Pluronic L64 Micelles near Cloud Point: Investigating the Role of Micellar Growth and Interaction in Critical Concentration Fluctuation and Percolation

Insights into the Phase Diagram of Pluronic L64 Using Coarse-Grained Molecular Dynamics Simulations

We investigate the concentration-dependent phase diagram of pluronic L64 in aqueous media at 300 and 320 K using coarse-grained (CG) molecular dynamics (MD) simulations. The CG model is derived by adapting the Martini model for nonbonded interactions along with the Boltzmann inversion (BI) of bonded interactions from all-atom (AA) simulations. Our derived CG model successfully captures the experimentally observed micellar-, hexagonal-, lamellar-, and polymer-rich solution phase. The end-to-end distance reveals the conformational change from an open-chain structure in the micellar phase to a folded-chain structure in the lamellar phase, increasing the orientational order. An increase in temperature leads to expulsion of water molecules from the L64 moiety, suggesting an increase in L64 hydrophobicity. Thermodynamic analysis using the two-phase thermodynamics (2PT) method suggests the entropy of the system decreases with increasing L64 concentration and the decrease in free energy (F) with temperature is mainly driven by the entropic factor (-TS). Further, the increase in aggregation number at lower concentrations and self-assembly at very high concentrations is energetically driven, whereas the change from the micellar phase to the lamellar phase with increasing L64 concentration is entropically driven. Our model provides molecular insights into L64 phases which can be further explored to design functionality-based suprastructures for drug delivery and tissue engineering applications.

Impact of Two Water-Miscible Ionic Liquids on the Temperature-Dependent Self-Assembly of the (EO)6-(PO)34-(EO)6 Block Copolymer

There are many studies on the self-assembly of triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers in aqueous solution. These polymers display a rich phase diagram as a function of block length, concentration, temperature, and additives. Here, we present a small-angle neutron scattering study of the impact of two water-miscible ionic liquids, 1-butyl-3-methylimidazolium chloride ([C₄C₁mim][Cl]) and 1-butyl-3-methylpyrrolidinium chloride ([C₄C₁pyrr][Cl]), on the temperature-dependent self-assembly of (EO)₆-(PO)₃₄-(EO)₆, also known as L62 Pluronic, in aqueous solution. Both ionic liquids depress the temperatures of the various structural transitions that take place, but ([C₄C₁pyrr][Cl]) has a stronger effect. The structures that the triblock copolymer self-assembles into do not dramatically change nor do they significantly change the series of structures that the system transitions through as a function of temperature relative to the various transition temperatures.

Topology and Sequence-Dependent Micellization and Phase Separation of Pluronic L35, L64, 10R5, and 17R4: Effects of Cyclization and the Chain Ends

The topology effects of cyclization on thermal phase transition behaviors were investigated for a series of amphiphilic Pluronic copolymers of both hydrophilic–hydrophobic–hydrophilic and hydrophobic–hydrophilic–hydrophobic block sequences. The dye solubilization measurements revealed the lowered critical micelle temperatures (T_CMT) along with the decreased micellization enthalpy (ΔH_mic) and entropy (ΔS_mic) for the cyclized species. Furthermore, the transmittance and dynamic light scattering (DLS) measurements indicated a block sequence-dependent effect on the clouding phenomena, where a profound decrease in cloud point (T_c) was only found for the copolymers with a hydrophilic–hydrophobic–hydrophilic block sequence. Thus, the effect of cyclization on these critical temperatures was manifested differently depending on its block sequence. Finally, a comparison of the linear hydroxy-terminated, methoxy-terminated, and cyclized species indicated the effect of cyclization to be unique from a simple elimination of the terminal hydrophilic moieties.

