Influence of pH-Responsive Monomer Content on the Behavior of Di-Block Copolymers in Solution and as Stabilizers of Pickering Latex Particle Emulsifiers

Abstract

In this study, diblock copolymers poly(methyl methacrylate)-block-poly (2-dimethylaminoethyl methacrylate) (pMMA-b-pDMAEMA) are investigated for the steric stabilization of latex particles and the subsequent use of these latex particles as Pickering emulsifiers. Solution properties of the diblock copolymers highlight that the pDMAEMA block length influences the critical micelle concentration (CMC) and micelle hydrodynamic diameter in response to changes in pH and the pK_a. The block length can also be used as a way to control the particle size of sterically stabilized polystyrene latex particles prepared via emulsion polymerization. The suspension properties of these latex particles are also presented. Emulsion studies using these latex particles as emulsifiers show that both continuous phase pH and electrolyte concentration affect emulsion stability to coalescence. At high pH, stable emulsions are formed due to the affinity of the particles to the interface. At low pH, protonation of the amine groups reduces the affinity and thus droplet coalescence is observed. Increasing the electrolyte concentration improves emulsion stability, but causes an increase in droplet size due to adsorption of flocculated/aggregated particles. Finally, it is shown that these latex particles can be used in conjunction with membrane emulsification techniques to produce emulsions with low polydispersity.

Keywords: Pickering emulsions; core-shell particles; membrane emulsification; pDMAEMA; pH-responsive polymer.

Figure 1

Variation in the diblock copolymer (or the formed micellar aggregates) hydrodynamic diameter as a function of polymer concentration for four different DMAEMA block lengths; (A) 20, (B) 54, (C) 108, and (D) 245. The polymer solutions are prepared at pH 4 in the presence of 0.01 M KNO₃ (T = 25°C).

Figure 2

Changes in (A) hydrodynamic diameter and (B) degree of polymer protonation as a function of solution pH for the four diblock copolymers prepared at a concentration of 1,000 ppm (above the CMC). The polymer solutions are prepared in the presence of 0.01 M KNO₃, (T = 25°C). The pK_aof the copolymers obtained are; DMAEMA₂₀ = 7.41, DMAEMA₅₄ = 7.16, DMAEMA₁₀₈ = 7.08, and DMAEMA₂₄₅ = 6.92.

Figure 3

Influence of pH and electrolyte concentration on the properties of particle dispersions obtained from emulsion polymerizations conducted in the presence of the different diblock copolymers (A) LP-DMAEMA₅₄, (B) LP-DMAEMA₁₀₈ and (C) LP-DMAEMA₂₄₅. Electrolyte concentrations used for the measurements were () 0 M (i.e., no added) KNO₃, () 0.01 M KNO₃, () 0.11 M KNO₃, and () 1 M KNO₃.

Figure 4

Hexadecane in water emulsions stabilized by (A) LP-DMAEMA₅₄, (B) LP-DMAEMA₁₀₈ and (C) LP-DMAEMA₂₄₅ as a function of pH. Digital micrographs of the emulsions 24 h after production (top) and the droplet diameter distribution based on volume % (bottom). No background electrolyte was added to the continuous phase and measurements were performed a 25°C.

Figure 5

Digital images (top) and droplet size data (bottom) for hexadecane in water emulsions stabilized by (A) LP-DMAEMA₅₄, and (B) LP-DMAEMA₁₀₈, and (C) LP-DMAEMA₂₄₅ as a function of pH in the presence of 0.01 M KNO₃. Measurements were performed at 25°C.

Figure 6

Variation in mean droplet size of hexadecane in water emulsions as a function of membrane rotation speed at 2 and 4 wt% particle loading of LP-DMAEMA₂₄₅. The continuous phase contained an electrolyte concentration of 0.01 M KNO₃ at pH 9. The oil injection rate was fixed at 0.01 mL min⁻¹.

Figure 7

Variation in mean droplet size of hexadecane in water emulsions as a function of oil injection rate at 2 and 4 wt% particle loading of LP-DMAEMA₂₄₅. The continuous phase contained an electrolyte concentration of 0.01 M KNO₃ at pH 9. The membrane rotation speed was fixed at 1,000 rpm.