Oriented film including polystyrene polymer having syndiotactic configuration

An orineted polystyrene film including a polystyrene polymer having a syndiotactic configuration, the film having a thickness of 25 .mu.m or more, and a rectangular sample of the film having a value C expressed by equation (I) of 0.007 or less:

• ##EQU8## wherein the rectangular sample is obtainable from (A) or (B):
• (A) a rolled film extending in a longitudinal direction and having a width in a width direction perpendicular to the longitudinal direction, two sides of the rectangular sample in sample in the longitudinal direction of the film each being 1,000 mm in length and the other two sides of the rectangular sample each having a size equal to the width of the film;
• or
• (B) a sheet of any shape, the rectangular sample constituting an inscribed rectangle having an area larger than any other inscribed rectangle,
• wherein A.sub.max is obtained by designating four squares respectively in four apices of the rectangular sample, each square having sides of 100 mm in length, and setting an orientation direction of molecules of a first square to 0.degree., A.sub.max being an absolute value of a maximum angle among angles defined by the orientation direction of molecules of the first square and an orientation direction of molecules of each of the other three squares,
• R is expressed by equation (II) but is defined as 1 when exceeding 1:
• R=D/1000 (II)
• D is a distance (mm) between a center of gravity of the first square and a center of gravity of the square having the orientation direction which defines A.sub.max with the first square, and
• HS.sub.max is a size change ratio of a square sample which is cut from the rectangular sample and dry-heated at a temperature of 150.degree. C. for 30 minutes, the square sample being designated in an apex of the rectangular sample and having sides of 100 mm in length.