Don’t worry about your phone being “bent”! Learn about the “gold film” that makes your phone bend without bending.

Recently, Samsung released the fifth generation of foldable screen mobile phone, which attracted many people's attention. The biggest advantage of foldable screen mobile phone is flexible folding. The screen can be bent many times without deformation, and it is also rigid. What material gives foldable screen mobile phone this characteristic?

Flexible displays can be divided into flexible substrates, display media and thin film packaging according to their structure. The key to folding technology lies in the development of flexible substrates. How to use materials and technologies to achieve foldability is the key technology in the research process. The core material used in folding screen mobile phones is transparent polyimide. The molecular structure of polyimide contains very stable aromatic heterocycles, and the molecular structure is compact, with strong intermolecular and intramolecular forces. These advantages give it excellent properties that cannot be replaced by other polymer materials, such as high and low temperature resistance, low dielectric constant, and easy realization of fine graphic circuit processing. Polyimide film is considered to be one of the organic polymer materials with the best comprehensive performance due to its own advantages, and has been included in the medium and long-term development plans of many developed countries.

The material used in folding screen mobile phones is not the traditional polyimide film, but a transparent polyimide film obtained after a certain processing. Because there is a strong intermolecular bonding force in the molecular structure of polyimide, it is easy to form a charge transfer complex. This substance is one of the culprits causing the material to absorb light, causing the traditional polyimide film to appear light yellow, and it is obviously not suitable for direct use on folding screen mobile phones. Researchers optimize polyimide by introducing other groups into the molecular structure of polyimide, reducing the intermolecular force to reduce the formation of charge transfer complexes, thereby preparing a colorless and transparent polyimide film for use in folding screen mobile phones. This technical improvement not only overcomes the shortcomings of traditional polyimide film in color, but also retains its excellent performance. This is also the secret of transparent polyimide becoming the core material of folding screen mobile phones.

Polyimide film itself has many excellent properties, coupled with its high price and high technical barriers, it is called "golden film". At present, as one of the special engineering materials, polyimide is known as "the most promising engineering plastic in the 21st century" and has outstanding performance in aerospace, microelectronics, fiber and other fields.

Polyimide has a tight molecular structure and strong intermolecular bonding, so it has a high glass transition temperature and can still maintain high molecular mobility in low temperature environments, thereby maintaining good physical properties. The vacuum and high and low temperature environments in space place stringent requirements on the protective outerwear of spacecraft, and polyimide film has become the preferred material for spacecraft outerwear due to its own advantages.

If the integrated circuit installed on the missile is exposed to radiation, the computer connected to the missile may have a logical error. The chemical resistance and excellent radiation resistance of polyimide stand out among many materials. With the development of the electronics industry, high-performance polyimide film has become a key material for microelectronics manufacturing and packaging, and is widely used in ultra-large-scale integrated circuits and flexible packaging substrates.

In terms of fibers, polyimide fibers are mainly used in the military market, and the civilian market is being developed. Polyimide itself has high flame retardant properties and low smoke emission, and is a self-extinguishing material. Non-woven fabrics woven from polyimide fibers are the most ideal fiber material for fire-retardant clothing. In daily life, polyimide materials are also the best choice for flame-retardant work clothes and fire blankets. The simple improvement of the multi-layer insulation material used on spacecraft is the civilian version of the emergency blanket, which has the same function as on spacecraft and can effectively reflect infrared rays emitted by the human body to maintain human body temperature in an emergency.

Since its discovery, polyimide has been a "hot commodity" among polymer materials. It can be seen in many applications and has made great contributions to the development of science and technology and human life. However, compared with other polymers, its preparation cost and technical barriers are still too high, so finding ways to reduce costs and develop R&D is one of the key directions for future research.

(The first author is a professor and doctoral supervisor at Northwest Normal University, and the second author is a master's student at Northwest Normal University)