Redispersible Polymer Colloids Using Carbon Dioxide as an External Trigger

CO2-Switchable colloids

The development and design of CO2-switchable colloidal particles is described. A presentation of the principles of CO2 switching, especially as they apply to colloids, is followed by recent progress in the preparation of several types of colloidal particles (polymer nanoparticles, metal-organic frameworks (MOFs), quantum dots, graphene, cellulose nanocrystals, carbon nanotubes) for various applications (Pickering stabilizers, catalysts, latexes), and our perspective on future opportunities.

Polymer Colloids: Current Challenges, Emerging Applications, and New Developments

Polymer colloids are complex materials that have the potential to be used in a vast array of applications. One of the main reasons for their continued growth in commercial use is the water-based emulsion polymerization process through which they are generally synthesized. This technique is not only highly efficient from an industrial point of view but also extremely versatile and permits the large-scale production of colloidal particles with controllable properties. In this perspective, we seek to highlight the central challenges in the synthesis and use of polymer colloids, with respect to both existing and emerging applications. We first address the challenges in the current production and application of polymer colloids, with a particular focus on the transition toward sustainable feedstocks and reduced environmental impact in their primary commercial applications. Later, we highlight the features that allow novel polymer colloids to be designed and applied in emerging application areas. Finally, we present recent approaches that have used the unique colloidal nature in unconventional processing techniques.

CO2-Responsive Water-Soluble Conjugated Polymers for In Vitro and In Vivo Biological Imaging

Water-soluble conjugated polymers (WCPs) composed of a hydrophobic polythiophene main chain with hydrophilic tertiary amine side-chains can directly self-assemble into sphere-like nano-objects in an aqueous solution due to phase separation between the hydrophilic and hydrophobic segments of the polymeric structure. Due to the presence of gas-responsive tertiary amine moieties in the spherical structure, the resulting polymers rapidly and reversibly tune their structural features, surface charge, and fluorescence performance in response to alternating carbon dioxide (CO₂) and nitrogen (N₂) bubbling, which leads to significantly enhanced fluorescence and surface charge switching properties and a stable cycle of on and off switching response. In vitro studies confirmed that the CO₂-treated polymers exhibited extremely low cytotoxicity and enhanced cellular uptake ability in normal and tumor cells, and thus possess significantly improved fluorescence stability, distribution, and endocytic uptake efficiency within cellular organisms compared to the pristine polymer. More importantly, in vivo assays demonstrated that the CO₂-treated polymers displayed excellent biocompatibility and high fluorescence enhancement in living zebrafish, whereas the fluorescence intensity and stability of zebrafish incubated with the pristine polymer decreased linearly over time. Thus, these CO₂ and N₂-responsive WCPs could potentially be applied as multifunctional fluorescent probes for in vivo biological imaging.

Tuning CO2-induced reversible redispersion or irreversible destabilisation for latex separation

HYPOTHESIS

The CO₂-sensitive dispersion/precipitation transition of polymer latexes fabricated based on a responsive emulsifier is a promising way to conveniently acquire bulk polymer materials. Nevertheless, the tedious synthesis procedures for switchable surfactants and the harsh operating requirements for the sensitive latexes constrain the applicability of the approach for latex preparation. Therefore, a new strategy for generating latexes with tunable CO₂ responsiveness in a maneuverable way is urgently needed.

EXPERIMENTS

In this work, a CO₂-switchable electrostatic interaction is introduced to construct responsive latexes. A series of lightly crosslinked poly(diethylaminoethyl methacrylate-styrene) [P(DEA-St)] latexes with different PDEA contents were fabricated via one-pot emulsion copolymerization, with divinylbenzene and sodium dodecylsulfate (SDS) used as the crosslinker and anionic emulsifier, respectively. The influence of the DEA feeding ratio on the resulting P(DEA-St) colloids was characterized by Fourier transform infrared spectroscopy and scanning electron microscopy. Then, a cyclic CO₂/N₂ input was introduced to verify the response transitions of polymer latexes.

FINDINGS

Accompanied by the stepwise decrease of DEA feeding ratio, the morphology of the resulting copolymerized nanoparticles changed from an ambiguous bulge to the typical spherical pattern. In addition, the P(DEA-St) latexes treated by cyclic CO₂/N₂ exhibit four different types of response modes, namely (i) CO₂-switchable swelling/deswelling transition, (ii) CO₂-reversible dispersion/coagulation transition, (iii) CO₂-induced irreversible destabilisation and (iv) CO₂-insensitive latexes. The CO₂-responsive destabilisation is highly applicable in the separation and transportation fields of commercial latex products, such as poly(methyl methacrylate), poly(n-butyl acrylate) and poly(butyl methacrylate) colloids.

