Charged Copolymers Research Articles

Growing polyion complex micelles: kinetics and mechanism of electrostatic template polymerization and assembly

HypothesisElectrostatic-templated polymerization (ETP) is a recently developed strategy for robust fabrication of stable polyion complex (PIC) micelles with regulated size and morphology, yet the kinetics and mechanism about this one pot process remain elusive. ExperimentsIn ETP method, ionic monomers are polymerized in the presence of an oppositely charged ionic-neutral diblock copolymer as template. We investigate the monomer conversion and electrostatic assembly as a function of time, under different polymerization conditions of ionic strength, pH, template/monomer molar ratio and the presence of a cross-linker. FindingsThe template copolymer accelerates the monomer conversion and formation of PIC micelles dependent on ionic strength and pH. The process follows the “Pick-up” mechanism, where monomers first convert into oligomers which complex with template to induce further chain growth and electrostatic assembly. Introducing cross-linker hardly impacts the reaction kinetics and “Pick-up” route, while it creates PIC micelles containing cross-linked ionic network assembly with the template. Further removing the template by concentrated salt provides purified ionic nanogels. The disclosed findings not only provide a better understanding of the polymerization-assembly process, but also optimize the controls of electrostatic-templated polymerization for the rational design and fabrication of diverse PIC micelles and polyelectrolyte particles.

Mixing Order Asymmetry in Nanoparticle-Polymer Complexation and Precipitation Revealed by Isothermal Titration Calorimetry.

In recent years, there has been a renewed interest in complex coacervation, driven by concerted efforts to offer novel experimental and theoretical insights into electrostatic charge-induced association. While previous studies have primarily focused on polyelectrolytes, proteins, or surfactants, our work explores the potential of using cerium (CeO2) and iron (γ-Fe2O3) oxide nanoparticles (NPs) to develop innovative nanomaterials. By combining various charged species, such as polyelectrolytes, charged neutral block copolymers, and coated NPs, we study a wide variety of complexation patterns and compare them using isothermal titration calorimetry, light scattering, and microscopy. These techniques confirm that the titration of oppositely charged species occurs in two steps: the formation of polyelectrolyte complexes and subsequent phase (or microphase) separation, depending on the system studied. Across all examined cases, the entropic contribution to the total free energy surpasses the enthalpic contribution, in agreement with counterion release mechanisms. Furthermore, our investigation reveals a consistent asymmetry in the reaction enthalpy associated with the secondary process, with exothermic profiles observed upon the addition of cationic species to anionic ones and endothermic profiles in the reverse case.

Bridge-rich and loop-less hydrogel networks through suppressed micellization of multiblock polyelectrolytes

Most triblock copolymer-based physical hydrogels form three-dimensional networks through micellar packing, and formation of polymer loops represents a topological defect that diminishes hydrogel elasticity. This effect can be mitigated by maximizing the fraction of elastically effective bridges in the hydrogel network. Herein, we report hydrogels constructed by complexing oppositely charged multiblock copolymers designed with a sequence pattern that maximizes the entropic and enthalpic penalty of micellization. These copolymers self-assemble into branched and bridge-rich network units (netmers), instead of forming sparsely interlinked micelles. We find that the storage modulus of the netmer-based hydrogel is 11.5 times higher than that of the micelle-based hydrogel. Complementary coarse grained molecular dynamics simulations reveal that in the netmer-based hydrogels, the numbers of charge-complexed nodes and mechanically reinforcing bridges increase substantially relative to micelle-based hydrogels.

In Situ Polymerized Zwitterionic Copolymer Ionic Gel Electrolytes with High Performance for Lithium-Ion Batteries.

Gel electrolytes are a promising research direction due to their high safety. However, its poor room temperature conductivity along with complex preparation process hinder its practical application. In this article, a type of zwitterionic gel electrolyte is prepared by in situ polymerization. The introduction of charged but nonmigrating zwitterionic copolymer in the polymer chain is beneficial to the dissociation of the lithium salt, improving the ion transport of the electrolyte on this account. At room temperature, the conductivity of lithium ion reaches 9.1 × 10-4 S cm-1, which contributes to achieve excellent electrochemical performance at high rates. The assembled Li|LiFePO4 cell also shows a capacity retention rate of 90.5% after 150 cycles at 0.5 C at room temperature as well as remarkable cycle stability at 1 C. These offer a novel tactic for the efficient and safe commercial application of lithium-ion batteries.

