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At least 127 records · Page 7Linked to original sources
PubMed1996

Self-Assembly in a Mixture of Two Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) Copolymers in Water

The self-assembly behavior in water of a mixture of two poly (ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymers, (EO)13(PO)30(EO)13 (L64) and (EO)37(PO)58(EO)37 (P105), was explored at 25°C. The phase boundaries were established using 2H-NMR and inspection under polarized light; the structure of the various lyotropic liquid crystalline (LLC) phases was determined with small-angle X-ray scattering, while viscosity and differential scanning calorimetry measurements were used to probe the isotropic water-rich solution region. Isotropic regions, similar to the neat polymers, are stable at high polymer content. The addition of water induces structure in the amphiphilic block copolymer system. An extended lamellar (D) LLC phase is formed at 20-25% water content; a hexagonal (E) and a cubic (I) LLC phases supersede D at higher water contents. In addition to the above, a narrow isotropic region (L') is observed on the L64-water binary axis, in equilibrium with the E and the D phases. The hexagonal and lamellar LLC phases extended all the way from the L64-rich to the P105-rich side of the ternary L64-P105-water phase diagram; the characteristic hexagonal and lamellar structural dimensions varied linearly with P105 content in the L64-P105 mixture at a constant water concentration. An isotropic (micellar) solution phase (L1) dominates the high-water content corner of the ternary phase diagram. Viscosity measurements in this region provided evidence for increased interactions between the micelles as the boundary to the LLC phases was approached.

Journal Article↗
PubMed2006

Dynamics of micelles of the triblock copolymers poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) in aqueous solution.

A number of results reported on the kinetics of exchange of triblock copolymers poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, between micelles and the intermicellar aqueous solution are reviewed and analyzed to extract the rate constants k(+) for the entry of a copolymer into a micelle and k(-) for the exit of a copolymer from a micelle. Contrary to what is generally observed for conventional surfactants, the rate constant for the entry of a copolymer into a micelle is slower to much slower than for a diffusion-controlled process and decreases as the degree of polymerization of the PO block, n(PO), increases. The effect of the degree of polymerization of the EO block, n(EO), on the two rate constants is significant only for low values of n(EO). The variation of k(-) with n(PO) strongly suggests that the free copolymer molecule adopts a conformation where the PO block is tightly coiled with little contact with water and not a fully extended and hydrated conformation, in contrast to what is commonly assumed when analyzing the dependence of the cmc on the polymerization degree of the hydrophobic block.

Journal Article↗
PubMed2004

Effects of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers on structure and stability of liposomal dioleoylphosphatidylethanolamine.

The effects caused by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO; Pluronic) copolymers on the structure and stability of dioleoylphosphatidylethanolamine (DOPE) liposomes were studied by means of turbidity, leakage, and cryo-transmission electron microscopy investigations. The results show that by inclusion of Pluronics in the DOPE dispersion it is possible to stabilize the lamellar Lalpha phase and to produce liposomes that are stable and nonleaky at low pH (pH 5). The stabilizing capacity was observed to depend critically on the molecular composition of the Pluronics. Block copolymers with comparably long PPO and PEO segment lengths, such as F127 and F108, most effectively protected DOPE liposomes prepared at high pH from aggregation and subsequent structural rearrangements induced by acidification. A sufficiently long PPO block was found to be the most decisive parameter in order to obtain adequate coverage of the liposome surface at low Pluronic concentrations. Upon increasing the copolymer concentration, however, Pluronics with comparably short PPO and PEO segment lengths, such as F87 and P85, could also be used to stabilize the DOPE liposomes. Essentially the same trends were observed when the Pluronics were added to preformed DOPE liposomes instead of being included in the preparation mixture. In this case the least effective copolymers failed, however, to completely prevent the DOPE liposomes from releasing encapsulated hydrophilic markers.

Cryoelectron Microscopy↗
PubMed2004

Characterization and demulsification of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymers.

Four poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymers with different molecular weights and PPO/PEO composition ratios were synthesized. The characterization of the PEO-PPO-PEO triblock copolymers was studied by surface tension measurement, UV-vis spectra, and surface pressure method. These results clearly showed that the CMC of PEO-PPO-PEO was not a certain value but a concentration range, in contrast to classical surfactant, and two breaks around CMC were reflected in both surface tension isotherm curves and UV-vis absorption spectra. The range of CMC became wider with increasing PPO/PEO composition ratio. Surface pressure Pi-A curves revealed that the amphiphilic triblock copolymer PEO-PPO-PEO molecule was flexible at the air/water interface. We found that the minimum area per molecule at the air/water interface increased with the proportion of PEO chains. The copolymers with the same mass fractions of PEO had similar slopes in the isotherm of the Pi-A curve. From the demulsification experiments a conclusion had been drawn that the dehydration speed increased with decreased content of PEO, but the final dehydration rate of four demulsifiers was approximate. We determined that the coalescence of water drops resulted in the breaking of crude oil emulsions from the micrograph.

