Effect of Salt on Self-Assembly in Charged Block Copolymer Micelles

Overlooked Activation Role of Sulfite in Accelerating Hydrated Electron Treatment of Perfluorosulfonates

Photoexcitation of sulfite (SO32-) is often used to generate hydrated electrons (eaq-) in processes to degrade perfluoroalkyl and polyfluoroalkyl substances (PFASs). Conventional consensus discourages the utilization of SO32- concentrations exceeding 10 mM for effective defluorination. This has hindered our understanding of SO32- chemistry beyond its electron photogeneration properties. In contrast, the radiation-chemical study presented here, directly producing eaq- through water radiolysis, suggests that SO32- plays a previously overlooked activation role in the defluorination. Quantitative 60Co gamma irradiation experiments indicate that the increased SO32- concentration from 0.1 to 1 M enhances the defluorination rate by a remarkable 15-fold, especially for short-chain perfluoroalkyl sulfonate (PFSA). Furthermore, during the treatment of long-chain PFSA (C8F17-SO3-) with a higher concentration of SO32-, the intermediates of C8H17-SO3- and C3F7-COO- were observed, which are absent without SO32-. These observations highlight that a higher concentration of SO32- facilitates both reaction pathways: chain shortening and H/F exchange. Pulse radiolysis measurements show that elevated SO32- concentrations accelerate the bimolecular reaction between eaq- and PFSA by 2 orders of magnitude. 19F NMR measurements and theoretical simulations reveal the noncovalent interactions between SO32- and F atoms, which exceptionally reduce the C-F bond dissociation energy by nearly 40%. As a result, our study offers a more effective strategy for degrading highly persistent PFSA contaminants.

Multivalent Salt-Induced Self-Assembly of Amphiphilic Polyelectrolytes of Different Charge Fractions: A Coarse-Grained Molecular Dynamics Simulation Study

Amphiphilic polymers with both hydrophobic and hydrophilic blocks are of great interest for their potential applications in drug delivery. Their self-assembly behavior in response to environmental factors like ion charge and multivalent salt concentration has been the subject of recent investigation. Our study utilizes coarse-grained molecular dynamics simulations to investigate the aggregation behavior of amphiphilic copolymers upon introducing tetravalent salt at varying charge fractions. We identify a critical concentration, Cs*, where the aggregation number reaches its maximum for each charge fraction, followed by a subsequent decrease at the excessive salt regime. This study reveals distinct morphological transitions in response to increasing salt concentration and decreasing charged fractions, namely, (i) stable dispersed micelles, (ii) a singular micelle comprising all copolymer chains, and (iii) redispersed micelles, particularly evident at lower charged fractions. Our study highlights the significant influence of tetravalent salt and charge fractions of polyelectrolyte chains on the self-assembly behavior of polyelectrolyte copolymers.

Ionic Strength-Dependent Structure of Complex Coacervate Core Micelles

Salt concentration-dependent structure of complex coacervate core micelles (C3Ms), formed by polyether-based block copolyelectrolytes containing cationic ammonium (A) or anionic sulfonate (S) groups in aqueous media, is investigated by light scattering and small-angle X-ray/neutron scattering (SAX/NS). As the salt concentration increases, both a core radius (Rcore) and an aggregation number (Nagg) significantly decrease, but a corona thickness (Lcorona) is nearly unchanged. Larger salt concentrations can lower the interfacial tension between the coacervate cores and aqueous media, resulting in an increased interfacial area per chain and a more relaxed conformation of the core blocks. Based on the structure characterization, the scaling relationship between structure parameters (i.e., Rcore, Nagg, and Lcorona) and salt concentration is obtained and compared to the theoretical description estimated by the free energy balance between the entropic penalty of core stretching and the interfacial energy. We propose that the free energy contribution of the core block stretching is not negligible in C3Ms because of the highly swollen cores caused by water.

