Shell Formation Mechanism for Direct Microencapsulation of Nonequilibrium Pure Polyamine Droplet

Abstract

The understanding of the shell formation mechanism for the novel encapsulation technique via integrating microfluidic T-junction and interfacial polymerization is not only important to fabricate high-quality polyamine microcapsules but also of practical and theoretical significance to the wide application of this method based on nonequilibrium droplets. Herein, using pure polyamine as a targeting core, the shell formation mechanism was investigated by studying the achieved shell structures of the preliminary and final microcapsules and the behavior of nonequilibrium polyamine droplets in the coflow solvent. It reveals the shell has a multilayered structure, i.e., a rough outer layer, a porous middle layer, and a thin but dense inner layer, and the porous middle layer consists of pores with two size levels. This shell structure was correlated to fractal geometry of the polyamine droplet before being encapsulated, which was generated attributed to the interactions between the nonequilibrium polyamine droplet and the coflow solvent, i.e., interdiffusion and spontaneous emulsification. To achieve robust microcapsules, shell thickness and tightness were also studied by varying the composition of the reaction solution in terms of diisocyanate concentration and type of solvent with different polarity. The effect of these two key parameters on shell in this method is very similar to that in the traditional interfacial polymerization. In addition, the influence of the shell-forming stage and shell-growth stage on the robustness of microcapsule was discussed, indicating the former is decisive.

Fabrication and Property Regulation of Small-Size Polyamine Microcapsules via Integrating Microfluidic T-Junction and Interfacial Polymerization

“…It can be seen that the microcapsule only has a thin, smooth inner wall and a thick rough outer wall, with the absence of the porous middle layer. This is consistent with our previous results when pure TEPA was encapsulated [ 36 ]. With decreasing the droplet size for the polyamine, the Laplace pressure exerted on the droplet increases, suppressing the self-emulsification of the polyamine and the generation of the secondary polyamine micro-droplets around.…”

“…However, it also reduces the production efficiency of the microencapsulation technique to fabricate microcapsules. According to our previous studies, [ 36 , 37 ] the use of smaller tubing to feed polyamines can also generate smaller droplets and thus prepare small-size polyamine microcapsules. Therefore, in this investigation, the T-junction T0/T2, manufactured using Teflon tubing T0 with a smaller I.D.…”

“…It can be seen from Figure 2 that the outer wall of the prepared microcapsules is thick, rough, and fluffy when the optimized condition for preparing large-size microcapsules was used [ 36 , 40 ]. In addition, the microcapsule collection contains more debris.…”

“…According to our previous research, [ 36 ] the shell-forming monomer (HMDI) in the reaction solution significantly influences the thickness and compactness of the microcapsule shell. While a lower concentration of HMDI causes the microcapsule shell to be too thick and loose, a higher one results in a thinner and too dense shell, which cannot completely cover the secondary microcapsules formed by self-emulsification.…”

“…The fabrication of small-size polyamine microcapsules was carried out using the technique via integrating microfluidic T-junction and interfacial polymerization, which was successfully demonstrated in our group previously [ 10 , 36 ]. Briefly, the co-flow solvent, C16 with 1.0 wt% surfactant Arlacel P135, and the polyamine mixture, 25 wt% TEPA and 75 wt% JEFFAMINE T403 (25TEPA75T403), were separately injected through the T-junction unit at feeding rates of 15.0 mL/h and V mL/h, respectively using two syringe pumps (Lead Fluid, TYD02) to generate polyamine droplets, as shown in Figure 1 .…”