Emulsions↗
PubMed2004

Association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers with cationic surfactants in aqueous solution.

The association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers in aqueous solution with hexadecyltrimethylammonium bromide (HTAB), tetradecyltrimethylammonium bromide (TTAB), and dimethylene bis(decyldimethylammonium bromide) (10-2-10), was studied by fluorescence, viscosity, and Krafft temperature measurements. It has been observed that (EO)18(PO)31(EO)18 interacts more strongly than (EO)2(PO)15.5(EO)2 and (EO)2.5(PO)31(EO)2.5 with HTAB/TTAB due to synergistic interactions. A stronger capability of (EO)18(PO)31(EO)18 to interact with cationic surfactants arises from the greater number of electronegative EO units (total 36 EO units) than of (EO)2(PO)15.5(EO)2 (total 4 EO units) and (EO)2.5(PO)31(EO)2.5 (total 5 EO units). The antagonistic mixing behavior of present triblock polymers has been observed with 10-2-10. A difference in the mixing behavior of the latter from that of HTAB/TTAB has been attributed to its dimeric nature, which may create steric hindrances with triblock polymer components at the head group region in the mixed state.

Journal Article↗
PubMed2005

Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-g-poly(vinyl pyrrolidone): synthesis and characterization.

Pluronic poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers are grafted with poly(vinyl pyrrolidone) by free radical polymerization of vinyl pyrrolidone with simultaneous chain transfer to the Pluronic in dioxane. This modified polymer has both thermal responsiveness and remarkable capacity to interact with a wide variety of hydrophilic and hydrophobic pharmaceutical agents which is very attractive for medical applications. The chemical structure of the graft copolymers was characterized by FTIR and 1H NMR spectroscopy. Polymerization conditions such as initiators, feed ratio, and reaction times are studied to obtain the ideal graft copolymer.

Journal Article↗
PubMed2005

Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)-g-poly(vinylpyrrolidone): association behavior in aqueous solution and interaction with anionic surfactants.

In this work, we aimed to study the association and interaction behavior of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) block copolymers grafted with poly(vinylpyrrolidone). Critical micellization concentrations were determined using fluorescent probes (pyrene) and critical micellization temperatures characterizing temperature-dependent transitions from monomers to multimolecular micelles were measured. The thermal responsiveness of the copolymer is not affected by the grafting. The hydrodynamic radius of the graft copolymer micelles is found to be greater than that of the original copolymer micelles. The graft copolymer is found to form anisotropic aggregates. The structure of the graft copolymer micelles is less disrupted by the anionic surfactant sodium dodecyl sulfate, compared to the ungraft copolymer.

Journal Article↗
PubMed2003

Effect of sodium chloride on association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer in aqueous solutions.

The effect of sodium chloride (NaCl) upon the thermally induced association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer, Pluronic P103, has been investigated using pyrene fluorescence spectroscopy. The critical micellization temperature (CMT) of Pluronic P103 in aqueous solution is decreased by the addition of NaCl. The standard enthalpy and entropy of micellization for Pluronic P103 in water are increased in the presence of small amounts of NaCl, but further addition of NaCl decreases the standard enthalpy and entropy of micellization. The I1/I3 ratio of pyrene in aqueous Pluronic P103 solutions at temperature below the CMT decreases with increases of NaCl concentration, which is related to the decrease of PPO solubility. The decrease in polarity of the PPO shifts the CMT toward lower temperature.

Journal Article↗
PubMed2001

Understanding drug release from poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) gels.

Experimental and mathematical studies were performed to understand the release mechanism of small molecular weight compounds from poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) polymer gels (trademarked Pluronic by BASF Corp.) of various concentrations. Studies of the diffusion coefficient of solutes in the polymer gels were performed using a novel technique to predict movement of drugs within the gel as release occurs. Studies were also performed to determine the diffusion coefficient of water in the polymer gel, as it is this parameter that controls the dissolution rate of the polymer, and in turn, the drug release rate. A model was formulated and solved numerically to determine the controlling release mechanism. By parameter modification, this algorithm for determining the overall mass of drug released from a drug loaded gel can be used for a number of drugs and for a wide range of initial polymer concentrations. Drug release data were obtained with a novel experimental setup and were used to verify the accuracy of the overall solution of the model. The results of the model indicate that although the rate of polymer dissolution ultimately controls the drug release, about 5% of the release is due to diffusion at the gel/liquid interface, giving rise to a slightly non-linear release. It was also found that agitation speed greatly affects the dissolution rates of these polymer gels.

