Preparation of uniform micrometer-sized polymer particles with closed-cell porous architecture made by limited coalescence of a double emulsion

• Uniform micrometer-sized porous polymer particles with closed-cell pores.
• Particle size control: limited coalescence of a Pickering (W1/O/W2) double emulsion.
• Use of ionized hydrocolloid in W1 for colloidal stability of W1/O.
• High pressure homogenization of double emulsion without losing W1 features.
• Control of osmotic pressure in W1 and W2 to tailor size of pores and porosity.

We describe a new and practical process for making narrow size distribution polymer particles in the 4–20 μm size range that contain uniform multiple closed-cell pores with excellent simultaneous predictive regulation of micrometer and sub-micrometer sized features. Our approach involves making a water-in-oil-in-water (W1/O/W2) double emulsion through a sequence of controlled emulsification and droplet solidification steps with several differentiating aspects from the standard double emulsion method. Firstly, providing colloidal stability to the inner emulsion (W1/O) through the use of an ionized hydrocolloid in W1, where the oil phase (O) is a high molecular weight branched polyester in ethyl acetate. Secondly, control of particle size and size distribution using the limited coalescence (LC) process after high pressure homogenization of the water-in-oil-in-water (W1/O/W2) emulsion through an orifice plate, delivering rapid pressure drop, constant back pressure and large extensional deformation to break up the oil phase droplets of the W1/O/W2 premix without destroying the integrity of the inner emulsion. While Pickering double emulsions have been made, conventional wisdom regarding the robustness of such emulsions, has precluded high pressure homogenization required in the LC process to make <20 μm, narrow size distribution particles. In this work we present a material system and an emulsification technique that were co-designed to overcome this limitation. Finally, the ionized hydrocolloid enables manipulation of osmotic pressure of the inner and outer water phases across the oil membrane, tuning the size of the internal features from hundreds of nanometers to several microns while maintaining the closed-cell architecture of the final particle. While it is especially challenging to achieve such control in structurally complex microparticles, this paper demonstrates a simple, yet truly versatile, adaptable and scalable solution for making functional, closed-cell porous polymer particles. Representative images of whole and fractured porous particles are shown in the Graphical Abstract.

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