title: Microfluidic approach to the formation of internally porous polymer particles by solvent extraction.
abstract: We report the controlled formation of internally porous polyelectrolyte particles with diameters ranging from tens to hundreds of micrometers through selective solvent extraction using microfluidics. Solvent-resistant microdevices, fabricated by frontal photopolymerization, encapsulate binary polymer (P)/solvent (S1) mixtures by a carrier solvent phase (C) to form plugs with well-defined radii and low polydispersity; the suspension is then brought into contact with a selective extraction solvent (S2) that is miscible with C and S1 but not P, leading to the extraction of S1 from the droplets. The ensuing phase inversion yields polymer capsules with a smooth surface but highly porous internal structure. Depending on the liquid extraction time scale, this stage can be carried out in situ, within the chip, or ex situ, in an external S2 bath. Bimodal polymer plugs are achieved using asymmetrically inverted T junctions. For this demonstration, we form sodium poly(styrenesulfonate) (P) particles using water (S1), hexadecane (C), and methyl ethyl ketone (S2). We measure droplet extraction rates as a function of drop size and polymer concentration and propose a simple scaling model to guide particle formation. We find that the extraction time required to form particles from liquid droplets does not depend on the initial polymer concentration but is rather proportional to the initial droplet size. The resulting particle size follows a linear relationship with the initial droplet size for all polymer concentrations, allowing for the precise control of particle size. The internal particle porous structure exhibits a polymer density gradient ranging from a dense surface skin toward an essentially hollow core. Average particle porosities between 10 and 50% are achieved by varying the initial droplet compositions up to 15 wt % polymer. Such particles have potential applications in functional, optical, and coating materials.
authors.affiliation: Department of Chemical Engineering, Imperial College London , London SW7 2AZ, U.K.
authors.forename: Takaichi
authors.initials: T
authors.lastname: Watanabe
authors.forename: Carlos
authors.initials: C
authors.lastname: G Lopez
authors.forename: Jack F
authors.initials: JF
authors.lastname: Douglas
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Ono
authors.forename: João T
authors.initials: JT
authors.lastname: Cabral
mesh_terms: D000473:Alkanes / Q000737:chemistry*; D002074:Butanones / Q000737:chemistry*; D004573:Electrolytes / Q000138:chemical synthesis / Q000737:chemistry / Q000302:isolation & purification; D046210:Microfluidic Analytical Techniques; D010316:Particle Size; D011137:Polystyrenes / Q000138:chemical synthesis* / Q000737:chemistry / Q000302:isolation & purification; D016062:Porosity; D012997:Solvents / Q000737:chemistry; D013499:Surface Properties; D014867:Water / Q000737:chemistry*
chemical_list: D000473:Alkanes; D002074:Butanones; D004573:Electrolytes; D011137:Polystyrenes; D012997:Solvents; D014867:Water; C005222:methylethyl ketone; C003321:polystyrene sulfonic acid; C007932:n-hexadecane