Homopolymer and multi-block Diels-Alder polyphenylenes: Synthesis, physical properties, X-ray diffraction, and gas transport

A gas transport and X-ray structural analysis is reported for a methylated poly(phenylene) (MPP) and non-methylated poly(phenylene) (PP) homopolymer, a random copolymer, and MPP-PP block copolymers synthesized using Diels-Alder chemistry. These highly rigid and aromatic polymers display Tg's of 400 °C and 382 °C for PP and MPP. Multi-block copolymerization of PP and MPP had a significant effect on the spatial arrangement of polymer chains as revealed by changes in the X-ray diffraction amorphous peak. The membranes displayed a narrower fractional free volume (FFV) from 0.288 to 0.304. The gas permeability, solubility, diffusivity and selectivity data for He, H₂, O₂, CO₂, N₂, and CH₄ were determined for this polymer series. Gas permeability variation between PP and MPP was 1.5% for H₂ (134 Barrers) and 16% for N₂ (7.11 Barrers). However, the multi-block MPP-PP-B2 led to permeability increases between 22.4% for H₂ (164 Barrers) and 61.7% for N₂ (11.5 Barrers). This work reveals clear relationships between the ordering of the amorphous region and gas diffusivity and solubility. The permselectivity trade-off did not correlate well to gas kinetic diameter (KD), polymer chemistry or composition, and depended largely on the gas pair. This poly(phenylene) family displayed high permeability and moderate ideal gas selectivity that were near the upper bound. Permselectivity results are compared to polyphenylene oxide (PO), functionalized PO using ([sbnd]NH₂ and [sbnd]NO₂), 6FDA-DABA, and 6FDA-mPDA.