Aggregation behavior of pluronic triblock copolymer in 1-butyl-3-methylimidazolium type ionic liquids

Three amphiphilic poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) ethers triblock copolymers, denoted Pluronic L61 (PEO3PPO30PEO3), Pluronic L64 (PEO13PPO30PEO13), and Pluronic F68 (PEO79PPO30PEO79) were shown to aggregate and form micelles in ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6). The surface tension measurements revealed that the dissolution of the copolymers in ILs depressed the surface tension in a manner analogous to aqueous solutions. The cmcs of three triblock copolymers increase following the order of L61, L64, F68, suggesting that micellar formation was driven by solvatophobic effect. cmc and gamma cmc decrease with increasing temperature because hydrogen bonds between ILs and hydrophilic group of copolymers decrease and accordingly enhance the solvatophobic interaction. Micellar droplets of irregular shape with average size of 50 nm were observed. The thermodynamic parameters DeltaGm0, DeltaHm0, DeltaSm0 of the micellization of block copolymers in bmimBF4 and bmimPF6 were also calculated. It was revealed that the micellization is a process of entropy driving, which was further confirmed by isothermal titration calorimetry (ITC) measurements.     

Micellisation and gelation of block-copoly(oxyethylene/oxybutylene/oxyethylene) E58 B17 E58

A triblock-copoly(oxybutylene/oxyethylene), denoted E₅₈ B₁₇ E₅₈ (80 wt-% E), was prepared by sequential anionic copolymerisation and its micellar and gelation properties in aqueous solution were determined. The oxybutylene/oxyethylene copolymer had the advantage over comparable oxypropylene/oxyethylene copolymers of greater uniformity of composition and chain length. The methods generally used were surface tension, light scattering intensity, photon correlation spectroscopy, together with observation of gelation. Measurements were made at 30°, 40° and 55° C over a full concentration range from dilute solution into the gel (>200 g dm^–3) and at specified concentrations, including the gel, over the temperature range from 10° to 70° C. The lower and upper temperature boundaries of the gel region were identified, and the occurrence of syneresis was noted. The properties investigated and discussed include critical micelle and gel concentrations, micellar weights and sizes, and structures and mobilities in the gel.     

Phase transitions in aqueous triblock copolymers: NMR relaxation studies

Longitudinal NMR relaxation times were used to investigate the thermal transitions occurring in an aqueous triblock copolymer of the poly(oxyethylene)–poly(oxypropylene)–poly(oxyethylene) family. In such a system molecule–micelle and micelle–liquid crystal transitions are observed, depending on temperature and composition. The longitudinal relaxation time, R ₁, significantly changes when the aforementioned phase transitions take place. In the case of molecule–micelle equilibrium, changes in R ₁ values of the methyl (and methynic) group signal, located in the lipophilic portion of block copolymers, are observed. The effect is ascribed to a significant dehydration of the poly(oxypropylene) chains, as a consequence of micelle formation. Conversely, the thermal transitions from micelles to liquid-crystalline phases are associated with significant changes in the relaxation time of poly(oxyethylene) units. The latter effect is tentatively ascribed to a partial dehydration and/or interdigitation of the poly(oxyethylene) moieties in the block copolymer.     

Micellization of block copolymer P94 in aqueous solution

P94 is a triblock copolymer of poly(oxyethylene) and poly(oxypropylene), type EPE. Eluent gel permeation chromatography has been used to demonstrate the formation of large micelles in solutions of the copolymer at temperatures immediately above the critical micelle temperature. Values of the critical micelle temperature (cmt) obtained in this way are in excellent agreement with results from differential scanning calorimetry and surface tensiometry. A van't Hoff plot was used to define the concentration dependence of the cmt for dilute solutions (c < or =2 wt%) and to obtain an apparent value of the standard enthalpy of micellization. Using published data, comparison is made with the thermodynamics of micellization in concentrated solution (c > or =20 wt%).     

