Semipermeable Elastic Microcapsules for Gas Capture and Sensing

Abstract

Monodispersed microcapsules for gas capture and sensing were developed consisting of elastic semipermeable polymer shells of tuneable size and thickness and pH-sensitive, gas selective liquid cores. The microcapsules were produced using glass capillary microfluidics and continuous on-the-fly photopolymerisation. The inner fluid was 5-30 wt% K 2 CO 3 solution with m-cresol purple, the middle fluid was a UV-curable liquid silicon rubber containing 0-2 wt% Dow Corning ® 749 fluid, and the outer fluid was aqueous solution containing 60-70 wt% glycerol and 0.5-2 wt% stabiliser (polyvinyl alcohol, Tween 20 or Pluronic ® F-127). An analytical model was developed and validated for prediction of the morphology of the capsules under osmotic stress based on the shell properties and the osmolarity of the storage and core solutions. The minimum energy density and UV light irradiance needed to achieve complete shell polymerisation were 2 J•cm -2 and 13.8 mW·cm -2 , respectively. After UV exposure, the curing time for capsules containing 0.5 wt% Dow Corning ® 749 fluid in the middle phase was 30-40 min. The CO 2 capture capacity of 30 wt% K 2 CO 3 capsules was 1.6-2 mmol/g depending on the capsule size and shell thickness. A cavitation bubble was observed in the core when the internal water was abruptly removed by capillary suction, whereas a gradual evaporation of internal water led to buckling of the shell. The shell was characterised using TGA, DSC, and FTIR. The shell degradation temperature was 450-460°C.

Deformation and water loss from solvent filled microcapsules under compressive loads

“…The capsules had an average density, ρ s = 1164 kg/m 3 , and diameter, d = 480 μm. The capsules were fabricated with a ratio of shell volume to total capsule volume, , resulting in a shell thickness of L ≅ 30 μm . The mechanical properties of the shell material (Tego Rad™ 2650) were not measured but are assumed to be the same as values reported for Semicosil™, another polymer used for encapsulation ( E = 150 kPa, ν = 0.5). The shell's permeability to water is .…”

“…The third term inside the parenthesis, p shell , is the internal pressure caused by stretching or buckling of the elastic shell material as the capsule's internal volume changes (due to mass transfer). In our earlier work this term was modeled using the theory of Nabavi et al for inflated spherical shells, and the semi‐empirical numerical simulation results of Quilliet for buckled capsules. This allowed us to represent the effect of both capsule hydration (volume increase) and dehydration (volume decrease) on mass transfer rates.…”

“…Several groups have now demonstrated the ability to manufacture small (100–600 μm diameter), solvent filled capsules using capillary microfluidic devices . The polymers used for the thin (10–50 μm) capsule shells are highly elastic after being UV cured, with Young's modulus and are also highly permeable to water. The former allows them to survive osmotic swelling to four times their original diameter and compressive strains up to 80% without rupture.…”

“…The polymers used for the thin (10–50 μm) capsule shells are highly elastic after being UV cured, with Young's modulus and are also highly permeable to water. The former allows them to survive osmotic swelling to four times their original diameter and compressive strains up to 80% without rupture. Such impressive deformability may be considered an asset in the lab, but it does raise concerns about the suitability of microcapsules in industrial scale absorbers.…”

“…Each packed bed was discretized into N y = 100 layers along the vertical axis, and an equal number of capsules were assumed to exist in each layer. The compressive force acting on a capsule in each layer was determined by Equations (3),(5), and(6). This force was then used as input to Taber's capsule deformation theory (Supporting information), which provided values of the internal pressure and capsule deformation.…”