Hydrophobically Associating Polyacrylamide "Water-in-Water" Emulsion Prepared by Aqueous Dispersion Polymerization: Synthesis, Characterization and Rheological Behavior.

The hydrophobically associating polyacrylamide (HAPAM) is an important kind of water-soluble polymer, which is widely used as a rheology modifier in many fields. However, HAPAM products prepared in a traditional method show disadvantages including poor water solubility and the need for hydrocarbon solvents and appropriate surfactants, which lead to environmental pollution and increased costs. To solve these problems, we reported a novel kind of HAPAM "water-in-water" (w/w) emulsion and its solution properties. In this work, a series of cationic hydrophobic monomers with different alkyl chain lengths were synthesized and characterized. Then, HAPAM w/w emulsions were prepared by the aqueous dispersion polymerization of acrylamide, 2-methylacryloylxyethyl trimethyl ammonium chloride and a hydrophobic monomer. All these emulsions can be stored more than 6 months, showing excellent stability. An optical microscopy observation showed that the particle morphology and the particle size of the HAPAM emulsion were more regular and bigger than the emulsion without the hydrophobic monomer. The solubility tests showed that such HAPAM w/w emulsions have excellent solubility, which took no more than 180 s to dilute and achieve a homogeneous and clear solution. The rheology measurements showed that the HAPAM association increases with a hydrophobe concentration or the length of hydrophobic alkyl chains, resulting in better shear and temperature resistances. The total reduced viscosity was 124.42 mPa·s for cw101, 69.81 mPa·s for cw6-1, 55.38 mPa·s for cw8-0.25, 48.95 mPa·s for cw12-0.25 and 28 mPa·s for cw16-0.25 when the temperature increased from 30 °C to 90 °C. The cw8-2.0 that contains a 2 mol% hydrophobe monomer has the lowest value at 19.12 mPa·s due to the best association. Based on the excellent stability, solubility and rheological properties, we believe that these HAPAM w/w emulsions could find widespread applications.

An insight into the thermo-thickening behavior of wormlike micellar solutions based on ultra-long-chain surfactants.

Generally, the solution viscosity of wormlike micelles (WLMs) assembled from common surfactants decreases upon an increase in the temperature, following the Arrhenius law. However, abnormal thermo-thickening behavior has been repeatedly observed for WLMs formed by ultra-long-chain (≥C18) surfactants. It would be useful to unravel the mechanism behind this phenomenon. Here, three C22-tailed surfactants with an erucyl tail, two of them containing carboxylate or sulfonate head groups (UC22DAB and UC22DAS) and a cationic one with an iodide counterion (UC22DAI), and two C18-tailed betaines with olemido- and stearamidopropyl hydrophobic chains (UC18DAS and C18DAS) were characterised in terms of their viscosity and viscoelastic behavior with increasing temperature to examine the roles of head groups, tail length and tail nature. Their micellar structures were elucidated at various temperatures using small angle neutron scattering (SANS), small angle neutron X-ray scattering (SAXS) and molecular dynamics simulation. It is found that the thermo-thickening behavior of ultra-long-chain surfactants is ascribed to the prolonged persistence length and increased entanglement points. When the temperature is increased, an increase in viscosity is always accompanied by a longer persistent length, thus a larger hydrodynamic volume. The iceberg structure around the hydrophobic tail of surfactants can be destroyed at high temperatures leading to the self-assembly of these surfactants. In this self-assembly process, compared to cationic surfactants, zwitterionic surfactants can form WLMs more readily due to weak electrostatic repulsions between their head groups; the longer tails give the surfactants enhanced hydrophobicity to form WLMs with a long breaking time; the cis-unsaturation and the resulting kink in the hydrophobic tails give the surfactants good solubility, which is not conducive to the formation of micelles. In brief, the zwitterionic, longer tail and saturated tail surfactants can form WLMs with a prolonged persistence length at elevated temperatures.

CO2-Triggered ON/OFF Wettability Switching on Bioinspired Polylactic Acid Porous Films for Controllable Bioadhesion.