Supramolecular formation of triblock copolymers in polar/nonpolar solvents

Triblock copolymers could form supramolecules in either polar or nonpolar solvents at appropriate concentration and temperature ranges or in the presence of additives. The association properties and the structure of supramolecules of PEO-PPO-PEO and PPO-PEO-PPO (PEO and PPO refer to poly(oxyethylene) and poly(oxypropylene), respectively) triblock copolymers in xylene and/or water were investigated by using light scattering, small-angle neutron scattering, and small-angle X-ray scattering. The association process of aqueous solution or water-rich ternary systems was entropy driven and temperature played an important role. The additive, e.g., water in the oil-rich ternary system, played a very important role on the micellization of PEO-PPO-PEO, e.g., Pluronic L64, in xylene. The micelles had a core-shell structure and the micellar shell was rather heavily solvated. At high copolymer concentrations, large aggregates with a lamellar structure was formed and the amount of large aggregates increased with increasing copolymer concentration before gel formation.     

Aggregation Behavior of Pluronic-L64 in Presence of Sodium Dodecyl Sulphate in Water

The effect of sodium dodecyl sulfate (SDS) on the micellization and aggregation behavior of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) amphiphilic copolymer (Pluronic L64: EO₁₃ PO₃₀ EO₁₃) have been investigated by various techniques like, cloud point, viscosity, isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC), fluorescence spectroscopy, room temperature phosphorescence (RTP), and small angle neutron scattering (SANS). Addition of SDS in L64 solutions shows mark alteration of different properties. We observed synergistic interaction between SDS and Pluronic L64. The changes in the critical micelle concentration (CMC), critical micelle temperature (CMT), cloud point (CP), micelle size, and shape has been correlated and reported in terms of structure dynamics and mechanics. The ITC titrations have been used to explore the different stages of binding and interactions of SDS with L64. The enthalpies of aggregation for copolymer-SDS aggregates binding, organizational change of bound aggregates, and the threshold concentrations of SDS in the presence of copolymer were estimated directly from ITC titration curves. The effect of temperature on enthalpy values has been reported in terms of different aggregation state. Fluorescence and RTP for L64 were used to investigate the change in micellar environment on the addition of SDS at different temperature. Appearance and shifting of SANS peaks have been used to monitor the size and inter micellar interaction on addition of SDS in L64 solution. Cloud point and viscosity elaborate the penetration of SDS molecule in L64 micelle and hence changing the micellar architect.     

Microenvironment in the corona region of triblock copolymer micelles: temperature dependent solvation and rotational relaxation dynamics of coumarin dyes

Dynamic Stokes' shift and fluorescence anisotropy measurements using coumarin-153 (C153) and coumarin-151 (C151) as the fluorescence probes have been carried out in aqueous poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123) and poly(ethylene oxide)100-poly(propylene oxide)70-poly(ethylene oxide)100 (F127) block copolymer micelles with an aim to understand the water structures and dynamics in the micellar corona region. It has been established that the probes reside in the micellar corona region. It is indicated that the corona regions of P123 and F127 micelles are relatively less hydrated than the Palisade layers of neutral micelles like Triton-X-100 and Brij-35. From the appraisal of total Stokes' shift values for the probes in the two block copolymer micelles, it is inferred that the F127 micelle is more hydrated than the P123 micelle. It is observed that the dynamic Stokes' shift values for both of the probes remain more or less similar at all the temperatures studied in the P123 micelle. For C153 in F127, however, the observed Stokes' shift is seen to decrease quite sharply with temperature, though it remains quite similar for C151. Moreover, the fraction of the unobserved initial dynamic Stokes' shift is appreciably higher for both the probes in the F127 micelle compared to that in P123. Over the studied temperature range of 293-313 K, the spectral shift correlation function is described adequately by a bi-exponential function. Rotational relaxation times for C153 in both the micelles show a kind of transition at around 303 K. These results have been rationalized assuming collapse of the poly(ethylene oxide) (PEO) blocks and formation of water clusters in the corona region due to dehydration of poly(ethylene oxide) blocks with an increase in temperature. A dissimilar probe location has been inferred for the differences in the results with C153 and C151 probes in F127. Comparison of the microviscosity and the hydration of the block copolymer micelles has also been made with those of the other commonly used neutral micelles, for a better understanding of the results in the block copolymer micelles.     

Formation of block copolymer micelles in solutions of a linear homopolymer in a good solvent

The existence of micelles of polystyrene-block-poly(ethylene/propene) in solutions of polystyrene in toluene was investigated. Toluene is a good solvent of both copolymer blocks whereas polystyrene and poly(ethylene/propene) are immiscible polymers. The presence of homopolystyrene at high enough concentration can induce the micellization of polystyrene-block-poly(ethylene/propene) in solution of a good solvent such as toluene. The thermodynamics of this new micelle system at a given polystyrene concentration was studied. Light scattering measurements were carried out in order to determine the critical micelle temperature (CMT) of different micellar solutions. Standard Gibbs energy, enthalpy and entropy of micellization were estimated from CMT and concentration data. The numerical values found were less negative than those found for micelle systems consisting in a block copolymer dissolved in a single selective solvent.     