Light and latex: advances in the photochemistry of polymer colloids

Unparalleled temporal and spatial control of colloidal chemical processes introduces immense potential for the manufacturing, modification, and manipulation of latex particles.

CO2 activation through C–N, C–O and C–C bond formation

CO₂ fixation reactions have been explored using different organic substrates upon activation with a superbase.

Dual CO2/temperature-responsive diblock copolymers confer controlled reversible emulsion behavior

Dual-stimuli responsive diblock copolymers possessing unique temperature-sensitive and CO₂/N₂-switching ability were successfully developed to promote efficient manipulation of reversible emulsification processes.

Gas-Responsive Polymers

Gas-responsive polymers have inspired much interest over the past ten years. Gas triggers can interact with functionalities on polymer chains and, thus, modulate their chain structures, architectures, and aggregation states. This review summarizes the latest research progresses in the theme of developing different gas triggers for fine control over some critical properties of polymers, as well as their potential applications in various areas. We focus on the interactions/reactions between gases and gas-responsive functionalities of polymers and highlight some state-of-art developments, which provided good insight and understanding of each particular gas-responsive polymer. We also offer a perspective point of view on future research directions on gas-responsive polymers, both in fundamental studies and in potential application developments.

pH Switchable Emulsions Based on Dynamic Covalent Surfactants

Dynamic covalent surfactants were designed to prepare pH switchable emulsions. A dynamic covalent bond between nonamphiphilic building blocks (polyethylenimine (PEI) and benzaldehyde (B)) was introduced to form the dynamic covalent surfactant PEI-B. The dynamic nature of covalent bond in PEI-B was confirmed by ¹H NMR and fluorescence probe analysis. Stable emulsions were successfully prepared with interfacial active PEI-B at pH 7.8 with various water/paraffin oil ratios under sonication. When lowering the pH to 3.5, a complete phase separation was observed as a result of breaking dynamic covalent bond in the interfacial active PEI-B. After tuning the pH back to 7.8, stable emulsion was obtained again due to the reformation of the dynamic covalent bond and hence interfacial active PEI-B. The emulsification and demulsification were dependent on the formation and breaking of dynamic covalent bond in PEI-B. Such pH-triggered emulsification and demulsification can be switched at least three times. Application of dynamic covalent surfactants will open up a novel route for preparing responsive emulsions.

CO2-Responsive fluorescent hyperbranched poly(ether amine)s

Hyperbranched polymers bearing a fluorophore display gas-controlled fluorescence switching as a result of reversible protonation/deprotonation of amino groups in their structure.

CO2-Responsive polymer materials

This paper reviews the chemical fundamentals of CO₂-responsive polymers as well as the latest reported “smart” material systems switched by CO₂.

CO2/N2 triggered switchable Pickering emulsions stabilized by alumina nanoparticles in combination with a conventional anionic surfactant

Switchable n-decane-in-water Pickering emulsions were prepared using positively charged alumina nanoparticles in combination with a trace amount of the anionic surfactant sodium dodecyl sulfate (SDS) and equal moles of a CO₂/N₂ switchable surfactant.

Pickering Emulsions Responsive to CO2/N2 and Light Dual Stimuli at Ambient Temperature

A dual stimuli-responsive n-octane-in-water Pickering emulsion with CO2/N2 and light triggers is prepared using negatively charged silica nanoparticles in combination with a trace amount of dual switchable surfactant, 4-butyl-4-(4-N,N-dimethylbutoxyamine) azobenzene bicarbonate (AZO-B4), as stabilizers. On one hand, the emulsion can be transformed between stable and unstable at ambient temperature rapidly via the N2/CO2 trigger, and on the other hand, a change in droplet size of the emulsion can occur upon light irradiation/rehomogenization cycles without changing the particle/surfactant concentration. The dual responsiveness thus allows for a precise control of emulsion properties. Compared with emulsions stabilized by specially synthesized stimuli-responsive particles or by stimuli-responsive surfactants, the method reported here is much easier and requires a relatively low concentration of surfactant (≈1/10 cmc), which is important for potential applications.

Oxygen and Carbon Dioxide Dual Gas-Switchable Thermoresponsive Homopolymers

Herein, we report the first oxygen (O₂) and carbon dioxide (CO₂) dual gas-switchable thermoresponsive polymers based on a newly synthesized monomer N-(2-fluoroethyl amide)-N-(2-(diethylamino)ethyl) acrylamide, that is, AM(F1EA-DEAE), which bears both O₂-switchable fluorinated ethyl amide (F1EA) and CO₂-switchable N,N-diethylamino ethyl (DEAE) moieties on its side chain. PolyAM(F1EA-DEAE) samples prepared from reversible addition-fragmentation chain transfer (RAFT) polymerization exhibited good temperature-responsive properties. Their inherent low critical solution temperature (LCST) could be reversibly tuned to different levels by respectively purging O₂ or CO₂ into its aqueous solution. The O₂-treatment shifted LCST to a higher temperature, while the CO₂-treatment made the polymer fully water-soluble. The polymer could be readily recovered to its initial state by washing off the trigger gas with an inert gas such as nitrogen. This work provides an effective monomer design approach for the preparation of O₂ and CO₂ dual gas-responsive polymers.

