Study On The Crystal Morphology And Thermodynamic Property Regulation Of Polyetherketoneketone

Carbon fiber reinforced resin matrix composites have excellent mechanical properties,good heat resistance,lightweight,recyclable and other characteristics.Polyether ketone ketone(PEKK)is a semi-crystalline thermoplastic polymer,and the melting point can be controlled by changing the ratio of p-benzene(T)and m-benzene(I)structures in the molecular chain.During heat treatment,the properties of pure resin and composite materials are affected by temperature,holding time and pressure.In this paper,PEKK sheet and carbon fiber/polyether ketone ketone(CF/PEKK)composites were prepared by molding method.Scanning electron microscopy(SEM),nano indentation test,tensile property test,dynamic mechanics analysis(DMA),differential scanning calorimetry(DSC),X-ray diffraction(WAXD)and other testing methods were used to study the effects of heat treatment conditions and the structure of PEKK on the crystallization behavior and thermodynamic properties of PEKK,which provided data for the manufacturing optimization of its related composites.The first part studied the effects of preheating treatment on the crystallization behavior of PEKK and the thermodynamic properties of PEKK.It was found that preheating treatment can promote the growth of crystal type I and inhibit the growth of crystal type II,enhance the tensile strength and rigidity of PEKK,and reduce the thermal and oxygen stability.The effect is most obvious when the heat treatment temperature is 380 °C.Compared with the unpreheated samples,the tensile strength and Young's modulus are increased by 5 times and 10.7% respectively.The hardness is reduced,by 3% compared with the unpreheated sample.The effects of annealing process and cooling mode on PEKK properties were studied on the basis of preheating treatment.It is found that the rigidity and toughness can be customized by means of natural cooling,heat preservation in high temperature zone and rapid cooling with flowing water.The different annealing treatment methods can induce the spontaneous formation of crystal type II in the middle and high temperature region,resulting in the decrease of material rigidity and plastic improvement,or hardening and embrittlement effect through the rapid cooling process.Young's modulus can reach more than 2.8 GPa,which is increased by 7%.Pressure holding cooling can improve the thermal oxygen stability and rigidity of the sample,but the crystallinity and tensile strength of the sample are decreased.The second part studied the effects of the ratio of p-benzene/m-benzene(T/I)on the thermodynamic properties of PEKK materials.It was found that the melting temperature,glass transition temperature and thermal stability of PEKK increased with the increase of the T/I ratio.The mechanical properties of PEKK with T/I ratio of 60/40 are better,which provides a reference for optimizing the processing technology of PEKK and its composites.In the third part,solution impregnation method was used to prepare CF/PEKK prepreg strip,and then the CF/PEKK composite material was prepared by molding method.It was found that the preparation process of heat treatment temperature 360 °C,preheating for 20 min,and holding pressure for 20 min could obtain the composite material with uniform combination and good performance.On this basis,the influence of the addition of CF on the PEKK composite material was further analyzed.It was found that the addition of CF inhibited the crystallization growth of PEKK,reduced the degree of crystallinity and crystallization perfection,improved the thermal stability,and significantly increased the fracture strength and elastic modulus.In this experiment,the influence mechanism of thermal history and T/I ratio on PEKK crystallization behavior and thermodynamic properties was studied in depth,which can provide experimental and theoretical guidance for predicting and optimizing the mechanical properties and processing technology of composite parts.