Micellisation and gelation of diblock copolymers of ethylene oxide and propylene oxide in aqueous solution, the effect of P-block length

The aqueous solution properties of five diblock copolymers prepared by sequential anionic copolymerisation (i.e. E₁₀₂P₃₇, E₁₀₄P₅₂, E₉₂P₅₅, E₁₀₄P₆₀ and E₉₈P₇₃ where E denotes oxyethylene and P denotes oxypropylene) were studied across a wide range of concentration. The techniques used to study micellisation and micellar properties in dilute solution were static and dynamic light scattering, surface tension, and eluent gel-permeation chromatography. The gelation of concentrated solutions was also investigated. As expected, the critical micelle concentration (CMC) was lowered and the association number of the micelles was increased by an increase in P-block length. In contrast, the critical gel concentration was unchanged, consistent with the constant E-block length leading to micelles with essentially identical E-block fringes. Comparison of the CMCs of the diblock copolymers with those of triblock E_mP_nE_m copolymers with the same P-block length shows the diblock copolymers to micellise more efficiently. A similar comparison of the CMCs of the diblock copolymers with those of E_mB_n copolymer (B denotes oxybutylene) shows the hydrophobicity of a P unit to be one-sixth that of a B unit. The possibility is explored of correlating the limiting association number of a spherical micelle with the hydrophobe block length of its constituent copolymer. Of the five copolymers, only dilute solutions of E₉₈P₇₃ were predominantly micellar at both room temperature and body temperature, and this copolymer must be a prime candidate in any consideration of the potential application of E_mP_n copolymers in the solubilisation and controlled release of drugs.     

Continuous synthesis of CdSe(x)Te(1-x) nanocrystals: chemical composition gradient and single-step capping

Thermosensitive poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer, Pluronic F68, containing a hydrophobic unit, oligo-(lactic acid)(oligo-LA) or oligo-caprolactone (oligo-CL), 2-META and RGD as side groups was successfully synthesized and characterized by (1)H NMR, FTIR, and elemental analysis. Their aqueous solution displayed special gel-sol-gel phase transition behavior with increasing temperature from 10 to 70°C, when the polymer concentration was above critical micelle concentration (CMC). The gel-sol phase diagram was investigated using tube inversion method, rheological measurement, and dynamic light scattering. Based on these results, the gelation properties of modified F68 were affected by several factors such as the composition of the substituents, chain length of oligo L-LA or oligo ε-CL, and the concentration of the polymer solutions. The unique phase transition behavior with temperature was observed by modified F68 triblock copolymer, composed of the PPO blocks core and the PEO blocks shell in aqueous solution. This phenomenon was elucidated using (1)H NMR data; the alteration of hydrophobic interaction and chain mobility led to the formation of transparent gel, coexistence of gel-sol, and opaque gel. These hydrogels may be useful in drug delivery and tissue engineering.     

Effect of chain lengths of PEO-PPO-PEO on small unilamellar liposome morphology and stability: an AFM investigation

The morphology and stability of small unilamellar egg yolk phosphatidylcholine (EggPC) liposomes modified with the Pluronic copolymer (poly (oxyethylene)-poly (oxypropylene)-poly (oxyethylene) (PEO-PPO-PEO)) with different compositions on mica surface have been investigated using atomic force microscopy. Morphology studies reveal significant morphological changes of liposomes upon incorporating the Pluronic copolymer. Bilayers are observed for Pluronic with small hydrophilic (PEO) chain lengths such as L81 [(PEO)2(PPO)40(PEO)2] and L121 [(PEO)4(PPO)60(PEO)4]; bilayer and vesicle coexistence is observed for P85 [(PEO)26(PPO)39.5(PEO)26] and F87 [(PEO)61.1(PPO)39.7(PEO)61.1]; and stable vesicles are observed for F88 [(PEO)103.5(PPO)39.2(PEO)103.5], F127 [(PEO)100(PPO)65(PEO)100], and F108 [(PEO)132.6(PPO)50.3(PEO)132.6]. The micromechanical properties of Pluronic-modified EggPC vesicles were studied by analyzing AFM approaching force curve. The bending modulus (k(c)) of the Pluronic-modified EggPC vesicles increased several-fold compared with that of the pure EggPC vesicles. The significant difference is due to the enhanced rigidity of the EggPC vesicles as a result of the incorporation of PPO molecules and PEO chains. Based on the analysis of onset point by AFM and diameters of vesicles by light scattering, it was concluded that the favorable model to describe the polymer-bilayer interaction is the membrane-spanning model.     

