Preparation And Characterization Of Crosslinking-/Compounding-Modified Polyimide Films

Aromatic polyimides（PIs）are a class of polymer materials that are radiation-resistant,heat-resistant,chemically stable and of excellent mechanical properties.They have been widely used in aerospace,microelectronics and other high-tech fields.With the advancement of science and technology and the rapid development of industry,higher requirements have been placed on the thermal and mechanical properties of polyimide materials.Modification by appropriate methods can improve the performance of polyimide in certain aspects,such as processability,gas separation performance,heat resistance,and mechanical properties.In this dissertation,a series of modified polyimide films were prepared by the modification methods of chemical crosslinking,compounding with inorganic compounds,combination of crosslinking and compounding.And these as-prepared high performance polyimide-based film materials were tested for characterization and performance studies.This work is mainly divided into six parts as follows:1.PAA-BPO crosslinked films were prepared by solution blending and radical reaction using BPO as crosslinking agent,and then PI-BPO crosslinked films were obtained by heat treatment.The structure,thermal and mechanical properties of PI-BPO crosslinked films were characterized and studied using FT-IR,TGA,DMA and tensile test.The results show that the PI-BPO crosslinked film prepared at 330?C with 9%BPO by weight has a better performance.The glass transition temperature（T_g）is 305?C,which is 24?C higher than that of the pure PI film;and the tensile modulus is up to 2.72±0.10 GPa,increasing by 49%compared to the pure PI film.Chemical crosslinking can restrict the movement of polymer chains and increase the heat resistance and modulus of PI.2.4,4′-oxybis（azidobenzene）（AD）was prepared using 4,4′-oxybis（azidobenzene）and sodium azide by azide reaction.The AD-crosslinked polyimide（PI-AD）films were obtained by solution blending of AD and PAA,followed by film casting and thermal imidization.The structure,thermal and mechanical properties of the AD-crosslinked PI films were investigated using FT-IR,TGA,DMA and tensile test.The results show that AD-crosslinked PI film formed at 330?C with 13%AD by weight has the highest heat resistance,with T_g value up to 321?C,which is 61?C higher than that of the pure PI film.When the amount of AD is 11%,the mechanical properties of AD-crosslinked PI film are optimum.The tensile stress and modulus are173.12±16.06 MPa and 1.98±0.17 GPa,respectively,which are 27%and 30%higher than those of the pure PI film.The thermal stability of the AD-crosslinked PI films is higher than that of the pure PI film.The crosslinking modification can improve the thermal and mechanical properties of PI film.3.Phosphorus-doped polyimide（PI-P）films were prepared by film casting and heat treatment using diphenyl phosphate（DPhP）and PAA（BPDA/BIA/BPA=10/8/2）as materials.The structure,thermal properties and mechanical properties of PI-P composite films were characterized using FT-IR,TGA,DMA,EDS and tensile test.The results show that the thermal and mechanical properties of PI-P composite films prepared at 370°C with 0.6%DPhP addition are the best.The 5%weight loss temperatures in air and argon atmospheres and the T_g value are 577?C,587?C and405?C,respectively,which are 50?C,35°C,and 16°C higher than those of the pure PI film.The tensile stress,modulus,and toughness are respectively 47%,57%and76%higher than those of the pure PI.Since phosphorus compounds can form a protective layer on the polymer surface at high temperature,and have a strong interface force with the PI matrix,a small amount of diphenyl phosphate can significantly increase the thermal and mechanical properties of the PI-P composite films.4.The PI-B₂O₃（PI-B）composite films were prepared by in-situ thermal conversion of H₃BO₃ in a PAA blend,which was blended using two polyamic acids,PAA-I（BPDA/BPA）and PAA-II（BPDA/ODA）,in an equal ratio.The structure,thermal properties and mechanical properties of the as-prepared PI-B composite films were characterized by using FT-IR,XRD,TGA,DMA,EDS and tensile test.The results show that the thermal properties and mechanical properties of PI-B composite film prepared at 370°C with 5%H₃BO₃ addition are optimum.The 5%weight loss temperatures in air and argon atmospheres and the T_g value are 583°C,598°C and315°C,respectively,which are 69°C,74°C and 15°C higher than those of the pure PI film.The tensile stress,modulus,elongation at break,and toughness are 1.70,1.57,1.31 and 2.44 times of those of the pure PI film.The in situ thermally formed B₂O₃glassy nanoparticles reduce the free volume of PI and have the function of thermal insulation and oxygen blocking,which significantly improves the thermal properties of PI-B composites.In addition,the increase in the mechanical properties of the PI-B composite films may be caused by the strong interfacial interaction between B₂O₃ and PI matrix.5.The melamine（MA）-crosslinked polyimide/B₂O₃ composite films（PI-B₂O₃-MA）were prepared by in-situ thermal conversion using H₃BO₃,MA and the PAA blend of PAA-I（BPDA/BPA）and PAA-II（BPDA/ODA）in an equal ratio as raw materials.The structure,mechanical properties and thermal properties of PI-B₂O₃-MA composite films were investigated using FT-IR,TGA,DMA and tensile test.The results show that the mechanical properties and thermal stability of PI-B₂O₃-MA composite film with 3%MA addition is the best,and the tensile stress and modulus are 342.45±5.73 MPa and 4.89±0.05 GPa,respectively,which are71%and 124%higher than those of the pure PI.The 5%weight loss temperatures in air and argon atmospheres are 591°C and 607°C,respectively.Compared with pure PI,PI-MA-3 and PI-B₂O₃,PI-B₂O₃-MA composite films have higher thermal stability.The PI-B₂O₃-MA composite film with 4%MA addition has the highest heat resistance,and its T_g value is up to 330°C,which is 30°C,11°C and 15°C higher than that of the pure PI,PI-MA-3 and PI-B₂O₃ films,respectively.The combined effect of compounding-and crosslinking-modification significantly improves the thermal properties and modulus of the composite films.6.MA-crosslinked phosphorus-doped polyimide composite films（PI-MA-P）were prepared by solution blending and thermal imidization using diphenyl phosphate（DPhP）,MA and the PAA blend of PAA-I（BPDA/BPA）and PAA-II（BPDA/ODA）in an equal ratio as raw materials.The structure,thermal and mechanical properties of PI-MA-P composite films were characterized by using FT-IR,TGA,DMA,EDS and tensile test.The results show that the thermal stability and mechanical properties of PI-MA-P composite film with 0.4%DPhP addition are optimum.The 5%weight loss temperature in air and nitrogen atmosphere is 585°C and 587°C respectively,which are 71°C and 63°C higher than those of the pure PI film,42°C and 22°C higher than those of the PI-MA-3 film.The tensile stress and modulus of PI-MA-P composite film increase respectively by 57%and 28%compared to those of the pure PI film,and by 31%and 16%compared to those of the PI-MA-3 film.The heat resistance of PI-MA-P composite film with 0.7%DPhP addition is the best,and the T_g value is as high as 333°C,which are 33°C and 14°C higher than those of the pure PI and PI-MA-3 films,respectively.Crosslinking and doping modification can synergistically enhance the thermal and mechanical properties of PI films.