Abstract
A facile method for the preparation of hierarchically porous spherical particles using high internal phase water-in-oil-in-water (w/o/w) double emulsions via the photopolymerization of the water-in-oil high internal phase emulsion (w/o HIPE) was developed. Visible-light photopolymerization was used for the synthesis of microspherical particles. The HIP emulsion had an internal phase volume of 80% and an oil phase containing either thiol pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) or trimethylolpropane tris(3-mercaptopropionate) (TMPTMP) and acrylate trimethylolpropane triacrylate (TMPTA). This enabled the preparation of microspheres with an open porous morphology, on both the surface and within the microsphere, with high yields in a batch manner. The effect of the thiol-to-acrylate ratio on the microsphere diameter, pore and window diameter, and degradation was investigated. It is shown that thiol has a minor effect on the microsphere and pore diameter, while the acrylate ratio affects the degradation speed, which decreases with increasing acrylate content. The possibility of free thiol group functionalization was demonstrated by a reaction with allylamine, while the microsphere adsorption capabilities were tested by the adsorption of methylene blue.
Highlights
Porous polymers are an increasingly researched class of materials due to their specific characteristics and wide applicability
In order to achieve kinetical stability of the high internal phase emulsion, which is necessary for retaining the unique morphology during the polymerization, the polarity of the aqueous phase is usually increased by the addition of salts
Thiol-ene photopolymerization was chosen as the polymerization mechanism as it has been shown to be efficient for the preparation of porous monolithic polyHIPE materials [32,33]
Summary
Porous polymers are an increasingly researched class of materials due to their specific characteristics and wide applicability. PolyHIPE materials possess a unique interconnected cellular porous macroscopic structure, which is the result of the formation of the precursor high internal phase emulsion. In such emulsions, droplets of the internal phase are packed in the closest possible proximity, while the surfactant molecules prevent both phase separation and phase inversion. When the polymerization of the continuous phase is initiated, and the viscosity and the density of the continuous phase increase, resulting in areas of high pressure at the thinnest points of droplet contact This results in polymer film rupture and the formation of the unique polyHIPE morphology, where primary pores (usually termed cavities or voids) are the result of the emulsion internal phase droplets, while the interconnecting pores (usually termed windows) are the result of the density changes during the polymerization. Suspension polymerization produces a wider size distribution of the microspheres, as the polymerizable phase is mixed with the suspension phase, which results in the formation of droplets of different sizes [21]
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