NOVELTY - A self-crosslinking polyamic acid (P1) comprising a condensation reaction product of melamine-based material, an acid anhydride, and a diamine compound is new.    USE - In the preparation of self-crosslinking polyimide used in the preparation of self-crosslinking polyimide film (claimed); in flexible display devices for transmitting various types of visualized information; in various applications such as in display devices, as a material for waveguides, protection coating film for solar cells, in radio frequency identification and electronic devices; as an optical film, a compensation film for liquid crystal display devices or an organic light emitting diodes; as an alignment layer for liquid crystal display devices, a material for waveguides; and as a protection layer of other devices.    ADVANTAGE - The display device comprising the polyimide film is thin and light and has low power consumption. The display device containing the polyimide film exhibits improved overall performance, reliability and low cost. The polyimide exhibits good thermal resistance; has excellent light transmittance and thermal characteristics. The self-crosslinking polyimide film prepared using the polyimide is transparent. The self-crosslinking polyimide film has an average light transmittance of greater than or equal to 80% (preferably greater than or equal to 85%, especially greater than or equal to 90%) at a wavelength of about 380 to about 800 nm measured using a UV-VIS spectrometer and based on a thickness of 10 to 200 mu m of the polyimide film; has a yellowness index (YI) of less than or equal to 15 (preferably 0.1-5) based on a thickness of 10 to 200 mu m of the polyimide film; has an average linear expansion coefficient (also referred to as the coefficient of thermal expansion), of less than or equal to 100 ppm/ degrees C (preferably 3-80 ppm/ degrees C) measured at a temperature of 50 to 200 degrees C, based on a thickness of 10 to 200 mu m of the polyimide film; and has a permittivity of less than or equal to 3.0 at 1 GHz. The self-crosslinking polyimide film has excellent thermal stability, electrical characteristics, and mechanical properties, and is transparent, and thus can be used as an optical film, a compensation film for liquid crystal display devices or an organic light emitting diodes, an alignment layer for liquid crystal display devices, a material for waveguides, a protection coating film for solar cells, or a radio frequency identification (RFID) substrate, and also as a protection layer of other devices.    DETAILED DESCRIPTION - A self-crosslinking polyamic acid (P1) comprising a condensation reaction product of melamine-based material, an acid anhydride of formula (I), and a diamine compound is new.    Cy=a trivalent or a quadrivalent organic group selected from 4-20C cyclic group, 6-20C monocyclic aromatic group, 2-20C condensed polycyclic aromatic group, and/or interconnected 2-20C non-condensed polycyclic aromatic group interconnected by an aromatic group (all optionally substituted).    INDEPENDENT CLAIMS are included for the following:    (1) new self-crosslinking polyimide (P2) comprising a condensation and imidization product of melamine-based material, an acid anhydride (I) and a diamine compound;    (2) preparation of self-crosslinking polyamic acid (P1) and self-crosslinking polyimide (P2); and    (3) a self-crosslinking polyimide film comprising the self-crosslinking polyimide.