Polyarylene sulfide copolymer and method of producing the same

A polyarylene sulfide copolymer having a glass transition temperature of 95° C. or higher and 190° C. or lower, and having a melting point of 300° C. or lower, as measured by differential scanning calorimetry,

• wherein the copolymer comprises as a structural unit arylene sulfide units having a number average molecular weight (Mn) of 1,000 or more and 10,000 or less,
• the arylene sulfide units are represented as repeating units —(Ar—S)— and a copolymeric component,
• the copolymer contains 80 mol % or more of the repeating units represented by —(Ar—S)—, and
• a crystallized film of the polyarylene sulfide copolymer prepared by sandwiching a sample of the polyarylene sulfide copolymer and a spacer sandwiched between polyimide films,
• sandwiching the sample together with polyimide films between press molds that are heated to a melting point of the sample+30° C., and pressurized for 1 minute, after one-minute pressurization and retention, removing the sample together with the polyimide films and sandwiched between metal plates that is heated to a glass transition temperature+30° C. and pressurized for 5 minutes for crystallization to obtain the crystallized film, is immersed in N-methyl-2-pyrrolidone (NMP) that is heated to 80° C., and after keeping the liquid temperature at 80° C. for 20 minutes, the crystallized film is insoluble.