Antioxidant Mechanism Of Carbon Nanotubes And Their Application In Polyethylene

Nowadays, polyolefin/carbon nanotube (CNT) nanocomposites has aroused wide attention of researchers due to their prominently improved mechanical, thermal, optical and physic-chemical properties comparing with the pure or conventional filler-reinforced polyolefin. Generally, the researches are focused on two aspects:(a) the reinforced effect and mechanism of CNTs in polyolefin, which help us to understand and optimize the performance of the composites;(b) the chemical modification of the CNTs, which means to improve their dispersion state in the polymer matrix and thus achieve better physical properties of the composites.Based on the two aspects, the thermal oxidative stability of polyethylene/CNT nanocomposites and the antioxidant mechanism of CNTs are first studied in this thesis. After that, three kinds of hindered phenol grafted CNTs are synthesized to achieve enhanced antioxidant ability and better dispersion state in the polymer matrix. The main work and results are as follows.(1) The thermal oxidative degradation of high density polyethylene (HDPE) was investigated during oven aging at100℃.1H low-field solid-state nuclear magnetic resonance (NMR) was used to characterize the behavior of molecular chains and the changes in phase content of HDPE during aging. The prolongation of aging led to a progressive increase in the amount of the crystalline phase at the expense of the other two phases. A slight decrease in chain mobility of the crystalline phase and interphase was observed simultaneously. The results obtained from other traditional technical approaches are also discussed in the context of the molecular dynamics properties revealed by NMR. The reduction in molecular weight, in chain mobility and the increase in crystallinity during thermo-oxidation were the main factors which caused the loss of mechanical performance of HDPE.(2) The isothermal and non-isothermal crystallization behavior of polyethylene containing various zero, one, and two dimensional (0-D,1-D, and2-D) carbon nanofillers were investigated by means of differential scanning calorimetry. For a given temperature, the isothermal crystallization rate got slower with the addition of lower dimensional carbon nanofillers. The values of Avrami and Tobin exponents indicated that the isothermal crystallization of polyethylene followed two-dimensional crystal growth in the presence of2-D/1-D carbon nanofillers, while exhibited three-dimensional heterogeneous crystal growth in the presence of0-D carbon nanofillers. Contrary to the isothermal study, the non-isothermal crystallization of polyethylene was accelerated in the presence of lower dimensional nanofillers. Ozawa and Mo methods were used to analyze the non-isothermal crystallization data. It was observed that only Mo approach could successfully describe the non-isothermal crystallization process of polyethylene/carbon nanocomposites.(3) The influence of Single-walled carbon nanotubes (SWCNTs), multiple-walled carbon nanotubes (MWCNTs), and hydroxylated multiple-walled carbon nanotubes (MWCNTs-OH) on the thermal oxidative degradation of polyethylene was studied respectively. The thermal oxidative stability of polyethylene was enhanced with the addition of CNTs. Based on the oxidation induction experiments, it was found that the antioxidant capacity of the CNTs was in the following order:MWCNTs-OH> MWCNTs> SWCNTs. The antioxidant ability and mechanism of CNTs were further examined by electron spin resonance spectra and Raman spectra. It was observed that the antioxidant behavior of CNTs obeys a free radical scavenging mechanism. The difference in their defect concentration was one of the factors which caused different antioxidant capacity between these CNTs.(4) To improve the antioxidant ability and dispersion state of CNTs in the polymer matrix, a hindered phenolic antioxidant (AO) has been covalently grafted onto the surface of MWCNTs using a silane coupling agent. Two contrasting routes, two-step functionalization and one-step functionalization of MWCNTs, were developed. The corresponding polyethylene/MWCNT composites were prepared by melt blending. Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, thermal gravimetric analysis, transmission electron microscopy, and Raman spectroscopy confirmed the successful functionalization of MWCNTs. Electron spin resonance spectra revealed that the free radical scavenging activity of MWCNTs was greatly increased after functionalization. The resultant MWCNTs exhibited improved dispersion and antioxidant efficiency in the polyethylene matrix. Compared with the two-step functionalized MWCNTs, the one-step functionalized MWCNTs were more efficient in preventing polyethylene from thermal oxidative degradation.(5) The first and second generation of dendritic polyols (G1and G2) with butanediamine as core and acrylic acid as repeating unit are synthesized and used as linkage to covalently connect MWCNTs and AO. The resultant CNTs (MWCNTs-G1-AO and MWCNTs-G2-AO) and corresponding polyethylene/CNT nanocomposites were prepared. It was found that G2are more efficient in grafting AO than Gl. Due to a higher loading of antioxidant groups, the antioxidant ability of MWCNTs-G2-AO are stronger than MWCNTs-G1-AO. Comparing with the pristine CNTs, MWCNTs-G2-AO can further extent the oxidation induction time (200℃) of the polyethylene by93.6min. Furthermore, it was found that there is a significant antioxidant synergy between MWCNTs-G2-AO and Irgafos168.(6) To further improve the loading of AO on the surface of the CNTs, a hyperbranched linkage was proposed. Firstly, the hyperbranched polyglycerol was grown from the surface of MWCNTs by ring-opening polymerization. After that, AO was grafted onto the MWCNTs through esterification. It showed that the obtained MWCNTs-HPG-AO were coated with an organic layer of about4nm. The result indicated that the loading of AO was2.23mmol for per gram MWCNTs, which increased about an order of magnitude comparing with CNTs functionalized through other methods. Polyethylene nanocomposites incorporated with different kinds of functionalized CNTs were prepared respectively. The MWCNTs-HPG-AO were separately dispersed in the polyethylene. Among all the nanocomposites, polyethylene/MWCNT-HPG-AO nanocopmosite was the most stable during thermal oxidation. It showed that the oxidation activation energy of polyethylene was improved with the addition of MWCNTs-HPG-AO.