Thermal Decomposition And Crystallization Melting Behavior Of Poly (Ether Ketone Ketone) Copolymers

Poly（ether ketone ketone）（PEKK）is a thermoplastic special engineering plastic with excellent heat resistance,mechanical properties and corrosion resistance,which is usually polymerized with terephthaloyl chloride（TPC）,isophthaloyl chloride（IPC）and diphenyl ether（DPE）.The melting point and crystallization ability of PEKK copolymers can be adjusted by controlling the ratio of terephthaloyl chloride to isophthaloyl chloride（the so-called T/I）during the polymerization process.Due to its adjustable crystallization rate and wider processing window,PEKK is considered to be an ideal material as the matrix for high-performance thermoplastic composites.To date,the raw materials used in most of the research on PEKK copolymers were synthesized by a traditional two-step method.Therefore,the study of thermal decomposition and crystallization melting behavior of PEKK copolymer synthesized by one-step method is helpful to understand the relationship between its structure and properties,and provide a theoretical guidance for its molding processing and new product development.First of all,the molecular structure and thermal decomposition behavior of PEKK copolymers were studied.The structure of PEKK copolymers were characterized by FT-IR,¹H-NMR and ¹³C-NMR,and the obtained results were consistent with the expected molecular structure of polymer.Due to the similar chemical environment of some hydrogens on the benzene ring of the PEKK copolymers,the peaks overlap,which makes it difficult to accurately determine the T/I ratio in the structure by ¹H-NMR.The T/I ratio can be determined by ¹³C-NMR.The pyrolysis behavior of PEKK was studied by DSC,TGA and TG-IR-GC/MS.With the prolongation of high-temperature treatment time,the polymer is cross-linked,so the crystallinity and melting point decrease,and the glass transition temperature increases.Regardless of the ratio of T/I,the 5%weight loss temperature of PEKK copolymer is between559-565°C,and they still have a high carbon residue rate at 800°C,which indicates that the PEKK copolymers have good thermal stability.PEKK80 was selected as the research object,the thermal decomposition mechanism of PEKK was analyzed by TG-IR-GC/MS.It is inferred that the chain scission of PEKK mainly occurs at the ketone bond and ether bond,forming benzene,ether or carbonyl free radicals,and then through the recombination of free radicals and free radicals,hydrogen transfer and other reactions,small molecular aromatic compounds and volatile products such as H₂,CO and CO₂are generated.Then,in order to establish the relationship between the crystallization and melting behavior of PEKK copolymers with different T/I ratios,through DSC,POM,WAXD and TMDSC,the non-isothermal crystallization,isothermal crystallization,spherulite morphology,polymorphism and multiple melting behavior were studied in detail.With the addition of meta isomers,the crystallization ability and melting point of PEKK copolymers reduced,as confirmed by non-isothermal and isothermal crystallization kinetics analysis.For melt-crystallized PEKK samples and unmelt-processed PEKK powder samples,different crystal forms were observed by WAXD.The DSC heating scan curve after isothermal crystallization of PEKK showed double melting peaks,and the low temperature melting peak showed the characteristics of the annealing peak.Simultaneously,for the TMDSC scans,a glass transition and an endothermic peak were observed on the reversible and non-reversible heat flow curves,respectively,at the position of the low temperature melting peak.These results demonstrate that the double melting behavior is mainly related to the superimposed with rigid amorphous devitrification and enthalpy relaxation.In addition,the thermal classification of PEKK copolymers were carried out for the first time by using the successive self-nucleation and annealing（SSA）technology and thermal fractionation was successfully achieved.