Abstract: A separation method for a liquid mixture, which comprises removing water from said liquid mixture by pervaporation using a membrane, the main constituent material therefor being a copolyimide having the structure represented by the constitutional repeating unit of the following general formula (I): ##STR1## wherein R represents ##STR2## for from 10 to 30 mol % of said repeating unit, while R also represents ##STR3## for from 90 to 70 mol % of said repeating unit, or a copolyamide-imide comprising from 90 to 10 mol % of the constitutional repeating unit having the structure represented by the formula (II): ##STR4## and from 10 to 90 mol % of the constitutional repeating unit having the structure represented by the formula (III): ##STR5##

Abstract: A copolyamide acid comprising repeating units represented by the general formulas: ##STR1## in which the repeating unit (A) and the repeating unit (B) are present in a molar ratio from 20:80 to 80:20, and which has a logarithmic viscosity number (.eta..sub.inh) of 0.5 dl/g--10 dl/g measured in N,N-dimethylformamide at a concentration of 0.5 g/dl and at a temperature of 30.degree. C. and a copolyimide comprising repeating units represented by the general formulas: ##STR2## in which the repeating unit (C) and the repeating unit (D) are present in a molar ratio from 20:80 to 80:20 and which has a logarithmic viscosity number (.eta..sub.inh) of o.5 dl/g--10 dl/g measured in 97% concentrated sulfuric acid at a concentration of 0.5 g/dl and at a temperature of 30.degree. C.