Poly(acrylonitrile) ultrafiltration membranes. II. Membrane morphology and permeation characteristics

Abstract

The rheology and phase‐boundary characteristics of various solutions comprising three polyacrylonitrile (PAN) grades dissolved in solutions of N,N‐dimethylformamide + salt (LiCl, ZnCl2, or AlCl3) additives were correlated with the resulting membrane morphology as determined by microscopy and permeability measurements. The phase separation characteristics of the dope solution were not markedly affected by the PAN molecular weight (MW); however, they were affected by the salt additive. For higher MW grades, the effect of salt addition can also be masked by the increased self‐association tendency of the polymer chains. PAN‐B and ‐C membranes were clearly less asymmetric in structure than the lower MW PAN‐A–based membranes. This is attributed to the higher viscosity/lower diffusivity of the PAN‐B and ‐C solutions, which results in slower solvent–nonsolvent exchange during the phase inversion process. Two factors reduce the incidence of surface defects (increased bubble points): (a) higher solution viscosity dampens surface perturbations during phase inversion, and (b) phase inversion pathways resulting in more homogenous morphology lead to membranes with higher bubble points. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2074–2085, 2005

Effects of polymer molecular weight, concentration, and role of polyethylene glycol as additive on polyacrylonitrile homopolymer membranes

“…Effect of molecular weight of PAN is very scant in literature. Only one study on PAN copolymer is reported . In this study, effects of molecular weight of PAN homoplymer are studied using molecular weight of PAN, namely 50, 100, 150, and 250 kDa, at 15 wt%.…”

“…Effects of polyvinylpyrrolidone (PVP) and other organic and inorganic chemicals as additives on PAN are available. A summary of these studies is presented in Table . Jung et al studied the effect of molecular weight of PVP in PAN copolymer with dimethylsulfoxide (DMSO) as solvent and characterized the membranes in terms of morphology, flux, and rejection characteristics.…”

“…Effectiveness of the membranes was enhanced by treating with sodium hypochlorite solution that resulted higher water permeability compared to untreated membranes. Phadke et al prepared and characterized PAN UF membranes of three grades in N , N ‐dimethyl formamide (DMF) with salts like (LiCl, ZnCl 2 , or AlCl 3 ) as additives. It was found that the phase separation characteristics of the dope solution were not markedly affected by the PAN molecular weight (MW) but were affected by the additive.…”

“…Hence, various methods are introduced to modify the membrane surface to reduce fouling. These include blending , addition of inorganic salts , coating, adsorption , chemical grafting , radiation induced‐grafting with UV , plasma treatment , and immobilization of hydrophilic polymers or bio‐macromolecules . Practically, it is difficult to reduce the pore size of the PAN membranes by phase inversion technique due to its poor solubility in various solvents.…”

Phase behavior and mechanism of formation of protofiber morphology of solution spun poly(acrylonitrile) copolymers in DMF‐water system

“…Though molecular weight in the range tested above, did not seem to affect the phase diagram or the composition of the critical point, it can not be generalized for all molecular weights. Phase diagrams for polymers with very low molecular weights or for systems with very low polymer concentrations may have significant dependence on the changes in molecular weight 28, 29…”