Process for treating liquid crystal polymer film

A heat-treated polymer film made by the process comprising the steps of:

• (a) heating a film made by the extrusion molding of a polymer capable of forming an optically anisotropic melt phase while contacting at least one surface of said film with a supporting body, to a temperature sufficient to melt said polymer;
• (b) cooling the melted polymer to form a solidified polymer layer;
• and
• (c) separating said solidified polymer layer from said supporting body;
• said heat-treated polymer film having an intra-layer peeling strength measured by conducting a 90.degree. peeling test in accordance with JIS C6471 of at least 2 kg/cm;
• a tensile strength measured in accordance with JIS C2318 of at least 5 kg/mm.sup.2 ;
• a thermal expansion coefficient that is positive when measured along four different lines in the film plane, wherein each of said four lines passes through a common point on the film plane, with the second line being at an angle of 45.degree. with respect to the first line, with the third line being at an angle of 45.degree. with respect to the second line, and with the fourth line being at an angle of 45.degree. with respect to the third line;
• and a dimensional change ratio when heated of not more than 0.1% in its absolute value, wherein said ratio is ›(L.sub.1 -L.sub.0)/L.sub.0 !.times.100%, where L.sub.0 is the length of a film sample measured at 25.degree. C. and L.sub.1 is the length of the same film sample measured at 25.degree. C. after being heated in air at 190.degree. C. for 30 minutes under no tension;
• and a degree of planar orientation of 50 to 70% as measured by x-ray diffractometry.