CO2-responsive polymeric materials: synthesis, self-assembly, and functional applications

CO2 is an ideal trigger for switchable or stimuli-responsive materials because it is benign, inexpensive, green, abundant, and does not accumulate in the system. Many different CO2-responsive materials including polymers, latexes, solvents, solutes, gels, surfactants, and catalysts have been prepared. This review focuses on the preparation, self-assembly, and functional applications of CO2-responsive polymers. Detailed discussion is provided on the synthesis of CO2-responsive polymers, in particular using reversible deactivation radical polymerization (RDRP), formerly known as controlled/living radical polymerization (CLRP), a powerful technique for the preparation of well-defined (co)polymers with precise control over molecular weight distribution, chain-end functional groups, and polymer architectural design. Self-assembly in aqueous dispersed media is highlighted as well as emerging potential applications.

Oxygen-switchable thermo-responsive random copolymers

In this paper, we report the synthesis of oxygen (O₂)-switchable thermo-responsive random copolymers based on fluorinated acrylamide monomer homologues: N-(2-fluoroethyl)acrylamide (F1EA), N-(2,2-difluoroethyl)acrylamide (F2EA), and N-(2,2,2-trifluoroethyl)acrylamide (F3EA).

Responsive Aqueous Foams Stabilized by Silica Nanoparticles Hydrophobized in Situ with a Conventional Surfactant

In the recent past, switchable surfactants and switchable/stimulus-responsive surface-active particles have been of great interest. Both can be transformed between surface-active and surface-inactive states via several triggers, making them recoverable and reusable afterward. However, the synthesis of these materials is complicated. In this paper we report a facile protocol to obtain responsive surface-active nanoparticles and their use in preparing responsive particle-stabilized foams. Hydrophilic silica nanoparticles are initially hydrophobized in situ with a trace amount of a conventional cationic surfactant in water, rendering them surface-active such that they stabilize aqueous foams. The latter can then be destabilized by adding equal moles of an anionic surfactant, and restabilized by adding another trace amount of the cationic surfactant followed by shaking. The stabilization-destabilization of the foams can be cycled many times at room temperature. The trigger is the stronger electrostatic interaction between the oppositely charged surfactants than that between the cationic surfactant and the negatively charged particles. The added anionic surfactant tends to form ion pairs with the cationic surfactant, leading to desorption of the latter from particle surfaces and dehydrophobization of the particles. Upon addition of another trace amount of cationic surfactant, the particles are rehydrophobized in situ and can then stabilize foams again. This principle makes it possible to obtain responsive surface-active particles using commercially available inorganic nanoparticles and conventional surfactants.

Oxygen and carbon dioxide dual gas-responsive and switchable microgels prepared from emulsion copolymerization of fluoro- and amino-containing monomers

We report herein the design and preparation of microgels that are responsive to both O2 and CO2 gases. The microgels were synthesized through soap-free emulsion copolymerization of O2-responsive monomer 2,3,4,5,6-pentafluorostyrene (FS) and CO2-responsive monomer 2-(diethylamino)ethyl methacrylate (DEA) with N,N'-methylenebis(acrylamide) (BisAM) as the cross-linker. The P(DEA-co-FS) microgels dispersed in aqueous solution could undergo volume phase transitions triggered by O2 and/or CO2 aeration. The particles were very responsive to CO2, while their responsivity to O2 was moderate. Microgels having different levels of the responsivity could be designed and prepared by varying the FS content in the copolymer. The phase transitions were also highly reversible, and the initial states of microgels could be easily recovered by "washing off" the trigger gases with N2. Multicycle O2, CO2, and N2 aerations were applied, and no loss in the dual gas responsivity and switchability was observed.

Morphology and size control of calcium carbonate crystallized in a reverse micelle system with switchable surfactants

Sub-micron CaCO₃ particles with different morphologies were prepared in reverse micelles based on a CO₂/N₂ switchable surfactant (N′-dodecyl-N,N-dimethyl acetamidine bicarbonate).