Volume Phase Transition of Thermoresponsive Microgels Scrutinized by Dynamic Light Scattering and Turbidity: Correlations Depend on Microgel Homogeneity.

Thermoresponsive microgels experience a volume phase transition triggered by temperature changes, a phenomenon often analyzed using dynamic light scattering to observe overall size alterations via the diffusion coefficient. However, local structural changes are typically assessed using more intricate and expensive techniques like small-angle neutron or X-ray scattering. In our research, we investigate the volume phase transition of poly-N-isopropylacrylamide (PNIPAM)-based microgels by employing a combination of temperature-dependent dynamic light scattering and simpler, faster, and more efficient attenuation measurements. We utilize attenuation at a fixed wavelength as a direct measure of dispersion turbidity, linking the absolute changes in hydrodynamic radius to the absolute changes in turbidity. This approach allows us to compare "classical" PNIPAM microgels from precipitation polymerization, charged copolymer microgels from precipitation copolymerization, and core-shell microgels from seeded precipitation polymerization. Our study includes a systematic analysis and comparison of 30 different microgels. By directly comparing data from dynamic light scattering and attenuation spectroscopy, we gain insights into structural heterogeneity and deviations from the established fuzzy sphere morphology. Furthermore, we demonstrate how turbidity data can be converted to swelling curves.

Streamlined Formation and Manipulation of Charged Polymersomes.

Charged polymersomes are attractive for advanced material applications due to their versatile encapsulation capabilities and charge-induced functionality. Although desirable, the pH-sensitivity of charged block copolymers adds complexity to its self-assembly process, making it challenging to produce charged polymersomes in a reliable manner. In this work, a flow approach to control and strike a delicate balance between solvent composition and pH for self-assembly is used. This allowsfor the identification of a phase window to reliably produce of charged polymersomes. The utility of this approach to streamline downstream processes, such as morphological transformation or in-line purification is further demonstrated. As proof-of-concept, it is shown that the processed polymersomes can be used for surface modifications facilitated by charge complexation.

Tuning physicomechanical properties of PEG-interpenetrated anionically modified semi-IPN cryogels functionalized with carboxylate groups

Anionically modified interpenetrated polymer network (semi-IPN) cryogels containing linear polyethylene glycol chains (PEG) were prepared via in situ synthesis and directed assembly of the negatively charged copolymer poly(acrylamide-co-sodium acrylate) (PAN) network template. The influence of reaction conditions; the concentration of linear PEG chains, and polymerization temperature on the elasticity and swelling of these IPNs was analyzed in detail. The interpenetrating structure of PEG chains and PAN host network have been confirmed by FTIR, TGA and XRD analysis. The equilibrium water content, oscillatory swelling/deswelling kinetics, and water transport mechanism were analyzed as a function of PEG content. The results showed that semi-IPN gels exhibited different swelling behavior as the reaction conditions varied. Addition of 13.9% (w/v) PEG resulted in a 25-fold increase in the swelling ratio of semi-IPN hydrogels. By performing compression tests on semi-IPN gels to determine the change of compression properties, the effective cross-linking density of semi-IPN PAN/PEG was expressed as a second-order polynomial depending on the amount of PEG. The swollen modulus of semi-IPN cryogels was much higher than that of semi-IPN hydrogels for all PEG concentrations. Semi-IPNs showed reversible on-off switching with remarkable sensitivity to external pH stimulus. Structural parameters, degree of swelling, volume fraction of cross-linked polymer in the swollen state, and diffusion coefficients due to pH-induced swelling of semi-IPN gels were determined. The presence of linear PEG chains in the structure promoted swelling by both increasing the swelling rate and significantly reducing the time required to reach the equilibrium-state. The swelling process was Fickian diffusion, and the diffusion coefficients for swelling of PEG-interpenetrated semi-IPNs increased with PEG content. The effectiveness of removing methylene blue (MB) dye from aqueous solutions by semi-IPNs was evaluated. Batch adsorption experiments were performed as a function of the gelation temperature, amount of PEG, contact time, and initial dye concentration. The adsorption isotherm and kinetics were described well by Langmuir isotherm model and pseudo-second-order model, respectively. Adsorption of MB was a multi-step process involving surface adsorption followed by intra-particle diffusion. Semi-IPN PAN/PEG cryogels prepared under cryocondition with high adsorption capacities and fast adsorption rates have been developed as new adsorbents effective in removing dyes and other toxic substances with higher mechanical strength in water and wastewater treatment.