Solution and Film Self-Assembly Behavior of a Block Copolymer Composed of a Poly(ionic Liquid) and a Stimuli-Responsive Weak Polyelectrolyte

Cu(0)-mediated atom transfer radical polymerization was used to synthesize a poly(ionic liquid), poly[4-vinylbenzyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] (PVBBImTf2N), a stimuli-responsive polyelectrolyte, poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA), and a novel block copolymer formed from these two polymers. The synthesis of the block copolymer, poly[2-(dimethylamino) ethyl methacrylate]-block-[poly(4-vinylbenzyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] (PDMAEMA-b-PVBBImTf2N), was examined to evaluate the control of "livingness" polymerization, as indicated by molecular weight, characterizations of degree of polymerization, and 1HNMR spectroscopy. 2D DOSY NMR measurements revealed the successful formation of block copolymer and the connection between the two polymer blocks. PDMAEMA-b-PVBBImTf2N was further characterized for supramolecular interactions in both the bulk and solution states through FTIR and 1H NMR spectroscopies. While the block copolymer demonstrated similar intermolecular behavior to the PIL homopolymer in the bulk state as indicated by FTIR, hydrogen bonding and counterion interactions in solution were observed in polar organic solvent through 1H NMR measurements. The DLS characterization revealed that the PDMAEMA-b-PVBBImTf2N block copolymer forms a network-like aggregated structure due to a combination of hydrogen bonding between the PDMAEMA and PIL group and electrostatic repulsive interactions between PIL blocks. This structure was found to collapse upon the addition of KNO3 while still maintaining hydrogen bonding interactions. AFM-IR analysis demonstrated varied morphologies, with spherical PDMAEMA in PVBBImTf2N matrix morphology exhibited in the region approaching the film center. AFM-IR further revealed signals from silica nano-contaminates, which selectively interacted with the PDMAEMA spheres, demonstrating the potential for the PDMAEMA-b-PVBBImTf2N PIL block copolymer in polymer-inorganic nanoparticle composite applications.

The ins and outs of membrane bending by intrinsically disordered proteins

Membrane curvature is essential to diverse cellular functions. While classically attributed to structured domains, recent work illustrates that intrinsically disordered proteins are also potent drivers of membrane bending. Specifically, repulsive interactions among disordered domains drive convex bending, while attractive interactions drive concave bending, creating membrane-bound, liquid-like condensates. How might disordered domains that contain both repulsive and attractive domains affect curvature? Here, we examined chimeras that combined attractive and repulsive interactions. When the attractive domain was closer to the membrane, its condensation amplified steric pressure among repulsive domains, leading to convex curvature. In contrast, when the repulsive domain was closer to the membrane, attractive interactions dominated, resulting in concave curvature. Further, a transition from convex to concave curvature occurred with increasing ionic strength, which reduced repulsion while enhancing condensation. In agreement with a simple mechanical model, these results illustrate a set of design rules for membrane bending by disordered proteins.

Advances in the Structural Design of Polyelectrolyte Complex Micelles

Polyelectrolyte complex micelles (PCMs) are a unique class of self-assembled nanoparticles that form with a core of associated polycations and polyanions, microphase-separated from neutral, hydrophilic coronas in aqueous solution. The hydrated nature and structural and chemical versatility make PCMs an attractive system for delivery and for fundamental polymer physics research. By leveraging block copolymer design with controlled self-assembly, fundamental structure-property relationships can be established to tune the size, morphology, and stability of PCMs precisely in pursuit of tailored nanocarriers, ultimately offering storage, protection, transport, and delivery of active ingredients. This perspective highlights recent advances in predictive PCM design, focusing on (i) structure-property relationships to target specific nanoscale dimensions and shapes and (ii) characterization of PCM dynamics primarily using time-resolved scattering techniques. We present several vignettes from these two emerging areas of PCM research and discuss key opportunities for PCM design to advance precision medicine.

Interactions between an Associative Amphiphilic Block Polyelectrolyte and Surfactants in Water: Effect of Charge Type on Solution Properties and Aggregation

The study of interactions between polyelectrolytes (PE) and surfactants is of great interest for both fundamental and applied research. These mixtures can represent, for example, models of self-assembly and molecular organization in biological systems, but they are also relevant in industrial applications. Amphiphilic block polyelectrolytes represent an interesting class of PE, but their interactions with surfactants have not been extensively explored so far, most studies being restricted to non-associating PE. In this work, interactions between an anionic amphiphilic triblock polyelectrolyte and different types of surfactants bearing respectively negative, positive and no charge, are investigated via surface tension and solution rheology measurements for the first time. It is evidenced that the surfactants have different effects on viscosity and surface tension, depending on their charge type. Micellization of the surfactant is affected by the presence of the polymer in all cases; shear viscosity of polymer solutions decreases in presence of the same charge or nonionic surfactants, while the opposite charge surfactant causes precipitation. This study highlights the importance of the charge type, and the role of the associating hydrophobic block in the PE structure, on the solution behavior of the mixtures. Moreover, a possible interaction model is proposed, based on the obtained data.