Drug Delivery Systems↗
PubMed2002

Room temperature phosphorescence of 6-bromo-2-naphthol in micelles of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers and in their mixed aggregates with sodium dodecyl sulfate.

Room temperature phosphorescence (RTP) of 6-bromo-2-naphthol has been investigated in aqueous micellar solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers as well as in their mixed aggregates with sodium dodecyl sulfate. RTP of the phosphorophor was enhanced to some extent in the micelles of the block copolymers. However, marked enhancement of RTP was observed in the mixed aggregates. The enhancement of RTP is attributed to effective incorporation of the phosphorophor into the micelles and the aggregates, resulting in suppression of nonradiative deactivation of the phosphorescent state.

Micelles↗
PubMedDate not supplied

Biodegradable polymeric nanospheres formed by temperature-induced phase transition in a mixture of poly(lactide-co-glycolide) and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer.

The mixture of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer(F-127) and PLGA (poly(lactide-co-gycolide)) forms a liquid state above their phase transition temperatures, and the phase-separated state is induced by decreasing the temperature below the phase transition temperature. On the basis of the temperature-induced phase transition behavior in the mixture of F-127 and PLGA, a novel method for the preparation of drug-loaded PLGA nanospheres was designed and characterized by measuring the loading amount, the encapsulation efficiency, and the drug release pattern. Paclitaxel, used as a potent anticancer drug, was selected as a model drug.

Antineoplastic Agents↗
PubMed2006

Self-consistent-field analysis of the micellization of carboxy-modified poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers.

The micellization properties of carboxy-modified Pluronics P85 (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers) are investigated by means of a molecularly realistic self-consistent-field theory. We consider the, so-called, carboxylic acid end-standing P85 (CAE-85) case where the carboxylic group is located at the end of both PEO parts and the carboxylic acid center-standing P85 (CAC-85) case where each of the carboxylic group presents between the PEO and PPO blocks. The micellization of these copolymers depends on the pH, the added electrolyte concentration phis, and the temperature. It is shown that the aggregation number (Nagg) decreases, whereas the critical micellization concentration (CMC) increases with pH. For the case of increasing phis, the Nagg increases and the CMC decreases. The critical micellization temperature (CMT) and cloud point temperature (CPT) increase with pH at low phis and decrease at increasing phis. The changing from CAE-85 to CAC-85 leads to increasing CMC and CMT, but lower CPT.

Journal Article↗
PubMed2005

Microviscosity in multiple regions of complex aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide).

Aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO109-PPO41-PEO109) copolymers are nonionic surfactants that self-organize to form aggregate structures with increasing temperature or concentration. We have studied two concentrations over a range of temperatures so that the copolymers are in one of three microphases: unimers, micelles, or hydrogels formed from body centered cubic aggregates of micelles. Three different coumarin dyes were chosen based on their hydrophobicity so that different aggregate regions could be probed independently-water insoluble coumarin 153 (C153), hydrophobic coumarin 102 (C102), and the hydrophilic sodium carboxylate form of coumarin 343 (C343-). Fluorescence anisotropy experiments provide detailed information on the local microviscosity. C153 experiences a fourfold increase in reorientation time and hence microviscosity with increasing temperature through the microphase transition from unimers to micelles. C102 also shows an increase in microviscosity with temperature but smaller in magnitude and with the microphase transition shifted to higher temperature relative to C153. C343- shows only a slight sensitivity to the microphase transition. For any of the three coumarin probes, fluorescence anisotropies do not show any correlation with the microphase transition to form cubic hydrogels.

Molecular Structure↗
PubMed2005

Small-angle neutron scattering and theoretical investigation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) stabilized oil-in-water microemulsions.

The aim of this study is to determine the effects of oil solutes and alcohol cosolvents on the structure of oil-in-water microemulsions stabilized by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers. The systems investigated involved the solubilization of 1,3,5-trimethylbenzene or 1,2-dichlorobenzene by P123 (EO(20)-PO(70)-EO(20)) pluronic surfactant micelles in water and water + ethanol solvents. The structures of these swollen micelles were determined by small-angle neutron scattering (SANS). A thermodynamic model was employed to interpret the characterization data. The results of the thermodynamic model for micellization agreed well with the SANS data from samples of micelles swollen by both oils. The model predicted the size of the micelles within 5% accuracy using only one fitting parameter, the micelle polydispersity. Ethanol had significantly different effects on the polymer micelles that contained solubilized oil compared to pure polymer micelles. For pure polymer micelles, the addition of ethanol increased the solubility of the polymer and, therefore, decreased the total volume fraction of micelles, while for polymer-oil aggregates, ethanol tended to have a positive effect on the volume fraction of micelles. SANS results showed that the greatest divergence from pure aqueous solvent results occurred at oil concentrations above the microemulsion stability limit.