Criteria Governing Rod Formation and Growth in Nonionic Polymer Micelles

Self-assembled wormlike micelles (WLMs) are widely studied in small-molecule surfactants due to their unique ability to break and recombine; however, less is known about the structure and dynamics of nonionic polymer WLMs. Here, solutions of seven triblock poloxamers, composed of poly(propylene oxide) (PPO) midblocks and poly(ethylene oxide) (PEO) end blocks, are comprehensively examined to determine the role of poloxamer composition, temperature, and inorganic salt type and concentration on rod formation and subsequent elongation into WLMs. Phase separation and sphere-to-rod transition temperatures were quantified via cloud point measurements and shear rheology, respectively, and corroborated with small-angle neutron scattering (SANS). The local microstructure of resulting rodlike micelles is remarkably similar across poloxamer type and sodium fluoride (NaF) or sodium chloride (NaCl) content. Salt addition reduces transition temperatures, with the most pronounced effects for poloxamers with high PEO molecular weights and PEO fractions. Between these two temperatures, several poloxamers elongate into WLMs, where shear rheology detects increases in viscosity up to 6 orders of magnitude. Despite similar local microstructures, poloxamer identity and salt content impact micelle growth substantially, where large poloxamers with lower PEO fractions exhibit the highest viscosities and longest relaxation times. While sodium fluoride has little impact on micelle growth, increasing NaCl concentration dramatically increases the WLM viscosity and relaxation time. This result is explained by different interactions of each salt with the micelle: whereas NaF interacts primarily with PEO chains, NaCl may also partition to the PPO/PEO interface in low levels, increasing micelle surface tension, scission energy, and contour length.

Unusual Growth and Hydration Characteristics of Oil Solubilized Micelles in Aqueous Pluronic Systems

Lipophile induced modulations of self-assembly characteristics in aqueous Pluronic systems merit attention because of wide-ranging uses of Pluronics as solubilizing agents of lipophilic substances. In this paper, we report unusual evolutions of structural and hydration properties in lavender essential oil (LO) solubilized Pluronic P85 aqueous micellar systems as a function of micellar volume fraction and temperature. Our DLS, SANS, and viscometry studies show that the spherical-to-wormlike micellar structural transition observed in 1% P85 solutions upon solubilization of LO quite unexpectedly gets suppressed with increased P85 concentration to ≥5%. Detailed SANS studies reveal that the core sizes of the oil solubilized micelles cannot attain the threshold value required for the onset of structural transition at higher copolymer concentrations due to their progressive shrinking with an increase in P85 concentration. Oil solubilized P85 solutions show two cloud points and very interestingly exhibit micellar growth upon cooling to their lower cloud points. Steady state fluorescence studies explain this based on increasing dehydration of micellar corona with a decrease in temperature, very much opposite to what is observed in pure aqueous Pluronic systems. The results give new insight into viscous flow properties and low temperature storage possibilities of oil solubilized aqueous Pluronic systems.

Complexation of Pluronic L62 (EO6)-(PO34)-(EO6)/aerosol-OT (sodium bis(2-ethylhexyl)sulfosuccinate) in aqueous solutions investigated by small angle neutron scattering

We investigate the phase behaviours of Pluronic L62 in aqueous solution in the presence of aerosol-OT (AOT) molecules by small angle neutron scattering (SANS). The presence of AOT significantly changes the micellization phenomenon of L62 micelles in aqueous solution, including their critical micelle temperature (CMT), global size, and asphericity. The origin of these observations is attributed to the complexation between the neutral L62 surfactants and the ionic AOT molecules, which additionally provides charge to the mixed micelles: we analyse the data and extract meaningful information using the Ornstein-Zernike integral formalism. As a result, we observe that the co-micellization of L62 and AOT is very stable across a wide temperature range.

Small molecule-mediated self-assembly behaviors of Pluronic block copolymers in aqueous solution: impact of hydrogen bonding on the morphological transition of Pluronic micelles

The influence of hydrogen bonding on the self-assembly behaviors of Pluronic P123 micelles is experimentally and theoretically investigated by introducing three small molecules, i.e. propyl benzoate (PB), propyl paraben (PP) and propyl gallate (PG) into the aqueous solution. It is discovered that the number of phenolic hydroxyl groups and concentration of the tested small molecules exhibit a profound impact on the micellar morphology. Although all the small molecules increase the size and polydispersity of Pluronic micelles in a concentration-dependent manner, the micellar morphologies induced by them vary considerably as demonstrated by DLS and cryo-TEM measurement. PB, without phenolic hydroxyl, cannot bring about the morphological change of P123 micelles, while PP induces a series of morphological transitions from spheres to long worm-like micelles and then to unilamellar vesicles by increasing the PP content. Upon increasing the number of phenolic hydroxyls in small molecules, i.e. PG, the fusion of the intermicellar core takes place, resulting in the formation of large micelles and micellar clusters. A qualitative study by NMR reveals that the different locations of small molecules within the micelles are attributed to the balance of hydrogen bonding and hydrophobic interaction between small molecules and copolymers. In addition, molecular dynamics simulations (MDS) are performed to further confirm the experimental results and provide quantitative information on intermolecular interaction strength. It is supposed that the mechanism of micellar morphological transition mediated by small molecules is ascribed to the hydrogen bonding interactions with varying strengths between the PEO blocks and their phenolic hydroxyls, which governs their locations in micelles, affecting the free energies from different regions of micelles, and consequently leads to the varying micellar morphologies. This study deepens our understanding of the role of hydrgen bonding in the self-assembly behaviors of Pluronic micelles and provides an alternative strategy for manipulating the nanostructure of Pluronic micelles.

