Preparation and Properties of Liquid Crystal Polyesterimides Terminated with Benzocyclobutene
Combining features of liquid crystal polyester(LCP)and benzocyclobutene(BCB),two BCB-containing compounds,2-(dicyclo[4.2.0]oct-1,3,5-triene-3-yl)-5-hydroxy-1H-isoindole-1,3(2H)-diketone(BCB-HP)and 2,3-dihydro-1,3-dioxo-2-(dicyclo[4.2.0]oct-1,3,5-triene-3-yl)-1H-isoindole-5-carboxylic acid(BCB-CP),were designed and prepared as terminal capping compounds for LCP synthesis.Liquid crystal polyesterimides(LCPEIs)were synthesized by solution condensation copolymerization of the phenolic BCB-HP,the carboxylic BCB-CP,4-hydroxybenzoic acid(4-HBA),and 6-hydroxy-2-naphthoic acid(HNA),along with N-(3-carboxyphenyl)-4-hydroxyphthalimide(3-CHP),in the presence of esterification reagents and conditions,diphenyl chlorophosphate(DPCP),LiCl and pyridine at room to reflux temperature,and subsequent methanol precipitation and drying as fine powder.Films of LCPEI-22/44-BCB(with 22 mol％/44 mol％3-CHP)were generated by hot-pressing and curing procedures,and the curing temperature is around 270 ℃,which is favorably lower than those of 3-ethynylaniline(3-EA,310 ℃)and 4-phenylethynylphthalic anhydride(PEPA,370 ℃),two alkyne-type thermal reactive terminal groups used in literature.These BCB-type LCPEIs have good thermal stabilities(Td,5％=416.7-454.8 ℃)in thermo-gravimetric analysis(TGA)under nitrogen gas.Compared with the commercially available Vectra A950 LCP and non-BCB-containing LCPEI-22/44,LCPEI-22/44-BCB have better solubility(4/1 V/V chloroform-pentafluorophenol),lower melting temperature(Tm,200 ℃),higher glass transition temperature(Tg=149-167 ℃),and lower isotropic temperature(Ti,350 ℃),indicating processing convenience and application potential.Compared to LCPEI-22,film of LCPEI-22-BCB has a favorably lower linear thermal expansion coefficient(CTE,41.13 versus 73.29x10-6 K-1),a higher storage modulus(E',3.48 versus 0.53 GPa),higher elastic modulus(Er,10.83 versus 9.40 GPa),and higher hardness(H,0.66 versus 0.58 GPa).Film of LCPEI-22-BCB also has better hydrophobicity with increased water contact angle(124.7° versus 114.3°).Film of LCPEI-22-BCB exhibits good and comparable low dielectric constant(Dk=3.364)and excellent low dielectric loss(Df=0.0019)at 10 GHz high frequency and room temperature.Merits of the above features would make the BCB-type LCPEIs promising materials for flexible copper clad laminates.
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