Towerlike SBA-15: base and (10)-specific coalescence of a silicate-encased hexagonal mesophase tailored by nonionic triblock copolymers

Hydrothermal templating of mesoporous molecular sieves by nonionic triblock copolymers [poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) (PEO-PPO-PEO)] at specific block lengths of EO(20)PO(70)EO(20) and selected 2 M HCl dosage (pH -0.3) caused the formation of micrometer-sized SBA-15 hexagons with well-ordered hexagonal pore channels (pore size and wall thickness of approximately 6 nm and pore-to-pore distance of approximately 12 nm) after template removal. For a beneficial lower surface energy, these {10} laterally coalesced hexagons tend to stack imperfectly over the base into towerlike entities, leaving dislocations and faults within the single domain thus formed. Evidence for the mechanism of Brownian motion/coalescence of the hexagonal-mesophase particulates, previously suggested for MCM-41 accretion in the presence of cationic surfactant, is more clearly identifiable in the present low-pH case of amphiphilic block copolymer templates and linear silica oligomers.     

A thermally induced transition from a body-centred to a face-centred cubic lattice in a diblock copolymer gel

 A transition from a body-centred cubic structure at room temperature to a face-centred cubic structure at higher temperature has been observed in a solution of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer. The ordered micellar structures were confirmed using small-angle X-ray scattering by preparing oriented domains via steady shearing in a Couette cell. The effect of shear in generating highly oriented, twinned cubic domains is discussed. Received: 13 January 1998 Accepted: 22 January 1998     

Poly(ethylene oxide)-poly(styrene oxide)-poly(ethylene oxide) copolymers: Micellization, drug solubilization, and gelling features

Two new poly(ethylene oxide)-poly(styrene oxide) triblock copolymers (PEO-PSO-PEO) with optimized block lengths selected on the basis of previous studies were synthesized with the aim of achieving a maximal solubilization ability and a suitable sustained release, while keeping very low material expense and excellent aqueous copolymer solubility. The self-assembling and gelling properties of these copolymers were characterized by means of light scattering, fluorescence spectroscopy, transmission electron microscopy, and rheometry. Both copolymers formed spherical micelles (12-14 nm) at very low concentrations. At larger concentration (>25 wt%), copolymer solutions showed a rich phase behavior, with the appearance of two types of rheologically active (more viscous) fluids and of physical gels depending on solution temperature and concentration. The copolymer behaved notably different despite their relatively similar block lengths. The ability of the polymeric micellar solutions to solubilize the antifungal drug griseofulvin was evaluated and compared to that reported for other structurally-related block copolymers. Drug solubilization values up to 55 mg g⁻¹ were achieved, which are greater than those obtained by previously analyzed poly(ethylene oxide)-poly(styrene oxide), poly(ethylene oxide)-poly(butylene oxide), and poly(ethylene oxide)-poly(propylene oxide) block copolymers. The results indicate that the selected SO/EO ratio and copolymer block lengths were optimal for simultaneously achieving low critical micelle concentrations (cmc) values and large drug encapsulation ability. The amount of drug released from the polymeric micelles was larger at pH 7.4 than at acidic conditions, although still sustained over 1 day.     

Phase behavior of a mixture of poly(isoprene)-poly(oxyethylene) diblock copolymer and poly(oxyethylene) surfactant in water