Molecular Simulation Study of the Kinetics and Mechanism of Micellization of Charged and Uncharged Block Copolymer in Aqueous Solution: Polystyrene-Block-Poly(acrylic acid) PS-b-PAA

The micellization kinetics, dynamics, and diffusional properties of poly(styrene)-block-poly(acrylic acid) copolymer chains in salt-free aqueous solution have been investigated as a function of the copolymer composition (XPS ) for the cases of un-ionized (charge density f = 0) and ionized PAA blocks (charge density f = 1), for the first time to our knowledge via atomistic molecular dynamic simulations. The micelle formation mechanism was inspected by tracking the population of unimers and clusters across the simulation trajectory; it confirmed that the asymmetric copolymer micelle formation followed a combined approach of unimer insertion and cluster fusion mechanisms, while symmetric micelle formation followed the unimer insertion method exclusively. Micelle formation took a longer time for copolymers having charged (ionized) PAA blocks (f > 0) and relatively short PS blocks (number fraction of PS repeat units in copolymer XPS < 0.5) due to the presence of a greater number of hydrophobic groups, in agreement with the micellization kinetics observed for model copolymer micelles via DPD (Dissipative Particle Dynamics) simulation studies in literature. The conformational dynamics were studied using the relaxation of backbone dihedral angles and radius of gyration, for the core and corona blocks, respectively, showed that the PS blocks relaxed slower as compared to the soluble PAA blocks in solution due to their hydrophobic nature. The conformational relaxation time increased linearly for the PS blocks and was invariant for the PAA blocks with respect to XPS. The interaction dynamics of the PS-b-PAA copolymer micelles studied via the relaxation times of the PAA-PAA inter-chain (τHB,PP) and PAA-water inter-molecular hydrogen bonds (τHB,PW) showed the increase of τHB,PP, and the decrease of τHB,PW in solution with the increase in XPS. The PAA-water H-bond relaxes slower at f = 1 than that at f = 0 due to the stronger affinity of ionized PAA units with water molecules. The diffusivity of the copolymer chains decreased exponentially with an increase in XPS value, which is in agreement with the theoretical and experimental observations described in the literature.

Phase Behavior of Charged Star Block Copolymers at Fluids Interface

AbstractThe phase behavior of block copolymers (BCPs) at the water‐oil interface is influenced by the segmental interaction parameter ( ) and chain architecture. We synthesized a series of star block copolymers (s‐BCPs) having polystyrene (PS) as core and poly(2‐vinylpyridine) (P2VP) as corona. The interaction parameters of block‐block ( PS‐P2VP) and block‐solvent ( P2VP‐solvent) were varied by adjusting the pH of the aqueous solution. Lowering pH increased the fraction of quaternized‐P2VP (Q‐P2VP) with enhanced hydrophilicity. By transferring the equilibrated interfacial assemblies, morphologies ranging from bicontinuous films at pH of 7 and 3.1 to nanoporous and nanotubular structure at pH of 0.65 were observed. The nanoporous films formed hexagonally packed pores in s‐BCP matrix, while nanotubes comprised Q‐P2VP as corona and PS as core. Control over pore size, d‐spacing between pores, and nanotube diameters was achieved by varying polymer concentration, molecular weight, volume fraction and arm number of s‐BCPs. Large‐scale nanoporous films were obtained by freeze‐drying emulsions. Remarkably, the morphologies of linear BCPs were inverted, forming hexagonal‐packed rigid spherical micelles with Q‐P2VP as core and PS as corona in multilayer. This work provides insights of phase behaviors of BCP at fluids interface and offer a facile approach to prepare nanoporous film with well‐controlled pore structure.

Phase Behavior of Charged Star Block Copolymers at Fluids Interface.

