Janus Microgels Produced from Functional Precursor Polymers
"Janus Microgels Produced from Functional Precursor Polymers"
Langmuir 26 14842–14847 (2010).
Entry by Meredith Duffy, AP 225, Fall 2011
Janus particles, which exhibit contrasting properties of one or more kinds on opposite sides, have previously been synthesized with high monodispersity via droplet microfluidics. This system has consisted of simultaneously injecting two monomer streams into a microfluidic channel, periodically breaking the stream into droplets using a second phase, and immediately curing the droplets. As a result, particle formation and polymerization occur essentially at the same time, limiting the ability to independently tune the material properties and morphology of the particles.
Seiffert et al. present a new method for the production of Janus microgels by injecting solutions of preformulated crosslinkable polymers instead of monomers into the microchannel and curing them through polymer-analogous gelation, not chain-growth gelation. In doing so, they decouple polymerization and particle formation, allowing for greater control over the properties of the two halves of the particle. Not only is contrast greater between the two halves, due to the reduced mixing before curing of high molecular weight polymers compared to monomers, but additionally higher-order morphologies such as spherical capsules are made possible. The technique is demonstrated using both differentially dyed and magnetically polarized Janus microgels (Fig. 1a) and microcapsules (Fig. 1b) made from precursor PNIPAAm solutions.
Methods and Results
Polymer solutions of poly(N-isopropylacrylamide) (PNIPAAm) were prepared before injection. PNIPAAm was chosen for its ability to be functionalized in several ways including copolymerization and polymer-analogous modification, which is due to its solubility in a range of solvents both polar and nonpolar. Here, for proof of principle, they simply tagged two solutions with fluorescent dyes, red and green, and left a third untagged, intended for the center of the particle. All three polymer solutions were also mixed with a photocrosslinker. By injecting the three solutions simultaneously into side-by-side input channels, they generated the side-by-side laminar flow characteristic of microfluidics. Then, oil-surfactant solution was injected periodically on both sides of the channel via a cross-junction, pinching off monodisperse droplets of PNIPAAm. These were photo-gelated using focused UV light.
By varying the input flow rates of the outer polymer solutions relative to that of the inner one, they could tune the inner morphology of the gels (Figure 2), making the dyed polymers permeate almost to the center at high outer to inner flow rate ratio, or concentrating the dyes (or, by extension, other functionalizations of the polymer solutions) only on the outsides of the particle by inverting the ratio. Moreover, by inserting another oil phase between the dyed solutions in place of the undyed polymer, hollow Janus shells were produced (Figure 1b). These shells have high potential for encapsulation applications, among other uses.
Finally, magnetically polarized microgels were created by mixing magnetic nanobeads into the center PNIPAAm solution and subjecting the pinched-off polymer beads to a lateral magnetic field orienting the nanobeads before UV gelation. Under no magnetic field, the resultant microgels orient themselves randomly (Figure 5a), but when subjected to a field, gels exhibit identical orientation of the red and green halves with about 90% consistency (Figure 5b/c).
- The Janus microgels presented here, especially the dyed magnetic gels and the hollow gels, demonstrate the potential for Janus particles with more functionalities than just a single Janus property like color. In the magnetic gel case, the properties exhibited are just contrasting dyes and a permanent magnetic moment, but particles could potentially be formed with different surface structures, electric charges, chemical functionalizations and other modifications all at once. Likewise, by using a more functional Janus property than simply dye color and encapsulating a drug or reagent, the hollow gels could be used for drug delivery or other similar applications to selectively deliver material upon demand. Overall, this droplet microfluidics device takes advantage of the properties of emulsions and of polymer solutions/gels to produce novel soft materials.