NOVEL WHOLLY AROMATIC POLYAMIDE-HYDRAZIDES .6. DEPENDENCE OF MEMBRANEREVERSE-OSMOSIS PERFORMANCE ON PROCESSING PARAMETERS AND POLYMER STRUCTURAL VARIATIONS

Integrally skinned phase inversion membranes were successfully cast fr om dimethylacetamide solutions of a series of novel wholly aromatic po lyamide-hydrazides for reverse osmosis performance. All the membranes were characterized for their salt rejection (percentage) and water per meability (cm(3) cm(-2) day(-1)) of 0.5 N aqueous sodium chloride feed solution at 3924 kPa operating pressure. The effect of polymer struct ural variations together with several processing parameters to achieve the best combination of high selectivity and permeability were discus sed. The polymers structural variations were obtained by varying their para-and meta-oriented phenylene rings content. The latter was change d from 0 to 50mol%. The processing variables included temperature and period of the solvent evaporation of the cast membranes, coagulation t emperature of the thermally treated membranes and annealing of the coa gulated membranes, casting solution composition, membrane thickness an d the operating pressure. During the thermal treatment step the asymme tric structure of the membranes with a thin dense skin surface layer s upported on a more porous layer was established. The former layer seem s to be responsible for the separation performance. The results reveal ed that the membrane performance depended strongly on the conditions o f its processing as well as the structure of the polymer from which it is cast. Under identical preparation condition, substitution of p-phe nylene rings for m-phenylene ones within the polymer series resulted i n an increase in salt rejection capability of the membranes. This may be attributed to an increase in their chain symmetry associated with i ncreased molecular packing and rigidity through enhanced intermolecula r hydrogen bonding. This produces a barrier with much smaller pores th at would efficiently prevent the solute particles from penetration. Fo r a given membrane, the higher the temperature and the longer the peri od of the solvent evaporation would result in a membrane of lower solv ent content and with a thicker skin layer and consequently led to high er salt rejection at lower water permeability. Further, annealing in d eionized water at 100 degrees C produced membranes with optimum salt r ejection. Upon annealing, the membrane shrinks resulting in decreasing its pore size particularly in the skin layer. This membrane morpholog y change improved the salt rejection, Addition of lithium chloride to the casting solution produced a membrane with increased porosity and i mproved 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 a t nearly 4000 kPa operating pressure. Membranes showed rejection up to 99.5% at water permeability 13 cm(3) cm(-2) day(-1). (C) 1997 Elsevie r Science Ltd.