Wholly aromatic polyamide‐hydrazides. V. Preparation and properties of semipermeable membranes from poly[4‐(terephthaloylamino) benzoic acid hydrazide]

Effects of several processing parameters on the properties of poly[4‐ (terephthaloylamino) benzoic acid hydrazide] semipermeable membranes were investigated. The membranes were prepared using a polymer phase inversion method that consisted of the following sequence of steps: casting of thin, neutralized polymer solution layers on suitable solid support; thermal treatment of the obtained layers to form viscous liquid protomembranes; coagulation of the protomembranes into solid semipermeable membranes; and, when that was additionally desired, posttreatment of the obtained membranes in order to further affect their final separation properties. The processing parameters investigated included temperature and time of exposure to thermal treatment during the preparation of protomembranes, nature of the employed medium, time of exposure, and temperature of the posttreatment of coagulated membranes. In addition, particular attention was devoted to the effect of polymer molecular weight on the properties of the membranes, and for that purpose a series of five polymer samples with molecular weights that ranged from about 17,000 to about 64,500 was prepared using the previously described viscometrical method to monitor the polymer forming polycondensation reactions. The results obtained showed that membrane performance was very much influenced by all of the examined processing variables and that products with considerably different properties could be obtained from the same polymer sample by using different processing parameters. Thus, the use of higher temperatures and longer exposure times in the protomembrane forming thermal treatment step gave higher rejection and lower permeability of the finished membranes, although there seems to exist limiting “ceiling conditions” above which this trend rather abruptly ceases to continue any further. It appears that these conditions correspond to the onset of irreversible changes, either in the polymer chemical structure or in the membrane supermolecular organization. Posttreatment of the prepared membranes was found essential for useful properties in the single‐stage separation applications, which require high membrane selectivity. Of the investigated possibilities, annealing in boiling deionized water was crucial, while exposure to concentrated acids was also important in the case of HCl but rather deleterious in the case of H 2 SO 4 . Most significantly, the membrane properties clearly depended on the polymer molecular weight, and it was found that there was a critical polymer molecular weight above which this dependence ceased to be important for practical purposes. It is suggested that this may be a general phenomenon, characteristic for all membrane forming polymer materials, and a possible explanation of its nature is tentatively offered.