The phase behavior of block copolymers (BCPs) at the water-oil interface is influenced by the segmental interaction parameter ( ) and chain architecture. We synthesized a series of star block copolymers (s-BCPs) having polystyrene (PS) as core and poly(2-vinylpyridine) (P2VP) as corona. The interaction parameters of block-block ( PS-P2VP ) and block-solvent ( P2VP-solvent ) were varied by adjusting the pH of the aqueous solution. Lowering pH increased the fraction of quaternized-P2VP (Q-P2VP) with enhanced hydrophilicity. By transferring the equilibrated interfacial assemblies, morphologies ranging from bicontinuous films at pH of 7 and 3.1 to nanoporous and nanotubular structure at pH of 0.65 were observed. The nanoporous films formed hexagonally packed pores in s-BCP matrix, while nanotubes comprised Q-P2VP as corona and PS as core. Control over pore size, d-spacing between pores, and nanotube diameters was achieved by varying polymer concentration, molecular weight, volume fraction and arm number of s-BCPs. Large-scale nanoporous films were obtained by freeze-drying emulsions. Remarkably, the morphologies of linear BCPs were inverted, forming hexagonal-packed rigid spherical micelles with Q-P2VP as core and PS as corona in multilayer. This work provides insights of phase behaviors of BCP at fluids interface and offer a facile approach to prepare nanoporous film with well-controlled pore structure.

Engineering soft nanochannels in isoporous block copolymer nanofiltration membranes for ion separation

Membranes with high ion selectivity are important for the separation of ionic species at the Ångstrom scale. Here, we report on positively charged isoporous block copolymer (BCP) nanofiltration membranes with designed soft nanochannels. The isoporous BCP membranes were obtained from the combination of BCP self-assembly and non-solvent-induced phase separation. The well-defined soft nanochannels were engineered via a versatile post-modification approach using a combination of di- and monofunctional alkyl halide. The water permeance and effective pore size of the membranes were fine-tuned within the nanofiltration regime. Both single- and binary-salt retentions demonstrate the capability of separating mono-/divalent cations based on the ionic size difference in Ångstrom scale, charge, and energy to strip away the hydration shells. Especially the real selectivity from the binary-salt systems is > 2 times higher than the selectivity obtained from the single-salt systems, e.g., the real selectivity of K+/Mg2+ is up to 15 for the 10 mM binary-salt mixture with a mole ratio of 1:1. Such enhancement arises from the competition effect regarding energy barrier, size of hydrated ion, and diffusivity. The Na+/Mg2+ binary-salt system with simulating the salt concentration ratio of the seawater illustrates the potential of the resulting membranes to recover the valuable ionic species from water, e.g. Mg, classified as a “critical raw material” in Europe. The Na+/Mg2+ real selectivity is up to 42 with a permeance of 15 L m−2 h−1 MPa−1 at a salt concentration of 10 mM.

Solvent-Free Synthesis of Multifunctional Block Copolymer and Formation of DNA and Drug Nanocarriers.

The synthesis of well-defined multifunctional polymers is of great importance for the development of complex materials for biomedical applications. In the current work, novel and multi-amino-functional diblock copolymer for potential gene and drug delivery applications was successfully synthesized. A highly efficient one-step and quantitative modification of an alkyne-functional polycarbonate-based precursor was performed, yielding double hydrophilic block copolymer with densely grafted primary amine side groups. The obtained positively charged block copolymer co-associated with DNA, forming stable and biocompatible nanosized polyplexes. Furthermore, polyion complex (PIC) micelles with tunable surface charge and decorated with cell targeting moieties were obtained as a result of direct mixing in aqueous media of the multi-amino-functional block copolymer and a previously synthesized oppositely charged block copolymer bearing disaccharide end-group. The obtained well-defined nanosized PIC-micelles were loaded with the hydrophobic drug curcumin. Both types of nanoaggregates (polyplexes and PIC-micelles) were physico-chemically characterized. Moreover, initial in vitro evaluations were performed to assess the nanocarriers' potential for biomedical applications.