Sequence of Polyurethane Ionomers Determinative for Core Structure of Surfactant-Copolymer Complexes

The core of micelles self-assembled from amphiphiles is hydrophobic and contains little water, whereas complex coacervate core micelles co-assembled from oppositely charged hydrophilic polymers have a hydrophilic core with a high water content. Co-assembly of ionic surfactants with ionic-neutral copolymers yields surfactant-copolymer complexes known to be capable of solubilizing both hydrophilic and hydrophobic cargo within the mixed core composed of a coacervate phase with polyelectrolyte-decorated surfactant micelles. Here we formed such complexes from asymmetric (PUI-A2) and symmetric (PUI-S2), sequence-controlled polyurethane ionomers and poly(N-methyl-2-vinylpyridinium iodide)29-b-poly(ethylene oxide)204 copolymers. The complexes with PUI-S2 were 1.3-fold larger in mass and 1.8-fold larger in radius of gyration than the PUI-A2 complexes. Small-angle X-ray scattering revealed differences in the packing of the similarly sized PUI micelles within the core of the complexes. The PUI-A2 micelles were arranged in a more ordered fashion and were spaced further apart from each other (10 nm vs. 6 nm) than the PUI-S2 micelles. Hence, this work shows that the monomer sequence of amphiphiles can be varied to alter the internal structure of surfactant-copolymer complexes. Since the structure of the micellar core may affect both the cargo loading and release, our findings suggest that these properties may be tuned through control of the monomer sequence of the micellar constituents.

Visible light-induced apoptosis activatable nanoparticles of photosensitizer-DEVD-anticancer drug conjugate for targeted cancer therapy

The therapeutic efficacy of photodynamic therapy (PDT) in cancer treatment is attributed to the conversion of tumor oxygen into reactive singlet oxygen (¹O₂) using photosensitizers. However, poor tissue penetration and rapid oxygen depletion have limited the effectiveness of PDT. Therefore, we have developed visible light-induced apoptosis activatable nanoparticles of the photosensitizer (Ce6)-caspase 3 cleavable peptide (Asp-Glu-Val-Asp, DEVD)-anticancer drug monomethyl auristatin E (MMAE) conjugate, resulting in Ce6-DEVD-MMAE nanoparticles. The average size of self-assembled Ce6-DEVD-MMAE nanoparticles was 90.8 ± 18.9 nm. Compared with conventional PDT based on high-energy irradiation, the new therapy uses lower-energy irradiation to induce apoptosis of cancer cells, and activation of caspase 3 to successfully cleave the anticancer drug MMAE from the Ce6-DEVD-MMAE nanoparticles, resulting in strong cytotoxic effects in cancer cells. Notably, the one-time activation of MMAE in the Ce6-DEVD-MMAE nanoparticles further amplified the cytotoxic effect resulting in additional cell death in the absence of visible light irradiation. Furthermore, Ce6-DEVD-MMAE nanoparticles passively accumulated in the targeted tumor tissues via enhanced permeation and retention (EPR) effect in mice with squamous cell carcinoma (SCC7). The high levels of toxicity were retained after exposure to lower-energy irradiation. However, Ce6-DEVD-MMAE nanoparticles did not show any toxicity in the absence of exposure to visible light irradiation, in contrast to the toxicity of free MMAE (1-10 nM). Thus, the light-induced therapeutic strategy based on apoptotic activation of Ce6-DEVD-MMAE nanoparticles can be used to treat solid tumors inaccessible to conventional PDT.

Physical Principles of Membrane Shape Regulation by the Glycocalyx

Cells bend their plasma membranes into highly curved forms to interact with the local environment, but how shape generation is regulated is not fully resolved. Here, we report a synergy between shape-generating processes in the cell interior and the external organization and composition of the cell-surface glycocalyx. Mucin biopolymers and long-chain polysaccharides within the glycocalyx can generate entropic forces that favor or disfavor the projection of spherical and finger-like extensions from the cell surface. A polymer brush model of the glycocalyx successfully predicts the effects of polymer size and cell-surface density on membrane morphologies. Specific glycocalyx compositions can also induce plasma membrane instabilities to generate more exotic undulating and pearled membrane structures and drive secretion of extracellular vesicles. Together, our results suggest a fundamental role for the glycocalyx in regulating curved membrane features that serve in communication between cells and with the extracellular matrix.