Surfactant-Mediated and Morphology-Controlled Nanostructured LiFePO4/Carbon Composite as a Promising Cathode Material for Li-Ion Batteries

The synthesis of morphology-controlled carbon-coated nanostructured LiFePO4 (LFP/Carbon) cathode materials by surfactant-assisted hydrothermal method using block copolymers is reported. The resulting nanocrystalline high surface area materials were coated with carbon and designated as LFP/C123 and LFP/C311. All the materials were systematically characterized by various analytical, spectroscopic and imaging techniques. The reverse structure of the surfactant Pluronic® 31R1 (PPO-PEO-PPO) in comparison to Pluronic® P123 (PEO-PPO-PEO) played a vital role in controlling the particle size and morphology which in turn ameliorate the electrochemical performance in terms of reversible specific capacity (163 mAh g-1 and 140 mAh g-1 at 0.1 C for LFP/C311 and LFP/C123, respectively). In addition, LFP/C311 demonstrated excellent electrochemical performance including lower charge transfer resistance (146.3 Ω) and excellent cycling stability (95 % capacity retention at 1 C after 100 cycles) and high rate capability (163.2 mAh g-1 at 0.1 C; 147.1 mAh g-1 at 1 C). The better performance of the former is attributed to LFP nanoparticles (<50 nm) with a specific spindle-shaped morphology. Further, we have also evaluated the electrode performance with the use of both PVDF and CMC binders employed for the electrode fabrication.

Synthesis of Prolate-Shaped Au Nanoparticles and Au Nanoprisms and Study of Catalytic Reduction Reactions of 4-Nitrophenol

The growth into anisotropic one or two dimensions is important in plasmonic gold nanomaterials because extinction occurs along multiple axes and sometimes the resonance extends to the near-infrared region. The surfactant mixture of cetyltrimethylammonium bromide (CTAB) and Pluronic triblock copolymers has been recently demonstrated to be efficient anisotropic soft templates for the growth of noble metal nanomaterials. Seed-mediated growth of two types of anisotropic Au nanoparticles is achieved in this study. One is one-dimensional prolate-shaped Au nanoparticles with the average aspect ratios of 2.29 and 2.59, and the other is two-dimensional gold nanoprisms with the average edge length of 50.4 nm. These anisotropic structures are believed to be produced by the tendency of Pluronic copolymers to be micellized anisotropically at the elevated temperatures and by the preference for being lamellar mesophases in the phase diagrams when the concentration is highest. When prepared in the surfactant mixture of CTAB and L-64 (17.9%), Au nanoparticles containing spherical particles (27.9 nm) as the major products show the best catalytic performances in the reduction reactions of 4-nitrophenol.

Ag(i)-mediated self-assembly of anisotropic rods and plates in the surfactant mixture of CTAB and Pluronics

One-dimensional (1D) metallogels are commonly observed in metal-coordinated complexes, but there are not many examples of soft crystalline solids which are generated by the self-assembly of metal–polymer complexation in a non-gel state. In a continued effort to obtain 1D materials by utilizing the tendency of Pluronic triblock copolymers to be micellized anisotropically at an elevated temperature, we investigate Ag(i)-mediated self-assembly of the surfactant mixture of Pluronic copolymers and cetyltrimethylammonium bromide (CTAB). At sufficiently high temperature, Pluronic copolymers are known to organize into many crystalline mesophases, such as body-centered-cubic, hexagonal, and lamellar phases. Four Pluronics of L-31, L-64, P-123, and F-108 were studied, and at the concentration of 17.9%, macroscale 1D rods with the aspect ratios ranging from 3.07 to 12.8 are obtained. At the concentration of 35.7%, anisotropic two dimensional (2D) planar plates are observed. These planar structures were believed to be generated from 2D lamellar mesophases, which is consistent with the general phase diagram of Pluronic copolymers that shows lamellar phase with the highest concentration. In the absence of ascorbic acid, rods and plates of larger size are produced. Rather than as a reductant, ascorbic acid is thought to play the roles of an agent to increase the hydrophilicity, and as a mediator to determine the dimension of rods and plates by hindering the long range self-assembly of alkyl chains. Dehydration by the addition of AgNO₃, and the increase of hydrophobicity enable self-assembly of alkyl groups of CTAB and Pluronics and promote the formation of crystalline soft solids.