Bioinspired honeycomb-like porous films with switchable properties have drawn much attention recently owing to their potential application in scenarios in which the conversion between two opposite properties is required. Herein, the CO2-gas-triggered ON/OFF switching wettability of biocompatible polylactic acid (PLA) honeycomb porous films is fabricated. Highly ordered porous films with diameters between 2.0 and 2.8 µm are separately prepared from complexes of nonresponsive PLA and a CO2-sensitive melamine derivative [N2,N4,N6-tris(3-(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine, MET] via the breath figure method. The hydrophilic CO2-sensitive groups can be precisely arranged in the pore's inner surface and/or top surface of the films by simply changing the PLA/MET ratio. The sensitive groups in the pore's inner surface act as a switch triggered by CO2 gas controlling water to enter the pores or not, thus resulting in ON/OFF switching wettability. The largest response of the water contact angle of honeycomb films reaches 35°, from 100 to 65°, leading to an obvious hydrophobic-hydrophilic conversion. The improved surface wettability enhances the interaction between the cell and honeycomb film surface, thus resulting in a better cell attachment. Such smart properties accompanying the biocompatible polymer and biological gas trigger facilitate possible biomedical and bioengineering applications in the future for these films.

Tunable Viscoelastic Properties of Sodium Polyacrylate Solution via CO2-Responsive Switchable Water.

Upon stimulus by CO2, CO2-switchable viscoelastic fluids experience a deliberate transition between non-viscous and highly viscous solution states. Despite attracting considerable recent attention, most such fluids have not been applied at a large- scale due to their high costs and/or complex synthesis processes. Here, we report the development of CO2-switchable viscoelastic fluids using commercially available sodium polyacrylate (NaPAA) and N,N-dimethyl ethanol amine (DMEA)-based switchable water. Upon bubbling CO2, into the solutions under study, DMEA molecules are protonated to generate quaternary ammonium salts, resulting in pronounced decreases in solutions viscosity and elasticity due to the influence of increased ionic strength on NaPAA molecular conformations. Upon removal of CO2 via introduction of N2, quaternary salts are deprotonated to tertiary amines, allowing recovery of fluid viscosity and elasticity to near the initial state. This work provides a simple approach to fabricating CO2-switchable viscoelastic fluids, widening the potential use of CO2 in stimuli-responsive applications.

Cryogenic wormlike micelles.

Cryogenic wormlike micellar solutions which possess a freezing point far below 0 °C while retaining rheological properties similar to those of ordinary wormlike micellar solutions are fabricated from the self-assembly of a C22-tailed monounsaturated zwitterionic surfactant, erucyl dimethyl amidopropyl betaine, in an ethylene glycol/water co-solvent at subzero temperature. Such fluids may find potential applications in fields where viscoelasticity is highly desired at subfreezing temperatures.

Correction: Cryogenic wormlike micelles.

Correction for 'Cryogenic wormlike micelles' by Hongyao Yin et al., Soft Matter, 2019, 15, 2511-2516.

Direct formation of hydrophilic honeycomb film by self-assembly in breath figure templating of hydrophobic polylacticacid/ionic surfactant complexes.

Honeycomb-patterned porous films with good surface wettability have great potential applications in various areas. However, hydrophilic honeycomb films are difficult to obtain using the direct self-assembly of pure (co)polymers. Thus, additional and special treatments are required to improve film wettability, which makes the procedure complicated and difficult to access. In this study, a facile way to prepare hydrophilic honeycomb-structured porous films is proposed that uses the direct self-assembly of complexes of biocompatible hydrophobic poly(l-lactic acid) and dodecyltrimethylammonium chloride by breath figure templating. The addition of ionic surfactant not only improves film quality but also confers good wettability. The obtained hydrophilic pore arrays were found to effectively promote cell attachment. Such a hydrophilic honeycomb-patterned porous film could find potential applications where pore wetting is required, including tissue engineering, lithography, and nanoparticle embedding.

Directed Self-Assembly in "Breath Figure" Templating of Melamine-Based Amphiphilic Copolymers: Effect of Hydrophilic End-Chain on Honeycomb Film Formation and Wetting.

