Study On Transparent Semi-aromatic Polyimide And Its Strengthening And Toughening Modification

Recently,the development of flexible display technology is very fast.Polyimide(PI)is considered to be one of the ideal materials for flexible display substrate due to the excellent transparency,light weight and flexibility.However,most of the traditional PI materials have poor transparency,which can not meet the requirements of flexible display industry.Therefore,monomers containing alicylic structure,which can effectively weaken the interaction between molecular chains and reduce the formation of charge transfer complexes(CTC)was used to prepare semi-aromatic PI films with higher optical transparency.Furthermore,nano-silica(SiO₂)particles and dibenzo-crown ether(DB-18-6)were introduced into the above semi-aromatic PI matrix to improve its mechanical properties.Two semi-aromatic PI films from 1,4-diaminocyclohexane(t-CHDA)and 3,3',4,4'-biphenyl tetracarboxylic dianhydride(BPDA)(PI_t-CHDA-BPDA),4,4'-diaminobiphenyl(BZD)and cyclobutane tetracarboxylic dianhydride(CBDA)(PI_BZD-CBDA)were prepared by traditional two-step method.And the wholly aromatic PI film from BZD and BPDA(PI_BZD-BPDA)was prepared for comparison.The UV-Vis spectra results showed that the transmittance at 450nm(T₄₅₀)of PI_t-CHDA-BPDAfilm and PI_BZD-BPDAfilm were 85.0%and 53.5%respectively,which was much higher than that of PI_BZD-BPDAfilm(T₄₅₀=3.1%).Moreover,the temperature of 5%weight loss(T_5%)and glass transition(T_g)of PI_t-CHDA-BPDAfilm were 497.4?and 365.3?,respectively.That was to say,PI_t-CHDA-BPDAfilm possessed excellent optical performance and thermal stability at the same time.Aano-silica(SiO₂)nanoparticles were introduced into PI_t-CHDA-BPDAfilm by a simple and efficient solution blending method to obtain a series of PI/SiO₂nanocomposite films(the SiO₂content was 0.1 wt%,0.25 wt%,0.5 wt%or 1 wt%).The mechanical properties and thermal stability of PI/SiO₂nanocomposite films were increased to a certain degree with the increase of SiO₂content.When the loading of SiO₂nanoparticles reached 0.25 wt%,Young's modulus and tensile strength of PI-0.25-SiO₂composite films were increased significantly to7.7 GPa and 174.8 MPa,respectively.Besides,the T_gvalue and the coefficient of thermal expansion of PI-0.25-SiO₂were 371.2?and 13.8 ppm·K^-1,respectively.Moreover,the transmittance at 500nm of PI-0.25-SiO₂composite films was over 70%,indicating the excellent optical transparency.The complex of dibenzo 18-crown ether-6(DB-18-6)with t-CHDA was used to prepare PI/dibenzo-crown ether(PI/DB-18-6)composite films via the polycondensation with BPDA.The host-guest inclusion structure was formed between DB-18-6 molecule and the semi-aromatic PI backbone.The strength and toughness of the composite film increase significantly with the increase of the DB-18-6 amount.When the molar ratio of DB-18-6 to t-CHDA was 0.1,the tensile strength and elongation at break of PI-1-CE were 127.2 MPa and2.0%,respectively.When the molar ratio of DB-18-6 to t-CHDA reached 0.3,the tensile strength and elongation at break of PI-3-CE composite films were increased to 192.2 MPa and7.3%,which were increased by 51%and 265%compared to PI-1-CE,respectively.The above results showed that the effect of strengthening and toughening for PI films was further improved.