Synthesis And Characterization Of Alternating Copolyamides With High Heat Resistance

In recent years,with the emergence of new application fields,higher requirements have also been put on the properties of polyamide,especially the high temperature resistance.However,traditional polyamides exhibit limited heat resistance and rather high water absorption,the humidity has a significant impact on their heat resistance,dimensional stability and mechanical properties.Polyoxamides feature a special trans-arranged ortho-dicarbonyl structure,which allows them to form strong adjacent hydrogen bonding between the molecular chains.The unique double hydrogen bonding endows it with excellent performance,such as high heat resistance,low water absorption,good crystallization properties together with mechanical properties.As a result,polyoxamides have attracted more and more attention in the last few decades.However,the high T_m(close to its decomposition temperature)of some polyoxamides(PA62,et al.)creates an obstacle for processing and copolymerization is often needed to alter the melting point.Random copolymerization generally destroys the regularity of the molecular chain and leads to the decrease of crystallinity,which results in a drop in mechanical properties.In contrast to the random counterpart with the same composition,alternating copolyamide usually has a more regular molecular chain arrangement and displays higher crystallinity as well as better mechanical properties.Up to now,there are few studies focused on alternating copolyamide and their preparation methods,and the corresponding relationship between their structures and properties and the similarities and differences with random copolyamide of the same composition has not been completely clarified.In view of this,a series of novel high performance alternating copolyamides,especially alternating copolyoxamides,with high heat resistance and low water absorption were synthesized in this thesis by solution/solid-state polymerization method,one-pot copolymerization method and direct solid-state polymerization of amide salt.On the basis of the systematic study of the structures and properties,the similarities and differences between the alternating and random copolyamide with the same composition were compared,and the amide exchange kinetics were investigated.The main research contents of this thesis are as follows:(1)Novel alternating copolyamides PAn T-alt-n2 were successfully synthesized by n Tn-diamines and dibutyl oxalate via solution/solid-state polymerization.Dimethyl terephthalate and different diamines(1,5-pentanediamine,1,6-hexanediamine,1,10-diaminodecane)were used to prepare n Tn-diamines,then the PAn T-alt-n2 oligomers were prepared by pre-polymerized of n Tn-diamines with dibutyl oxalate in solution.The high molecular weight PAn T-alt-n2 was finally obtained after solid-state polymerization of the oligomer.The alternating structures and properties of PAn T-alt-n2 were characterized and evaluated by FT-IR,~1H NMR,¹³C NMR,DSC,TGA,etc.The results revealed that PAn T-alt-n2 possessed quite regular alternating structures and exhibited good thermal properties(T_m>275?.T₅>415?),high crystallinity(X_c>32%)and low saturated water absorption(<3.5 wt%),which were far better than the random counterpart.(2)Oxalate-terminated bisester-bisoxalamide(262-diester)was firstly synthesized by the reaction of dibutyl oxalate and 1,6-hexanediamine.High molecular weight PAn2-alt-62s were then prepared by 262-diester and different diamines(1,5-pentanediamine,1,10-diaminodecane,m-xylylenediamine)via solution/solid-state polymerization(SSP).Their properties were evaluated and compared to the random copolyoxamides PAn2-ran-62.PAn2-alt-62 with regular alternating structures had high melting point(T_m>280?)and enthalpy(?H_m?60 J/g),which were better than the random copolyoxamide with the same composition.The alternating structures and adjacent double hydrogen bonds in the molecular chain endowed PAn2-alt-62 with high crystallinity(?35%)and low saturated water absorption(<3.5 wt%).The crystallization morphologies of PAn2-alt-62 were observed by polarized light optical microscopes,the crystals of PA52-alt-62 and PAMXD2-alt-62 displayed three-dimensional growth and spherulitic-type morphology,while the crystals of PA102-alt-62 showed two-dimensional growth and crystallized in irregular crystals morphology.The study of isothermal crystallization kinetics indicated that PAn2-alt-62 had excellent crystallization performance,their t_1/2 were all less than 165 s.The non-isothermal crystallization kinetics of PAn2-alt-62 were also investigated and fitted by Jeriorny and Mo method,respectively.The results demonstrated that the non-isothermal crystallization rate of PA102-alt-62was the fastest while PAMXD2-alt-62 was the slowest,which was consistent with the isothermal crystallization results.DMA and tensile tests shown that the storage modulus of PAn2-alt-62 at 130?remained higher than 1 GPa.Meanwhile,PAn2-alt-62 also exhibited high tensile strength(89?95MPa).These results suggested that PAn2-alt-62 had a good application prospect in the field of high heat-resistant engineering plastics.(3)This part focused on the one-pot polymerization method for preparing PA102-alt-62 and its amide exchange reaction kinetics.A comprehensive investigation was conducted into the effects of polymerization temperature and time on the alternating structure,melting point and relative viscosity of PA102-alt-62.The regular alternating structure in the molecular chain was gradually destroyed and tended to become random with the increase of temperature and time during the heat treatment process.Arrhenius'equation was used to estimate the activation energy of the amide exchange reaction,which was calculated to be 165 k J/mol for the first 50 minutes while 59 k J/mol after 50 minutes.This result should indicate two different mechanisms of amide exchange for different stages.Compared to the early stage(<50 min)of the heat treatment,both the proportion of random structures in PA102-alt-62 and its molecular weight gradually increased in the late stage(>50min).These factors may promote the amide exchange reaction to a certain extent,so that the rate of amide exchange in late stage is higher than that in the early stage.(4)5T5n salts were prepared from 5T5-diamine and different aliphatic diacids(adipic acid,sebacic acid)and then subjected to direct solid-state polymerization,resulting in PA5T-alt-5n with high molecular weight and perfect alternating structure.The resultant PA5T-alt-5n had high crystallinity(X_c?36%)and exhibited superior performance over PA5T-ran-5n with the same composition in terms of thermal properties(T_m:PA5T-alt-56 284?,PA5T-alt-510 258?;vs,PA5T-ran-56 278?,PA5T-ran-510 202?)and crystallization properties.Meanwhile,due to the introduction of 5T units and the alternating structure,the thermal properties of PA5T-alt-5n had been greatly improved compared to the corresponding homopolymers PA56 and PA510.Furthermore,the introduction of the para-benzene ring also made the saturated water absorption of PA5T-alt-56 was much lower than that of PA56.In summary,a series of high heat-resistant alternating copolyamides with excellent properties were designed and successfully prepared,and the corresponding relationship between their structures and properties as well as the amide exchange reaction kinetics were further studied in this thesis.On the one hand,it tries to provide a new idea for the design and preparation of high heat resistant polyamides,on the other hand,it also provides a theoretical basis for the polymerization and processing of alternating copolyamides.