Synthesis and Properties of a Series of CO2 Switchable Gemini Imidazolium Surfactants

Novel switchable gemini imidazolium surfactants with different carbon atoms in hydrophobic group were successfully synthesized by condensation of fatty acid with triethylene tetramine, then the intermediates were reacted with CO2 to give the imidazolinium bicarbonates. The structures of intermediates and products were identified by IR and 1H-NMR spectra. As the results show, the structures of the products obtained correspond to the target compounds designed. By surface tension measurements, these Gemini surfactants have excellent surface activity with low cmc and surface tension. The conductivity and surface tension cycles show that these surfactants could be switched from imidazoline (neutral form) to imidazolium bicarbonate (charged form) reversibly and repeatedly.

Highly CO2/N2-switchable zwitterionic surfactant for pickering emulsions at ambient temperature

Cross-linked polymer particles were prepared via surfactant-free emulsion copolymerization of 2-(diethylamino)ethyl methacrylate (DEAEMA) and sodium methacrylate (SMA) using N,N'-methylenebis(acrylamide) (MBA) as a cross-linker. Generated particles are zwitterionic, possessing unique isoelectric points in the pH range of 7.5-8.0, which is readily tunable through CO2/N2 bubbling. The particles were found to be highly responsive to CO2/N2 switching, dissolving in water with CO2 bubbling and precipitating with N2 bubbling at room temperature. Pickering emulsions of n-dodecane were prepared using these particles as the sole emulsifier. These emulsions can be rapidly demulsified with CO2 bubbling, resulting in complete oil/water phase separations. Nitrogen bubbling efficiently re-emulsifies the oil with the aid of homogenization. The rapid emulsification/demulsification using CO2/N2 bubbling at room temperature provides these cross-linked zwitterionic particles with distinct advantages as functional Pickering surfactants.

Block copolymer self-assembly controlled by the “green” gas stimulus of carbon dioxide

CO₂ can act as a “green” stimulus to precisely tune the self-assembly of block copolymers.

Novel Pickering emulsifiers based on pH-responsive poly(2-(diethylamino)ethyl methacrylate) latexes

The emulsion copolymerization of 2-(diethylamino)ethyl methacrylate (DEA) with a divinylbenzene cross-linker in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) at 70 °C afforded near-monodisperse, sterically stabilized PEGMA-PDEA latexes at 10% solids. Dynamic light scattering studies indicated intensity-average diameters of 190 to 240 nm for these latexes at pH 9. A latex-to-microgel transition occurred on lowering the solution pH to below the latex pKa of 6.9. When dilute HCl/KOH was used to adjust the aqueous pH, a systematic reduction in the cationic microgel hydrodynamic diameter of 80 nm was observed over ten pH cycles as a result of the gradual buildup of background salt. However, no such size reduction was observed when using CO2/N2 gases to regulate the aqueous pH because this protocol does not generate background salt. Thus, the latter approach offers better reversibility, albeit at the cost of slower response times. PEGMA-PDEA microgel does not stabilize Pickering emulsions when homogenized at pH 3 with n-dodecane, sunflower oil, isononyl isononanoate, or isopropyl myristate. In contrast, PEGMA-PDEA latex proved to be a ubiquitous Pickering emulsifier at pH 10, forming stable oil-in-water emulsions with each of these four model oils. Lowering the solution pH from 10 to 3 resulted in demulsification within seconds. This is because these pH-responsive particles undergo a latex-to-microgel transition, which leads to their interfacial desorption. Six successive demulsification/emulsification cycles were performed on these Pickering emulsions using HCl/KOH to adjust the solution pH. Demulsification could also be achieved by purging the emulsion solution with CO2 gas to lower the aqueous pH to 4.8. However, complete phase separation required CO2 purging for 4 h at 20 °C. A subsequent N2 purge raised the aqueous pH sufficiently to induce a microgel-to-latex transition, but rehomogenization did not produce a stable Pickering emulsion. Presumably, a higher pH is required, which cannot be achieved by a N2 purge alone.

A conventional surfactant becomes CO2-responsive in the presence of switchable water additives

We have developed a new benign means of reversibly breaking emulsions and latexes by using "switchable water", an aqueous solution of switchable ionic strength. The conventional surfactant sodium dodecyl sulfate (SDS) is not normally stimuli-responsive when CO2 is used as the stimulus but becomes CO2 -responsive or "switchable" in the presence of a switchable water additive. In particular, changes in the air/water surface tension and oil/water interfacial tension can be triggered by addition and removal of CO2 . A switchable water additive, N,N-dimethylethanolamine (DMEA), was found to be an effective and efficient additive for the reversible reduction of interfacial tension and can lower the tension of the dodecane/water interface in the presence of SDS surfactant to ultra-low values at very low additive concentrations. Switchable water was successfully used to reversibly break an emulsion containing SDS as surfactant, and dodecane as organic liquid. Also, the addition of CO2 and switchable water can result in aggregation of polystyrene (PS) latexes; the later removal of CO2 neutralizes the DMEA and decreases the ionic strength allowing for the aggregated PS latex to be redispersed and recovered in its original state.