An introduction to the principles and fundamentals of CO2-switchable polymers and polymer colloids

Developing aerogel surfaces via switchable-hydrophilicity tertiary amidine coating for improved oil recovery

Blanket aerogels (i.e., Cabot™ Thermal Wrap® (TW) and Aspen™ Spaceloft® (SL)) with surfaces that have controllable wettability are promising advanced materials for oil recovery applications, where high oil uptake during deployment could be coupled with high oil release to enable reusability of recovered oil. The study presented here details the preparation of CO₂-switchable aerogel surfaces through the application of switchable tertiary amidine (i.e., tributylpentanamidine (TBPA)) onto aerogel surfaces using drop casting, dip coating, and physical vapor deposition techniques. TBPA is synthesized via two step processes: (1) synthesis of N, N-dibutylpentanamide, (2) synthesis of N, N-tributylpentanamidine. The deposition of TBPA is confirmed by X-ray photoelectron spectroscopy. Our experiments revealed that surface coating of TBPA onto aerogel blankets was partially successful within limited set of process conditions (e.g., 290 ppm CO₂ and 5500 ppm humidity for PVD, 106 ppm CO₂ and 700 ppm humidity for drop casting and dip coating), but that the post-aerogel modification strategies yielded poor, heterogeneous reproducibility. Overall, more than 40 samples were tested for their switchability in the presence of CO₂ and water vapor, respectively, and the success rate was 6.25 %, 11.7 % and 18 % for PVD, drop casting, and dip coating, respectively. The most likely reasons for unsuccessful coating onto aerogel surfaces are: (1) the heterogeneous fiber structure of the aerogel blankets, (2) poor distribution of the TBPA over the aerogel blanket surface.

CO2-responsive gels

CO₂-responsive materials undergo a change in chemical or physical properties in response to the introduction or removal of CO₂. The use of CO₂ as a stimulus is advantageous as it is abundant, benign, inexpensive, and it does not accumulate in a system. Many CO₂-responsive materials have already been explored including polymers, latexes, surfactants, and catalysts. As a sub-set of CO₂-responsive polymers, the study of CO₂-responsive gels (insoluble, cross-linked polymers) is a unique discipline due to the unique set of changes in the gels brought about by CO₂ such as swelling or a transformed morphology. In the past 15 years, CO₂-responsive gels and self-assembled gels have been investigated for a variety of emerging potential applications, reported in 90 peer-reviewed publications. The two most widely exploited properties include the control of flow (fluids) via CO₂-triggered aggregation and their capacity for reversible CO₂ absorption-desorption, leading to applications in Enhanced Oil Recovery (EOR) and CO₂ sequestration, respectively. In this paper, we review the preparation, properties, and applications of these CO₂-responsive gels, broadly classified by particle size as nanogels, microgels, aerogels, and macrogels. We have included a section on CO₂-induced self-assembled gels (including poly(ionic liquid) gels).

Reversible stability of colloids switched by CO2 based on polyhexamethylene guanidine

The stability of a colloid, including emulsion and polymer latex, can be destroyed irreversibly by the addition of salt. Using the CO₂ stimulus, amines can be converted into organic ammonium salts reversibly, which can access the switching of colloids. Polyhexamethylene guanidine (PHMG), was chosen as a switchable amine. The conductivity of PHMG aqueous solution switched by adding and removing CO₂. Surface tension measurements verified that, under CO₂, the critical micelle concentration of sodium dodecyl benzene sulfonate (SDBS) decreased from 1.0 × 10^-3 to 5.0 × 10^-4 M with the addition of PHMG. The crude oil emulsion containing SDBS and PHMG was destroyed and restored reversibly by the treatment with CO₂ and N₂. The polystyrene latex also occurred an obvious stratification after sparging with CO₂ and returned a homogeneous phase upon bubbling N₂. This study is intended to pave the way for colloids which has reversible stability in response to CO₂ stimulation.

