Retardation film

2:

• A phase difference film having a coefficient of linear thermal expansion of 100 ppm/° C. or less in absolute value, a photoelastic coefficient of 5×10−12 m2/N to 30×10−12 m2/N and a water absorption rate of 2.0 wt % or less.

4:

• The phase difference film according to claim 1, wherein the phase difference film has a thickness of 50 μm or less and reverse wavelength dispersion Re(450)/Re(550) of 0.50 to 0.99.

6:

• The phase difference film according to claim 5, wherein the cellulose derivative comprises (a) the organosilyl group and (b) the acyl group.

7:

• The phase difference film according to claim 5, wherein the organosilyl group is a trisubstituted organosilyl group.

8:

• The phase difference film according to claim 5, wherein the organosilyl group has at least one selected from the group consisting of a tertiary butyl group, a tertiary hexyl group and an isopropyl group.

9:

• The phase difference film according to claim 5, wherein the acyl group is a 1-naphthoyl group or a 2-naphthoyl group.

10:

• The phase difference film according to claim 1, wherein the phase difference film has a water absorption rate of 0.1 wt % or more and 2.0 wt % or less.

11:

• A circular polarizing plate, comprising the phase difference film according to claim 1.

12:

• An image display device comprising the circular polarizing plate according to claim 11.

13:

• The phase difference film according to claim 8, wherein the organosilyl group is selected from the group consisting of a tertiary butyldimethylsilyl (TBDMS) group, a tertiary butyldiphenylsilyl (TBDPS) group, a tertiary hexyldimethylsilyl (THDMS) group and a triisopropylsilyl (TIPS) group.

14:

• The phase difference film according to claim 5, wherein the cellulose derivative comprises (a) the organosilyl group and (b) the second aliphatic group.

15:

• The phase difference film according to claim 5, wherein the polymer material contains one cellulose derivative represented by the formula (1).

16:

• The phase difference film according to claim 5, wherein the polymer material contains at least two cellulose derivatives each represented by the formula (1).

17:

• The phase difference film according to claim 5, wherein the cellulose derivative has a number average molecular weight of 10,000 to 400,000.

18:

• The phase difference film according to claim 5, wherein the phase difference film further comprises at least one additive selected from the group consisting of a plasticizer, a heat stabilizer, an ultraviolet stabilizer, an in-plane retardation increasing agent and a filler.