Journal Article↗
PubMed2004

Heat capacity of transfer of (Ethylene oxide)13-(propylene oxide)30-(ethylene oxide)13 from water to the aqueous anionic surfactant solutions at 298 K. A quantitative treatment.

Heat capacities of transfer (DeltaCpt) of unimeric (ethylene oxide)13-(propylene oxide)30-(ethylene oxide)13 from water to the aqueous surfactant solutions as functions of the surfactant concentrations (mS) were determined at 298 K. The surfactants investigated are sodium hexanoate, sodium heptanoate, sodium octanoate, sodium undecanoate, and sodium dodecanoate. For short alkyl chain surfactants, the profiles of the DeltaCpt versus mS curves show maxima and minima; for long alkyl chain surfactants, the maximum becomes sharper and moved to lower mS values whereas the minimum tends to disappear. These experimental trends are different from those of the enthalpy in agreement with the fact that heat capacity, being the derivative of enthalpy with respect to temperature, reflects additional terms generated by temperature change on the equilibria in solution. On the basis of a thermodynamic model recently proposed by us for properties first derivatives of Gibbs free energy, a quantitative treatment of the experimental data was done. Such an approach assumes that even in the dilute surfactant region monomers of surfactant associate with unimeric copolymer generating surfactant-copolymer aggregation complexes and, whenever the surfactant achieves the conditions for the micellization, the formation of copolymer-micelle mixed aggregates takes place. The equation derived for the heat capacity of transfer is more complex than that for the enthalpy because it contains five additional terms due to the shift of the equilibria induced by the temperature change. It turned out that these contributions, evaluated by using the equilibrium constants and the associated enthalpies, cannot be neglected for a quantitative treatment of the experimental data. The minimizing procedure provided the heat capacity changes for the formation of the surfactant-copolymer aggregation complexes and the copolymer-micelle mixed aggregates.

Journal Article↗
PubMed1998

Effects of inorganic salts on solubilization of estriol in an aqueous solution of poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) triblock copolymer.

The effects of inorganic salts on the solubilization of estriol was studied in an aqueous solution of a poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) triblock copolymer (Pluronic L-64). The solubility of estriol increased with increasing Pluronic L-64 and salt concentrations. Furthermore, two clear transition points corresponding to the polymolecular micelle formation and a pronounced change in micellar structure of Pluronic L-64 were found on the solubility curves. The effectiveness of inorganic salts for increasing solubility of estriol followed approximately the lyotropic or Hofmeister series, except Li+.

Drug Compounding↗
PubMed1997

[X-ray fluorescence spectrometrlc determination of nine heavy rare earth elements in the enriched mixture of thulium oxide, ytterbium oxide and lutecium oxide by asming thin film technique].

A method for the determination of nine heavy rare earth elements in the enriched mixture of thulium oxide ytterbium oxide and lutecium oxide by XRF method has been studied in this poper. The samples were prepared by ashing thin film. This method is characterized as it is low cost, economic and simple in standards preparation, and standards can be easily preserved. The accuracy and precision of the results were satisfactory for the demands of prodnction.

English Abstract↗
PubMed1995

Reduction of nitric oxide synthase activity in human neutrophils by oxidized low-density lipoproteins. Reversal of the effect of oxidized low-density lipoproteins by high-density lipoproteins and L-arginine.

Oxidized low-density lipoproteins (ox-LDL) inhibit vascular relaxation by decreasing the synthesis or rapid degradation of endothelium-derived relaxing factor (EDRF), now identified to be nitric oxide (NO). We examined the regulation of NO synthase activity in human neutrophils, which also generate NO, by lipoproteins. Isolated human neutrophils were incubated with native-LDL, ox-LDL (10-50 micrograms protein/mL), high-density lipoproteins (HDL, 100 micrograms protein/mL) or HDL+ox-LDL, and NO synthase activity was measured as conversion of [3H]L-arginine to [3H]L-citrulline. Ox-LDL, but not native-LDL or HDL, significantly decreased NO synthase activity in human neutrophils. This effect of ox-LDL was incubation time and concentration dependent. The incubation of cells with HDL or L-arginine diminished the effects of ox-LDL on NO synthase activity. Thus, ox-LDL decreases the activity of NO synthase enzyme, and this effect of ox-LDL can be modified by HDL and L-arginine.

Arginine↗