The effect of hard segments on the gas separation properties of polyurethane membranes

Effect of hard segment content was investigated on the gas permeability characteristics of polyether based urethanes. These polymers were synthesized by using 2000 molecular weight polypropylene-glycol (PPG 2000) with toluene diisocyanate (TDI). Conversion of prepolymer to a final PU was carried out by using 1,4-butane diol (BD) as a chain extender. Four different block ratios were chosen in order to prepare polyurethanes with different hard segment contents. The PPG1000 polyol based PU with the same content of hard segment was also synthesized to evaluate the effect of soft segment length on the gas separation properties of the polyurethanes. The prepared polymers were characterized and the permeability of gases was investigated. Permeation measurements of polymers have revealed that the permeabilities of gases decrease with increasing the hard segment content by which the selectivity of gases in polymers increases as well. The polymer containing 42.7 wt% hard segment has shown that its CO 2 permeability coefficients and ideal separation factors relative to N2 are higher than38.2 barrer (1 barrer =1×10-10 [cm3 (STP) cm/cm2.s.cmHg]) and 33.21, respectively. While, the polymer containing 26 wt% hard segment showed 79.5 barrer and 20.9 values, respectively. The gas permselectivities of polymers containing the same hard segment content with different soft segment lengths have shown that by increasing the soft segment length the permeability towards gases increases, meanwhile their gas selectivity decreases.