Smart superplasticizers based on redox-responsive polymers for rheology control of cementitious materials

A new concept is introduced for active rheology control of cementitious materials using stimuli-responsive polymers. RAFT polymerization of methacrylic acid (MAA) and 2,2,6,6, tetramethyl-4-piperidyl methacrylate (TMPMA) was employed to prepare poly(MAA-co-TMPMA) as precursor for the preparation of poly(MAA-co-TEMPO), a redox-responsive copolymer containing 2,2,6,6-tetramethylpiperidine-1-oxyl group. Poly(MAA-co-TEMPO) is soluble in alkaline water and undergoes a redox switch when electrically triggered, oxidizing the TEMPO group to the oxoammonium cations (TEMPO+). The observed active stiffening control is hypothesized to result from adsorption of the negatively charged copolymer with the TEMPO side chains on cement particles and (partial) desorption of the zwitterionic copolymer with the TEMPO+ side chains. The new chemical admixtures could enhance flowability of cement pastes at low W/C ratio and sharply increase storage modulus when applying an electric potential of 1 V. Poly(MAA-co-TEMPO) acts as smart superplasticizer that can actively alter the rheology of cementitious materials upon applying an electric voltage.

Synthesis of a novel next-generation positively charged polymer and its in-situ grafting into thin film composite membranes to enhance the performance for desalination

High efficiency of the polyamide membranes can be achieved by rationally tailoring the active layer, which empowers the membrane with better anti-fouling characteristics, high flux, and separation efficiencies. Herein, a next-generation anti-fouling positively charged copolymer of diallyldimethylammonium chloride (DADMAC) and N1, N1-diallyldodecane-1,12-diammonium chloride (DADAC) was synthesized and thoroughly characterized by TGA, FTIR, proton (1H) and the carbon (13C) NMR. The synthesized poly(DADMAC-co-DADAC), after conversion to poly(DADMAC-co-DADA) having free NH2 group, was in-Situ grafted while forming the active layer due to its rationally designed anchoring points of NH2 group. The in-Situ grafting resulted in a cross-linked polyamide active layer containing positively charged brushes of poly (DADMAC-co-DADA) at regular intervals. The AFM analysis has shown that the polyamide layer's surface roughness increased with the poly-(DADMAC-co-DADA) grafting concentration, which positively impacts the membranes' flux. When exposed to the water, the grafted poly-(DADMAC-co-DADA) expanded and spread a positively quaternary ammonium network on the membrane surface, effectively repelling the cations. The introduced membrane has shown high performance compared to the pristine polyamide membrane. The functionalized membranes have displayed a 3.5 times flux for 2000 ppm of NaCl while maintaining a good rejection at 93.1%. The anti-fouling behavior of the membranes was evaluated against the model positively charged CTAB (cetyltrimethylammonium bromide) foulants (2000 ppm). Poly- (DADMAC-co-DADA) grafted membranes have shown a high anti-fouling tendency. After 600 min (10 h) of continuous operation, the flux declined less than 15%, whereas the pristine polyamide membrane lost 99%. The poly-(DADMAC-co-DADA) grafted membranes recovered more than 93% of their maximum flux. Thus, the in-Situ grafting of positively charged polymer is a possible opportunity to develop the next generation of high-performance polyamide membranes.

Preparation of Water-Soluble Polyion Complex (PIC) Micelles with Random Copolymers Containing Pendant Quaternary Ammonium and Sulfonate Groups.

Cationic random copolymers (PCm) consisting of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC; P) with methacroylcholine chloride (MCC; C) and anionic random copolymers (PSn) consisting of MPC and potassium 3-(methacryloyloxy)propanesulfonate (MPS; S) were prepared via a reversible addition-fragmentation chain transfer method. "m" and "n" represent the compositions (mol %) of the MCC and MPS units in the copolymers, respectively. The degrees of polymerization for the copolymers were 93-99. Water-soluble MPC unit contains a pendant zwitterionic phosphorylcholine group whose charges are neutralized in pendant groups. MCC and MPS units contain the cationic quaternary ammonium and anionic sulfonate groups, respectively. The stoichiometrically charge-neutralized mixture of a matched pair of PCm and PSn aqueous solutions resulted in the spontaneous formation of water-soluble PCm/PSn polyion complex (PIC) micelles. These PIC micelles have the MPC-rich surface and MCC/MPS core. These PIC micelles were characterized using 1H NMR, dynamic and static light scattering, and transmission electron microscopic measurements. The hydrodynamic radius of these PIC micelles depends on the mixing ratio of the oppositely charged random copolymers. The charge-neutralized mixture formed maximum-size PIC micelles.

