Non-surface activity and micellization of ionic amphiphilic diblock copolymers in water. Hydrophobic chain length dependence and salt effect on surface activity and the critical micelle concentration

Strong Anionic/Charge-Neutral Block Copolymers from Cu(0)-Mediated Reversible Deactivation Radical Polymerization

Despite recent developments in controlled polymerization techniques, the straightforward synthesis of block copolymers that feature both strong anionic and charge-neutral segments remains a difficult endeavor. In particular, solubility issues may arise during the direct synthesis of strong amphiphiles and typical postpolymerization deprotection often requires harsh conditions. To overcome these challenges, we employed Cu(0)-mediated reversible deactivation radical polymerization (Cu(0)-RDRP) on a hydrophobic isobutoxy-protected 3-sulfopropyl acrylate. Cu(0)-RDRP enables the rapid synthesis of the polymer, reaching high conversions and low dispersities while using a single solvent system and low amounts of copper species. These macromolecules are straightforward to characterize and can subsequently be deprotected in a mild yet highly efficient fashion to expose their strongly charged nature. Furthermore, a protected sulfonate segment could be grown from a variety of charge-neutral macroinitiators to produce, after the use of the same deprotection chemistry, a library of amphiphilic, double-hydrophilic as well as thermoresponsive block copolymers (BCPs). The ability of these various BCPs to self-assemble in aqueous media was further studied by dynamic light scattering, ζ-potential measurements as well as atomic force and electron microscopy.

6:2 Chlorinated polyfluoroalkyl ether sulfonate as perfluorooctanesulfonate alternative in the electroplating industry and the receiving environment

Electroplating industry is an important application field of per- and polyfluoroalkyl substances (PFASs) as the chromium mist suppressants. 6:2 chlorinated polyfluoroalkyl ether sulfonate (6:2 Cl-PFAES) and perfluorooctanesulfonate (PFOS) have been the two widely used mist suppressants, and after the ban of PFOS, 6:2 Cl-PFAES will become the dominant suppressant. The behavior and mechanisms of 6:2 Cl-PFAES in the electroplating industry and the receiving environment were studied and compared with PFOS. 6:2 Cl-PFAES behaved similarly with PFOS due to their similar chemical structure. However, some difference exists for the relatively stronger hydrophobicity of 6:2 Cl-PFAES. Up to 35.7 mg/L of PFOS and 13.4 mg/L of 6:2 Cl-PFAES were found in the industrial wastewater influents, and were effectively reduced to 0.3-0.8 mg/L by the interaction with chromium hydroxide through hydrophobic interaction and ligand exchange. The stronger hydrophobicity of 6:2 Cl-PFAES than PFOS resulted in its accumulation in the surface of foams and comparable or less removal during the industrial and municipal wastewater treatment. 6:2 Cl-PFAES exhibited higher bioaccumulation potential than PFOS in the surface water. 6:2 Cl-PFAES emitted by both mists and water may pose health risks to humans. More attentions towards 6:2 Cl-PFAES are needed after the replacement of PFOS by it in the electroplating industry as a global contaminant of emerging concerns.

The effect of macromolecular structure on the rheology and surface properties of amphiphilic random polystyrene-r-poly(meth)acrylate copolymers prepared by RDRP

In this work rheological and surface properties of various random copolymers of styrene and sodium (meth)acrylate, prepared using reversible deactivation radical polymerization (RDRP), were studied. It is shown that the properties of these polymers in water solution, relevant for several applications, are affected by their chemical structure and molecular weight. Cryo-TEM images of their concentrated water solutions do not show the presence of nano-objects as micelles, however the existence of some aggregates seems to be confirmed by fluorescence measurements using pyrene as a hydrophobic probe and by surface tension measurements. Moreover, interesting results are displayed about the viscosity as well as the surface tension of these water polymer solutions, due probably to different interactions at the molecular level as suggested by fluorescence measurements.

