Influence of solid-state polymerization on structure and properties of naphthalene ring structure aromatic liquid crystal copolyester
Objective Degradation,cross-link side reaction and poor processing ability are caused by high reaction temperature and melt viscosity of liquid crystal copolyesters in the final stage of melt polycondensation.In order to solve such problems,low molecular weight of liquid crystal copolyesters was prepared by one-pot melting transesterification method.The influences of solid-state polymerization time and temperature on thermal properties,crystallization properties and melt index of liquid crystal copolyesters were investigated.Method In this study,the low molecular weight liquid crystal copolyester derived from 6-hydroxy-2-naphthalene carboxylic acid(HNA),2,6-naphthalene dicarboxylic acid(NDA),terephthalic acid,4,4'-dihydroxy biphenyl(BP)were prepared by in-situ one-pot melt polymerization method.After solid-state polymerization,the structure and properties of liquid crystal polyesters were analyzed by Differential scanning calorimeter,Thermogravimetric analyzer,polarized microscope,X-ray diffraction and melt index.Results The melting temperature of liquid crystal copolyester(Tm)and the glass transition temperature(Tg)were found to increased and then decreased with the increase of polymerization time.Crystallization enthalpy △Hc and melting enthalpy △Hm reached the maximum after 12 h of polymerization,and were decreased gradually with the increase of solid-state polymerization time △Hc and △Hm.Clear marble and textured liquid crystal texture were still observed 8 h after solid-state polymerization,showing nematic liquid crystal behavior,but with the increase of polymerization time,birefringence and texture gradually got blurred.When the polymerization time was increased to 36 h,the crystal structure did not change,remaining in the state of orthogonality.The results showed that the phase transition temperature and crystallization rate of liquid crystal copolyesters were greatly affected by polymerization time and temperature.The crystallization performance of liquid crystal copolyesters was better when the polymerization temperature was 305 ℃ and the polymerization time was about 12 h.The initial degradation temperature of liquid crystal copolyesters after solid state polymerization was generally higher than that of the initial sample,and the temperature corresponding to the maximum mass loss was somewhat lower than that of the sample.The carbon residue of liquid crystal copolyesters at 700 ℃ remained between 42％and 44.5％,indicating that the molecular weight and thermal stability of liquid crystal copolyesters were improved during the polymerization process but the possibility of thermal degradation was increased by long-term high temperature polymerization.With the increase of polymerization time,the melt index at different polymerization temperatures began to decrease rapidly.When the polymerization temperature was 315 ℃,the melt index of copolyesters was decreased the fastest.After 24 h solid-state polymerization,the melt index of copolyesters did not change significantly.Considering that high molecular weight products were obtained under low energy consumption,8-24 h solid-state polymerization time was appropriate.Conclusion The structure and properties of liquid crystal copolyester are influenced by the operation parameters of solid-state polymerization,such as temperature,time,nitrogen flow rate and particle size.This research was focused on the influence of temperature and time on the thermal and crystallization properties of liquid crystal copolyester.Owing to the continued reaction of the terminal group,the molecular chain of liquid crystal copolyester after solid-state polymerization increases,the molecular weight increases,and the melting temperature,glass transition temperature and thermal stability are improved.However,after solid-state polymerization for more than 12 h and the temperature is higher than 305 ℃,the concentration of the reactive terminal group inside the particles decreases.When the molecular weight reaches the equilibrium limit,the melting point of the copolyester decreases gradually,and the degradation reaction of the molecular chain gradually dominates,and the thermal stability and crystallinity decrease.The results show that the thermal and crystalline properties of liquid crystal copolyesters are improved significantly after solid state polymerization at 305 ℃ for 12 h,which is conducive to its processing and application.
thermotropic liquid crystal copolyestermelt polymerizationsolid state polymerizationthermal propertycrystal structure
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