBS/MWCNTs/PLLA and mMWCNTs-g-PLLA/PLLA composites.