Gas permeation in polyarylates: effects of polarity and intersegmental mobility

Permeability coefficients for He, Ar, O2, N2, CH4, and CO2 in five polyarylates synthesized from a mixture of iso: terephthalic acids (1:1) and a bridge substituted bisphenol were measured at a pressure of 1.0 × 106 N/m2 and temperature of 35°C. Bisphenols used for the syntheses of these polyarylates were prepared by substituting one of the bridge methyl groups (-CH3) in bisphenol-A with one of the following functional groups: ethyl (-CH2-CH3), isobutyl (-CH2-CH(CH3)2), phenyl (-C6H5), or methyl propionate (-CH2-CH2-COOCH3). This type of substitution does not affect the polymer packing significantly. The calculated Hildebrand solubility parameter for these polyarylates also varies only in a narrow range. With these critical parameters being constant, the polyarylate permeabilities correlate well with increasing chain stiffness as indicated by secondary transition temperatures (Tτ°C). Substitutions which increased polarity had a negative effect on transport properties

A 2<SUP>7-3</SUP> fractional factorial optimization of polybenzimidazole based membrane electrode assemblies for H<SUB>2</SUB>/O<SUB>2</SUB> fuel cells

We describe the usefulness of a statistical fractional factorial design to obtain consistent and reproducible behavior of a membrane-electrode-assembly (MEA) based on a phosphoric acid (PA) doped polybenzimidazole (PBI) membrane, which allows a H<SUB>2</SUB>/O<SUB>2</SUB> fuel cell to operate above 150°C. Different parameters involved during the MEA fabrication including the catalyst loading, amount of binder, processing conditions like temperature and compaction load and also the amount of carbon in the gas diffusion layers (GDL) have been systematically varied according to a 2<SUP>7-3</SUP> fractional factorial design and the data thus obtained have been analyzed using Yates's algorithm. The mean effects estimated in this way suggest the crucial role played by carbon loading in the gas diffusion layer, hot compaction temperature and the binder to catalyst ratio in the catalyst layer for enabling continuous performance. These statistically designed electrodes provide a maximum current density and power density of 1,800 mA cm<SUP>-2</SUP> and 280 mW cm<SUP>-2</SUP>, respectively, at 160°C using hydrogen and oxygen under ambient pressure

Gas permeation in polyarylates: effect of bisphenol and acid substitution symmetry

The effect of bisphenol substitution symmetry on gas permeation as well as other relevant structural properties of polyarylates has been investigated both experimentally and by molecular modelling. Asymmetric or di-substitution of methyl groups on the phenyl rings of bisphenol-A or phenolphthalein resulted in polyarylates with similar packing density and permeability and with increased permselectivity compared to the corresponding polymers with the unsubstituted bisphenols. Symmetric or tetra-substitution of methyl groups on the bisphenol-A phenyl rings led to polyarylates with decreased packing density, increased permeability and similar selectivities as the corresponding polymers with the unsubstituted bisphenol. Molecular modelling studies of the chain conformation gave further insight into the mechanism by which substitution symmetry affects the polymer properties. The differences in the minimum energy chain conformation of symmetrically and asymmetrically substituted bisphenol-A polyarylate chains help in explaining the variation in packing density and permeation properties. Calculations of relative bond flexibility and the energy barrier for bond rotation of specific moieties in the minimized energy chain conformation correlate with molecular mobility as measured by sub-Tg transition temperatures. The diacid used for polyarylate synthesis was also varied in order to investigate the effect of acid linkage symmetry. Polyarylates based on the above bisphenols and asymmetrically linked isophthalic acid were compared with the corresponding polymers based on symmetrically linked terephthalic acid or 2,6-naphthalene dicarboxylic acid. Isophthalic acid-based polyarylates had higher packing density, chain mobility, and permselectivity and lower permeability than their terephthalic acid-based counterparts. Incorporation of the naphthalene acid along with the terephthalic acid also results in polyarylates with lower packing density and higher chain rigidity than the isophthalic acid-based polymers