Mixtures of amphiphilic polymers and surfactants are used in a wide range of applications, e.g., pharmaceuticals, detergents, cosmetics, and drug delivery systems. Still, many questions remain on how ...the structure and, in particular, the kinetics of block copolymer micelles are affected in the presence of surfactants and what controls the solubilization kinetics. In this work, we have studied the stability and solubilization kinetics of block copolymer micelles upon the addition of the surfactant sodium dodecyl sulfate (SDS) using small-angle X-ray/neutron scattering. The ability of the surfactant to dissolve polymer micelles or form mixed micelles has been investigated using two types of amphiphilic polymers, poly(ethylene-alt-propylene)–poly(ethylene oxide) (PEP1-PEO20) and n-alkyl-functionalized PEO (C28-PEO5). The exchange kinetics of C28-PEO5 micelles are in the order of hours, while PEP1-PEO20 micelles are known to be frozen on a practical timescale. In this work, we show that the addition of SDS to PEP1-PEO20 provides virtually no solubilization, even after an extended period of time. However, upon adding SDS to C28-PEO5 micelles, we observe micellar dissolution and formation of mixed micelles occurring on the timescale of hours. Using a coexistence model of mixed and neat micelles, the SAXS data were analyzed to provide detailed structural parameters over time. First, we observe a fast fragmentation/fission step followed by a slow reorganization process. The latter process is essentially independent of concentration at low volume fraction but is greatly accelerated at larger concentrations. This might indicate a crossover from a predominance of molecular exchange to fusion/fission processes.
We study the self-assembly of mixtures of n-alkyl mono- and difunctionalized poly(ethylene oxide) (PEO) chains in the dilute concentration regime. The monofunctional PEOs were prepared by living ...anionic polymerization with varying n-alkyl length (n = 14, 16, 22, 28) and constant PEO molecular weight of 5 kg/mol. The difunctional materials were obtained through end-to-end coupling of two of the monofunctionalized PEOs via their terminal hydroxyl groups. The chosen synthetic pathway yields well-defined model compounds with narrow molecular weight distribution and complete end-group functionalization. By using both small-angle neutron scattering (SANS) and dynamic light scattering (DLS) combined with theoretical data modeling, we have systematically investigated both the global and inner structure of the self-assembled micellar structures. For short n-alkyl chain-ends, we find a formation of clustered micelles with a finite size whereas, intriguingly, at longer n-alkyls, we observe a crossover to flowerlike micelles. This was confirmed both by DLS, which is very sensitive to formation of larger clusters, as well as with SANS, which also showed a clear transition from attractive to repulsive intermicellar interactions upon increasing n-alkyl length. We attribute this to the balance between the hydrophobic enthalpic terms that favor anchoring of both chain-ends to the core and the entropic cost associated with the bending of the polymer chains. For short n-alkyls, exposure of the chain-ends in the corona structure leads to net dominance of the attractive interactions while for longer hydrophobic chains it leads to a stabilization of loops and consequently flowerlike micellar morphology. Using contrast-variation SANS, the contribution of mono- and difunctional chains could be separated, confirming the flowerlike micellar structure.
Understanding self-assembly of amphiphilic copolymers in aqueous solution is an important issue in many areas, e.g., in order to tailor-make carriers for drugs and genes. We have synthesized modified ...versions of the copolymer of type PEO-PPO-PEO (Pluronic, F127), with short (PCL(5)) or long (PCL(11)) PCL blocks at both ends. Turbidity, dynamic light scattering (DLS), small angle neutron scattering (SANS), and rheology measurements were carried out on dilute aqueous solutions of these polymers to investigate their self-assembly behavior. The DLS results clearly show that both micellization and inter-micellization can be controlled by polymer concentration, temperature, and length of the PCL block. The interplay between unimers, micelles, and clusters of micelles could be monitored and the size and size distribution of the species were determined. The SANS data could be portrayed by a spherical core-shell model at all considered conditions of temperature and concentration for F127 and PCL(5) apart from F127 at the lowest temperature measured. The SANS data for PCL(11) were described by a spherical core-shell model at low temperatures, whereas at elevated temperatures asymmetric sub-structures appeared and a cylindrical core-shell model was employed in the analysis of the data. The appearance of pronounced correlation peaks at elevated temperatures signalizes marked intermicellar interactions. The shear viscosity data revealed a minor shear thinning effect, suggesting that the interchain structures are rather stable and not easily disrupted. The work shows that PCL-modification of Pluronic has a large influence on the self-assembly process and on the final structure of the assemblies.