Amphiphilic copolymers are widely used in the fabrication of hierarchically honeycomb-structured films through a "breath figure" (BF) process because the hydrophilic block plays a key role in stabilising water templating. However, the hydrophilic monomers reported are mainly confined to acrylic acid and its derivatives, which largely limits understanding of the formation of BF arrays and the introduction of additional functions on porous films. The relationship between polymer composition, film microstructure and surface properties are also less documented. Herein, a novel melamine-based hydrophilic moiety, N-[3-({3-[(4,6-bis{[3-(dimethylamino)propyl]amino}-1,3,5-triazin-2yl)amino]propyl}(methyl)amino)propyl]methacrylamide (ANME), was incorporated into polystyrene (PS) chains by combining atom-transfer radical polymerisation and post-modification to afford three well-defined end-functionalised PS-PANME derivatives. These polymers were used to fabricate honeycomb films through the BF technique. Both inner and outer microstructures of the films were characterised by optical microscopy, AFM and SEM. Polymer hydrophilicity is enhanced upon increasing the PANME content, which results in variation of the film microstructure and porosity, and provokes a transition from Cassie-Baxter to Wenzel behaviour. Furthermore, the surface wettability of as-prepared honeycomb films and corresponding pillared films is mainly governed by film morphology, rather than by the properties of the polymers. Knowledge of the relationships between polymer composition and film structure, as well as surface wettability, is beneficial to design and prepare hierarchically porous films with desirable structures and properties.

CO2 -Induced Morphological Transition of Co-Assemblies from Block-Random Segmented Polymers.

The co-assembly process is an effective approach to construct hierarchically nanostructured soft materials, but morphological transition of co-assemblies upon external stimuli, particularly the "green" trigger CO2 , is not unraveled yet. Here, a segmented copolymer, poly(styrene)-block-poly[(4-vinyl pyridine)-random-((2-(diethylamino)ethyl methacrylate)] (P1), is used to co-assemble in the mixed solvent of dimethyl formamide and water with poly(ethylene oxide)-block-poly[(4-vinyl pyridine)-random-((2-(diethylamino)ethyl methacrylate)] (P2) and poly(ethylene oxide)-block-poly(acrylic acid) (P3), respectively. It is found that Janus micelles are generated from the P1-P2 pair in the presence of ferric ion, while wormlike micelles are formed from the P1-P3 duad. Upon stimulation with CO2 , Janus and wormlike aggregates are transferred into core-shell and spherical micelles, respectively.

A CO2-switchable amidine monomer: synthesis and characterization.

Smart system employed CO2 gas as new trigger has been attracting enormous attention in recent years, but few monomers that are capable of switching their hydrophobicity/hydrophility upon CO2 stimulation have been reported. A novel CO2 responsive monomer, 4-vinylbenzyl amidine, is designed and synthesized in this work with N,N-dimethylacetamide dimethyl acetal and 4-vinylbenzyl amine that is prepared through the Gabriel reaction. In bi-phase solvent of n-hexane and water, the monomer dissolves in n-hexane first and then transforms into water upon the CO2 treatment, indicating a hydrophobic to hydrophilic transition. This transformation is demonstrated as reversible by monitoring the conductivity variation of its wet dimethyl formamide solution during alternate bubbling/removing CO2. The protonation of 4-vinylbenzyl amidine upon CO2 treatment is demonstrated by 1H NMR which also accounts for the dissolubility change. The reversible addition-fragmentation chain-transfer polymerization of this monomer is also performed, finding the reaction only occurs in glacial acetic acid. The reason can be ascribed to the different radical structure produced in different solvent.

CO2-Induced Reversible Dispersion of Graphene by a Melamine Derivative.

Smart graphene with stimuli-responsive dispersity has great potential for applications in medical and biochemical fields. Nevertheless, reversible dispersion/aggregation of graphene in water with biocompatible and removable trigger still represents a crucial challenge. Here, we report CO2-induced reversible graphene dispersion by noncovalent functionalization of reduced graphene oxide with N(2),N(4),N(6)-tris(3-(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine (MET). It was demonstrated that MET can be strongly adsorbed on graphene surface through van der Waals interaction to facilitate dispersing graphene in water. Moreover, reversible aggregation/dispersion of graphene can be achieved simply by alternately bubbling CO2 and N2 to control the desorption/adsorption of MET on graphene surface.

Solvent-Driven Formation of Worm-Like Micelles Assembled from a CO2-Responsive Triblock Copolymer.