Macro- and Microphase Separated Protein-Polyelectrolyte Complexes: Design Parameters and Current Progress

Protein-containing polyelectrolyte complexes (PECs) are a diverse class of materials, composed of two or more oppositely charged polyelectrolytes that condense and phase separate near overall charge neutrality. Such phase-separation can take on a variety of morphologies from macrophase separated liquid condensates, to solid precipitates, to monodispersed spherical micelles. In this review, we present an overview of recent advances in protein-containing PECs, with an overall goal of defining relevant design parameters for macro- and microphase separated PECs. For both classes of PECs, the influence of protein characteristics, such as surface charge and patchiness, co-polyelectrolyte characteristics, such as charge density and structure, and overall solution characteristics, such as salt concentration and pH, are considered. After overall design features are established, potential applications in food processing, biosensing, drug delivery, and protein purification are discussed and recent characterization techniques for protein-containing PECs are highlighted.

Self-Assembly of Lysine-Based Dendritic Surfactants Modeled by the Self-Consistent Field Approach

Implementing a united atom model, we apply self-consistent field theory to study structure and thermodynamic properties of spherical micelles composed of surfactants that combine an alkyl tail with a charged lysine-based dendritic headgroup. Following experiments, the focus was on dendron surfactants with varying tail length and dendron generations G0, G1, G2. The heads are subject to acetylation modification which reduces the charge and hydrophilicity. We establish a reasonable parameter set which results in semiquantitative agreement with the available experiments. The critical micellization concentration, aggregation number, and micelle size are discussed. The strongly charged dendronic surfactants micelles are stable for generation numbers G0 and G1, for progressively higher ionic strengths. Associates of G2 surfactants are very small and can only be found at extreme surfactant concentration and salt strengths. Micelles of corresponding weaker charged acetylated variants exist up to G2, tolerate significantly lower salt concentrations, but lose the spherical micelle topology for G0 at high ionic strengths.

Effect of Counterions on the Self-Assembly of Polystyrene-Polyphosphonium Block Copolymers

The ability to manipulate block copolymers on the nanoscale has led to many scientific and technological advances. These include nanoscale ordered bulk and thin films and also solution phase components; these are promising materials for making smaller ordered electronics, selective membranes, and also biomedical applications. The ability to manipulate block copolymer material architectures on such small scales has risen from thorough investigations into the properties that affect the architectures. Polyelectrolytes are an important class of polymers that are used to make amphiphilic block copolymers. In this context the authors synthesized polystyrene-b-polyphosphonium block copolymers with different anions coordinated to the polyphosphonium block in order to study the effect of the anion on the aqueous self-assembly of the polymers. The anions play an important role in the solubility of the monomeric materials which results in differences in the self-assembly observed through dynamic light scattering and transmission electron microscopy.

pH and Salt Effects on Surface Activity and Self-Assembly of Copolymers Containing a Weak Polybase

Copolymers with well-defined architectures, controlled molecular weights, and narrow molar mass dispersities (Đ) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The resultant polymers contain different combinations of the pH-responsive monomer 2-(diethylaminoethyl) methacrylate (DEAEMA), the hydrophobic comonomer butyl methacrylate (BMA), and a neutral hydrophilic stabilizing monomer polyethylene glycol monomethyl ether methacrylate (designated O950). Surface tension and cryo-TEM measurements of native and heavy-atom stained samples were used to characterize the pH and salt responsiveness of the different polymers as a function of their composition. These studies indicate that while the polymers predominately self-assemble to form spherical micelles, a narrow size distribution is observed in aqueous solutions of poly(O950)-b-(BMA) and poly(O950)-b-(DEAEMA-co-BMA), whereas a broad size distribution characterizes the assemblies of poly(O950)-b-(DEAEMA) and poly(DEAEMA-co-BMA). In the latter case, micelles having diameters around 15-25 nm are found along with smaller aggregates (about 10 nm) mostly arranged in elongated necklace-like structures. The pH and salt-responsiveness of the DEAEMA residue, as indicated by the surface activity of the copolymers, was found to depend on the nature of the additional components: covalently linked hydrophobic groups (BMA) moderated the pH response of the copolymer as compared to nonionic and hydrophilic groups as in poly(O950)-b-(DEAEMA). These results suggest that mutual interactions among the building blocks of self-assembling copolymers should be taken into account when designing responsive copolymers.

The electrostatic co-assembly in non-stoichiometric aqueous mixtures of copolymers composed of one neutral water-soluble and one polyelectrolyte (either positively or negatively charged) block: a dissipative particle dynamics study

The electrostatic co-assembly in non-stoichiometric aqueous mixtures of diblock copolymers.

