Effect of Hydroxyl-Containing Fragments on the Structure and Properties of Membrane-Forming Polyamide-Imides.

The structural features and thermophysical and transport properties of dense nonporous membranes of the casting type from (co)polyamide-imides synthesized by the polycondensation of the diacid chloride of 2-(4-carboxyphenyl)-1,3-dioxoisoindoline-5-carboxylic acid and diamines 5,5'-methylene-bis (2-aminophenol) (DADHyDPhM) and 4,4'-methylenebis(benzeneamine) (DADPhM), taken in molar ratios of 7:3, 1:1, and 3:7, have been studied. The effect of hydroxyl-containing modifying fragments of dihydroxy diphenylmethane introduced in various amounts into the main polymer chain on the pervaporation properties of the formed films is discussed. It has been shown that the presence of the residual solvent N-methyl-2-pyrrolidone in the films not only has a plasticizing effect on the characteristics of film membranes but also promotes the preferential transmembrane transport of polar liquids, primarily methanol (permeation rate over 2 kg for a copolymer with a ratio of DADHyDPhM:DADPhM = 7:3). The removal of the residual solvent from the polymer film, both thermally (heating to 200 °C) and by displacement with another solvent as a result of sequential pervaporation, led to a significant decrease in the rate of transfer of polar liquids and a decrease in the selectivity of the membrane. However, the dehydrocyclization reaction resulted in more brittle films with low permeability to penetrants of different polarities. The results of our comprehensive study made it possible to assume the decisive influence of structural changes in membranes occurring in connection with the competitive formation of intra- and intermolecular hydrogen bonds.

New Membrane-Forming Aromatic Co-Poly(amide-imide)s: Influence of the Chemical Structure on the Morphological, Thermal and Transport Properties.

Polymer film membranes are used to solve specific separation problems that dictate structural requirements. Structural and morphological parameters of film membranes based on glassy polyheteroarylenes can be controlled in the process of preparation from solutions that opens up prospects for obtaining structured membranes required for targeted separation. In the case of aromatic poly(amide-imide)s, the possibility of controlling film formation and structure virtually has not been studied. In the present work, a series of homologous co-poly(amide-imide)s differing in the number of repeating units with carboxyl-substituted aromatic fragments was synthesized by polycondensation. Comparative analysis of the processes of formation of membranes with different morphologies based on these polymers under equal conditions was performed. New information was obtained about the influence of the amounts of carboxyl groups and the residual solvent on structural properties of asymmetric membranes. The influence of these factors on transport properties of dense membranes under pervaporation conditions was studied. It was demonstrated that in the case of carboxyl-containing poly(amide-imide)s, the domains formed during film preparation had a significant effect on membrane properties.

Pervaporation membranes of a simplex type with polyelectrolyte layers of chitosan and sodium hyaluronate.

Self-supporting multilayer films containing a polyelectrolyte complex (PEC) were prepared by the sequential layering of sodium hyaluronate (HA, MW 5.4 × 104) and chitosan (CS, MW 1.6 × 105, the degree of deacetylation 0.80) in different orders. Imaging with low-voltage scanning electron microscopy (LVSEM) showed that the CS/HA films had a multilayer structure, while X-ray diffraction (XRD) indicated significant structuring of the CS layer near the PEC-CS region. Analysis of the thermal properties of the CS/HA films revealed differences in the structural organization and morphological features of the polymer layers and high thermal stability of the PEC layer. Testing of the transport properties of the CS/HA film in pervaporation (PV) separation using different compositions of ethanol-water mixtures indicated that the multilayer membrane was selective across a wide range of concentrations in the feed. Separation of an azeotropic ethanol-water mixture containing 5 wt% water yielded a permeate consisting of about 100 wt% water. LVSEM revealed that the membrane microstructure changed during the PV process due to membrane swelling and changes in the arrangement of the macromolecules during transport of the penetrant. The results support the use of CS/HA composite films as highly effective PV membranes. In addition to pervaporation separation, CS/HA multilayer films can also be used for drug delivery, tissue engineering, and wound healing applications.

Pervaporation multilayer membranes based on a polyelectrolyte complex of λ-carrageenan and chitosan.

A polyelectrolyte complex (PEC) was prepared from chitosan (CS) and λ-carrageenan (λ-CAR) using a layer-by-layer deposition of polyion solutions on a plated nonporous support. This material was then used as a multilayer membrane for the pervaporation separation of aqueous ethanol solutions. The fabricated complex film (25-30µm thick) was a multilayer system (λ-CAR-PEC-CS) containing a polycation CS (MW 3.1×105, DDА 0.93), a polyanion λ-CAR (MW 3.5×105, extracted from the alga Chondrus armatus), and a PEC layer formed between the two polyion layers. X-ray diffraction indicated a significant structuring of the film in the region of the composite PEC-CS bilayer. The structural and morphological characteristics of the CS surface in the multilayer membrane, as revealed by atomic force microscopy, were close to the characteristics of the dense CS film. However, this structure changed following pervaporation (i.e., the distinct spherical structures on the surface disappeared). Similarly, the initially loose surface of λ-CAR in the composite changed to an ordered domain after pervaporation. The transport properties of the pervaporation membranes were tested by examining the separation of ethanol-water mixtures of different compositions. The flux increased with an increase in the weight percentage of water in the feed mixture, but the separation capacity of the membrane was unchanged. In a range of feed concentrations of 50-94wt%, the membrane mainly releases water with a corresponding concentration in the permeate of 99.9-99.8wt% and substantial fluxes of 0.003-1.130kgm-2h-1 at 40°C. The obtained results indicate significant prospects for the use of non-gelling type CARs for the formation of highly effective pervaporation membranes.

Two-Ply Composite Membranes with Separation Layers from Chitosan and Sulfoethylcellulose on a Microporous Support Based on Poly(diphenylsulfone-N-phenylphthalimide).

Two-ply composite membranes with separation layers from chitosan and sulfoethylcellulose were developed on a microporous support based on poly(diphenylsulfone-N-phenylphthalimide) and investigated by use of X-ray diffraction and scanning electron microscopy methods. The pervaporation properties of the membranes were studied for the separation of aqueous alcohol (ethanol, propan-2-ol) mixtures of different compositions. When the mixtures to be separated consist of less than 15 wt % water in propan-2-ol, the membranes composed of polyelectrolytes with the same molar fraction of ionogenic groups (-NH3⁺ for chitosan and -SO3- for sulfoethylcellulose) show high permselectivity (the water content in the permeate was 100%). Factors affecting the structure of a non-porous layer of the polyelectrolyte complex formed on the substrate surface and the contribution of that complex to changes in the transport properties of membranes are discussed. The results indicate significant prospects for the use of chitosan and sulfoethylcellulose for the formation of highly selective pervaporation membranes.