Novel wholly aromatic polyamide-hydrazides: 6. Dependence of membrane reverse osmosis performance on processing parameters and polymer structural variations

Integrally skinned phase inversion membranes were successfully cast from dimethylacetamide solutions of a series of novel wholly aromatic polyamide-hydrazides for reverse osmosis performance. All the membranes were characterized for their salt rejection (percentage) and water permeability (cm3 cm−2 day−1) of 0.5 N aqueous sodium chloride feed solution at 3924 kPa operating pressure. The effect of polymer structural variations together with several processing parameters to achieve the best combination of high selectivity and permeability were discussed. The polymers structural variations were obtained by varying their para- and meta-oriented phenylene rings content. The latter was changed from 0 to 50 mol%. The processing variables included temperature and period of the solvent evaporation of the cast membranes, coagulation temperature of the thermally treated membranes and annealing of the coagulated membranes, casting solution composition, membrane thickness and the operating pressure. During the thermal treatment step the asymmetric structure of the membranes with a thin dense skin surface layer supported on a more porous layer was established. The former layer seems to be responsible for the separation performance. The results revealed that the membrane performance depended strongly on the conditions of its processing as well as the structure of the polymer from which it is cast. Under identical preparation condition, substitution of p-phenylene rings for m-phenylene ones within the polymer series resulted in an increase in salt rejection capability of the membranes. This may be attributed to an increase in their chain symmetry associated with increased molecular packing and rigidity through enhanced intermolecular hydrogen bonding. This produces a barrier with much smaller pores that would efficiently prevent the solute particles from penetration. For a given membrane, the higher the temperature and the longer the period of the solvent evaporation would result in a membrane of lower solvent content and with a thicker skin layer and consequently led to higher salt rejection at lower water permeability. Further, annealing in deionized water at 100°C produced membranes with optimum salt rejection. Upon annealing, the membrane shrinks resulting in decreasing its pore size particularly in the skin layer. This membrane morphology change improved the salt rejection. Addition of lithium chloride to the casting solution produced a membrane with increased porosity and improved its water permeability. The effects of coagulation temperature and thickness of the membrane on the separation efficiency were also discussed. The optimum salt separation of the membranes was attained at nearly 4000 kPa operating pressure. Membranes showed rejection up to 99.5% at water permeability 13 cm3 cm−2 day−1. © 1997 Elsevier Science Ltd.