Polymer worm-like micelles (WLMs) are difficult to target due to the narrow composition window. In this work, we report polymer WLMs self-assembled from a linear ABC triblock copolymer consisting of an intermediate fluorinated block of poly(2,2,3,4,4,4-hexafluorobutyl methacrylate) (F), a hydrophilic segment of poly(ethylene oxide) (O) and a CO2-responsive flank of poly(2-(diethylamino)ethyl methacrylate) (E). In the mixed solvent of water and ethanol, the polymer aggregates evolve from spheres to short rods, then long cylinders and finally WLMs when the volume ratio of water increases from 0 to 50%. Upon the stimulus of CO2, the E block is protonated, thus transforms from hydrophobic to hydrophilic. However, the WLMs just partially return back to spheres even the protonation degree of E block is up to 95%. The closely packed arrangement of fluorinated block caused by the increasing interfacial tension of the fluorinated blocks and solvent could account for the formation of WLMs and its shape alternation under CO2 stimulus.

Insights into the relationship between CO2 switchability and basicity: examples of melamine and its derivatives.

Owing to its wide availability, nontoxicity, and low cost, CO2 working as a trigger to reversibly switch material properties, including polarity, ionic strength, hydrophilicity, viscosity, surface charge, and degree of polymerization or cross-linking, has attracted an increasing attention in recent years. However, a quantitative correlation between basicity of these materials and their CO2 switchability has been less documented though it is of great importance for fabricating switchable system. In this work, the "switch-on" and "switch-off" abilities of melamine and its amino-substituted derivatives by introducing and removing CO2 are studied, and then their quantitative relationship with basicity is established, so that performances of other organobases can be quantitatively predicted. These findings are beneficial for forecasting the CO2 stimuli-responsive behavior of other organobases and the design of CO2-switchable materials.

Self-Assembling Anti-Freezing Lamellar Nanostructures in Subzero Temperatures.

The requirement for cryogenic supramolecular self-assembly of amphiphiles in subzero environments is a challenging topic. Here, the self-assembly of lamellar lyotropic liquid crystals (LLCs) are presented to a subzero temperature of -70 °C. These lamellar nanostructures are assembled from specifically tailored ultra-long-chain surfactant stearyl diethanolamine (SDA) in water/glycerol binary solvent. As the temperature falls below zero, LLCs with a liquid-crystalline Lα phase, a tilted Lß phase, and a new folded configuration are obtained consecutively. A comprehensive experimental and computational study is performed to uncover the precise microstructure and formation mechanism. Both the ultra-long alkyl chain and head group of SDA play a crucial role in the formation of lamellar nanostructures. SDA head group is prone to forming hydrogen bonds with water, rather than glycerol. Glycerol cannot penetrate the lipid layer, which mixes with water arranging outside of the lipid bilayer, providing an ideal anti-freezing environment for SDA self-assembly. Based on these nanostructures and the ultra-low freezing point of the system, a series of novel cryogenic materials are created with potential applications in extremely cold environments. These findings would contribute to enriching the theory and research methodology of supramolecular self-assembly in extreme conditions and to developing novel anti-freezing materials.

Selenium-decorated biocompatible honeycomb films with redox-switchable surface for controlling cell adhesion/detachment.

HYPOTHESIS: Selenium (Se)-containing compound is sensitive to redox stimulation, showing hydrophobic-hydrophilic reversible transition. Introduction of such compound into honeycomb film could confer on it redox-switchable surface wettability, which is expected to control cell adhesion/detachment behavior. EXPERIMENTS: Didodecyl selenide was designed and mixed with polystyrene to prepare honeycomb films using "breath figure" method. The film microstructures were characterized by scanning electron microscope and atomic force microscopy, and the arrangement of Se atoms in honeycomb film was determined by X-ray photoelectron spectroscopy and energy dispersive spectrometry. The variation of film wettability upon the alternating stimulation of H2O2 and Vc was examined. Then the cell adhesion, proliferation, and controlled detachment on honeycomb films were conducted. FINDINGS: The introduction of didodecyl selenide helps to form ordered honeycomb film, and Se atoms were found to located on the bottom, pore walls, and top surface of the film. The presence of didodecyl selenide not only greatly improves film biocompatibility by enhancing cell thioredoxin reductase activity, but also imparts the film with H2O2-/vitamin C-regulated tunable wettability that controls cell adhesion and detachment. H2O2 treatment produces a hydrophilic surface for cell adhesion and proliferation, whereas the addition of vitamin C generates hydrophobic surfaces and allows cells to detach while remaining alive with high activity.

