Retardation film and production method for retardation film

A phase difference film composed of a resin C containing a copolymer P including a polymerization unit A and a polymerization unit B wherein the polymerization unit A is an aromatic vinyl-based unit and the polymerization unit B is a hydrogenated chain conjugated diene-based unit,

• the phase difference film including a lamellar phase separation structure that generates a structural birefringence,
• the phase separation structure including a phase (A) having the polymerization unit A as a main component and a phase (B) having the polymerization unit B as a main component,
• an in-plane direction retardation Re(550) measured at a wavelength of 550 nm being 0 nm or more and 10 nm or less,
• a thickness-direction retardation Rth(550) measured at a wavelength of 550 nm being 80 nm or more and 300 nm or less, and
• the phase difference film satisfying the following formulae (1A) and (2), or the following formulae (1B) and (2),
• f(A)>0.5 (1A)
• f(B)>0.5 (1B)
• D(A)>D(B) (2)
• wherein
• f(A) represents a total weight ratio of the polymerization unit A in the copolymer P,
• f(B) represents a total weight ratio of the polymerization unit B in the copolymer P,
• D(A)=ReA(450)/ReA(550),
• D(B)=ReB(450)/ReB(550),
• ReA(450) represents an in-plane direction retardation (nm) of a film (A) formed from a polymer (A) composed of the polymerization unit A measured at a wavelength of 450 nm,
• ReA(550) represents an in-plane direction retardation (nm) of the film (A) measured at a wavelength of 550 nm,
• ReB(450) represents an in-plane direction retardation (nm) of a film (B) formed from a polymer (B) composed of the polymerization unit B measured at a wavelength of 450 nm, and
• ReB(550) represents an in-plane direction retardation (nm) of the film (B) measured at a wavelength of 550 nm.