| The complexity of the thermal behavior of semi-crystalline polymers has been the subject of extensive studies. In order to describe the temperature dependent crystallization or reorganization of the semi-crystalline structure, different models have been proposed. In terms of semi-crystalline polymers like PEEKK, its Tg in the interior crystalline region is generally affected by its crystallinity. As a result,characteristics of the entire crystals may be affected by the Tg of amorphous region,which is related to the correlation of characteristics between amorphous regions and crystalline regions in the semi-crystalline structure. Generally, a semi-crystalline structure can be thought of as a metastable system being far from equilibrium. The term “metastable ” implies the existence of energetic barriers and related kinetic constraints on the system’s path toward equilibrium, i.e. toward a higher crystallinity.Therefore, it is of great importance to research the relationship between cause as well as impact factor “metastable” produced and its melting behavior.Poly(aryl ether ketones) have been called high performance polymers as they exceed other thermoplastic polymers in their mechanical and physical properties as well as in their chemical resistance, which combine an extremely high thermal resistance up to 260 ℃ with good mechanical properties, elevated Young ’ s modulus and high impact strength. PEEKK is one of the polymers with typical characteristics in the Poly(aryl ether ketones) family. It is a thermoplastic materialwith a melting point of 370 ℃ and can thus be processed by conventional injection molding machines, calenders of dies to molded parts, films or fibres. Usually, the polymer is partially crystalline and depending on the processing conditions, the crystallinity ranges between 0% and 40%. At the same time, the crystallinity of PEEKK can be varied widely, depending on its thermal history. With injection or compression molding, well crystallized material can be obtained which merely retains its shape and mechanical properties at elevated temperatures above Tg. For that case,it is an important prerequisite for the choice of the most suitable processing parameters tailored for a particular application.In present thesis, PEEKK is used as raw materials and relevant characterization methods such as differential scanning calorimetric analysis(DSC) 、 dynamic mechanical analysis(DMA)、WAXD、universal testing machine and tensile shock testing machine are used to analyze and research the double melting behavior of PEEKK film samples and its influence on the material properties.The DSC results show that the only samples isothermal cold crystallization processed appear double melting behavior. In terms of different heat treatment temperature, low melting temperature will be about 20 ℃ higher than its heat treatment temperature. When Tm-Tc<10 ℃, low melting temperature melting peak will coincide with low melting temperature melting peak and its range increases linearly. On one hand, in terms of different heat annealing time, low melting temperature will gradually move to the higher temperature with the extension of the heat annealing time as well as the peak area will be gradually increased. Similar conclusion is made like the extension of the heat annealing time with increase of DSC heating rate. At the same time, cold crystallization peak of PEEKK film samples will gradually move to the lower temperature with increase of DSC cooling rate and the peak area will be gradually increased also. On the other hand, however, the double melting behavior of PEEKK Samples is not affected by its thickness and high melting temperature is not affected by the heat annealing temperature, time and DSC heating rate.The experimental phenomena above can be explained with our proposed“three-phase model”.The WAXD results show that the diffraction peaks will occur in the only samples isothermal cold crystallization and melting crystallization processed. And the diffraction summit of samples isothermal cold crystallization processed will gradually increase cold crystallization processed will gradually decline with the increase of the heat annealing temperature as well as tanδ peak will gradually move to the lower temperature, whose peak area will reduce correspondingly. At the same time, the low temperature melting peak appears above Tgcorresponding to heat annealing temperature. Similar conclusion has been made in terms of the research of heat annealing time. The phenomenon happened above can be explained with “three-phase model” and it proves the double melting behavior of PEEKK does affect the mechanical properties.The tensile properties test show that the tensile strength and tensile modulus of the melting crystallization samples are maximum, whose elongation at break is minimum, followed by isothermal cold crystallization Samples. The tensile strength and tensile modulus of isothermal cold crystallization Samples will gradually increase with the extension of the heat annealing temperature and time while elongation at break tends unchanged. Above phenomenon is due to the existence of “three-phase model” and different crystallinity of the transition phase.The tensile impact properties test show that the tensile impact strength of amorphous samples is maximum followed by isothermal cold crystallization samples.The tensile impact strength of isothermal cold crystallization Samples will gradually decrease with the extension of the heat annealing temperature and time. It proves that the higher the crystallinity of PEEKK film samples have, the worse their toughness are, which is also due to different crystallinity of the transition phase. |
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