Application of a Zwitterionic Hydrophobic Associating Polymer with High Salt and Heat Tolerance in Brine-Based Fracturing Fluid

ZID₁₆PM, a zwitterionic hydrophobic associating polymer, has equivalent positive and negative charges and some hydrophobic monomers with twin-tailed long hydrophobic chains. It exhibits a great heat resistance and salt tolerance to the common salt in formation brine (MgCl₂, CaCl₂, NaCl, and KCl), which is attributed to its anti-polyelectrolyte effect and strong association force. High-salinity water (seawater or formation water) can be prepared as a fracturing fluid directly. In this paper, the formation water of the West Sichuan Gas Field is directly prepared into fracturing fluid with a concentration of 0.3% ZID₁₆PM (Fluid-1), and the seawater of the Gulf of Mexico is directly prepared into fracturing fluid with a concentration of 0.3% ZID₁₆PM (Fluid-2). Finally, rheological measurements, proppant suspension tests, and core matrix permeability damage rate tests for the Fluid-1 and Fluid-2 are conducted. Results show that after 120 min of shearing at 140 and 160 °C, respectively, the viscosity of Fluid-1 remains in the range of 50–85 mPa·s, and the viscosity of Fluid-2 remains in the range of 60–95 mPa·s. And the wastewater produced by an oilfield in Shaanxi, Xinjiang, and Jiangsu are also prepared into fracturing fluids with a concentration of 0.3% ZID₁₆PM, the viscosity of these fracturing fluids can remain 32, 42, and 45 mPa·s, respectively, after 120 min of shearing at 160 °C. All results demonstrate that the polymer ZID₁₆PM displays prominent performance in fracturing fluids.

Effects of the Hydrophilic-Lipophilic Balance of Alternating Peptides on Self-Assembly and Thermo-Responsive Behaviors

A series of N-substituted poly(Gly–alter–Val) peptides were successfully synthesized for the systematic evaluation of the micellization behavior of alternating peptides. Three-component polymerization employing an aldehyde, a primary ammonium chloride, and potassium isocyanoacetate afforded four alternating peptides in excellent yields. We investigated the dependence of the hydrophilic–lipophilic balance of alternating peptides on the micellization behavior. All the aqueous solutions of alternating peptides exhibited upper critical solution temperature (UCST) behaviors, strongly indicating that the alternating binary pattern would mainly contribute to the UCST behaviors. The cloud points of alternating peptides shifted to higher temperatures as the side chains became more hydrophilic, which is opposite to the trend of typical surfactants. Such unusual micellization behaviors appeared to be dependent on the quasi-stable structure of single polymer chains formed in water.

Anion amphiphilic random copolymers and their performance as stabilizers for O/W nanoemulsions

A series of anionic amphiphilic random copolymers with sodium p-styrene sulfonate and dodecyl methacrylate were synthesized via free radical polymerization and their property in the formation and stabilization of nano-emulsions were investigated.