Small angle X-ray (SAXS) and neutron scattering techniques were combined to study mixed complexes formed between micelles of the nonionic amphiphilic PEO-PPO-PEO copolymer (P123) and the anionic bile ...salt (NaGDC) in aqueous solution. The purpose was to investigate the structural parameters of the charged complexes, such as size and internal structure, as well as their interparticle interactions in aqueous solution. The overall aim of this work was to gain understanding of how thermoresponsive PEO-PPO-PEO block copolymers interact with bile salts in order to make predictions as to whether they can be put forward as a new class of bile salt sequestrants in the treatment of bile-salt related diseases. The system was investigated at a constant P123 concentration of 1.74 mM and bile salt concentrations were varied up to a molar ratio n sub(NaGDC)/n sub(P123)nNaGDC/nP123 (MR) = 5.7. It was found that the NaGDC molecules preferentially associated to the PEO corona of the P123 micelle and due to their amphiphilic nature, close to the core/corona interface. Because of this association the micelles became charged causing their reciprocal interparticle repulsions in solution to increase. In parallel, the association caused a decrease in the core radius accompanied by dehydration, which in turn led to a decrease in total radius of the "P123 micelle-NaGDC" complexes. To elucidate the effect of the interactions on their diffusive motion, an interaction model based on a spherical particle with a hard-core interaction shell was employed using the fitted SAXS data. At higher molar ratios, the interparticle interaction was increasingly screened because of nonadsorbed bile salt in the surrounding solution. Meanwhile, a further decrease in total radial size of the P123 micelle-NaGDC complexes occurred due to a decrease in the aggregation number of P123 as the bile salt finally disintegrated the complexes. However, the micelles were found to be more stable and less prone to disintegration in D sub(2)O. This investigation demonstrated the importance of using small angle scattering techniques for studying intermolecular interactions in order to gain understanding of how natural surfactants influence the aggregation behavior of amphiphilic polymers.
A comprehensive experimental study on the interaction between the PEO–PPO–PEO block copolymer P123 (EO20PO68EO20) and the anionic bile salt sodium glycodeoxycholate (NaGDC) in water has been ...performed. The work was aimed at investigating the suitability of using P123 as bile salt sequestrant beside the fundamental aspects of PEO–PPO–PEO block copolymer–bile salt interactions. Various experimental techniques including dynamic and static light scattering, small-angle X-ray scattering, and differential scanning calorimetry (DSC) were employed in combination with electrophoretic mobility measurements. The system was investigated at a constant P123 concentration of 1.74 mM and with varying bile salt concentrations up to approximately 250 mM NaGDC (or a molar ratio n NaGDC/n P123 = 144). In the mixed P123–NaGDC solutions, the endothermic process related to the self-assembly of P123 was observed to gradually decrease in enthalpy and shift to higher temperatures upon progressive addition of NaGDC. To explain this effect, the formation of NaGDC micelles carrying partly dehydrated P123 unimers was proposed and translated into a stoichiometric model, which was able to fit the experimental DSC data. In the mixtures at low molar ratios, NaGDC monomers associated with the P123 micelle forming a charged “P123 micelle–NaGDC” complex with a dehydrated PPO core. These complexes disintegrated upon increasing NaGDC concentration to form small “NaGDC–P123” complexes visualized as bile salt micelles including one or a few P123 copolymer chains.
It is known that the reduction of blood cholesterol can be accomplished through foods containing a large number of dietary fibers; this process is partially related to the binding of bile salt to ...fibers. To gain new insights into the interactions between dietary fibers and bile salts, this study investigates the interactions between cationic hydroxyethyl cellulose (catHEC) and sodium deoxycholate (NaDC) or sodium cholate (NaC), which have a similar structure. Turbidity measurements reveal strong interactions between catHEC and NaDC, and under some conditions, macroscopic phase separation occurs. In contrast, the interactions with NaC are weak. At a catHEC concentration of 2 wt %, incipient phase separation is approached at concentrations of NaC and NaDC of 32.5 and 19.3 mM, respectively. The rheological results show strong interactions and a prominent viscosification effect for the catHEC/NaDC system but only moderate interactions for the catHEC/NaC system. Both cryogenic transmission electron microscopy and small-angle X-ray scattering results display fundamental structural differences between the two systems, which may explain the stronger interactions in the presence of NaDC. The surmise is that the extended structures formed in the presence of NaDC can easily form connections and entanglements in the network.
