Optical film

Abstract

An optical film having a desired wavelength dispersion characteristic, a low photoelastic coefficient and a desired Nz coefficient. The optical film is formed from a copolymer and meets the following conditions (i) to (v): (i) the thickness (d) is 20 to 80 μm; (ii) the following formulas (1) and (2) are satisfied: 0.6< R (450)/ R (550)<1  (1) 1.01< R (650)/ R (550)<1.40  (2) (R(450), R(550) and R(650) are retardations within the plane of the film at respective wavelengths); (iii) R(550) is 120 to 160 nm; (iv) the birefringence (Δn) is not less than 1.5×10 −3 ; and (v) the Nz coefficient represented by the following formula (4) is 1.18 to 2.40: Nz =( n x −n z )/( n x −n y )  (4) (n x , n y and n z are 3-dimensional birefringences of the film).

1 . An optical film which is formed from a copolymer and meets the following conditions (i) to (v):
 (i) the thickness (d) is 20 to 80 μm;   (ii) the following formulas (1) and (2) are satisfied:
   0.6< R (450)/ R (550)<1  (1)
 
   1.01< R (650)/ R (550)<1.40  (2)
 
   (R(450), R(550) and R(650) are retardations within the plane of the film at wavelengths of 450 nm, 550 nm and 650 nm, respectively);   (iii) R(550) is 120 to 160 nm (R(550) is a retardation within the plane of the film at a wavelength of 550 nm);   (iv) the birefringence (Δn) represented by the following formula (3) is not less than 1.5×10 −3 :
   Δ n=R (550)/( d× 10 3 )  (3)
 
   (d is the thickness (μm) of the film); and   (v) the Nz coefficient represented by the following formula (4) is 1.18 to 2.40:
     Nz =( n   x   −n   z )/( n   x   −n   y )  (4)
 
   (n y , ny and n z  are 3-dimensional birefringences of the film, n x  is the refractive index of an in-plane slow axis (x axis), n y  is a refractive index in a direction (y axis) orthogonal to the x axis in the in-plane direction, and n z  is a refractive index in a thickness direction (z axis) perpendicular to the plane including the x axis and the y axis).   
     
     
         2 . The optical film according to  claim 1  which is obtained by stretching a film comprising a copolycarbonate containing a unit (A) represented by the following formula: 
       
         
           
           
               
               
           
         
         (wherein R 1  and R 2  are each independently a hydrogen atom, hydrocarbon group having 1 to 10 carbon atoms which may contain an aromatic group, or halogen atom, R 3  and R 4  are each independently a hydrocarbon group having 1 to 10 carbon atoms which may contain an aromatic group, m and n are each independently an integer of 1 to 4, and p and q are each independently an integer of 0 or more) 
       
       and a unit (B) derived from an alicyclic aliphatic diol having a positive refractive index, 
       the molar ratio {(A):(B)} of the unit (A) to the unit (B) being 10:90 to 70:30. 
     
     
         3 . The optical film according to  claim 2 , wherein the copolycarbonate contains the unit (A) represented by the following formula: 
       
         
           
           
               
               
           
         
         (wherein R 1  and R 2  are each independently a hydrogen atom, hydrocarbon group having 1 to 10 carbon atoms which may contain an aromatic group, or halogen atom, R 3  and R 4  are each independently a hydrocarbon group having 1 to 10 carbon atoms which may contain an aromatic group, m and n are each independently an integer of 1 to 4, and p and q are each independently an integer of 0 or more) 
       
       and a unit (B1) represented by the following formula: 
       
         
           
           
               
               
           
         
         (wherein R 5  to R 8  are each independently a hydrogen atom or alkyl group having 1 to 10 carbon atoms), 
       
       the molar ratio {(A):(B1)} of the unit (A) to the unit (B1) being 10:90 to 70:30. 
     
     
         4 . The optical film according to  claim 3 , wherein the copolycarbonate contains a unit (A1) represented by the following formula: 
       
         
           
           
               
               
           
         
         (wherein R 1  and R 2  are each independently a hydrogen atom, hydrocarbon group having 1 to 10 carbon atoms which may contain an aromatic group, or halogen atom, and m and n are each independently an integer of 1 to 4), 
       
       and the unit (B1) represented by the following formula: 
       
         
           
           
               
               
           
         
         (wherein R 5  to R 8  are each independently a hydrogen atom or alkyl group having 1 to 10 carbon atoms), 
       
       the molar ratio {(A1):(B1)} of the unit (A1) to the unit (B1) being 10:90 to 50:50. 
     
     
         5 . The optical film according to  claim 4 , wherein the molar ratio {(A1):(B1)} of the unit (A1) to the unit (B1) of the copolycarbonate is 20:80 to 40:60. 
     
     
         6 . The optical film according to  claim 4 , wherein the copolycarbonate contains a unit (A2) represented by the following formula: 
       
         
           
           
               
               
           
         
       
       and a unit (B2) represented by the following formula: 
       
         
           
           
               
               
           
         
       
     
     
         7 . The optical film according to  claim 3 , wherein the copolycarbonate contains a unit (A3) represented by the following formula: 
       
         
           
           
               
               
           
         
       
       (wherein R 1  and R 2  are each independently a hydrogen atom, hydrocarbon group having 1 to 10 carbon atoms which may contain an aromatic group, or halogen atom, R 3  and R 4  are each independently a hydrocarbon group having 1 to 10 carbon atoms which may contain an aromatic group, m and n are each independently an integer of 1 to 4, and p and q are each independently an integer of 1 or more) and the unit (B1) represented by the following formula: 
       
         
           
           
               
               
           
         
         (wherein R 5  to R 8  are each independently a hydrogen atom or alkyl group having 1 to 10 carbon atoms), 
       
       the molar ratio {(A3):(B1)} of the unit (A3) to the unit (B1) being 20 to 80 to 70:30. 
     
     
         8 . The optical film according to  claim 7 , wherein the molar ratio {(A3):(B1)} of the unit (A3) to the unit (B1) of the copolycarbonate is 30:70 to 60:40. 
     
     
         9 . The optical film according to  claim 7 , wherein the copolycarbonate contains a unit (A4) represented by the following formula: 
       
         
           
           
               
               
           
         
       
       and the unit (B2) represented by the following formula: 
       
         
           
           
               
               
           
         
       
     
     
         10 . The optical film according to  claim 1 , wherein the absolute value of the photoelastic constant of the copolymer is not more than 30×10 −12  Pa −1 . 
     
     
         11 . The optical film according to  claim 1 , wherein the absolute value of “photoelastic constant×thickness” is not more than 1,700×10 −12  Pa −1 ·μm. 
     
     
         12 . The optical film according to  claim 1 , wherein the glass transition temperature of the copolymer is 120 to 170° C. 
     
     
         13 . A retardation film which is the optical film of  claims 1  to  12 . 
     
     
         14 . A liquid crystal display comprising the retardation film of  claim 13 .