Polyamide-based film, preparation method thereof, and cover window and display device comprising the same

A polyamide-based film, which comprises a polyamide-based polymer and has an RSRA value of 3.6 m/N to 5.0 m/N as represented by the following Equation 1 and an RSLA value of 3.7 m/N to 5.0 m/N as represented by the following Equation 2 based on the thickness of the film of 50 μm:

• RSRA=(RSRMD+RSRTD)/2 <Equation 1>
• in Equation 1, RSRMD is the loop stiffness value measured with the MD direction of the film as the longitudinal direction at room temperature, and RSRTD is the loop stiffness value measured with the TD direction of the film as the longitudinal direction at room temperature, wherein RSRMD is 3.35 m/N to 5.0 m/N, and RSRTD is 3.3 m/N to 5.0 m/N,
• RSLA=(RSLMD+RSLTD)/2 <Equation 2>
• in Equation 2, RSLMD is the loop stiffness value measured with the MD direction of the film as the longitudinal direction within 1 minute after the film is left at −20° C. for 2 hours, and RSLTD is the loop stiffness value measured with the TD direction of the film as the longitudinal direction within 1 minute after the film is left at −20° C. for 2 hours,
• wherein the polyamide-based polymer comprises an imide-based repeat unit and an amide-based repeat unit at a molar ratio of 0:100 to 7:93,
• wherein the film is manufactured by a method comprising,
• polymerizing a diamine compound, a dicarbonyl compound, and, optionally, a dianhydride compound in an organic solvent to prepare a polyamide-based polymer solution;
• casting the solution and then drying it to prepare a gel sheet;
• and
• thermally treating the gel sheet,
• wherein the step of thermally treating the gel sheet is carried out by fixing both ends of the gel sheet in the transverse direction with a fixing part, and thermally treating it while the width of the fixed gel sheet is changed,
• wherein the step of thermally treating the gel sheet comprises first thermal treatment through hot air supplied by at least one hot air blower;
• and second thermal treatment through at least one heater, and
• wherein when the section in which the first thermal treatment step is carried out is referred to as a first thermal treatment section,
• the section in which the second thermal treatment step is carried out is referred to as a second thermal treatment section,
• the maximum width of the gel sheet in the first thermal treatment section is referred to as Wa,
• the minimum width of the gel sheet is in the first thermal treatment section is referred to as Wb,
• the minimum width of the gel sheet in the first thermal treatment section and the second thermal treatment section is referred to as Wc, and
• the Wb/Wa value is 0.955 to 0.990, and the Wc/Wa value is 0.950 to 0.990.