Tailoring Interactions of Random Copolymer Polyelectrolyte Complexes to Remove Nanoplastic Contaminants from Water

We investigate the usage of polyelectrolyte complex materials for water remediation purposes, specifically their ability to remove nanoplastics from water, on which there is currently little to no prior research. We demonstrate that oppositely charged random copolymers are effective at quantitatively removing nanoplastic contamination from aqueous solution. The mechanisms underlying this remediation ability are explored through computational simulations, with corroborating quartz crystal microbalance adsorption experiments. We find that hydrophobic nanostructures and interactions likely play an important role.

Cross-Linkable Polymer-Based Multi-layers for Protecting Electrochemical Glucose Biosensors against Uric Acid, Ascorbic Acid, and Biofouling Interferences.

The lifetime of implantable electrochemical glucose monitoring devices is limited due to the foreign body response and detrimental effects from ascorbic acid (AA) and uric acid (UA) interferents that are components of physiological media. Polymer coatings can be used to shield biosensors from these interferences and prolong their functional lifetime. This work explored several approaches to protect redox polymer-based glucose biosensors against such interferences by designing six targeted multi-layer sensor architectures. Biological interferents, like cells and proteins, and UA and AA interferents were found to have individual effects on the current density and operational stability of glucose biosensors, requiring individual protection and treatment. Protection against biofouling can be achieved using a poly(2-methacryloyloxyethyl phosphorylcholine-co-glycidyl methacrylate) (MPC) zwitterionic polymer coating. An enzyme-scavenging approach was compared to electrostatic repulsion by negatively charged polymers for protection against AA and UA interferences. A multi-layer novel polymer design (PD) system consisting of a cross-linkable negatively charged polyvinylimidazole-polysulfostyrene co-polymer inner layer and a cross-linkable MPC zwitterionic polymer outer layer showed the best protection against AA, UA, and biological interferences. The sensor protected using the novel PD shield displayed the lowest mean absolute relative difference between the glucose reading without the interferent and the reading value with the interferent present and also displayed the lowest variability in sensor readings in complex media. For sensor measurements in artificial plasma, the novel PD extends the linear range (R2 = 0.99) of the sensor from 0-10 mM for the control to 0-20 mM, shows a smaller decrease in sensitivity, and retains high current densities. The application of PD multi-target coating improves sensor performance in complex media and shows promise for use in sensors operating in real conditions.

Self-Assembly of Charged Diblock Copolymers with Reduced Backbone Polarity

Charged block copolymers that can self-assemble into a host of nanostructures offer a great opportunity as next-generation battery electrolytes with outstanding ionic conductivity and mechanical robustness. The impact of ions on the self-assembly of charged block copolymers, however, remains to be fully understood. In this article, we report the phase behavior of charged-neutral block copolymers where a relatively nonpolar matrix was employed in the charged block to boost the strength of electrostatic interaction. The phase behavior was established using small-angle X-ray scattering (SAXS). We found that the overall shape of the phase boundary between ordered and disordered states is asymmetric, tilting toward the charged block-lean side of the phase portrait. However, the composition windows of the ordered phases, such as lamellae and hexagonally packed cylinders observed in this study, are comparable to those of neutral diblock copolymers and are not obviously affected by the incorporation of charges. The results obtained in this work provide insight into the impact of ions on the self-assembly of charged block copolymers as well as the design of nanostructured polymer electrolytes.

Assembly of polyelectrolyte star block copolymers at the oil-water interface.

