Effect of isopropylidene replacement on gas transport properties of polycarbonates

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Abstract
The gas sorption and transport properties of a series of polycarbonates in which the isopropylidene unit of bisphenol A polycarbonate has been replaced with another molecular group are presented. Two new materials, bisphenol of norbornane polycarbonate (NBPC) and bisphenol Z polycarbonate (PCZ), are compared with several polymers which have been studied previously in this laboratory, including bisphenol A polycarbonate (PC), hexafluorobisphenol A polycarbonate (HFPC), and bisphenol of chloral polycarbonate (BCPC). The effect of molecular structure on chain mobility and chain packing is related to the gas transport properties. Dynamic mechanical thermal analysis and differential scanning calorimetry are used to judge chain mobility, while x‐ray diffraction and free volume calculations give information about chain packing. Permeability measurements were made for He, H2, O2, N2, CH4, and CO2 at 35°C over a range of pressures up to 20 atm. Sorption experiments were also done for N2, CH4, and CO2 under the same conditions. The permeability coefficients of these polymers rank in the order HFPC ≫ NBPC>PC>BCPC ≫ PCZ for all of the gases. With the exception of BCPC, this order correlates well with fractional free volume. The low gas permeability of BCPC is attributed to a polarity effect. In general, bulky and relatively immobile substituents, as in HFPC and NBPC, can yield improved separation characteristics. The polar group of BCPC and the flexible cyclohexyl substituent of PCZ result in relatively low gas permeability.