Prediction of solvent-induced morphological changes of polyelectrolyte diblock copolymer micelles

Self-assembly processes of polyelectrolyte block copolymers are ubiquitous in industrial and biological processes; understanding their physical properties can also provide insights into the design of polyelectrolyte materials with novel and tailored properties. Here, we report systematic analysis on how the ionic strength of the solvent and the length of the polyelectrolyte block affect the self-assembly and morphology of the polyelectrolyte block copolymer materials by constructing a salt-dependent morphological phase diagram using an implicit solvent ionic strength (ISIS) method for dissipative particle dynamics (DPD) simulations. This diagram permits the determination of the conditions for the morphological transition into a specific shape, namely vesicles or lamellar aggregates, wormlike/cylindrical micelles, and spherical micelles. The scaling behavior for the size of spherical micelles is predicted, in terms of radius of gyration (R(g,m)) and thickness of corona (Hcorona), as a function of solvent ionic strength (c(s)) and polyelectrolyte length (NA), which are R(g,m) ∼ c(s)(-0.06)N(A)(0.54) and Hcorona ∼ c(s)(-0.11)N(A)(0.75). The simulation results were corroborated through AFM and static light scattering measurements on the example of the self-assembly of monodisperse, single-stranded DNA block-copolynucleotides (polyT50-b-F-dUTP). Overall, we were able to predict the salt-responsive morphology of polyelectrolyte materials in aqueous solution and show that a spherical-cylindrical-lamellar change in morphology can be obtained through an increase in solvent ionic strength or a decrease of polyelectrolyte length.

Light-triggered, self-immolative nucleic Acid-drug nanostructures

The simultaneous intracellular delivery of multiple types of payloads, such as hydrophobic drugs and nucleic acids, typically requires complex carrier systems. Herein, we demonstrate a self-deliverable form of nucleic acid-drug nanostructure that is composed almost entirely of payload molecules. Upon light activation, the nanostructure sheds the nucleic acid shell, while the core, which consists of prodrug molecules, disintegrates via an irreversible self-immolative process, releasing free drug molecules and small molecule fragments. We demonstrate that the nanostructures exhibit enhanced stability against DNase I compared with free DNA, and that the model drug (camptothecin) released exhibits similar efficacy as free, unmodified drugs toward cancer cells.

Self-assembly of a hydrophobically end-capped charged amphiphilic triblock copolymer: effects of temperature and salinity

This study elucidates the intricate interplay between hydrophobic and electrostatic interactions in aqueous solutions of a responsive charged triblock copolymer.

Effect of local chain deformability on the temperature-induced morphological transitions of polystyrene-b-poly(N-isopropylacrylamide) micelles in aqueous solution

The effect of temperature on the micellar morphology of two polystyrene-b-poly(N-isopropylacrylamide) (PS-b-PNIPAM) diblock copolymers in an aqueous solution was investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). At 25 °C, a mixture of vesicles and spheres are observed for the micelles of PS65-b-PNIPAM108, while PS65-b-PNIPAM360 exhibits mixed cylindrical and spherical micellar morphology. Upon increasing the temperature, the micellar morphology becomes spherical for PS65-b-PNIPAM108 at 60 °C and for PS65-b-PNIPAM360 at 40 °C. Such vesicle-to-sphere and cylinder-to-sphere transitions of micellar morphology are reversible when the micellar solutions are cooled back to 25 °C. However, these temperature-induced morphological transitions of the PS-b-PNIPAM micelles are contrary to the theoretical prediction. Qualitative analysis of the free energy shows that vesicular or cylindrical micelles tend to form at higher temperatures if only the overall volume change of the PNIPAM block is considered. The contradiction between the experimental results and theoretical prediction is interpreted in terms of the local deformability of the PNIPAM chains. At elevated temperatures, the collapsed PNIPAM globules are less deformable and must occupy larger areas at the micellar interface, although the overall volume is smaller at higher temperatures. This will lead to a larger repulsion between the PNIPAM globules and a remarkable increase in the free energy of the corona; thus, the formation of vesicles or cylinders at higher temperatures is prohibited.