Novel Hydrophobic Associating Polymer with Good Salt Tolerance

A hydrophobic associating polymer named DiPHAM (acrylamide/sodium acrylamide-2-methylpropanesulfonic/sodium acrylate/N,N-di-n-dodecylacrylamide) with good salt tolerance was synthesized via photo-initiation polymerization. The critical association concentration (CAC) of DiPHAM was determined by viscosity changes to be 490 mg/L with different DiPHAM concentrations and particle sizes varied under such dynamic conditions. The influences of aqueous metal ions with different charges on its aqueous solution were investigated by measuring apparent viscosity, viscoelasticity, thixotropy, rheology, and particle size, and by SEM observation. The apparent viscosity of the DiPHAM solution was affected by metal ions to some extent, but the viscosity of the polymer can be still maintained at 55 mPa·s under 20 × 10⁴ mg/L NaCl. Divalent metal ions show greater impact on DiPHAM aqueous solutions, but the polymer solutions showed resistance to the changes caused in viscosity, structure, and viscoelasticity by Ca2+ and Mg2+ ions. The salt tolerance of DiPHAM is due to the combination of hydrophobic association, the electrostatic shield, and double layer compression of the hydration shell. Increasing the ion concentration enhances the dehydration and further compresses the hydration shell, making the non-structural viscosity decrease, even "salting out". Measurements of rheological properties showed that DiPHAM solutions could maintain a relatively high viscosity (0.6%-71 mPa·s/0.3%-50 mPa·s) after 120 min of continuous shearing (170 s-1) at 140 °C. Under high-salinity (5000 mg/L Ca2+/3000 mg/L Mg2+) conditions, the solution with 0.6 wt% DiPHAM still maintained a high viscosity (50 mPa·s/70 mPa·s) after continuously shearing for 120 min at 120 °C and 170 s-1. The good salt tolerance of DiPHAM can lead to a variety of applications, including in fracturing fluids for enhanced oil recovery (EOR) and in sewage treatment.

Synthesis, self-assembly, and base-pairing of nucleobase end-functionalized block copolymers in aqueous solution

As a novel strategy, nucleobase-containing copolymers are created for molecular recognition and nucleobase releasing.

Surface Active to Non-Surface Active Transition and Micellization Behaviour of Zwitterionic Amphiphilic Diblock Copolymers: Hydrophobicity and Salt Dependency

We have synthesized a range of zwitterionic amphiphilic diblock copolymers with the same hydrophilic block (carboxybetaine) but with different hydrophobic blocks (n-butylmethacrylate (n-BMA) or 2-ethylhexylacrylate (EHA)) by the reversible addition⁻fragmentation chain transfer (RAFT) polymerization method. Herein, we systematically examined the role of hydrophobicity and salt concentration dependency of surface activity and micellization behaviour of block copolymer. Transition from surface active to non-surface active occurred with increasing hydrophobicity of the hydrophobic block of block copolymer (i.e., replacing P(n-BMA) by PEHA). Foam formation of block copolymer slightly decreased with the similar variation of the hydrophobic block of block copolymer. Block copolymer with higher hydrophobicity preferred micelle formation rather than adsorption at the air⁻water interface. Dynamic light scattering studies showed that block copolymer having P(n-BMA) produced near-monodisperse micelles, whereas block copolymer composed of PEHA produced polydisperse micelles. Zimm plot results revealed that the value of the second virial coefficient (A₂) changed from positive to negative when the hydrophobic block of block copolymer was changed from P(n-BMA) to PEHA. This indicates that the solubility of block copolymer having P(n-BMA) in water may be higher than that of block copolymer having PEHA in water. Unlike ionic amphiphilic block copolymer micelles, the micellar shape of zwitterionic amphiphilic block copolymer micelles is not affected by addition of salt, with a value of packing parameters of block copolymer micelles of less than 0.3.

Micellization of long-chain ionic liquids in deep eutectic solvents

The aggregation behavior of the ionic liquid (IL) 1-alkyl-3-methylimidazolium chloride with different alkyl chain lengths in a deep eutectic solvent (DES, composed of choline chloride and glycerol in a 1 : 2 mole ratio) was studied for the first time. The critical micellar concentration, micellar size and intermolecular interactions in IL/DES solutions were investigated by different techniques including the fluorescence probe technique, small angle X-ray scattering and Fourier transform infrared spectroscopy. The solvophobic effect dominates the micellization of CnmimCl in DES and the intermolecular hydrogen-bond interaction plays a positive role to promote micelle formation. The micellar solutions were utilized for the synthesis of the water-unstable metal-organic framework Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) at room temperature. X-Ray diffraction, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption-desorption isotherms confirm the formation of crystalline Cu3(BTC)2 nanocrystals with mesoporous structures. The morphologies and porosity properties of Cu3(BTC)2 nanocrystals can be modulated by varying the concentration of CnmimCl.