Ultra-sensitive detection of multiplexed heavy metal ions by MOF-derived carbon film encapsulating BiCu alloy nanoparticles in potable electrochemical sensing system.

In this work, we report the development of a new type of highly active and stable Bi-based electrode material, i.e., BiCu metal-organic frames (MOF) derived carbon film (CF) encapsulating BiCu alloy nanoparticles (BiCu-ANPs) for electrochemical sensing. The integration of Bi with Cu to form BiCu-ANPs can improve their electrocatalytic activity as well as the acid resistance. Meanwhile, the carbon film that encapsulates BiCu-ANPs not only guarantees the BiCu-ANPs with high electrical conductivity and fast electrochemical kinetics but also effectively alleviates the volume change during the adsorption and desorption of heavy metal (HM) ions. Therefore, the as-obtained CF encapsulating BiCu-ANPs (BiCu-ANPs@CF) exhibits fully exposed active sites, facile charge transfer, high stability and conductivity, which gives rise to enhanced sensitivity and stability for the electrochemical detection of HM ions. When integrated into a potable electrochemical sensing system for simultaneous detection of Pb2+, Cd2+ and Zn2+, the BiCu-ANPs@CF modified electrode exhibits low detection limit (i.e., 0.081 ppb for Pb2+, 0.95 ppb for Cd2+, 35 ppb for Zn2+), wide detection range (i.e., 0.5-700 ppb for Pb2+, 5-900 ppb for Cd2+, 150-600 ppb for Zn2+) and good anti-interference. Finally, the system has been used for on-site detection of multiplexed HM ions in human biological liquids and environmental water with a good spiked recovery rate, which demanstrates its promise application in the future for on-site monitoring of human health and pollutants in water quality.

Rock-on-a-chip: "Seeing" the association/disassociation of an adaptive polymer in solutions flowing through porous media.

The flow and transport of polymer solutions through porous media are ubiquitous in myriad scientific and engineering applications. With escalating interest in adaptive polymers, understanding the flow dynamics of their solutions is indispensable (yet lacking). Here, the hydrophobic-effect-driven reversible associations in a self-adaptive polymer (SAP) solution and its flow characteristics in a microfluidic-based "rock-on-a-chip" device have been analyzed. The hydrophobic aggregates were fluorescent labeled; this enabled a direct visualization of the in situ association/disassociation of the polymer supramolecular assemblies in pore spaces and throats. Furthermore, the influence of this adaptation on the macroscopic flow behavior of the SAP solution was analyzed by comparing its flow with that of two partially-hydrolyzed polyacrylamide (the molecular weight (MW)-equivalent HPAM-1 and ultrahigh-MW HPAM-2) solutions in the semi-dilute regime with similar initial viscosities. At low flow rates (with shear predominance), the SAP solution showed a low shear viscosity compared to HPAM-1, indicating a higher shear susceptibility for association than chain entanglement. Although the SAP exhibited the same elastic instability as the non-adaptive polymers above a threshold flow rate, the adaptable structure of the former advanced the onset of its viscoelastic-governed flow, providing a stronger flow resistance, possibly through an extension resistance. Furthermore, 3D-media analysis indicated that the reversible association/disassociation of SAP increased the accessible pore space during nonaqueous-liquid displacement, facilitating oil production.

Supramolecular self-assembly of robust, ultra-stable, and high-temperature-resistant viscoelastic worm-like micelles.