On the stability and morphology of complex coacervate core micelles: from spherical to wormlike micelles

We present a systematic study of the stability and morphology of complex coacervate core micelles (C3Ms) formed from poly(acrylic acid) (PAA) and poly(N-methyl-2-vinylpyridinium)-b-poly(ethylene oxide) (PM2VP-b-PEO). We use polarized and depolarized dynamic and static light scattering, combined with small-angle X-ray scattering, to investigate how the polymer chain length and salt concentration affect the stability, size, and shape of these micelles. We show that C3Ms are formed in aqueous solution below a critical salt concentration, which increases considerably with increasing PAA and PM2VP length and levels off for long chains. This trend is in good agreement with a mean-field model of polyelectrolyte complexation based on the Voorn-Overbeek theory. In addition, we find that salt induces morphological changes in C3Ms when the PAA homopolymer is sufficiently short: from spherical micelles with a diameter of several tens of nanometers at low salt concentration to wormlike micelles with a contour length of several hundreds of nanometers just before the critical salt concentration. By contrast, C3Ms of long PAA homopolymers remain spherical upon addition of salt and shrink slightly. A critical review of existing literature on other C3Ms reveals that the transition from spherical to wormlike micelles is probably a general phenomenon, which can be rationalized in terms of a classical packing parameter for amphiphiles.

Structure and dynamics of highly PEG-ylated sterically stabilized micelles in aqueous media

Molecular assemblies of highly PEG-ylated phospholipids are important in many biomedical applications. We have studied sterically stabilized micelles (SSMs) of self-assembled DSPE–PEG2000 in pure water and isotonic HEPES-buffered saline solution. The observed SSM sizes of 2–15 nm largely depend on the solvent and the lipid concentration used. The critical micelle concentration of DSPE–PEG2000 is 10 times higher in water than in buffer, and the viscosity of the dispersion dramatically increases with the lipid concentration. To explain the experimentally observed results, we performed atomistic molecular dynamics simulations of solvated SSMs. Our modeling revealed that the observed assemblies have very different aggregation numbers (N(agg) ≈ 90 in saline solution and N(agg) < 8 in water) because of very different screening of their charged PO4(–) groups. We also demonstrate that the micelle cores can inflate and their coronas can fluctuate strongly, thus allowing storage and delivery of molecules with different chemistries.

Formation and structure of ionomer complexes from grafted polyelectrolytes

We discuss the structure and formation of Ionomer Complexes formed upon mixing a grafted block copolymer (poly(acrylic acid)-b-poly(acrylate methoxy poly(ethylene oxide)), PAA₂₁-b-PAPEO₁₄) with a linear polyelectrolyte (poly(N-methyl 2-vinyl pyridinium iodide), P2MVPI), called grafted block ionomer complexes (GBICs), and a chemically identical grafted copolymer (poly(acrylic acid)-co-poly(acrylate methoxy poly(ethylene oxide)), PAA₂₈-co-PAPEO₂₂) with a linear polyelectrolyte, called grafted ionomer complexes (GICs). Light scattering measurements show that GBICs are much bigger (~70–100 nm) and GICs are much smaller or comparable in size (6–22 nm) to regular complex coacervate core micelles (C3Ms). The mechanism of GICs formation is different from the formation of regular C3Ms and GBICs, and their size depends on the length of the homopolyelectrolyte. The sizes of GBICs and GICs slightly decrease with temperature increasing from 20 to 65 °C. This effect is stronger for GBICs than for GICs, is reversible for GICs and GBIC-PAPEO₁₄/P2MVPI₂₂₈, and shows some hysteresis for GBIC-PAPEO₁₄/P2MVPI₄₃. Self-consistent field (SCF) calculations for assembly of a grafted block copolymer (having clearly separated charged and grafted blocks) with an oppositely charged linear polyelectrolyte of length comparable to the charged copolymer block predict formation of relatively small spherical micelles (~6 nm), with a composition close to complete charge neutralization. The formation of micellar assemblies is suppressed if charged and grafted monomers are evenly distributed along the backbone, i.e., in case of a grafted copolymer. The very large difference between the sizes found experimentally for GBICs and the sizes predicted from SCF calculations supports the view that there is some secondary association mechanism. A possible mechanism is discussed.

