pH-Switchable W/O Polymer Emulsion: A Promising Strategy for Rapid Dissolution of Drag Reducers.

Additional hydrophilic surfactants are generally introduced into W/O emulsion drag reducer systems to enhance the dissolution capacity of polymers. The hydrophilic surfactants may decrease the stability of W/O emulsion, which leads to deterioration of polymer emulsions in the storage and transport process instead. Herein, a pH-switchable surfactant, N-(2-morpholinoethyl) oleamide (NMEO) was designed for stabilizing a W/O emulsion drag reducer. The surface activity and solubility changes occurring at pH < 6 of NMEO guaranteed the phase inversion from W/O to O/W of emulsions upon pH stimulation. Based on optimal conditions (oil-water ratio of 0.429, NMEO concentration of 3 wt%, and pH of 6.5), the inverse emulsion polymerization of poly(acrylamide--acrylic acid--2-acrylamide-2-methylpropane sulfonic acid) was proceeded to obtain a W/O polymer emulsion with the pH-switchable behavior. It was demonstrated that the polymer emulsions were provided with prolonged storage stability by NMEO and could be stored for at least 30 days due to the absence of hydrophilic surfactants. The polymers were released and completely dissolved within 2.5 min by pH stimulation, compared with traditional emulsion polymers and powder polymers that require 4 and 17 min, respectively. In addition, the emulsion drag reducer prepared by NMEO provided drag-reduction performance of 64.67% at 0.021 wt% concentration. The pH-switchable behavior of NMEO promotes the validity of W/O polymer emulsions along with the capacity of rapid release and solubilization, which eliminates the imbalance between the long-term storage stability and rapid solubility of traditional drag reducers. Thus, NMEO-stabilized emulsion drag reducers are expected to be a promising alternative for traditional products.