To understand and resolve adsorption, reconfiguration, and equilibrium conformations of charged star copolymers, we carried out an integrated experimental and coarse-grained molecular dynamics simulation study of the assembly process at the oil-water interface. This is important to guide development of novel surfactants or amphiphiles for chemical transformations and separations. The star block copolymer consisted of arms that are comprised of hydrophilic-hydrophobic block copolymers that are covalently tethered via the hydrophobic blocks to one point. The hydrophobic core represents polystyrene (PS) chains, while the hydrophilic corona represents quaternized poly(2-vinylpyridine) (P2VP) chains. The P2VP is modeled to become protonated when in contact with an acidic aqueous phase, thereby massively increasing the hydrophilicity of this block, and changing the nature of the star at the oil-water interface. This results in a configurational change whereby the chains comprising the hydrophilic corona are significantly stretched into the aqueous phase, while the hydrophobic core remains solubilized in the oil phase. In the simulations, we followed the kinetics of the anchoring and assembly of the star block copolymer at the interface, monitoring the lateral assembly, and the subsequent reconfiguration of the star via changes in the interfacial tension that varies as the degree-of-protonation increases. At low fractions of protonation, the arm cannot fully partition into the aqueous side of the interface and instead interacts with other arms in the oil phase forming a network near the interface. These insights were used to interpret the non-monotonic dependence of pH with the asymptotic interfacial tension from pendant drop tensiometry experiments and spectral signatures of aromatic stretches seen in vibrational sum frequency generation (SFG) spectroscopy. We describe the relationship of interfacial tension to the star assembly via the Frumkin isotherm, which phenomenologically describes anti-cooperativity in adsorbing stars to the interface due to crowding. Although our model explicitly considers long-range electrostatics, the contribution of electrostatics to interfacial tension is small and brought about by strong counterion condensation at the interface. These results provide key insights into resolving the adsorption, reconfiguration, and equilibrium conformations of charged star block copolymers as surfactants.

Overcoming the Dilemma of Permeability and Stability of Polymersomes through Traceless Cross-Linking.

In nature, cells are highly compartmentalized into many organelles that are well separated from the rest of the cellular space by unique membrane structures, which are of crucial importance to allow cells to perform various physiological functions in such a small and crowded space. Learning from the ubiquitous membrane structures of cells and organelles has continuously inspired the development of artificial self-assembled nanostructures, with lipid vesicles (liposomes) and polymer vesicles (polymersomes) being the most representative examples. Similar to the membrane-bound structures of cells and organelles, both liposomes and polymersomes contain an aqueous interior enclosed by a bilayer membrane. Therefore, liposomes and polymersomes have been extensively investigated to mimic the fundamental structures and functions of living cells. For example, liposomes and polymersomes have been successfully engineered as nanocarriers, smart nanoreactors, artificial organelles, and so on. Notably, living cells can exchange both energy and materials with surrounding environments, benefiting from the selective permeability of lipid membranes. The permselectivity of cell membranes is thus an essential attribute of living organisms. Compared to liposomes, polymersomes have increased structural stability but low membrane permeability. Indeed, polymersomes are almost impermeable to small molecules, ions, and even water molecules. To improve the permeability of polymersomes, much effort has been devoted to the incorporation of channel proteins, the coassembly of oppositely charged block copolymers (BCPs), the development of stimuli-responsive BCPs, and so on. Despite great achievements, these approaches generally lead to decreased stability of polymersomes and, sometimes, polymersome disintegration. In this Account, we discuss our recent efforts to reconcile the stability and permeability of polymersomes via a traceless cross-linking approach. Although cross-linking reactions within bilayer membranes generally lead to decreased permeability, the traceless cross-linking approach can concurrently improve the stability and permeability of polymersomes. Specifically, stimuli-responsive polymersomes undergo either covalent cross-linking or noncovalent cross-linking reactions under specific stimuli to increase bilayer stability, while the cross-linking processes can concurrently permeabilize polymersome bilayers through cross-linking-driven hydrophobic-to-hydrophilic transitions. Notably, unlike conventional cross-linking processes requiring additional cross-linkers, the traceless cross-linking process does not involve extra cross-linking agents but takes full advantage of the in situ generated active moieties. By taking advantage of the simultaneous modulation of the stability and permeability of polymersomes via traceless cross-linking, these polymersomes can be further engineered as smart nanocarriers and nanoreactors. The robustness and generality of this approach have been validated by both extracellular and intracellular stimuli such as light irradiation, glutathione, and hydrogen peroxide. Moreover, many functional groups such as fluorescent dyes and contrast agents can be integrated into this versatile platform as well, enabling the construction of theranostic nanovectors capable of responding to pathological microenvironments. This Account provides a new approach to regulating the permeability of polymersomes while maintaining their structural stability.