Grafted ionomer complexes and their effect on protein adsorption on silica and polysulfone surfaces

We have studied the formation and the stability of ionomer complexes from grafted copolymers (GICs) in solution and the influence of GIC coatings on the adsorption of the proteins β-lactoglobulin (β-lac), bovine serum albumin (BSA), and lysozyme (Lsz) on silica and polysulfone. The GICs consist of the grafted copolymer PAA₂₈-co-PAPEO₂₂ {poly(acrylic acid)-co-poly[acrylate methoxy poly(ethylene oxide)]} with negatively charged AA and neutral APEO groups, and the positively charged homopolymers: P2MVPI₄₃ [poly(N-methyl 2-vinyl pyridinium iodide)] and PAH∙HCl₁₆₀ [poly(allylamine hydrochloride)]. In solution, these aggregates are characterized by means of dynamic and static light scattering. They appear to be assemblies with hydrodynamic radii of 8 nm (GIC-PAPEO₂₂/P2MVPI₄₃) and 22 nm (GIC-PAPEO₂₂/PAH∙HCl₁₆₀), respectively. The GICs partly disintegrate in solution at salt concentrations above 10 mM NaCl. Adsorption of GICs and proteins has been studied with fixed angle optical reflectometry at salt concentrations ranging from 1 to 50 mM NaCl. Adsorption of GICs results in high density PEO side chains on the surface. Higher densities were obtained for GICs consisting of PAH∙HCl₁₆₀ (1.6 ÷ 1.9 chains/nm²) than of P2MVPI₄₃ (0.6 ÷ 1.5 chains/nm²). Both GIC coatings strongly suppress adsorption of all proteins on silica (>90%); however, reduction of protein adsorption on polysulfone depends on the composition of the coating and the type of protein. We observed a moderate reduction of β-lac and Lsz adsorption (>60%). Adsorption of BSA on the GIC-PAPEO₂₂/P2MVPI₄₃ coating is moderately reduced, but on the GIC-PAPEO₂₂/PAH∙HCl₁₆₀ coating it is enhanced.

Grafted block complex coacervate core micelles and their effect on protein adsorption on silica and polystyrene

We have studied the formation and the stability of grafted block complex coacervate core micelles (C3Ms) in solution and the influence of grafted block C3M coatings on the adsorption of the proteins beta-lactoglobulin, bovine serum albumin, and lysozyme. The C3Ms consist of a grafted block copolymer PAA(21)-b-PAPEO(14) (poly(acrylic acid)-b-poly(acrylate methoxy poly(ethylene oxide)), with a negatively charged PAA block and a neutral PAPEO block and a positively charged homopolymer P2MVPI (poly(N-methyl 2-vinyl pyridinium iodide). In solution, these C3Ms partly disintegrate at salt concentrations between 50 and 100 mM NaCl. Adsorption of C3Ms and proteins has been studied with fixed-angle optical reflectometry, at salt concentrations ranging from 1 to 100 mM NaCl. In comparison with the adsorption of PAA(21)-b-PAPEO(14) alone adsorption of C3Ms significantly increases the amount of PAA(21)-b-PAPEO(14) on the surface. This results in a higher surface density of PEO chains. The stability of the C3M coatings and their influence on protein adsorption are determined by the composition and the stability of the C3Ms in solution. A C3M-PAPEO(14)/P2MVPI(43) coating strongly suppresses the adsorption of all proteins on silica and polystyrene. The reduction of protein adsorption is the highest at 100 mM NaCl (>90%). The adsorbed C3M-PAPEO(14)/P2MVPI(43) layer is partly removed from the surface upon exposure to an excess of beta-lactoglobulin solution, due to formation of soluble aggregates consisting of beta-lactoglobulin and P2MVPI(43). In contrast, C3M-PAPEO(14)/P2MVPI(228) which has a fivefold longer cationic block enhances adsorption of the negatively charged proteins on both surfaces at salt concentrations above 1 mM NaCl. A single PAA(21)-b-PAPEO(14) layer causes only a moderate reduction of protein adsorption.

Determining the morphology of polystyrene-block-poly(2-vinylpyridine) micellar reactors for ZnO nanoparticle synthesis

We report the use of reverse PS-b-P2VP diblock copolymer micelles as true nanoscale-sized reactor vessels to synthesize ZnO nanoparticles. The reverse micelles were formed in toluene and then sequentially loaded with zinc acetate dihydrate and tetramethylammonium hydroxide reactants. Moreover, high spatial resolution Z-contrast imaging and EDX spectroscopy techniques were used to confirm the segregation of the Zn cation to the core of the loaded micelles. Determining the chemical distribution with high nanoscale spatial resolution is shown to complement the less direct characterization by AFM, DLS and FTIR, thus demonstrating broader implications for the characterization of hybrid nanocomposite systems.