General considerations regarding the in vitro and in vivo properties of block copolymer micelle products and their evaluation

Block copolymer micelles are nanoparticles formed from block copolymers that comprise a hydrophilic polymer such as poly(ethylene glycol) and a poorly soluble polymer such as poly(amino acids). The design of block copolymer micelles is intended to regulate the in vivo pharmacokinetics, stability, and distribution profiles of an entrapped or block copolymer-linked active substance. Several block copolymer micelle products are currently undergoing clinical development; however, a major challenge in the development and evaluation of such products is identification of the physicochemical properties that affect the properties of the drug product in vivo. Here we review the overall in vitro and in vivo characteristics of block copolymer micelle products with a focus on the products currently under clinical investigation. We present examples of methods suitable for the evaluation of the physicochemical properties, non-clinical pharmacokinetics, and safety of block copolymer micelle products.

Photoresponsive block copolymer: synthesis, characterization, and surface activity control

Amphiphilic block copolymers bearing chromophores are used to achieve photoresponses upon exposure to suitable light, which alter molecular properties, but the photostimulus surface activity control of amphiphilic block copolymers remains to be elucidated. In this work, a series of novel amphiphilic block copolymers consisting of a carboxymethyl betaine monomer (called GLBT) and 4-ethoxy-4'-methacrylamide (EMAAB) with different block ratios have been synthesized using a reversible addition-fragmentation chain-transfer (RAFT) polymerization process. Copolymers were observed to be self-assembled in the aqueous solution above a critical micelle concentration, which was determined by static light scattering measurements and formed vesicles of 120-170 nm in diameter, at different pH values. Copolymers were found to be surface-active at pH 7 but exhibited non-surface activity at acidic and alkaline pH values. After being irradiated with 360 nm UV light, copolymers showed a significant photoresponse both at the surface and in bulk solution as a result of the photoinduced isomerization of azochromophores. The surface property of copolymers was significantly affected by UV irradiation at pH 7, and block copolymers became non-surface-active. The bulk properties changed considerably upon UV exposure where polymer vesicles transformed to micelles as a result of the polarity difference between two azo isomers (cis and trans isomers). All of these transitions were found to be reversible. A new method to control the surface active/nonactive and vesicle/micelle transitions by light and pH has been established by introducing an azobenzene chromophore and GLBT into amphiphlic diblock copolymers.

Chain length dependence of non-surface activity and micellization behavior of cationic amphiphilic diblock copolymers

The cationic and anionic amphiphilic diblock copolymers with a critical chain length and block ratio do not adsorb at the air/water interface but form micelles in solution, which is a phenomenon called "non-surface activity". This is primarily due to the high charge density of the block copolymer, which creates a strong image charge effect at the air/water interface preventing adsorption. Very stable micelle formation in bulk solution could also play an important role in the non-surface activity. To further confirm these unique properties, we studied the adsorption and micellization behavior of cationic amphiphilic diblock copolymers of poly(n-butyl acrylate)-b-poly(3-(methacryloyloxy)ethyl)trimethylammonium chloride) (PBA-b-PDMC) with different molecular weights of hydrophobic blocks but with the same ionic block length. These block copolymers were successfully prepared via consecutive reversible addition-fragmentation chain transfer (RAFT) polymerization. The block copolymer with the shortest hydrophobic block length was surface-active; the solution showed surface tension reduction and foam formation. However, above the critical block ratio, the surface tension of the solution did not decrease with increasing polymer concentration, and there was no foam formation, indicating lack of surface activity. After addition of 0.1 M NaCl, stable foam formation and slight reduction of surface tension were observed, which is reminiscent of the electrostatic nature of the non-surface activity. Fluorescence and dynamic and static light scattering measurements showed that the copolymer with the shortest hydrophobic block did not form micelles, while the block copolymers formed spherical micelles having radii of 25-30 nm. These observations indicate that micelle formation is also important for non-surface activity. Upon addition of NaCl, cmc did not decrease but rather increased as observed for non-surface-active block copolymers previously studied. The micelles formed were very stable, and their size decreased by only ∼5 nm after addition of 0.1 M NaCl.

