Polyacrylonitrile Layer Research Articles

Composite membranes with ultrathin separation layer prepared by self-assembly of polyelectrolytes

New composite membranes with ultrathin self-assembled polyelectrolyte separating layer and their use in gas, liquid and ion separation are described. Composite membranes were prepared by alternating electrostatic adsorption of polycations and polyanions on a porous PAN/PET supporting membrane (a polyethylenenterephthalate fleece coated with a thin layer of polyacrylonitrile). As the polycations poly(allylamine hydrochloride) (PAH), poly(ethylenimine) (PEI), poly(diallyldimethyl–ammonium chloride) (PDADMAC), poly(4-vinylpyridine) (P4VP) and chitosan (CHI) were used, the polyanion was always poly(styrene sulfonate sodium salt) (PSS). Studying CO 2 and N 2 permeation rates, selectivities for CO 2 ( α=1.5) were only found with P4VP/PSS separation layer. Studying ethanol–water pervaporation, separation factors α′ up to 80 were found, when the separation layer was made of PAH/PSS and annealed at T≥60°C. For non-annealed samples, the highest α′ value was 20. It was found for a separating membrane of PEI/PSS. Various effects of preparation and operation conditions on flux and separation factor are discussed. Studying permeation of NaCl and MgCl 2 in aqueous solution, a selective transport of the monovalent Na + was found, if the separation layer was PAH/PSS. The permeation rate for Na + was 15.1 times higher than for Mg 2+. Our study indicates that composite membranes with self-assembled polyelectrolyte separation layer are useful in materials separation, although individual polyions may exhibit a very different separation capability.

Concentration and desalting by membrane processes of a natural pigment produced by the marine diatom Haslea ostrearia Simonsen

The marine microalga Haslea ostrearia, also called ≪blue navicula⪢, presents the unique peculiar property among the diatoms, to produce at its extremities a blue hydrosoluble pigment called ≪marennine⪢. It is presented the concentration and the desalting of the exocellular pigment by membrane processes (ultrafiltration, nanofiltration, reverse osmosis). Nanofiltration is particularly developed given the potential of this type of application both for the concentration of molecules and for desalting. It is shown the effect of velocity and pressure on performances of nanofiltration membranes. Permeation flux superior to 100 l h −1 m −2 (at 14.10 5 Pa) are obtained with the Kiryat Weizmann membrane MP 20 (polyester coated with a polyacrylonitrile layer, cut-off 450 Da). For the desalting of the blue pigment solution, nanofiltration membranes present a few advantages: a low salt rejection (less than 10% at 14.10 5 Pa) and a high pigment rejection (the nanofiltration membrane MP 20 retains more than 95% of the pigment). This membrane used in diafiltration mode allows an acceptable speed of desalting (700 g of salt eliminated per hour and per m 2 at 25.10 5 Pa for a concentration of 18 g of salt per litre of solution).

Ultrathin self-assembled polyelectrolyte membranes for pervaporation

Ethanol–water pervaporation through new composite membranes with ultrathin self-assembled polyelectrolyte separating layer is described. The composite membranes were prepared by alternating electrostatic adsorption of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate sodium salt) (PSS) on a porous PAN/PET supporting membrane (a polyethylene terephthalate fleece coated with a thin layer of polyacrylonitrile). The sealing of the pores of the supporting membrane was studied by gas flow measurements. Pervaporation experiments were carried out under variation of the preparation and operation conditions. Generally it was found that the separation capability considerably increased, when the composite membrane was annealed at temperatures above 60°C, while the flux simultaneously decreased. The same was found, when the number of PAH/PSS layers was increased. Raising the pervaporation temperature led to both an increase of the flux and the separation factor. The highest separation factor of 70 was found at a low water content of the feed of 6.2% (w/w). The corresponding flux was 230 g m −2 h −1. Pervaporation was feasible up to a water content of 24% (w/w) in the feed. At higher values, hydrolysis set in resulting in partial desorption of the separating layer.