A complete chemical and morphological analysis of the evolution of battery electrode materials can be achieved combining different and complementary techniques. Operando small-angle X-ray scattering ...(SAXS) and wide-angle X-ray scattering (WAXS) were combined to investigate structural and electrochemical performances of an Na-ion battery, with amorphous red phosphorus in a carbon matrix (a-P/C) as the active anode material in a Swagelok-type cell. The charging process results in the formation of crystalline Na3P, while during discharging, the anode material returns to the initial a-P/C. From the analysis of the WAXS curves, the formation of crystalline phases appears only at the end of charging. However, SAXS data show that partial reorganization of the material during charging occurs at length scales nonaccessible with conventional X-ray diffraction, corresponding to a real space ordering distance of 4.6 nm. Furthermore, the analysis of the SAXS data shows that the electrode remains dense during charging, while it develops some porosity during the discharge phase. The presented results indicate that the combination of SAXS/WAXS adopted simultaneously, and nondestructively, on a working electrochemical cell can highlight new mechanisms of reactions otherwise undetected. This method can be applied for the study of any other solid electrode material for batteries.
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In biomedical applications, PEGylation is frequently utilized to enhance the nanoparticles (NPs) capability for long systemic circulation times in the blood and it is also crucial to ...stabilize the NPs and thereby minimize their ability to agglomerate. In this study, we have synthesized poly(N-isopropylacrylamide) (PNIPAAM) nanogels with covalently attached PEG chains of different length and PEG coating densities. It is observed that in the absence of PEG coating the nanogels aggregate at elevated temperatures. It is found from dynamic light scattering (DLS) that both increased PEG length and enhanced PEG coating density have crucial influence on the stability of the nanogels. The results show that long PEG chains have a stronger impact on the shielding ability of the PEG layer on the nanogels than a high coating density of short chains. The small angle neutron scattering (SANS) measurements on PEG-coated nanogels indicate that the coated layer contract at higher temperatures but still the particles are stabilized. The bare PNIPAAM nanogels can be electrostatically stabilized by adding a small amount of an ionic surfactant.
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Poly(vinyl alcohol)-g-poly(methyl methacrylate) (PVA-g-PMMA) amphiphilic copolymers self-assemble to form multi-micellar colloidal systems.
A PVA-g-PMMA copolymer containing 16–17% ...w/w of PMMA was synthesized by the free radical graft polymerization of methyl methacrylate on a PVA backbone by utilizing cerium(IV) ammonium nitrate as initiator and tetramethylethylenediamine (TEMED) as initiation activator. The aggregation behavior of the copolymer in water was comprehensively characterized by dynamic light scattering (DLS) and static light scattering (SLS), small angle neutron scattering (SANS), asymmetrical flow field-flow fractionation (A4F) and transmission electron microscopy (TEM). The colloidal stability before and after non-covalent crosslinking of PVA domains with boric acid was assessed by DLS. Finally, nanoparticles were nano spray-dried.
This copolymer self-assembles in water to form a complex nanostructure of low aggregation number particles (ca. 12–15 nm in diameter) that aggregate into larger ones with size ca. 60–80 nm, as determined by SANS and TEM. In addition, boric acid-crosslinking preserves the nanoparticle size, while conferring full physical stability under extreme dilution conditions. Nano spray-drying consolidates the crosslnking and enables the production of a dry flowing powder that upon re-dispersion in water regenerates the nanoparticles without major size changes.
The thermoresponsive amphiphilic block copolymer poly(d,l‐lactic acid‐co‐glycolic acid)‐block‐poly(ethylene glycol)‐block‐poly(d,l‐lactic acid‐co‐glycolic acid) (PLGA‐PEGn‐PLGA), which exhibits a ...reversible temperature‐induced sol–gel transition at higher polymer concentrations in aqueous solution has attached a great deal of interest because of its potential in biomedical applications. In the present work, the length of the hydrophobic PLGA blocks is kept constant, whereas the length of the hydrophilic PEG block is altered and this variation has a pronounced impact on the phase behavior of the aqueous samples and the structure of the polymer. A short PEG block promotes gelation at a low temperature, whereas a longer PEG block shifts the gelation point to higher temperature. By using a combination of turbidity, rheology, and small angle neutron scattering (SANS) methods, the authors have revealed dramatic temperature effects. In dilute solution, the SANS experiments expose asymmetric ellipsoid structures for the copolymer with the short PEG‐spacer, whereas spherical core–shell structure is observed for the polymer with long PEG‐spacer. In the semidilute concentration regime, SANS measurements disclose similar profiles for the two copolymers. In a broad temperature interval, the transition from spherical core–shell micelles to cylindrical structure and packing of cylinders is observed.
Thermo‐responsive amphiphilic block copolymers of poly(d,l‐lactic acid‐co‐glycolic acid)‐block‐poly(ethylene glycol)‐(PEG) block‐poly(d,l‐lactic acid‐co‐glycolic acid) type with different PEG length are prepared. These polymers have wide ranges of potential for biomedical applications. These copolymers exhibit very interesting structural features in both dilute and semidilute solutions. In the semidilute concentration regime, the structures change from core–shell to cylindrical shape as the temperature increases.
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