Non-surface activity and micellization behavior of cationic amphiphilic block copolymer synthesized by reversible addition-fragmentation chain transfer process

Cationic amphiphilic diblock copolymers of poly(n-butylacrylate)-b-poly(3-(methacryloylamino)propyl)trimethylammonium chloride) (PBA-b-PMAPTAC) with various hydrophobic and hydrophilic chain lengths were synthesized by a reversible addition-fragmentation chain transfer (RAFT) process. Their molecular characteristics such as surface activity/nonactivity were investigated by surface tension measurements and foam formation observation. Their micelle formation behavior and micelle structure were investigated by fluorescence probe technique, static and dynamic light scattering (SLS and DLS), etc., as a function of hydrophilic and hydrophobic chain lengths. The block copolymers were found to be non-surface active because the surface tension of the aqueous solutions did not change with increasing polymer concentration. Critical micelle concentration (cmc) of the polymers could be determined by fluorescence and SLS measurements, which means that these polymers form micelles in bulk solution, although they were non-surface active. Above the cmc, the large blue shift of the emission maximum of N-phenyl-1-naphthylamine (NPN) probe and the low micropolarity value of the pyrene probe in polymer solution indicate the core of the micelle is nonpolar in nature. Also, the high value of the relative intensity of the NPN probe and the fluorescence anisotropy of the 1,6-diphenyl-1,3,5-hexatriene (DPH) probe indicated that the core of the micelle is highly viscous in nature. DLS was used to measure the average hydrodynamic radii and size distribution of the copolymer micelles. The copolymer with the longest PBA block had the poorest water solubility and consequently formed micelles with larger size while having a lower cmc. The "non-surface activity" was confirmed for cationic amphiphilic diblock copolymers in addition to anionic ones studied previously, indicating the universality of non-surface activity nature.

Aggregation behavior of poly(ethylene glycol-bl-propylene sulfide) di- and triblock copolymers in aqueous solution

Block copolymers of poly(ethylene glycol)-bl-poly(propylene sulfide) (PEG-PPS) have recently emerged as a new macromolecular amphiphile capable of forming a wide range of morphologies when dispersed in water. To understand better the relationship between stability and morphology in terms of the relative and absolute block compositions, we have synthesized a collection of PEG-PPS block copolymers and quantified their critical aggregation concentration and observed their morphology using cryogenic transmission electron microscopy after thin film hydration with extrusion and after solvent dispersion from tetrahydrofuran, a solvent for both blocks. By understanding the relationship between aggregate character and block copolymer architecture, we have observed that whereas the relative block lengths control morphology, the stability of the aggregates upon dilution is determined by the absolute block length of the hydrophobic PPS block. We have compared results obtained with PEG-PPS to those obtained with poly(ethylene glycol)-bl-poly(propylene oxide)-bl-poly(ethylene glycol) block copolymers (Pluronics). The results reveal that the PEG-PPS aggregates are substantially more stable than Pluronic aggregates, by more than an order of magnitude. PEG-PPS can form a wide variety of stable or metastable morphologies in dilute solution within normal time and temperature ranges, whereas Pluronics can generally form only spherical micelles under the same conditions. On the basis of these results, block copolymers of PEG with poly(propylene sulfide) may present distinct advantages over those with poly(propylene glycol) for a number of applications.