Ultrathin membranes for gas separation and pervaporation prepared upon electrostatic self-assembly of polyelectrolytes

Composite membranes with ultrathin separation layer were prepared by alternating electrostatic adsorption of cationic and anionic compounds (polyelectrolytes and bolaamphiphiles) at porous supporting membranes such as Celgard 2400, PAN/PET (a polyethyleneterephthalate fleece coated with a thin layer of polyacrylonitrile) and Isopore (an etched ion-track polycarbonate membrane). As the ionic compounds poly(allylamine hydrochloride) (PAH), poly(styrenesulfonate sodium salt) (PSS), 1.4-diketo-3.6-diphenylpyrrolo-[3,4- c]-pyrrole-4,4′-disulfonic acid and 10,12-docosadiyne-1,22-disulfate disodium salt were used. The composite membranes were studied on their gas permeation and pervaporation properties. The adsorption of 20 layer pairs of PAH/PSS at PAN/PET membranes was sufficient to reduce the argon flow to 7% of the initial value, while 60 bilayers reduced the flow to 0.1%. Gas flow rates of oxygen, nitrogen and argon were nearly identical, while the flow rate of carbon dioxide was higher by a factor of up to 2.4. Toluene/heptane pervaporation through composite membranes of 60 bilayers of PAH/PSS on PAN/PET gave a toluene enrichment from 20% in the feed to 24% in the permeate, while for ethanol/water permeation a water enrichment from 3% in the feed up to 50% in the permeate was found.

Elaboration of heat-treated thin films of polyacrylonitrile for connector application

An original alternative to the different terminal coatings classically used for the protection of low-level connectors is presented, involving thin films of polymer treated in order to have adequate electrical and tribological properties. The authors show that polyacrylonitrile layers a few hundred Angstroms thick, grafted on electroplated nickel and rapidly heated in a lamp furnace, lead to organic coatings well suited for the aimed application. The evolution of the molecular structure of the polymer has been investigated by IRRAS and XPS techniques. A large series of contact resistance and friction coefficient measurements have been performed in a sphere/plane situation to characterize each type of polyacrylonitrile (PAN) derivative in conditions quite near real ones. The results bring out very promising performances for some of the obtained materials. >

Electrical properties of very thin heat-treated polyacrylonitrile layers electropolymerized on nickel for contact application

Recent investigations concerning weakly pyrolysed thin films of polyacrylonitrile (PAN) grafted and grown by electropolymerization on a nickel underplate are reported, which clearly attest the potential interest of the obtained material for low-level connector coating application. The electrical characterization of these organic layers in a contact situation requires various precautions, principally the use of surfaces as smooth as possible in order to avoid piercing the film and therefore metal/metal junctions. An original device for static contact resistance measurements is presented, simulating the real case of industrial connector parts by a sphere/plane contact with an adjustable normal load. The tested samples consist of Ni-electroplated plane brass strips covered by PAN layers about 550 Å thick and heated at different temperatures in the range 350–500 °C for 10 or 30 s; the pressing sphere is a 3.2 mm diameter smooth golden ball. IRRAS and XPS spectra of raw and thermally treated samples reveal a series of drastic changes in the molecular structure of the polymer and the correlative evolution of the electrical properties of the films is unambiguously observed. The most promising results concern the 400 °C/30 s heated coating, for which stable and reproducible contact resistance values lower than 10 mΩ are found under a 50 gf normal load.

Metastable de-excitation spectroscopy and UV photoelectron spectroscopy study of polyacrylonitrile layers

An electropolymerized polyacrylonitrile layer (200 nm thick) was studied by metastable de-excitation spectroscopy (He 2 3S) and UV photoelectron spectroscopy (HeI). We show that this layer has both insulating and conducting properties. Its electronic properties and its dielectric behaviour change appreciably after reticulation and with temperature. The importance of the nitrile group in the understanding of these properties is emphasized.