Reversible aggregation of responsive polymer-stabilized colloids and the pH-dependent formation of porous scaffolds

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Entry by Pichet Adstamongkonkul, AP 225, Fall 2011

Reference:

Title: Reversible aggregation of responsive polymer-stabilized colloids and the pH-dependent formation of porous scaffolds

Authors: Robert T. Woodward, Christopher Hight, Ufuk Yildiz, Nicolas Schaeffer, Esther M. Valliant, Julian R. Jones, Molly M. Stevens, and Jonathan V. M. Weaver

Journal: Soft Matter, 2011, Vol. 7, No. 16

Keyword: colloid, emulsion, surfactant, steric stabilization, electrosteric stabilization, assembly


Summary

Fine tuning interactions of colloids, such as switching the properties in response to stimuli and on-demand disassembly, is a very interesting aspect in the field and can have many applications. An alternative strategy of polymeric particles synthesis, in contrast with the in situ syntheses in poor solvent or self-assembly, is the isolation of pre-formed polymer within localized microscopic domains and template removal of solvent to form colloids, the so-called emulsion-solvent-evaporation (ESE) process. In this process, surfactant molecules play an important part in stabilizing the initial emulsion templates. In this study, the authors synthesized a pH-responsive, branched, amphiphilic copolymers of hydrophilic poly(ethylene glycol) methacrylate (PEGMA) and poly(methacrylic acid) (PMA), and 1-dodecanethiol-derived (DDT) hydrophobic chain ends, to be used as a surfactant. It has been shown that the resulting oil in water emulsion is stable at basic pH by both steric and electrostatic stabilization of both PEGMA and MA residues. However, at acidic pH, these colloids assemble as hydrogen bonds form between MA and EG repeat units, not only on the same droplets, but also between droplets. The research also demonstrated the reversible transition between the dispersed and aggregated states, only by the change in pH, without significant deformation or degradation. The fabrication of compacted monolith out of a collection of colloid additionally showed that one can control the macroporosity of such scaffold by the method of drying. This may be suitable for medical applications.

General scheme.jpg

Background

Polymeric particles are good as building blocks since they can be prepared with controllable sizes and properties, such as functional groups, encapsulation and release profile. On the other hand, amphiphilic copolymers are convenient surfactants, with many advantage points: interfacial adsorption, self-assembly, encapsulation capability, low toxicity, tunable properties, greater functionality, and highly stabilize emulsions. The branched amphiphilic copolymers have been shown to be extremely efficient emulsifiers, in that the emulsions can have excellent long term stabilities.

Results and Discussion

The oil-in-water emulsion droplet is made of poly(methyl methacrylic acid) (PMMA)/ethyl acetate solutions and an aqueous solution of PEGMA/PMA branched copolymer as surfactants. The removal of ethyl acetate solution resulted in colloids with the surface coated with the surfactant molecules. The adsorption of these molecules thus gave the colloidal surface the responsive properties. The high stabilization of the emulsion comes from the multiple hydrophobic end-groups of the surfactant. The colloids were found to be smaller than the initial emulsion droplets, because of the removal of ethyl acetate, and also the interfacial surface area is significantly smaller, given that the numbers of droplets and colloids are constant.

When the pH of the solution decreases, it was found that there was an immediate and significant increase in volume-averaged diameter of the colloids. This can be attributed to the protonization of the carboxylic groups on PMA repeat units, enabling them to form hydrogen bonds between MA and EG units, pulling the colloids together. The reverse process is also possible, by adding base into the solution, and the resulting colloids showed no sign of significant deterioration. However, too high the level of salts in the system would presumably limit the reversibility.

Cycle aggregation.jpg

The ratio of EG:MA in the surfactant can be used to control the hydrogen bonding of the units on the same droplets and between droplets. The authors found that excess EG residues compromise the hydrogen-bonding interactions due to the persistence of an excess steric stabilization sheath. The experimental results supported this hypothesis, as there was no change in the average particle diameters in the EG-rich system upon addition of acid. The limitation seems to be on the interactions on the same colloid surface (intra-colloid bonding) rather than between colloids (inter-colloid bonding). This is in turn the indication of the successful translation of polymer properties from liquid to solid surfaces.

The higher-order assembly was also examined, through centrifugation of the colloids to form a monolith, with the interstitial domains filled with trapped water. The study reported the stability of the monolith in acidic environment, but rapid dissolution of colloid assembly upon exposure to basic condition. In this experiment, different layers of monolith were incorporated with different hydrophobic dyes. After dissolution of monolith in basic solution and re-acidification, the colloid aggregation was induced and, with centrifugation, formed a new monolith with randomly distributed populations of dyes throughout the structure.

Monolith.jpg

The authors further investigated the hydration states of the monolith. The water loss was accompanied by volume reduction of the monolith, as the internal voids collapse. These voids can be preserved by sublimation or freeze-drying technique. The surface areas of the air and freeze-dried monoliths were found to be very similar, as the accessible surface areas are likely defined by the colloid size and number. As a result, the macroporosity of the monolith can be varied by changing the method of drying.