New Direction of PI Industry: Thin and Light, Low Temperature, Low Dielectric Constant, Transparent, Soluble, Low Expansion, etc.

Suzhou Kying Industrial Materials Co,Ltd.  2022-05-05 09:27:46 作者:SystemMaster
Direction 1: Low Temperature Synthesis of Polyimide PI
In general, PI is usually obtained by reacting diamine and dianhydride to form its prepolymer-polyamic acid (PAA), which must be imidized at high temperature (>300 °C), which limits its use in some application in the field. At the same time, volatile by-products are easily generated during the high-temperature imidization of PAA solution to synthesize PI, and it is not easy to store and transport. Therefore, it is necessary to study the synthesis of PI at low temperature. The current improved methods are: 1) one-step method; 2) molecular design; 3) adding low temperature curing agent.

Direction 2: Thin and Uniform Film
In order to meet the design requirements of lightness, thinness and high reliability of downstream application products, polyimide PI film is developing towards thinning, which puts forward higher requirements for its thickness uniformity, surface roughness and other properties. The improvement of key properties of PI films not only depends on the design of the molecular structure of the resin, but also the advancement of film forming technology is also crucial. At present, the preparation process of PI film is mainly divided into: 1. Impregnation method; 2. Casting method; 3. Biaxial orientation method.
With the application requirements of weight reduction, thinning and functionalization of devices in aerospace, electronics and other industries, ultra-thinning is an important trend in the development of PI films. According to the thickness (d), PI films can generally be divided into ultra-thin films (d≤8 μm), conventional films (8 μm<d≤50 μm, common film thicknesses are 12.5, 25, 50 μm), thick films (50 μm) <d≤125 μm, common thicknesses are 75, 125 μm) and ultra-thick films (d>125 μm). At present, the methods for preparing ultra thin PI films are mainly soluble PI resin method and blow molding method.
Soluble polyimide resin method: Conventional PI is generally insoluble and infusible, so only its soluble precursor PAA solution can be used for film preparation. Soluble PI resins are obtained by polymerizing dianhydride or diamine monomers containing large substituents, flexible groups or asymmetric and isomerized structures in their molecular structures, and their substituents or asymmetric structures can effectively reduce The strong interaction within or between molecular chains of PI increases the free volume between molecules, which is beneficial to the penetration and dissolution of the solvent.
Different from using PAA resin solution to prepare PI film, this process first directly prepares high molecular weight organic soluble PI resin, then dissolves it in DMAC to prepare a PI solution with suitable process viscosity, and finally casts and cures the solution on a steel belt. , PI film was obtained after biaxial stretching.
Blow molding method: The technology for preparing general-purpose polymer films by blow molding is very mature, and the film thickness can be easily adjusted by changing parameters such as hot air flow speed. The device is different from the traditional blow molding method for preparing polymer film, and the film is blown from top to bottom. The difficulty of this process lies in the transformation of the polymer from solution to bubbles and the process of forming the film from the bubbles through the pressure rollers. However, this process can directly use commercial polyamic acid solution or PI solution for film preparation, and avoid physical contact between the film and other substrates to the greatest extent; rollers are easier to perform surface polishing treatment than steel strips, and it is easier to achieve uniform heating , PI ultra-thin films with high strength and high thermal stability can be prepared.

Direction 3: Low Dielectric Constant Materials
With the rapid development of science and technology, the integrated circuit industry is increasingly developing towards low-dimensional, large-scale and even ultra-large-scale integration. When the size of electronic components is reduced to a certain size, the inductance-capacitance effect between wirings gradually increases, and the mutual influence of wire currents makes signal hysteresis very prominent, and signal hysteresis time increases. The delay time is proportional to the dielectric constant of the interlayer insulating material. Higher signal transmission speeds require the dielectric constant of the interlayer insulating material to be reduced to 2.0 to 2.5 (typically PI has a dielectric constant of 3.0 to 3.5). Therefore, in the context of the deep development of VLSI, reducing the dielectric constant of the interlayer material has become an important means to reduce the signal delay time.
At present, the methods for reducing the dielectric constant of PI thin films are divided into four categories: 1. fluorine atom doping; 2. fluorine-free/fluorine-containing copolymers; 3. siloxane-containing branched structure; 4. porous structured membranes
1. Fluorine atom doping: Fluorine atom has strong electronegativity, which can reduce the polarizability of electrons and ions of polyimide molecules and achieve the purpose of reducing the dielectric constant. At the same time, the introduction of fluorine atoms reduces the regularity of the molecular chain, making the stacking of the polymer chain more irregular, increasing the intermolecular space and reducing the dielectric constant.
2. Fluorine-free/fluorine-containing copolymer: the introduction of aliphatic copolymerization units can effectively reduce the dielectric constant. Alicyclic units also have low molar polarizability, which can destroy the planarity of molecular chains, suppress charge transfer and close packing of molecular chains at the same time, and reduce dielectric constant; at the same time, due to the dipole polarization of C-F bonds The ability is small and can increase the steric hindrance between molecules, so the introduction of C-F bonds can effectively reduce the dielectric constant. For example, the introduction of a bulky trifluoromethyl group can not only prevent the close packing of polymer chains, effectively reduce the intermolecular charge transfer of highly polarized dianhydride units, but also further increase the free volume fraction of the polymer to reduce purpose of the dielectric constant.
3. Siloxane-containing branched chain structure: cage-type molecule-polysilsesquioxane (POSS) has the advantages of uniform pore size, high thermal stability, and good dispersibility. The functional groups attached to the vertices of the POSS cage-type pore structure can be dispersed into the polyimide matrix to a certain extent after surface chemical modification such as polymerization, grafting and surface bonding to form a low dielectric constant compound with a pore structure film.

4. Porous structure membrane: Since the dielectric constant of air is 1, by introducing a large number of uniformly dispersed pore structures in the polyimide, the air volume rate is increased, and the porous foam material is formed to obtain a low-dielectric polyimide material. an effective way. At present, the methods for preparing porous polyimide materials mainly include thermal degradation method, chemical solvent method, introduction of hybrid materials with nano-hole structure, etc.

Direction 4: Transparent PI

The color of organic compounds is due to the fact that it absorbs certain wavelengths of visible light (400-700 nm) and reflects the remaining wavelengths, and the human eye perceives the reflected light. This absorption in the visible range is responsible for the coloration of aromatic polyimides. For aromatic polyimides, the chromophoric groups that cause light absorption can be as follows: a) two carboxyl groups on the imine ring; b) a phenyl group adjacent to the imine ring; c) diamine Residual groups and dianhydride residual groups contain functional groups.

Due to the strong intermolecular and intramolecular interactions in the molecular structure of polyimide, it is easy to form a charge transfer complex (CTC) between the electron donor (diamine) and the electron acceptor (diamine). The formation of CTC is the intrinsic reason for the material to absorb light.

To prepare colorless and transparent polyimide, it is necessary to reduce the formation of CTCs at the molecular level. The methods currently widely used mainly include:

1. Using monomers with side groups or asymmetric structures, the existence of side groups and asymmetric structures will also hinder the flow of electrons and reduce conjugation;

2. Introducing fluorine-containing substituents into the molecular structure of polyimide, using the electronegativity of fluorine atoms, the conjugation of electron clouds can be cut off, thereby inhibiting the formation of CTC;

3. Use alicyclic structure dianhydride or diamine monomer to reduce the content of aromatic structure in polyimide molecular structure.

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