Electrical Response of Poly(N-[3-(dimethylamino)Propyl] Methacrylamide) to CO2 at a Long Exposure Period

Amine-functionalized polymers (AFPs) are able to react with carbon dioxide (CO₂) and are therefore useful in CO₂ capture and sensing. To develop AFP-based CO₂ sensors, it is critical to examine their electrical responses to CO₂ over long periods of time, so that the device can be used consistently for measuring CO₂ concentration. To this end, we synthesized poly(N-[3-(dimethylamino)propyl] methacrylamide) (pDMAPMAm) by free radical polymerization and tested its ability to behave as a CO₂-responsive polymer in a transducer. The electrical response of this polymer to CO₂ upon long exposure times was measured in both the aqueous and solid phases. Direct current resistance measurement tests on pDMAPMAm films printed along with the silver electrodes in the presence of CO₂ at various concentrations reveal a two-region electrical response. Upon continuous exposure to different CO₂ flow rates (at a constant pressure of 0.2 MPa), the resistance first decreased over time, reaching a minimum, followed by a gradual increase with further exposure to CO₂. A similar trend is observed when CO₂ is introduced to an aqueous solution of pDMAPMAm. The in situ monitoring of pH suggests that the change in resistance of pDMAPMAm can be attributed to the protonation of tertiary amine groups in the presence of CO₂. This two-region response of pDMAPMAm is based on a proton-hopping mechanism and a change in the number of free amines when pDMAPMAm is exposed to various levels of CO₂.

Dual Gas-responsive Fluorescent Diblock Copolymer Synthesized via RAFT Polymerization

Stimulus-responsive polymers with luminescence properties have a wide range of applications in the fields of controlled drug release, fluorescent probes, and biological stents. In this paper, carbon dioxide (CO₂)/oxygen (O₂) dual-responsive fluorescent diblock copolymers were synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization method with two fluorescent monomers synthesized as its luminescence source, DEAEMA (CO₂ responsive monomer) and tFMA (O₂ responsive monomer). An experimental study demonstrated that the synthesized stimulus-responsive fluorescent polymer had a high sensitivity to CO₂; the double-responsive fluorescent diblock copolymer could form and achieve the reversal of polymer micelles in the aqueous solution when it was sequentially subjected to the introduction of CO₂ and O₂.

Molecular dynamics simulation of CO2-switchable surfactant regulated reversible emulsification/demulsification processes of a dodecane-saline system

CO2-Switchable surfactants are of great potential in a wide range of industrial applications related to their ability to stabilize and destabilize emulsions upon command. Molecular dynamics simulations have been performed to reveal the fundamental mechanism of the reversible emulsification/demulsification processes of a dodecane-saline system by a CO2-switchable surfactant that switches between active (i.e., N'-dodecyl-N,N-dimethylacetamidinium (DMAAH+)) and inactive (i.e., N'-dodecyl-N,N-dimethylacetamidine (DMAA)) forms. The density profiles indicate that DMAAH+ could increase the oil-water interfacial thickness to a greater extent compared to DMAA. DMAAH+ could sharply reduce the interfacial tension of the dodecane-saline system, while DMAA only exhibits a limited decrease, which is in accordance with the experimental observation that DMAAH+/DMAA can reversibly emulsify/demulsify alkane-water systems. Our simulations showed that both the number and lifetime of hydrogen bonds (HBs) between DMAA and water are almost equal to those between DMAAH+ and water. In DMAA, the N atom connecting with the alkyl tail acted as a HB acceptor, while the N atom attached by a proton in DMAAH+ acted as a HB donor. Furthermore, the HBs between DMAAH+ and HCO3- at the interfaces are relatively limited. Hence, it is deduced that the HBs are insufficient to achieve the CO2-switchability of DMAA/DMAAH+. The Lennard Jones and coulombic potentials between DMAA/DMAAH+ and other species show that the coulombic potentials between DMAAH+ and water or anions (i.e., Cl- and HCO3-) sharply decrease with the increase of DMAAH+ and are much lower than those in models with DMAA. The enhanced coulombic interactions between DMAAH+ and anions lead to a remarkable reduction in interfacial tension and the emulsification of the alkane-saline system. Therefore, coulombic interactions are of crucial importance to the reversible emulsification/demulsification processes regulated by CO2-switchable surfactants, namely DMAAH+/DMAA.