Macroscale anisotropic rods and plates were generated by the self-assembly of CTAB and Pluronics in the presence of AgNO₃.

Hard Pd Nanorods in the Soft Surfactant Mixture of CTAB and Pluronics: Seedless Synthesis and Their Self-Assembly

Seedless synthesis of Pd nanorods and their self-assembly into the layered smectic ordering are described. Aqueous Pluronic triblock copolymers (14.3-35.7%) are used as a soft template along with cetyltrimethylammonium bromide for inducing one-dimensional growth of Pd nanorods. Pluronic triblock copolymers are probably the most used polymer surfactants, and they are composed of poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-PEO triblocks. Neither pH adjustment nor AgNO₃ and other additives, such as poly(vinyl pyrrolidone) and ethylene glycol, are required to obtain Pd nanorods. Sonochemical synthesis at 43 °C, followed by thermal annealing for 1 h at 65 °C produces Pd nanorods with the aspect ratio from 3.1 (17.9%, Pluronic L-64) to 6.7 (35.7%, Pluronic P-123). Two-dimensional self-assembly of the nanorods is observed, and both nematic ordering between the mesogens and smectic ordering between the layers is identified. Micellar hydrophobic PPO with hydrated PEO coronas are known to self-assemble into many crystalline orders, including cubic, hexagonal, lamellar, and inverse hexagonal mesophases, which extend into cylindrical micelles with increasing temperature. Relatively small size of Pluronic copolymers with regard to general polymers, but rather large size of their micelles and their tendency to organize into crystalline mesophases are thought to contribute to the anisotropic growth of Pd nanorods.

TBP induced double cloud point in aqueous EO13PO30EO13 solutions: investigating the evolution of associated micellar characteristics as a function of temperature

TBP solubilized Pluronic L64 solutions exhibit inter-micellar attraction driven micellar cluster formation upon cooling, which is unique in non-ionic micellar systems.

Passive optical mapping of structural evolution in complex fluids

Low-coherence optical scattering allows probing the complex structure of self-assembling systems over extended ranges of the temperature and concentrations.

Rheological Properties and Reverse Micelles Conditions of PEO-PPO-PEO Pluronic F68: Effects of Temperature and Solvent Mixtures

The rheological properties of Pluronic F68 were dissolved in various water/organic liquid mixtures over a wide range of temperatures, all at a concentration of 20 mg/mL. We have considered the following binary mixtures: Pluronic F68/water, F68/p-xylene, and F68/phenol. Various conformational transitions were detected and interpreted. We have also shown that these mixtures retain a Newtonian behavior independently of temperature and conformational changes. For ternary F68/p-xylene/water, F68/phenol/water, and F68/water/phenol mixtures, the behaviour of the solution is intimately related to the temperature and the amount of water and organic solvent added.

Clouding and Aggregation Behavior of PPO-PEO-PPO Triblock Copolymer (Pluronic®25R4) in Surfactant Additives Environment

Aqueous solutions of commercially available Pluronic®R i. e. triblock copolymer composed of polypropylene oxide(PPO)-polyethylene oxide(PEO)-polypropylene oxide(PPO), (code named 25R4, BASF) in the presence of surfactant additives viz. hydrotropes (NaTS, NaXS), anionics (SDS, SDBS) and cationics (DTAB, CTAB) were investigated by cloud point(CP), surface tension and conductance techniques. All the techniques clearly indicated that Pluronic®25R4 interacts strongly with anionic surfactants but weakly with cationic surfactants. Surfactant additives used here, produced a marked increase in the CP of the copolymer. Elevation in CP is attributed to the solubility of PPO block of the copolymer and formation of Pluronic-surfactant complex. The effect of NaCl salt on the CP of Pluronic®25R4 at added SDS and CTAB was also studied and it was decreased due to the screening effect. At the presence of 25R4 in its non-associated form, the surface tension and conductance data is discussed with binding process and indicated strong interaction of Pluronic®25R4 with SDS, showing two break points (P1 and P2) in the plot and very weak interaction with CTAB observing single break point.

