Synthesis and gas transport properties of rigid block copolyaramides

Three block copolymers have been prepared by reacting two rigid polyaramide blocks, a high gas permeability rigid poly(hexafluoroisopropylidene-5-tert-butylisophthalamide), HFA/TERT block and a barrier aramide poly(benzophenone isophthalamide), DBF/ISO block. Blocks of each aramide were prepared by synthesizing two prepolymers by condensation and combining them to obtain the block copolyaramides, HFA/TERT-b-DBF/ISO, with different block lengths 9:9, 12:12 and 18:18 average repeating units on each block. Thermal and dynamic mechanical properties were measured. The onset of decomposition for these copolymers is above 480 °C with weight losses below 10 wt% at 500 °C. tan δ for the copolyaramides shows two transitions: one for the DBF/ISO block and the other corresponding to HFA/TERT block. DBF/ISO has very low gas permeability coefficients which are at least one order of magnitude below those of HFA/TERT. The block copolyaramides show permeability coefficients slightly lower than those of HFA/TERT well described by a parallel arrangement of block microdomains. They also show similar separation factors or an increase of them above those of the HFA/TERT homopolymer. It was found that shorter block segments (vg. HFA/TERT-b-DBF/ISO 9) present lower permeability coefficients than those with larger (HFA/TERT-b-DBF/ISO 18) block segments without an appreciable loss in selectivity for gas pairs O2/N2 and CO2/CH4 which are the industrial interest separations expected for these block copolymers, and O2/N2 ideal separation factors are better than those found in other copolymers with flexible polymer blocks.