Adsorption kinetics of amphiphilic diblock copolymers: from kinetically frozen colloids to macrosurfactants

We investigated the spontaneous adsorption properties of charged amphiphilic diblock copolymers on hydrophobic surfaces and explained the transition of behavior from depleting frozen colloids (that do not adsorb at all) to fast adsorbing macrosurfactants when the hydrophobicity of the nonsoluble block is reduced. Three copolymer families have been used with the same hydrophilic block poly(acrylic acid), a weak acid whose ionization alpha can be varied by changing the pH. The hydrophobic blocks polystyrene, PS, poly(n-butyl acrylate), PBA, and poly(diethylene glycol ethyl ether acrylate), PDEGA, have interfacial tensions with water gammacore/solvent, respectively, of 32, 20, and 3 mN/m. We were mainly interested in the regime of high ionization alpha > 0.3, where PAA chains have no affinity for hydrophobic surfaces, and we verified experimentally that micelles do not adsorb directly. With the three copolymer families we show that the adsorption kinetics at an early stage is driven by the self-assembly properties in bulk solution: adsorption is hampered for PS-b-PAA (physically/kinetically frozen micelles in solution), controlled by unimer extraction for PBA-b-PAA (nonequilibrium micelles in solution with very low CMC < 10-4 wt %), and controlled by unimer diffusion and electrostatic repulsion for PDEGA-b-PAA (micelles at equilibrium in solution with high CMC is approximately 1-5 wt %). This explains the power law dependences of adsorption with concentration as C-1 for PBA-b-PAA and C-2 for PDEGA-b-PAA. It is finally the interfacial tension with water of the nonsoluble block and not its glass transition that is the main control of bulk solution self-assembly and consequently of the adsorption kinetics properties of amphiphilic diblocks. We also proved by preparative GPC that the fraction of non-self-assembling diblock chains, which exists in all highly hydrophobic amphiphilic diblock systems, plays a negligible role in the adsorption properties. Finally, we investigated the intrinsic thermodynamic affinity between amphiphilic diblocks and hydrophobic surfaces. We show quantitatively that this affinity depends dominantly on the interfacial energies between the hydrophobic block, the surface, and water: diblocks with strongly hydrophobic nonsoluble blocks (PS, PBA) have a low affinity for weakly hydrophobic surfaces, and oppositely, diblocks with weakly hydrophobic nonsoluble block (PDEGA) have a universal affinity for hydrophobic surfaces (like small-molecule surfactants but for different physical reasons). Finally, we showed via surface rheology that when adsorption occurs anchoring is strong and irreversible for very hydrophobic diblocks (PBA-b-PAA) and weaker and (partially) reversible for less hydrophobic diblocks (PDEGA-b-PAA).

Chemical analysis and aqueous solution properties of charged amphiphilic block copolymers PBA-b-PAA synthesized by MADIX®

We have linked the structural and dynamic properties in aqueous solution of amphiphilic charged diblock copolymers poly(butyl acrylate)-b-poly(acrylic acid), PBA-b-PAA, synthesized by controlled radical polymerization, with the physico-chemical characteristics of the samples. Despite product imperfections, the samples self-assemble in melt and aqueous solutions as predicted by monodisperse microphase separation theory. However, the PBA core are abnormally large; the swelling of PBA cores is not due to AA (the Flory parameter chi(PBA/PAA), determined at 0.25, means strong segregation), but to h-PBA homopolymers (content determined by liquid chromatography at the point of exclusion and adsorption transition, LC-PEAT). Beside the dominant population of micelles detected by scattering experiments, capillary electrophoresis CE analysis permitted detection of two other populations, one of h-PAA, and the other of free PBA-b-PAA chains, that have very short PBA blocks and never self-assemble. Despite the presence of these free unimers, the self-assembly in solution was found out of equilibrium: the aggregation state is history dependant and no unimer exchange between micelles occurs over months (time-evolution SANS). The high PBA/water interfacial tension, measured at 20 mN/m, prohibits unimer exchange between micelles. PBA-b-PAA solution systems are neither at thermal equilibrium nor completely frozen systems: internal fractionation of individual aggregates can occur.