Solubilization of parabens in aqueous Pluronic solutions: investigating the micellar growth and interaction as a function of paraben composition

The influence of methyl paraben (MP) and butyl paraben (BP) on the aggregation characteristics of Pluronics in an aqueous medium has been investigated by DLS, SANS, viscometry, and fluorescence measurement techniques. Parabens are extensively used as preservatives in cosmetic, pharmaceutical, and food products. In this paper, we show that their influence on the restructuring and growth of Pluronics micelles vary quite significantly with their aqueous solubility and with the composition of Pluronics. In the case of P105 and P104, MP reduces the sphere-to-rod transition temperature down to room temperature, but BP with significantly less aqueous solubility than MP suppresses such micellar transition and leads to the formation of micellar clusters due to the onset of intermicellar attractive interaction. In the case of more hydrophobic Pluronic P103, on the other hand, both MP and BP are able to induce rapid room temperature sphere-to-rod micellar growth, which is not observed in the presence of water structure making salts like NaCl and Na(3)PO(4). These observations have been attributed to modulation of growth and restructuring processes of the Pluronic micelles arising due to different locations of parabens within the micellar corona as determined by their aqueous solubility and the hydrophobicity of the Pluronics.

Phase diagram of the Pluronic L64-H2O micellar system from mechanical spectroscopy

The linear viscoelastic properties of aqueous Pluronic L64 solutions have been investigated at high copolymer concentrations (25-62 wt%) using our modified low-frequency mechanical spectroscopy. The concentration-temperature phase diagram of the L64/H(2)O system was constructed by studying the evolution of the loss modulus and loss tangent as temperature is increased at a fixed frequency. A particular attention was focused on the dynamics approaching the beginning and ending points (39% and 60%) of the fusiform gel region in the phase diagram. The dynamics is found to have a similar viscoelastic behavior at the low and high concentrations, where a frequency scaling expected for a static percolated network is exhibited. Moreover, with increasing temperature, the system above the critical gel concentration undergoes a transition from a viscoelastic liquid to a solid gel through a percolated particle network. Therefore, our results suggest that the formation of the gel is dominated by the percolation of the particle clusters.

Role of hydration and water coordination in micellization of Pluronic block copolymers

Raman, attenuated total reflectance FTIR, near-infrared spectroscopy, and DFT calculations have been used in a study of aqueous solutions of three tri-block copolymers poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) or PEO-PPO-PEO with commercial names Pluronic PE6200, PE6400 and F68. It is shown that the process of micellization as a response to increased temperature is reflected in the hydroxyl stretching region of infrared and Raman spectra, which contains information both about restructuring of water and changes of polymer chains in polymer/water aggregates. Raman spectra exhibit differences between individual Pluronics even at temperatures below the critical micellization temperature (CMT). According to the attenuated total reflection (ATR) FTIR spectra, the same five water coordination types defined by the number of donated/accepted hydrogen bonds are present in interacting water as in bulk water. It indicates that models considering mixed states of water with different hydrogen bonding environments provide appropriate descriptions of bound water both below and above the CMT. Above the CMT, aggregate hydration increases in the order PE6400 < PE6200 < F68, although that does not fully correspond to the EO/PO ratio, and points to the differences in microstructure of aggregates formed by each copolymer. This study relates nanoscale phenomena (hydrophobic and hydrophilic hydration) with the mesoscale phenomenon of micellization.

Polymeric micelle-templated synthesis of hydroxyapatite hollow nanoparticles for a drug delivery system

Hydroxyapatite (HA) hollow nanoparticles (HNPs) have great potential in nanoscaled delivery devices due to their small size, excellent biocompatibility and expected high capacity. However, the preparation of HA HNPs for their application in a drug delivery system has rarely been reported because HA has a complicated crystal structure and it is difficult to obtain stable HA HNPs with hollows that are only nanoscaled in size. In the present study, HA HNPs were successfully produced through a novel polymeric micelle-templating method. The micelles were structured with completely insoluble Pluronic P123 molecules at cloud point as the core and Tween-60 molecules as the shell by the hydrophobic interaction of the alkyl chains with the insoluble P123 core. The morphology of the HA HNPs could be transformed from nanospheres to nanotubes by adding citric acid as a cosurfactant. The prepared HA HNPs had a much higher drug payload than traditional nanoparticles, using vancomycin as the model drug. Most importantly, the HA nanotubes were coupled with a layer of citrate molecules on the HA surfaces, which could further improve the drug load efficiency and could form an excellent pH-controlled open/closed gate for drug release with the addition of cationic polyelectrolytes.