Abstract
A series of ladder copolymers and a homopolymer were synthesized via aromatic nucleophilic substitution polycondensation of 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethylspirobisindane with tetrafluoroterephthalonitrile and a new monomer heptafluoro-p-tolylphenylsulfone as potential materials for membrane gas separation. Ladder polymers of this type comprising rigid and contorted chain structure have been commonly referred to as polymers of intrinsic microporosity (PIM) on account of their extraordinarily high fractional free volumes (FFV) and high gas permeability (P). The PIM polymer series of the present study was prepared in high molecular weight and low molecular weight distribution, using new experimental conditions of short reaction times and a high temperature of 160 degrees C. Polymer chain d-spacing was investigated using wide-angle X-ray diffraction. Polymer free volume was calculated from the polymer density and specific van der Waals volume. Gas permeabilities for oxygen and carbon dioxide decreased with increasing content of the trifluoromethylphenylsulfone versus the dinitrile monomer within the copolymer, while the selectivities of gases against nitrogen increased. The pendent phenylsulfone groups likely reside within the interchain free volume of the rigid and contorted ladder polymer, acting to reduce gas permeability and increase selectivity, though with no overall loss of performance relative the Robeson upper bound. Evidence for this is a reduction in d-spacing and fractional free volume with increasing content of the trifluoromethylphenylsulfone versus the dinitrile monomer in the copolymer series. The relationship between the gas permeability behavior and structures of the new PIM copolymers is discussed.