Patent ID: 7160926

Claim:
A membrane of an ion exchange fluorocarbon resin comprising a copolymer substantially composed of the following repeating units (A) and (B): where X is a F atom or a perfluoroalkyl group having 1 to 3 carbon atoms, n is an integer of 0 to 3, m is an integer of 1 to 3, Z is H, Cl, F or a perfluoroalkyl group having 1 to 3 carbon atoms and W′ is CO 2 H or SO 3 H, and having, after substituted with tributyl ammonium ions, a storage modulus (JIS K-7244) at a temperature of not less than Tc represented by the following general equations (1-1) to (1-3) of 0.8×10 6 Pa or more, and a heat of crystal fusion at 270 to 350° C. of 1 J/g or less: Tc (° C.)=55 +Tg ( EW< 750) (1-1) Tc (° C.)=0.444× EW− 278 +Tg (750 ≦EW< 930) (1-2) Tc (° C.)=135 +Tg ( EW≧ 930) (1-3) where Tg designates the peak temperature of loss tangent in the dynamic viscoelasticity measurement of the ion exchange fluorocarbon resin membrane (having SO 3 H as ends of side chains).