Crystallization Behavior Of Carbon Nanotubes-grafted Polymers With High Grafting Degrees

It is expected that the carbon nanotubes (CNTs)-grafted polymers with high grafting degrees may exhibit some unique crystallization behaviors, including crystallization kinetics and nucleation, due to the co-existence of structural confinement, stretched conformation of the grafted polymer chains and nucleation effect of CNTs. The crystallization behavior of CNTs-grafted polymers would also be affected by the molecular weight and grafting density of the grafted polymer chains. Moreover, the orientation mechanism of the grafted polymer crystals on the surface of CNTs is also interesting. In this dissertation, we focused on the crystallization behavior of multi-walled carbon nanotubes grafted poly(ε-caprolactone)(MWNTs-g-PCL) with various PCL molecular weights and grafting densities. The orientation of the PCL crystals on the surface of MWNTs after solution crystallization was studied as well.To facilitate studying the crystallization behavior of MWNTs-g-PCLs, the grafted PCL should possess both a high molecular weight and a high grafting degree. The modified [2+4] cyclo-addition reaction method was chosen for synthesis of MWNTs-grafted small molecules with a hydroxyl end group. Then in-situ ring-opening polymerization of ε-CL was carried out using CpTiCl3as the catalyst and the grafted small molecules having a hydroxyl end group as initiator. The MWNTs-g-PCLs with a grafting degree as high as95wt%could be obtained. Moreover, the molecular weight and grafting degree of the grafted PCL could be adjusted by the control of some reaction conditions, which will be beneficial to the further crystallization study.The isothermal crystallization kinetics of MWNTs-g-PCLs and PCL/MWNTs blend were first compared, and the results were discussed in terms of structural confinement and stretched conformation of the grafted PCL chains. It was observed that the structural confinement could intensify the heterogeneous nucleation of MWNTs, leading to an accelerated crystallizing rate but a reduced crystal size. The stretched conformation could greatly promote the crystallizing process of grafted PCL chains. The independent effects of PCL molecular weight and grafting density on the crystallization kinetics of MWNTs-g-PCLs were also studied. It was found that the crystallization rate of MWNTs-g-PCLs increased as the PCL molecular weight and grafting density increased. However, grafting density played a more important role in crystallization of MWNTs-g-PCLs. The self-nucleation behaviors of MWNTs-g-PCLs were also investigated by means of self-nucleation and annealing (SN) technique and the nucleation abilities of MWNTs in MWNTs-g-PCLs and PCL/MWNTs blend were compared. As mentioned above, the structural confinement intensified the heterogeneous nucleation of MWNTs, leading to much narrower temperature range of Domain II in MWNTs-g-PCLs, as compared with the neat PCL and PCL/MWNTs blend. The self-nucleation behavior of MWNTs-g-PCLs was also correlated with the grafting density and molecular weight of the grafted PCLs. At a relatively higher grafting density, the heterogeneous nucleation effect of MWNTs would become more evident. For some specific MWNTs-g-PCL samples, Domain Ⅱ might even disappear. The nucleation ability of MWNTs would be reduced for the grafted PCLs with a relatively lower molecular weight.In order to understand the crystallization mechanism of CNTs-grafted polymers at the molecular level, the isothermal crystallization process of multi-walled carbon nanotubes grafted poly(L-lactide)(MWNTs-g-PLLA) and PLLA/MWNTs with a relatively slower crystallization rate was monitored by in-situ FT-IR. Because of the crowding of polymer chains in MWNTs-g-PLLA, the PLLA chains would maintain stretched conformation to some extent even in the melt, which enabled it to form crystals by the coordinating movements of PLLA backbones followed by the motion of side groups at the very beginning of the crystallization process, while in PLLA/MWNTs, the side groups firstly interacted with the surface of MWNTs to form pre-ordered structures, then drived the movement of PLLA backbones to crystallize. Furthermore, the motion sequences of some characteristic groups during the crystallization process were obtained by two-dimensional correlation analysis of the FT-IR spectra, which agreed well with the crystallization processes obtained from the one-dimensional FT-IR results.The orientation of the PCL crystals in solution-crystallized MWNTs-g-PCLs and PCL/MWNTs blend was finally studied. After solution crystallization,"rod-like" crystals were formed for the PCL/MWNTs blend, which were different from the so-called "nano-hybrid shish-kebab"(NHSK) structure obtained in the polyethylene/CNTs blend. This indicated that crystallization of PCL could straighten MWNTs, but it only took effect for MWNTs with a length below5μm. When MWNTs were longer, they could not be straightened upon solution crystallization of PCL and the crooked MWNTs might exert a disadvantageous effect on crystallization of PCL. For MWNTs-g-PCLs, such a straightening effect was weakened due to structural confinement and thus lower crystallinity of the grafted PCL crystals. Only MWNTs with a smaller outer-diameter and a shorter length could be straightened by the grafted PCL crystals. The high-resolution TEM characterization revealed that the PCL chains in the crystals were not exactly parallel to the axis of MWNTs, which was the arrangement way of PE chains in the NHSK structure of solution-crystallized PE/CNTs. As a result, there was a component along the axis of MWNTs for the PCL crystallization force produced by crystal growth, which may straighten MWNTs.