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Processable Aromatic Polyimides

This review article deals with aromatic polyimides that are processable from the melt or soluble in organic solvents. Conventional aromatic polyimides represent the most important family of heat resistant polymers, but they cannot be processed in the melt, and their application in the state of soluble intermediates always involves a hazardous step of cyclo-dehydration and elimination of a non-volatile polar solvent. A major effort has therefore been devoted to the development of novel soluble and/or melt-processable aromatic polyimides that can be applied in the state of full imidation. The structural factors conducive to better solubility and tractability are discussed, and representative examples of monomers showing favourable structural elements have been gathered and listed with the chemical criteria. Experimental and commercial aromatic polyimides are studied and evaluated by their solubility, transition temperatures and thermal resistance. An example is also given of the methods of computational chemistry applied to the study and design of polyimides with improved processability.

Gas separation properties of aromatic polyimides

A series of aromatic polyimides have been investigated for their permeation properties to oxygen, nitrogen, helium, carbon dioxide and methane. The polymers are soluble, film-forming polyimides based on new anhydride monomers containing carbonyl groups as connecting linkages of phenyl rings and bulky side groups like phenyl and t-butyl. To assist in explaining the experimental results, molecular modeling was performed to calculate density, free volume and chain parameters that could account for the behavior of the polymers as selective barriers for gas penetrants. High values of O2/N2 selectivity and good permeabilites were observed for some polymers; their properties lie near the upper bound for this gas pair. Gas permeability typically increased with increasing free volume, and, in general, free volume could be related to the chemical composition of the polymer backbone and to the nature of the pendent groups.

Gas separation properties of aromatic polyamides containing hexafluoroisopropylidene groups

Abstract The synthesis and gas transport properties of aromatic polyisophthalamides (PIPAs), based on isophthaloyl chloride derivatives bearing pendent groups and hexafluoroisopropylidene (6F) linkages in the main chain, are reported and compared with properties of a similar series of PIPAs containing sulfonyl (SO2) rather than 6F in the main chain. All of those polymers exhibit high glass transitions temperatures. The polymers containing 6F groups were markedly more permeable and somewhat less selective than their sulfonyl analogs. Polymers containing a t-butyl pendent group at the 5 position of the isophthaloyl linkage were much more permeable than those bearing only a hydrogen atom at this position, although a strong decrease in permselectivity accompanied the large increase in permeability. CO2/CH4 solubility selectivity values of the 6F-containing polymers were similar to values reported for other polymetric and non-polymeric organic materials with similar concentrations of polar carbonyl linkages. In contrast, the CO2/CH4 solubility selectivity in SO2-containing variants of these polymers was substantially lower than expected based on total polar group concentration. The low CO2/CH4 solubility selectivity is believed to be related to the extremely efficient chain packing in the SO2-containing polymers, which may lead to strong amide-amide linkage interaction, thereby inhibiting carbonyl groups in the amide linkage from interactions with CO2 molecules to increase CO2/CH4 solubility selectivity.

Gas separation properties of aromatic polyamides with sulfone groups

Abstract The gas transport properties of three aromatic polyisophthalamides based on isophthaloyl chlorides and 4,4′-diaminodiphenylsulfone are reported at 35°C. The effects of bulky t-butyl and phthalimide substituents, at the 5 position of the isophthaloyl chloride moiety, on CO 2 , CH 4 , O 2 , N 2 , H 2 and He permeability, solubility and diffusivity were determined and correlated with chain packing and thermal properties of the polymers. Gas permeability was higher in substituted polyisophthalamides than in the unsubstituted analogue. Polymers containing the pendent t-butyl substituent have substantially higher permeability than polymers bearing the phthalimide substituent, despite the fact that the phthalimide substituent appears to be more bulky than the t-butyl group, based on van der Waals volume estimations. The strong polarity of the phthalimide moiety may act to increase chain-chain cohesive forces, which would tend to enhance chain packing, thereby reducing the packing-disrupting ability of the bulky phthalimide group. The permeability increase of the substituted polymers was accompanied by a permselectivity decrease.

Aromatic polyisophthalamides with iminobenzoyl pendant groups

Abstract Polyisophthalamides were prepared from aromatic diamines and 5-iminobenzoylisophthalic acid by the Yamazaki method of direct polyamidation catalyzed by triphenylphosphite. The properties of the polymers were measured and compared with the analogous unsubstituted polyisophthalamides. The incorporation of one iminobenzoyl pendant group per repeating unit gave rise to better solubility in strongly polar solvents. Higher content of amide groups per repeating unit allowed the modified polymers to absorb moisture to a greater extent than the parent polyisophthalamides. The glass transition temperatures were raised 20–30 by the presence of the pendant groups and they ranged from 290 to 317°. On the contrary, the substituted polymers showed lower initial decomposition temperatures, as measured by TGA, all of them beginning to decompose at about 410°. The mechanical properties of polymer films seemed not to be greatly affected by the pendant groups and only small differences were observed between substituted and unsubstituted polymers.