HYPOTHESIS: Worm-like micelles are susceptible to heating owing to the fast dynamic exchange of molecules between micelles. Inhibition of such exchange could afford robust worm-like micelles, which is expected to largely improve rheology properties at high temperatures. EXPERIMENTS: A cationic surfactant docosyl(trimethyl)azanium chloride (DCTAC) and a strongly hydrophobic organic counterion 3-hydroxy naphthalene-2-carboxylate (SHNC) were used for the worm-like micelles fabrication. The microstructure was characterized using cryogenic transmission electron microscopy and small-angle neutron scattering, and the interactions between DCTAC and SHNC were characterized using nuclear magnetic resonance spectroscopy. Rheometer was employed to measure the rheological properties of the solution. FINDINGS: SHNC/DCTAC at the molar ration of 1:2 forms ultra-stable worm-like micelles, whose viscosity remain stable at temperature up to 130 °C. SHNC is found to strongly adsorbs on DCTAC micelle with the orientation on the surface of micelle, keeping the naphthalene backbone entire penetration into the palisade layer while both carboxylic and hydroxyl groups protrude out of the micelle. With temperature increasing, this adsorption further strengthens, resulting in the growth contour length and accompanying the enhancement of rheological properties. One SHNC molecule and two DCTAC molecules are speculated to form a stable complex via multiple interactions including hydrophobic, cationic-π, and π-π interactions, which decreases the dynamic exchange of them between micelles. These findings are helpful to understand surfactant aggregates stability and assist the development of novel stable supramolecular nanostructures. Additionally, the excellent thermal stability of this worm-like micellar fluid makes it a potential high-temperature resistant clean fracturing fluid for deep oil reservoirs.

Comparative Studies on Hydraulic Fracturing Fluids for High-Temperature and High-Salt Oil Reservoirs: Synthetic Polymer versus Guar Gum.

The increasing energy demand has prompted engineers to explore deeper wells where rich oil and gas reserves exist. However, the high-temperature and high-salt conditions have impeded the further application of traditional water-based fracturing fluids in such reservoirs. Therefore, it is urgent to develop fracturing fluids that are suitable for such geographic characteristics. In this study, for the first time, a novel synthetic polymer, poly-(acrylamide-co-acrylic acid-co-2-acrylamido-2-methyl-1-propanesulfonic acid) (P3A), was investigated as a rheological modifier for water-based fracturing fluids in high-temperature and high-salt conditions and compared with a guar gum system. Results showed that the apparent viscosity increased with increasing P3A and guar gum concentrations, and the thickening ability of P3A was much better than that of guar gum. Despite the better shear and temperature resistance and proppant suspension ability of guar gum fluids in high-temperature and saturated salt conditions, plentiful solid residues after gel-breaking have prevented their progress in the petroleum industry. P3A fluids have no residues, but the unsatisfying proppant suspension capability and high dosage encourage us to promote their rheological performance via interaction with an organic zirconium crosslinker. Infrared spectroscopy and scanning electron microscopy were applied to guarantee the successful reaction of P3A with the crosslinker. The subsequent investigation indicated that the transformed fracturing fluid exhibited remarkably improved thickening capability and satisfying rheological performance in terms of temperature and shear resistance and proppant-carrying ability as well as gel-breaking results in a high-temperature and saturated salt environment. All of the above results suggest the potential application of crosslinked P3A in hydraulic fracturing for the reservoirs with hostile conditions, and this article also provides a new orientation for synthetic polymers utilized in the oil and gas industry.

Condensate Oil-Tolerant Foams Stabilized by an Anionic-Sulfobetaine Surfactant Mixture.

Foams are widely used to remove liquid loading at the bottom of gas wells to improve natural gas production. However, it becomes ineffective when a gas well contains a large amount of hydrocarbon condensate because oil will rupture the foams. In this work, condensate oil-tolerant foams were developed and stabilized by a mixture of cocamidopropyl hydroxyl sulfobetaine (CHSB) and sodium dodecyl sulfate (SDS). The foam properties are examined at different temperatures under atmospheric conditions and high pressures with various contents of condensate oil. It is found that the foam stability is improved when the oil content is increased; in addition, high temperature, high salinity, and high pressure are beneficial for foam stabilizing. To reveal the mechanism of stable foam in the presence of high content of oil, a confocal microscope was employed to visualize oil-foam interactions. It was observed that the high stability of the SDS-CHSB foams is ascribed to the formation of stable pseudoemulsion between oil droplets and the gas-liquid interface. Such condensate oil-tolerant foams show promising potential to be used in the foam-assisted lift process during natural gas production.