Thermoplastic acrylic resin and molded body for optical member

1. A thermoplastic acrylic resin comprising: 10 to 70% by weight of a repeating unit derived from a methacrylate monomer represented by the following formula (1); 5 to 40% by weight of a repeating unit derived from a vinyl aromatic monomer represented by the following formula (2); 0.1 to 3% by weight of a repeating unit derived from an aromatic group-containing methacrylate monomer represented by the following formula (3); and 20 to 50% by weight of a cyclic acid anhydride repeating unit represented by the following formula (4) or (5), said thermoplastic acrylic resin as a weight-average molecular weight in the average of 10,000 to 400,000 and a molecular weight distribution in the range of 1.8 to 3.0 determined by a GPC measurement method:

wherein R 1 represents a linear or branched alkyl group having 1 to 12 carbon atoms, or a cycloalkyl group having 5 to 12 carbon atoms,

wherein R 2 is a hydrogen and R 3 represents hydrogen, halogen, a hydroxyl group, an alkoxy group, a nitro group, or a linear or branched alkyl group having 1 to 12 carbon atoms, and I represents an integer of 1 to 3,

wherein R 4 represents hydrogen, halogen, a hydroxyl group, an alkoxy group, a nitro group, or a linear or branched alkyl group having 1 to 12 carbon atoms, m represents an integer of 1 to 3, and n represents an integer of 0 to 2,

wherein R 5 to R 8 may be the same or different and each represents hydrogen or a linear or branched alkyl group having 1 to 12 carbon atoms, and

wherein the thermoplastic acrylic resin further satisfies the following optical property (iii): (iii) an absolute value of retardation (Re) in the in-plane direction is not higher than 30 nm.

2. The thermoplastic acrylic resin according to claim 1 , characterized in that a molar ratio (B/A) of a content (B) of the cyclic acid anhydride repeating unit to a content (A) of the repeating unit derived from a vinyl aromatic monomer is larger than 1 and not larger than 10.

3. The thermoplastic acrylic resin according to claim 1 , wherein the repeating unit derived from a methacrylate monomer is derived from methyl methacrylate, the repeating unit derived from a vinyl aromatic monomer is derived from styrene, the repeating unit derived from an aromatic group-containing methacrylate monomer is derived from benzyl methacrylate, and the cyclic acid anhydride repeating unit is derived from maleic anhydride.

4. The thermoplastic acrylic resin according to claim 1 , wherein the thermoplastic acrylic resin satisfies the following optical property (i):

(i) an absolute value of a photoelastic coefficient is not higher than 3.0×10 −12 Pa −1 .

5. The thermoplastic acrylic resin according to claim 1 , wherein the thermoplastic acrylic resin satisfies the following optical property (ii):

(ii) a value of a slope K in the equation (a) of linear relationship between birefringence (Δn(S)) after drawing and a draw ratio (S) determined by least squares approximation satisfies the following expression (b):

Δ n ( S )= K×S+C   (a)

| K|< 0.30×10 −6   (b).

6. The thermoplastic acrylic resin according to claim 1 , wherein the thermoplastic acrylic resin further satisfies the following optical property (iv):

(iv) an absolute value of retardation (Rth) in the thickness direction is not higher than 30 nm.

7. The thermoplastic acrylic resin according to claim 1 , wherein the thermoplastic acrylic resin further satisfies the following optical property (v):

(v) a ratio (Rth/Re) of the retardation (Rth) in the thickness direction to the retardation (Re) in the in-plane direction satisfies the following expression (c):

0.1 <Rth/Re< 1  (c).

8. The thermoplastic acrylic resin according to claim 1 , characterized in that the thermoplastic acrylic resin further satisfies the following condition (vi):

(vi) the resin has a glass transition temperature (Tg) of 120° C. or higher.

9. The thermoplastic acrylic resin according to claim 1 , characterized in that the thermoplastic acrylic resin further satisfies the following condition (vii):

(vii) the resin has total light transmittance of 85% or higher.

10. A molding for optical material comprising a thermoplastic acrylic resin according to claim 1 .

11. The molding for optical material according to claim 10 , wherein the molding for optical material is an optical film.

12. The optical film according to claim 11 , characterized in that the optical film is a film which is formed by extrusion molding and drawn at a draw ratio of 0.1 to 300% at least in the uniaxial direction.

13. The optical film according to claim 11 , characterized in that the optical film is a film which is formed by cast molding and drawn at a draw ratio of 0.1 to 300% at least in the uniaxial direction.

14. A polarizing plate protective film comprising an optical film according to claim 11 .

15. A retarder film comprising an optical film according to claim 11 .

16. The molding for optical material according to claim 10 , wherein the molding for optical material is an optical lens.