Preparation of size tunable amphiphilic poly(amino acid) nanoparticles

Size tunable amphiphilic NPs composed of poly(gamma-PGA) and hydrophobic amino acids, such as Phe or Trp, were prepared. To prepare these size-regulated NPs, gamma-PGA-g-Phe or gamma-PGA-g-Trp dissolved in DMSO was added to various concentrations of NaCl solution. The gamma-PGA-Phe and gamma-PGA-Trp formed monodispersed NPs, and the size of NPs can be easily controlled by NaCl concentration. The different-sized NPs showed the same structure. The encapsulation of protein into the different-sized NPs was successfully achieved and the size of protein-encapsulated gamma-PGA-Phe NPs was increased when protein was encapsulated.

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).

Complex coacervate core micro-emulsions

Complex coacervate core micelles form in aqueous solutions from poly(acrylic acid)-block-poly(acrylamide) (PAAxPAAmy, x and y denote degree of polymerization) and poly(N,N-dimethyl aminoethyl methacrylate) (PDMAEMA150) around the stoichiometric charge ratio of the two components. The hydrodynamic radius, Rh, can be increased by adding oppositely charged homopolyelectrolytes, PAA140 and PDMAEMA150, at the stoichiometric charge ratio. Mixing the components in NaNO3 gives particles in highly aggregated metastable states, whose Rh remain unchanged (less than 5% deviation) for at least 1 month. The Rh increases more strongly with increasing addition of oppositely charged homopolyelectrolytes than is predicted by a geometrical packing model, which relates surface and volume of the particles. Preparation in a phosphate buffer - known to weaken the electrostatic interactions between PAA and PDMAEMA - yields swollen particles called complex coacervate core micro-emulsions (C3-μEs) whose Rh increase is close to that predicted by the model. These are believed to be in the stable state (lowest free energy). A two-regime increase in Rh is observed, which is attributed to a transition from more star-like to crew-cut-like, as shown by self-consistent field calculations. Varying the length of the neutral and polyelectrolyte block in electrophoretic mobility measurements shows that for long neutral blocks (PAA26PAAm405 and PAA39PAAm381) the ζ-potential is nearly zero. For shorter neutral blocks the ζ-potential is around -10 mV. This shows that the C3-μEs have excess charge, which can be almost completely screened by long enough neutral blocks.

Polyisobutylene-block-poly(methacrylic acid) diblock copolymers: self-assembly in aqueous media

Ionic amphiphilic diblock copolymer polyisobutylene-block-poly(methacrylic acid) (PIBx-b-PMAAy), with various lengths of nonpolar (x=25-75) and polyelectrolyte (y=170-2600) blocks, spontaneously dissolve in aqueous media at pH>4, generating macromolecular assemblies, the aggregation number of which depends on external stimuli (pH and ionic strength). Spherical micellar morphology with a compact core formed by the PIB blocks and a swollen corona built up from the PMAA blocks was deduced by cryogenic transmission electron microscopy. The micelles were further characterized by means of dynamic and static light scattering as well as small-angle neutron scattering. The critical micellization concentration, estimated by means of fluorescence spectroscopy with the use of pyrene as a polarity probe, is decisively determined by the length of the PIB block and is insensitive to changes in the length of the PMAA block.

Sphere-to-rod transition of non-surface-active amphiphilic diblock copolymer micelles: a small-angle neutron scattering study

Micellization behavior of amphiphilic diblock copolymers with strong acid groups, poly(hydrogenated isoprene)-block-poly(styrenesulfonate), was investigated by small-angle neutron scattering (SANS). We have reported previously (Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2005, 21, 9938) that this strongly ionic amphiphilic diblock copolymer shows almost no surface activity but forms micelles in water. In this study, the size, shape, and internal structures of the micelles formed by these unique copolymers in aqueous solution were duly investigated. The SANS data were well described by the theoretical form factor of a core-shell model and the Pedersen core-corona model. The micellar shape strongly depends on the hydrophobic chain length of the block copolymer. The polymer with the shortest hydrophobic chain was suggested to form spherical micelles, whereas the scattering curves of the longer hydrophobic chain polymers showed a q-1 dependence, reflecting the formation of rodlike micelles. Furthermore, the addition of salt at high concentration also induced the sphere-to-rod transition in micellar shape as a result of the shielding effect of electrostatic repulsion. The corona thickness was almost constant up to the critical salt concentration (around 0.2 M) and then decreased with further increases in salt concentration, which is in qualitatively agreement with existing theories. The spherical/rodlike micelle ratio was also constant up to the critical salt concentration and then decreased. The micelle size and shape of this unique polymer could be described by the common concept of the packing parameter, but the anomalously stable nature of the micelle (up to 1 M NaCl) is a special characteristic.