Helium and hydrogen selective carbon hollow fiber membranes: The effect of pyrolysis isothermal time

•The effect of the isothermal maintenance time on gas permeances was studied.•High separation coefficients of He/CH4, H2/CH4 and CO2/CH4 were achieved.•CMS hollow fiber membranes have been prepared by the pyrolysis of PI precursor.•The membrane which remained for 60min at 1173K exhibit lower gas permeances.•Real selectivities of He/CH4, H2/CH4, He/N2 and H2/N2 are higher than the calculated.