| Study on the structure control of polyolefin under the complex external fields for regulating its macroscopic properties, is a significant way to producing high performance polyolefins in real production. The structure of polyolefin includes chain structure and condensed state structure which determins the basic properties and service performance of polyolefin materials. The condensed state structure of polyolefin is not only determined by the intrinsic structure of polymer chains, but also greatly influenced by the processing conditions. For instance, the specific temperature fields and shear fields during processing have an evidently effect on the formation of the condensed state structure of polyolefin. Hence, further understanding of the relationship between the formation condition of the condensed state structure and macroscopic properties of polyolefin, is important for processing control and performance improvement.In the present dissertation, two semi-crystalline polyolefins, isotactic polypropylene (iPP) and impact-resistant polypropylene copolymer (IPC), were selected as the investigated subjects, their crystallizarion behavior, crystalline structure and macroscopic properties under shear field and temperature field have been well characterized. Considering the multi-composition and multi-phase characteristic of IPC, the evolution of phase morphology under external fields has also been systematically investigated. Based on the experimental results, the new knowledge about the structure-property relationship of these polyolefins has been acquired. The main research works and conclusions are introduced as follows:1. The phase morphology evolution and corresponding change of mechanical performance during thermal treatment in the molten state have been systematically investigated. The multi-phase impact-resistant polypropylene copolymer undergoes a significant dispersed phase coarsening upon annealing in the molten state, which follows a strong dependence on annealing time. With annealing time increasing, the dispersed phases of IPC gradually enlarge themselves. This phase evolution process is in accordance with Ostwald ripening theory, which strictly follows a d～t1/3time dependent law. Accordingly, the mechanical properties of IPC change with the evolution of phase morphology. Both impact strength and tensile elongation at break which represent the toughness of materials decrease observably with phase coarsening. and the decline trend is consistent with the increase of dispersed domain size. However. the tensile stress which mainly represents the rigidity of materials shows stability against phase coarsening. Due to toughness is greatly influenced by amorphous phase morphology, the enlargement of dispersed phase results in a significant decrease of toughness. On the other hand, the rigidity which is mainly determined by crystalline morphology remains steady during annealing because of the stable crystalline structure.2. The relaxation behavior of shear-induced nucleation precursor has been systematically investigated by means of ARES rheometer. The enhanced crystallization kinetics gradually faded away when samples were held at a high temperature above the melt point. The decline of shear effect with the increase of relaxation duration was observed both in pure iPP and β-nucleated iPP. However, the relaxation rate of shear-induced precursors in pure iPP is much faster than those in β-nucleated iPP. It suggests that foreign particles can stabilize the shear-induced precursors and consequently retard the relaxation process, thereby endowing the precursors in β-nucleated iPP with higher thermal stability. The effect of shear and subsequently relaxation on crystalline structure has been studied. It is found that the application of shear flow and β-nucleation agent has a counteraction effect on the formation of β-phase crystals, which caused a decrease of β-phase crystal content. However, the subsequent relaxation can free the restraint effect of shear flow on the formation of β-crystals, as well as restored the lost of β-phase crystal content caused by shear flow. Interestingly, the increase of β-phase content did not synchronize with the decay of shear effect. The influence of shear on crystal phase has disappeared completely, when shear-induced precursors were still present. We suggest that, right after the application of shear, the enhanced amount of a-nuclei induced the competitive growth of α-and β-crystals which caused the decrease of β-phase crystal content. With relaxation, the partially dissolved precursors leaves more space for the growth of β-crystals. so that mitigate the competition between α-and β-crystals and facilitate the formation of β-crystals. Meanwhile, the shish structure and some more perfect kebab structure are survived. Hence, the β-phase crystal content has restored to the maximum when shear enhanced effect still exists.3. Compared the variance of microstructure and mechanical property of two samples with different thermal history under the same final thermal treatment condition. The stepwise crystallized sample was directly cooled down to the thermal treatment temperature after isothermal crystallization; while the annealed sample was cooled down to room temperature after isothermal crystallization, than heated to the thermal treatment temperature. The mechanical property tests exhibit that the tensile strength of stepwise crystallized sample increases, the impact resistance of annealed sample increases. Structure investigation found that the a-relaxation peak which is related to the assignment of chains in rigid amorphous phase, moves to the high temperature region for stepwise crystallized sample, while it moves to the low temperature region for annealed sample during thermal treatment at120℃. It indicated that the rigid amorphous fraction (RAF) weakened and the thickness of lamellae increased in stepwise crystallized sample during thermal treatment, which caused the increase of tensile strength. However, for annealed sample, the RAF strengthened and lamellae structure hardly changed during thermal treatment. The newly formed rigid amorphous phase which contains some defects is in favor of energy dissipation, as a result, the impact resistance of annealed sample increased. |
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