Gas separation properties of pendent phenyl substituted aromatic polyamides containing sulfone and hexafluoroisopropylidene groups

Abstract The synthesis and gas transport properties of aromatic polyisophthalamides (PIPAs) based on isophthaloyl chloride (IPC) derivatives bearing a pendent phenyl group and a hexafluoroisopropylidene (6F) linkage in the main chain are reported. The properties of these polymers are compared with the properties of similar PIPAs containing sulfonyl (SO2) rather than 6F in the main chain. Polymers containing a phenyl pendent group at the five position of the isophthaloyl linkage are more permeable than those bearing only a hydrogen atom at this position, although increases in permeability are generally accompanied by decreases in selectivity. In the SO2-bearing polymer, the addition of a phenyl pendent group hinders chain packing more than in the 6F containing PIPAs. Consequently, permeability coefficients increase more upon addition of a pendent phenyl group in SO2-containing rather than 6F-containing PIPAs. The effect of amide linkage reversal on the gas transport properties of a polymer containing 6F linkages in the chain backbone and a hydrogen atom at the five position of the isophthaloyl linkage was minimal. All the PIPAs considered in this study were more permeable to nitrogen than to methane, some with nitrogen/methane selectivities of more than two.

Determination of some electrical parameters for two novel aliphatic-aromatic polyamide membranes

The effect of the chemical structure in the electrical response (membrane potential, Δo, salt diffusion, Ds, resistance, Rm, and capacitance, Cm) of two new aliphatic-aromatic polyamide membranes is considered. They are poly(ether-amide)s with an oxygen atom difference. Measurements were carried out with NaCl and MgCl2 solutions at different concentrations. Equivalent circuits were determined by impedance spectroscopy and a parallel RmCm association was obtained for both membranes and electrolytes. Under identical external conditions, significant differences for the membrane resistance values were found as a function of the membrane chemical nature, but the capacitance hardly depends on both membrane and electrolyte. From Δo and Rm values some membrane characteristic parameters such as transport numbers and ionic diffusion coefficients were determined. Concentration dependence of all these parameters was also studied.

Electrochemical parameters of sulfonated poly(ether ether sulfone) membranes in HCl solutions determined by impedance spectroscopy and membrane potential measurements

Abstract Electrical and electrochemical properties of different samples of sulfonated poly(ether ether sulfone) (SPEES) membranes with different sulfonation degrees (0%, 5% and 10%) were determined measuring the electrical resistance and concentration potential with the membranes in contact with HCl solutions at different concentrations. Impedance spectroscopy (IS) was used to determine the membrane resistance ( R m ) using equivalent circuits as models, and the results show how the sulfonation clearly affects the membrane electrical characteristics, strongly reducing the R m and also changing the type of circuit associated with the different membranes. The determination of both the proton transport number and the membrane permselectivity to H + ions in sulfonated samples was performed by measuring concentration potentials (Δ φ ). Proton diffusion in the sulfonated membranes were determined from R m and Δ φ values and a strong reduction in the membrane electrical resistance and an increase in the H + diffusion coefficient was observed with the increase of the sulfonation degree. A chemical characterization of the surface of nonsulfonated and sulfonated samples was also carried out by X-ray photoelectron spectroscopy (XPS).

A comparative analysis of flux limit models for ultrafiltration membranes

Abstract Here the permeate flux versus concentration characteristics of 0.1 % w/w to 8 % w/w aqueous solutions of dextran T-500, with a mean molecular weight of 500 000 dalton, are studied when they are tangentially filtered at 298 K through a new polyamide asymmetric membrane, with an applied pressure of 700 kPa, while the recirculation velocity in the retentate loop is kept constant in a range from 0.08 to 0.49 m/s. In such conditions, all these solutions are totally retained. The mass transfer coefficient is calculated, within the frame of the film theory for the concentration-polarization phenomenon, by studying the permeate flux as a function of concentration. It allows theevaluation of the membrane surface concentration as well. This concentration is also calculated by taking into account the osmotic limit theory. These two results for c m are tested by calculating the expected permeate fluxes in terms of the resistance model and comparing with the experimental ones. It can be concluded that the osmotic limit model reproduces the experimental permeate fluxes better, and in a wider range, than the gelation model.

Thermal behavior of aliphatic–aromatic poly(ether-amide)s

The thermal properties of a set of experimental aliphatic–aromatic polyamides containing ether linkages were examined as a function of their chemical structure. Variations of the glass transition temperature (Tg) and melting temperature (Tm) could be correlated with the length of the aliphatic spacers and with the orientation of the phenylene rings. Polymers with a high concentration of p-oriented phenylene units showed a higher Tg than those containing mainly m-oriented ones; Tg values ranged from 110 to 155°C. Surprisingly, a negligible dependence of Tgs on the nature of flexible spacers was observed. For all of the polymers, the thermal stability was virtually the same, about 440°C, when tested by dynamic thermogravimetric analysis (TGA). However, quite different levels of thermal stability were found by isothermal TGA analysis for polyamides with different flexible spacers. Moreover, the poly(ether-amide)s described here compare fairly well with wholly aromatic polyamides when measured by dynamic TGA; but isothermal TGA measurements clearly demonstrated that they decompose faster than aromatic polyamides. Treatment of the TGA curves by the method of McCallum provided kinetic data that confirmed a better long-term stability for poly(ether-amide)s with a higher proportion of para-oriented phenylene rings.