Viscosity Behavior of Microwave-Heated and Conventionally Heated Poly(ether sulfone)/ Dimethylformamide/Lithium Bromide Polymer Solutions Ani Idris, Iqbal Ahmed Department of Bioprocess Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia Received 18 June 2007; accepted 15 October 2007 DOI 10.1002/app.27590 Published online 27 December 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: This article investigates the viscosity behav- ior of new membrane dope solutions of poly(ether sulfone) (PES) and dimethylformamide with low-molecular-weight halogenated lithium bromide (LiBr) additives prepared with two different techniques: (1) a microwave (MW) tech- nique and (2) a conventional heating (CH) technique. In addition, the influence of different concentrations of anhy- drous halogenated LiBr additives (0–5 wt %) on the viscos- ity behavior is analyzed. The viscosity of the dope solu- tions was assessed with a conical rheometer equipped with a high-viscosity adapter. The results revealed that the pure PES solutions prepared by the MW and CH techniques exhibited pseudoplastic and Newtonian behavior, respec- tively. Both the MW and CH PES solutions containing the LiBr additives exhibited dilatant behavior, which obeyed the power law. The apparent viscosity of all the dope solu- tions prepared with the MW technique was lower than that of those prepared with the CH technique. Ó 2007 Wiley Peri- odicals, Inc. J Appl Polym Sci 108: 302–307, 2008 Key words: additives; halogenated; membranes; poly (ether sulfones); viscosity INTRODUCTION The introduction of a suitable additive to a casting solution is a convenient and efficient method of increasing the hydrophilic property of a membrane, and the compatibility of polymers with additives can often play a decisive role in the physical modifica- tion of fiber-forming polymers. The additives may be water, inorganic salts, low-molecular-weight organics, surfactants, polymers, mineral fillers, or blends of all of these. There are several mechanisms through which such additives can affect the final membrane properties, 1–6 and rheometry has been shown to be a very sensitive technique for detecting physical and chemical changes in these blends. 7 The presence of the additives in the dope not only changes the thermodynamic state of the dope but also influences the conformation and dynamics of the polymer, which in turn affects the kinetics of phase separation. 8 However, the latter is seldom mentioned in the literature. Nevertheless, the rheo- logical properties of multicomponent polymer sys- tems are related closely to the interaction between the components and the phase configuration; the rheological response can reflect a change in the con- formation of macromolecules exactly. 9 In a mem- brane-forming system, rheological properties are significantly affected by the quality of the solvent mixture, 10 which is usually degraded by additives in dimethylformamide (DMF). Although DMF is a dipolar, nonaqueous compound representative of amidic solvents, because of their ability to modify the potential reactivity of reacting states in electron- and proton-transfer reactions, these amides are widely used in settings such as solvent–reactivity relationships. DMF as a pure solvent is certainly to some extent associated by means of a nonspecific dipole–dipole interaction, and it is of particular in- terest because significant structural effects are absent on account of the lack of hydrogen bonds. Therefore, it may work as an aprotic, protophilic solvent of a large dipole moment (3.24 D) and a high dielectric constant (36.70) at 298.15 K. 11 Besides that, poly (ether sulfone) (PES) is an important polymeric mem- brane material because of its chemical resistance, mechanical strength, thermal stability, and transport properties, 12 with a low dielectric loss or tangent loss (0.0022 at 1 Hz). A change in the rheological properties of the dope, including a change in the viscosity, can influ- ence the nonsolvent–solvent exchange, the velocity of phase separation, and the gelation dynamics. 10 To some extent, the competition between the three Correspondence to: A. Idris (ani@fkkksa.utm.my). Contract grant sponsor: Ministry of Science, Technology, and Environment (through Intensification Research in Pri- ority Areas (IRPA) funding vote 79037). Journal of Applied Polymer Science, Vol. 108, 302–307 (2008) V V C 2007 Wiley Periodicals, Inc.