Design, Fabrication And Study Of Novel High Performance Ultra-thin Polyimide Films

Thermoplastic polyimide films (TPI) are widely used in the fields of microelectronic industry such as flexible printed circuit, tape automated bonding, interlayer dielectrics in multilevel large-scale integrated circuits and electronic skins for their considerable resistance to high temperatures, high dimensional accuracy and high tolerance against radiation. Recently, research on space exploration also takes advantages of TPI films for the fabrication of the base films for flexible solar cells, artificial satellites, solar films and space telescope.With the development of aerospace and electronic industry, higher speed, greater effective load, longer flying distance and lifetime are required for spacecraft, while the electronic products will be oriented to higher density integrated circuit with small volume and light weight. Thus the PI films should offer light weight, high strength, high modulus, thermal stability, dimensional stability, transparent, resistance to space environmental effects, low coefficient of thermal expansion (CTE), foldable, and so on. These properties are affected by the chemical structures and the aggregation structures of the PIs. The stretching method is commonly used to enhance the mechanical properties, optical properties, in-plane isotropy and reduce the CTE through inducing alignment of molecular chains in the direction of stretch and promote the crystallization of the polymers during the drawing processes. The commercial high performance PI films such as Kapton and Upilex, are prepared with uniaxial or biaxial streching to a thickness of at least 25μm and CTEs higher than 12 ppm·K-1. Traditional PIs are rigid for their strong interaction between polymer chains, stretching near T_g like other thermoplastic polymers, e.g. PET, PP, etc, are hardly achieved. To our knowledge, commercial high performance polyimide films are probably stretched with partially imidized poly (amic acid) precursor and imidized completely during the stretching process. However, the simultaneous solvent outgassing and the succeeding high-temperature imidization reaction during the stretching process are contaminative and also difficult to manage. The other potentially simpler and alternative approach is to stretch the thermoplastic and fully imidized polyimide films to prepare ultra-thin high performance films. However, for this stretching methodology, PIs should possess outstanding thermoplastic in nature and excellent rubbery properties at high temperature.A series of novel polyimides were prepared with designed diamine 1,3-bis(3-aminophenoxy-4'-benzoyl) benzene (BABB) containing ether and ketone moieties, meta-substituted linkages, and five kinds of commercial dianhydrides by two-step thermal imidization. By introducing flexible linkages and meta-substituted rings in the polymer chains, improved thermoplastic properties are anticipated. The ether and carbonyl groups may provide the polymer rubbery properties at high temperature owing to the CTC and cross links between the molecular chains, so that traditional stretching method could be applied to the fabrication of high performance polyimide films. Basic properties of these PI films, such as thermal properties, mechanical properties, CTE, aggregation, water absorption and optical properties are characterized and the relationship between the structures and properties are studied. All the PI films exhibited excellent thermoplastic properties owing to their very low glass transition temperature (<225℃) and drop of storage modulus at T_g (higher than 103). All the polyimides exhibited outstanding thermal stability (Td5 500～545℃), mechanical properties (2.7-3.0 GPa), optical properties (UV transmittance% about 88% at 800 nm), very low water absorption (<0.9%) and remarkable damping properties (tanδ2.0～3.0). The outstanding damping properties make these polyimides maybe useful as materials for shock absorption and sound insulation. All polyimides had excellent mechanical and rubbery properties at high temperature, especially, the PI-a exhibited the most remarkable rubbery properties and the largest strain (1600%), which indicated these polyimides can potentially be used to prepare ultra-thin film with ultra-high strength and ultra-high modulus by traditional stretching method as the PET polymer.Firstly, ultra thin and high performance PI films were obtained by hot stretching of the PI-a above T_g. The thickness of the films reduced at above 70%, and the thinnest locality is about 7μm. The mechanical properties of hot stretched films improved a lot, the tensile modulus increased from 2.7 GPa to 4.9 GPa, and the tensile strength enhanced from 109 MPa to 258 MPa. The hot stretched films are potentially used as flexible printed circuit due to appropriate CTEs with copper and silicon substrates. The stretched films showed excellent UV-visible transparence and optical birefringence, the transmittance enhanced from 84% to 89%.To prepare ultra high strength and ultra high modulus PI films, the PI-a film was uniaxially stretched at a temperature below its T_g. The stretched films exhibited outstanding mechanical properties, the tensile modulus was about 7.8 GPa and the tensile strength was about 386 MPa, which was about 3.5 times of that of undrawn film. The CTE of stretched films reduced from 52 ppm·K-1 to -1～-3 ppm·K-1. The stretched films exhibited higher mechanical properties and lower CTE as compared with that of Kapton. The stretched films also showed lower CTE than that of Upilex.The study of relationship between structures and properties of stretched films showed that the molecular chains developed a high degree of orientation along the stretching direction and promoted the crystallization during the stretch process, and the degree of orientation enhanced with the increase of stretch ratio. So the tensile properties improved and the CTE decreased along the stretch direction. The uniaxial stretch method also improved the optical properties and brought down the cost owing to the increased acreage.Not only does the introduction of the flexible linkages make the novel TPI has excellent thermoplastic properties but also make it possible to prepare high performance ultra thin films by stretching the imidized films, which is very valuable for the aerospace and micro-electronic industry.