FIELD OF DISCLOSURE

The field of this disclosure is polyimide films and electronic devices.

BACKGROUND OF THE DISCLOSURE

Polyimide films can potentially replace rigid glass cover sheets and other substrates which are currently used in display applications, such as organic light-emitting diode (OLED) displays. For example, aromatic polyimides are typically very thermally stable, with glass transition temperatures (Tg) of greater than 320° C., and have excellent foldability and rollability, a critical property needed for next-generation flexible displays. For polyimide films used in display applications, in addition to having high transmittance and low haze, the polyimide film also needs to be neutral in color. Typical specifications require that both a* and b* are no greater than 1 color unit from neutral (0) in CIE L*, a*, b* color space coordinates, i.e., the absolute values of a* and b* should be less than 1. The three coordinates of CIE L*, a*, b* represent: (1) the lightness of the color (L*=0 yields black and L*=100 indicates diffuse white), (2) its position between red/magenta and green (negative a* values indicate green, while positive values indicate magenta) and (3) its position between yellow and blue (negative b* values indicate blue and positive values indicate yellow).

Typical polyimides with fluorinated monomers, which are nearly colorless, still absorb light in the blue or violet wavelengths (400-450 nm) which gives the films a yellow appearance in transmission. The color of the polyimide films is mostly generated from charge transfer absorptions arising from HOMO-LUMO transitions which can occur both within the polymer chains and between polymer chains. Various approaches have been used to alter HOMO-LUMO transition energies or to frustrate interchain interactions. In one approach, a fluorinated monomer is used to alter the HOMO-LUMO transition energies of the polyimide polymer, but still some residual yellow color can be apparent in these polyimide films. Depending on the monomer composition in the polyimide, therefore, b* can be higher than 1. Since the CIE L*, a*, b* color measurement of a film is also dependent on its thickness, achieving a neutral appearance is even more difficult for thicker films, such as those greater than 25 μm.

Polyimides are generally stiff, highly aromatic materials; and the polymer chains tend to orient in the plane of the film as the film is being formed. This leads to differences in the refractive index in the plane of the film and perpendicular to the film. The difference in the parallel and perpendicular refractive indices results in optical retardation that can negatively impact display performance. This difference is measured as the retardation value (Rth) and is represented by the equation:
Rth=[(nx′+ny′)/2−nz′]×d

In the equation above, nx′, ny′ and nz′ respectively indicate refractive indices in an X-axis direction, a Y-axis direction and a Z-axis direction in the film. The X axis corresponds to an axial direction exhibiting the refractive index in the surface of the film, the Y axis corresponds to an axial direction perpendicular to the X axis within the surface, and the Z axis corresponds to a thickness direction perpendicular to the X axis and the Y axis. Further, d indicates the thickness of the protective layer in nm.

Touch Sensor Panel (TSP) substrates, positioned behind a circular polarizer in an OLED display, must have ultra-low optical retardation (Rth) in addition to visible transparency. Unlike polyimide films for OLED cover sheets, where high Tg's are desirable, the polyimide films for TSP substrates often need to be thermoformable (low Tg) while still maintaining low color. The thermoformability allows the manufactures to melt process the display components into the desired device form which can wrap around the device contours. Unfortunately, synthetic strategies employed to reduce color in polyimide films also typically raise their Tg.

SUMMARY

In a first aspect, a polyimide film includes a dianhydride, a fluorinated aromatic diamine and an aliphatic diamine. The polyimide film has a b* of less than one for a film thickness of at least 30 microns and a glass transition temperature of less than 300° C.

In a second aspect, an electronic device includes the polyimide film of the first aspect.

The foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as defined in the appended claims.