Reversible flocculation of nanoparticles by a carbamate surfactant

Fatty alkyldiamine readily reacts with CO₂ in aqueous solution at pH 12 to reversibly form surface active carbamate species. The carbamate can be reverted to the amine by exposure to N₂ and heat. In this work, a carbamate-based surfactant (Y12-carbamate) has been used to disperse and stabilize hydrophobic nanoparticles. This state could be regarded as the "on" state of a series of cycle. The nanoparticles were then flocculated when the carbamate groups were cleaved by exposure to N₂ and heating, corresponding to the "off" state. In a subsequent cycle, the nanoparticles were re-dispersed by exposure to CO₂, while the pH remained at 12. This cycle of re-dispersion and flocculation could be repeated two times without impairing the particle size. However, further cycles increased the particle size, indicating that all particles could not be completely re-dispersed. In addition, we also investigated the effect of pH on the colloidal stability with sodium Y12-carbamate, by measuring particle size and electrophoretic mobility. The results showed that pH strongly influenced the stability of the nanoparticles. Sodium Y12-carbamate stabilized the particles with a negative electrophoretic mobility at pH well above pKa whereas at pH close to pKa of Y12-amine (pKa = 9.0), the particles quickly flocculated, as a result of an ion-pair formation between Y12-ammonium and Y12-carbamate.

Formation, physicochemical and interfacial study of carbamate surfactants

Carbon dioxide is commonly used as pH regulator in switchable surfactant systems and in the formation of alkyl ammonium-alkyl carbamate ion-pair. Its use to form a meta-stable anionic surfactant has been less explored and can impart a cleavable character to the amphiphile. The reaction between CO₂ and an alkylamine, N,N-di(propylamino)dodecylamine (Y12-amine), under alkaline pH conditions, produced a stable anionic carbamate-based surfactant (Y12-carbamate). By heating and exposure to N₂, anionic Y12-carbamate could slowly be reverted into Y12-amine. The surface activity of Y12-amine and Y12-carbamate was investigated by surface tension measurements. To study the behavior of Y12-amine at the gas-water interface during CO₂ exposure, we used the pendant drop technique with a sealed chamber where the gas composition could be controlled. The Y12-carbamate had a higher CMC than Y12-amine at pH 12, and was also less surface active. The ion pair Y12-ammonium - Y12-carbamate, obtained at neutral pH, exhibited the lowest CMC and the highest surface activity. The interfacial formation of anionic Y12-carbamate induced an increase in surface tension. When CO₂ was exchanged to N₂, the migration from the bulk to the interface of Y12-amine induced a decrease in surface tension. The rate was dependent on the concentration of Y12-amine.

An eco-friendly approach for heavy metal adsorbent regeneration using CO2-responsive molecular octopus

Perennial problems of adsorption in wastewater treatment include adsorbent recycling, generation of waste sludge and secondary pollution because harmful concentrated acids, bases or strong chelators are often used for adsorbent regeneration and adsorbate recovery. We report, for the first time, an eco-friendly regeneration concept demonstrated with a CO₂-responsive octopus-like polymeric adsorbent. Various heavy metals can be scavenged at very high Q_e by such adsorbent through coordination. Most importantly, the rapid and complete regeneration of the adsorbent and recovery of the heavy metal ions can be readily achieved by CO₂ bubbling within a few minutes under mild conditions, i.e., room temperature and atmospheric pressure. The adsorbent can then be restored to its adsorptive state and reused upon removal of CO₂ by simply bubbling another gas. This eco-friendly, effective, ultra-fast and repeatable CO₂-triggered regeneration process using CO₂-responsive adsorbent with versatile structure, morphology or form can be incorporated into a sustainable closed-loop wastewater treatment process to solve the perennial problems.

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.

Grafting CO2-responsive polymers from cellulose nanocrystals via nitroxide-mediated polymerisation

CO₂-Responsive polymers are grafted onto cellulose nanocrystals using nitroxide-mediated polymerisation.

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.

Grafting well-defined CO2-responsive polymers to cellulose nanocrystals via nitroxide-mediated polymerisation: effect of graft density and molecular weight on dispersion behaviour

Tertiary polyamines and polyamides were grafted to cellulose nanocrystals (CNC) via Nitroxide-Mediated Polymerisation to yield well-defined CNC, which were examined for their CO₂-responsive dispersibility in water and oil.

Fluorescent and “breathable” CO2 responsive vesicles inspired from green fluorescent protein

CO₂ responsive fluorescent vesicles from a GFP chromophore labeled block-copolymer could change their size and fluorescence to mimic jellyfish breathing.

A guide to the selection of switchable functional groups for CO2-switchable compounds

CO₂-responsive species in water interconvert between neutral and bicarbonate forms, but only if the species has appropriate basicity.