Synthesis And Characterization Of Easy Processable Crosslinkable PBO Copolymer

The rigidity of the wholly aromatic polybenzoxazole(PBOs)molecular chain results in poor formability(insoluble in organic solvents and higher melting point than thermal decomposition temperature),which limits its application.In order to improve the processability of the moldability,side methyl groups and flexible alkylene groups are simultaneously introduced into the PBO molecular chain by molecular design.In the polyphosphoric acid reaction medium,by stepwise heating polycondensation reaction,two series of crosslinkable PBO copolymers(containing butylene and octylene soft segments,respectively)were synthesized with 2-methyl-4,6-diaminoresorcinol dihydrochlorate.Terephthalic acid,adipic acid and sebacic acid as comonomers.The copolymers were characterized by fourier transform infrared spectroscopy(FT-IR),nuclear magnetic resonance spectroscopy(1H-NMR),differential scanning calorimetry(DSC),thermogravimetric analyzer(TGA),ultraviolet-visible absorption spectroscopy(UV-Vis)and fluorescence spectroscopy.(1)The introduction of the side methyl group and the flexible alkylene segment obviously improved the processability of the molding.The copolymers could melt on heating,and the copolymer could be dissolved in the trifluoroacetic acid in addition to the concentrated sulfuric acid and methanesulfonic acid;(2)The DSC analysis results shown that with the content of the soft segment increasing,the melting point increased gradually and the melting range became wider.According to the TG test results,the copolymers had good thermal stability,and all the maximum thermal decomposition rate temperatures were above 400?,With the content of the soft segment increasing,the residual rate decreased;(3)The copolymer containing butylene segments was tested for TG at different heating rates.The thermogravimetric analysis could be divided into three stages:(a)the first stage was the process of cross-linking between the oxazole ring and the side methyl group in the small chain of the main chain;(b)The secondstage was a random breaking process of the butylene;(c)the third stage was the decomposition process of the benzoxazole.The second stage with the most obvious weight loss characteristics was studied in detail.The main curve method was used to obtain the dynamic three factors,and the activation energy obtained by the main curve method was accordance with those obtained by the equal conversion rate method(Flynn-Wall-Ozawa method and Kissinger method).It is verified that the obtained activation energy,pre-exponential factor and mechanism function were:E=204.46 kJ/mol,lnA=30.91 and[-ln(1-?)]0.65232.The function followed the random nucleation and subsequent growth mechanism,and in the second stage of weightlessness process,when the conversion rate reached 0.60,the activation energy was significantly improved.The third stage of weight loss began;(4)The ultraviolet-visible and fluorescence spectra of the copolymers in solution and the film were tested separately.In the trifluoroacetic acid solution,there were two UV-Vis absor:ption bands(peaks at around 290 nm and 340 nm),While in concentrated sulfuric acid solution,there were three UV-Vis absorption bands(peaks at about 290 nm,320 nm and Around 390 nm).The films also had three absorption bands in the UV-Vis region(peaks at about 290 n,340 nm,and 390 nm).The copolymer in solution and films were excited by excitation waves having wavelengths of 280 nm,340 nm,and 380 nm,respectively,and the fluorescence spectra were measured.Taking the excitation wave of 280 nm as an example,the emission peaks of aggregates appear in the samples of trifluoroacetic acid solution and concentrated sulfuric acid solution.In the solid film of the copolymer,the emission band exhibited a fine structure with three main emission peaks(peaks at about 340 im,400 nm and 475 nm),and they were from the emission of three chromophores in the molecular chain of the copolymer,respectively.The results of optical properties test shown that the molecular chains adopted unfolding conformation in the concentrated sulfuric acid solution,while the molecular chain in the trifluoroacetic acid solution adopted a random coil conformation with a certain aggregation between the segments.The distribution of the three chromophores M1,M2,and M3 on the macromolecular chain was relatively isolated and dispersed;(5)The introduction of soft segments on the molecular backbone to separated the luminescent group into isolated quantum dots,and the fluorescence properties of the copolymers could be adjusted by changing the length and content of the soft segment.