The phase behavior of a mixture of poly(isoprene)-poly(oxyethylene) diblock copolymer (PI-PEO or C250EO70) and poly(oxyethylene) surfactant (C12EO3, C12EO5, C12EO6, C12EO7, and C12EO9) in water was investigated by phase study, small-angle X-ray scattering, and dynamic light scattering (DLS). The copolymer is not soluble in surfactant micellar cubic (I1), hexagonal (H1), and lamellar (Lalpha) liquid crystals, whereas an isotropic copolymer fluid phase coexists with these liquid crystals. Although the PI-PEO is relatively lipophilic, it increases the cloud temperatures of C12EO3-9 aqueous solutions at a relatively high PI-PEO content in the mixture. Most probably, in the copolymer-rich region, PI-PEO and C12EOn form a spherical composite micelle in which surfactant molecules are located at the interface and the PI chains form an oil pool inside. In the C12EO5/ and C12EO6/PI-PEO systems, one kind of micelles is produced in the wide range of mixing fraction, although macroscopic phase separation was observed within a few days after the sample preparation. On the other hand, small surfactant micelles coexist with copolymer giant micelles in C12EO7/ and C12EO9/PI-PEO aqueous solutions in the surfactant-rich region. The micellar shape and size are calculated using simple geometrical relations and compared with DLS data. Consequently, a large PI-PEO molecule is not soluble in surfactant bilayers (Lalpha phase), infinitely long rod micelles (H1 phase), and spherical micelles (I1 phase or hydrophilic spherical micelles) as a result of the packing constraint of the large PI chain. However, the copolymer is soluble in surfactant rod micelles (C12EO5 and C12EO6) because a rod-sphere transition of the surfactant micelles takes place and the long PI chains are incorporated inside the large spherical micelles.     

Thermodynamics of micellization of tapered statistical copolymers of ethylene oxide and propylene oxide in water

Two tapered statistical copolymers were prepared by the oxyanionic polymerization of ethylene oxide and propylene oxide and characterized by gel permeation chromatography and 13C NMR spectroscopy. We denote the copolymers t-E/P38 and t-E/P30, where E = oxyethylene, OCH2CH2, and P = oxypropylene, OCH2CH(CH3), and the number denotes the mole percentage P. In each case the copolymer chain length was ca. 100 oxyalkylene units. The association of the copolymers to form micelles in aqueous solution was checked by dynamic light scattering. The critical micelle temperatures (cmt) of the copolymers at several concentrations were determined by static light scattering and dye solubilization, and values of the apparent standard enthalpy of micellization (DeltamicHapp0) were obtained. For both copolymers, a low value of DeltamicHapp0 was found when the copolymer concentration exceeded ca. 150 g dm(-3).     

Effect of SDS on the self-assembly behavior of the PEO-PPO-PEO triblock copolymer (EO)20(PO)70(EO)20

The mixed micellar system comprising the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)-based triblock copolymer (EO)(20)(PO)(70)(EO)(20) (P123) and the anionic surfactant sodium dodecyl sulfate (SDS) has been investigated in aqueous media by small-angle neutron scattering (SANS) and viscosity measurements. The aggregation number of the copolymer in the micelles decreases upon addition of SDS, but a simultaneous enhancement in the degree of micellar hydration leads to a significant increase in the micellar volume fraction at a fixed copolymer concentration. This enhancement in the micellar hydration leads to a marked increase in the stability of the micellar gel phase until it is destroyed at very high SDS concentration. Mixed micellar systems with low and intermediate SDS concentrations form the micellar gel phase in much wider temperature and copolymer concentration ranges than the pure copolymer micellar solution. A comparison of the observed results with those for the copolymers (EO)(26)(PO)(40)(EO)(26) (P85) and (EO)(99)(PO)(70)(EO)(99) (F127) suggests that the composition of the copolymers plays a significant role in determining the influence of SDS on the gelation characteristics of the aqueous copolymer solutions. Copolymers with high PO/EO ratios show an enhancement in the stability of the gel phase, whereas copolymers with low PO/EO ratios show a deterioration of the same in the presence of SDS.     

Micelles and gels of mixed triblock copoly(oxyalkylene)s in aqueous solution

The micellization in dilute aqueous solution of a 50/50 wt% mixture of two triblock copolymers, E45B14E45 and E62P39E62, and the gelation of concentrated micellar solutions have been investigated over a range of temperatures. Here E, B, and P denote oxyethylene, oxubutylene, and oxypropylene chain units. Comparison is made with aqueous solutions of the individual copolymers. The results of light scattering measurements are consistent with effectively separate micellization of the two copolymers in the mixture. Hard gel formed when the extent of micellization was high for both copolymers. Because of the relatively high critical micellization temperatures of copolymer E62P39E62, the low-temperature boundary of the hard gel was high for this copolymer and for the mixture. The minimum concentration for hard-gel formation was higher for the mixture than for either of the individual copolymers, as would be expected for packing of two distributions of micelles of different average size.