Preparation And Properties Of High Performance Polyimide Films Based On Benzimidazole And Benzoxazole Groups

Polyimide (PI) films have been widely applied in high technology fields, due tothe good combination of properties, including thermal resistance, mechanicalproperties, dimensional stability and electric properties. The most classical applicationis the flexible substrate materials. For example, PI films could be used as thesubstrates of the flexible printed circuit board (FPC) and tape automated bonding(TAB) in the field of microelectronics; they also could be the flexible substrates ofthin-films solar cells. High performance of PI films is required for these applications.For instance, when used as the substrates for FPC, CTE21values of PI films should bematched with that of copper, to avoid warpage or even crack of FPC due to themismatch of CTE values. Besides, CTE values should be smaller for the applicationof solar cells. Common PI films have CTE values of greater than30ppm/K. Therefore,properties of PI films are required to improve to adapt to the needs of high technologyapplications.Mechanical stretching is effective to improve the performance of PI films, whichwould promote molecular orientation in the stretching direction, to enhancemechanical properties and dimensional stability. Commercially available PI filmsusually have undergone mechanical stretching in the process of preparation. However,many parameters are involved in the stretching process, like viscosity of polymersolution, content of solvent, degree of pre-imidization, and the rate and ratio ofmechanical stretching. Another solution is to incorporate rigid groups into polyimide backbones to directly obtain high performance PI films, and avoid the use ofcomplex mechanical stretching.The objective of this thesis is to use diamine monomers containing rigidbenzimidazole and benzoxazole units,5,4’-diamino-2-phenyl benzimidazole (DAPBI)and5,4’-diamino-2-phenyl benzoxazole (DAPBO), to prepare high performance PIfilms and characterize the proeprties.DAPBO was polymerized with common dianhydrides to prepare a series of PIfilms based on benzoxazole units. These films showed good combination of properties:high thermal resistance with glass transition temperatures (Tg) up to412oC; superiormechanical properties over common PI films, with tensile modulus up to7.2GPa andstrength up to281MPa; extraordinarily low CTE values smaller than20ppm/K,demonstrating the promising potential in the fields of microelectronics andoptoelectronics. The introduction of benzoxazole unit has increased the rigidity of PIbackbones and the intermolecular interaction, greatly enhancing the properties of PIfilms.PI films based on DAPBI and DAPBO both showed outstanding properties, itwas observed that Tg values of DAPBI-based PI films always higher than those ofDAPBO type PI films, probably due to the stronger intermolecular interactionimparted by benzimidazole moiety. Thermal analysis and IR spectra were employedto manifest the strength of intermolecular interaction of the two types of PI films.Thermal properties of small-molecular weight model compounds proved thatDAPBI-based model compounds had higher degree of intermolecular interaction,which could be transferred to the polymer system; steric effects ofbiphenyltetracarboxylic dianhydride (BPDA) were applied: the intermoleculardistance increased when the degree of macromolecular distortion increased as theBPDA component changed, the effect of hydrogen bonding became less remarkablefor DAPBI type polyimides, leading to the gradual decrease of Tg difference (Tg)between DAPBI-and DAPBO-based PI films, and demonstrating the strongerintermolecular interaction for DAPBI type PI; Red shift of carbonyl IR band wasobserved for DAPBI-based PI compared to that of DAPBO type from IR spectra of PI films, this was due the formation of hydrogen bonding involved with carbonyl andbenzimidazole groups, giving direct evidence for the existence of hydrogen bondingof DAPBI-derived polyimides.CTE is an important parameter for PI films, which is closely related with thedegree of macromolecular in-plane orientation. In-plane orientation refers to thespontaneous orientation of polyimide backbones in the in-plane direction duringprebaking and imidization process, which means the anisotropy in the in-plane andout-of-plane directions. CTE values and the degree of in-plane orientation are notonly determined by the chemical structures of PI backbones, preparation conditionsalso have remarkable influences. DAPBO type PI films were selected as the researchtarget to investigate the effects of preparation conditions on CTE values. The resultsshowed that the preparation conditions, including film thickness, heating rate andsubstrate had significant influences on CTE, the values increased as a function of thefilm thickness and heating rate, and the substrate decreased CTE values. Poly(amicacid)(PAA) films with various solvent contents were prepared to study the effect ofresidual solvent. Thermal transition behavior of PAA films showed that imidizationand in-plane orientation of PI chains were remarkably affected by the residual solvent,which would change CTE values. Extraction of residual solvent of PAA by the use ofethanol led to extraordinarily low CTE values of PI films, displaying the negativeeffect of residual solvent in decreasing CTE.Certain PI films showed unusual thermal expansion behavior: negative in-planeCTE. PI films based on DAPBI and s-BPDA was selected as the research target toinvestigate the mechanism of the special thermal expansion behavior. When cured at400oC, the in-plane CTE was9.3ppm/K; however, the in-plane CTE turned negativewhen cured at350oC. A series of PI films cured at various temperatures were prepared,thermal expansion in the in-plane and out-of-plane directions were studied. It wasfound that the in-plane CTE increased as a function of the curing temperature andturned from negative to positive, while the out-of-plane CTE decreased as the curingtemperature increased. The morphological structures changed from amorphous tocrystalline at the same time, i.e. the expansion in the out-of-plane direction was preferential when PI films were amorphous, leading to the contraction in the in-planedirection and thus negative CTE value. Besides, PI system having negative CTE wasemployed as the matrix, by introducing a suitable amount of the third content, PI filmwith extraordinarily low thermal expansitivy was obtained.