Aggregation and self-assembly of amphiphilic block copolymers in aqueous dispersions of carbon nanotubes

The self-assembly (SA) of amphiphilic block copolymers (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)) was investigated in dispersions of single-walled and multiwalled carbon nanotubes (SWNT and MWNT, respectively) as a function of temperature. Differential scanning calorimetry (DSC) was used for characterization of the thermal behavior of the combined polymers-nanostructures system, and spin-probe electron paramagnetic resonance (EPR) was employed for probing the local dynamic and polarity of the polymer chains in the presence of nanostructures. It was found that SWNT and MWNT modify the temperature, enthalpy, and dynamic behavior of polymer SA. In particular, SWNT were found to increase the cooperativity of aggregating chains and dominate aggregate dynamics. MWNT reduced the cooperativity, while colloidal carbon black additives, studied for comparison, did not show similar effects. The experimental observations are consistent with the suggestion that dimensional matching between the characteristic radius of the solvated polymer chains and the dimensions of additives dominate polymer SA in the hybrid system.     

Structure and interactions of charged triblock copolymers studied by small-angle X-ray scattering: dependence on temperature and charge screening

A series of thermo-responsive cationic triblock copolymers composed of methoxy-poly(ethylene glycol) (MPEG, hydrophilic), poly(N-isopropylacrylamide) (PNIPAAM, temperature sensitive), and poly((3-acrylamidopropyl) trimethyl ammonium chloride) (PN(+), cationic) has been investigated as a function of temperature and ionic strength. In the MPEG-b-PNIPAAM-b-PN(+) copolymers, the MPEG block length is constant, and the lengths of the PNIPAAM and PN(+) blocks are varied. The solubility of the PNIPAAM block decreases with increasing temperature, and the triblock copolymer thus provides the possibilities of studying micelles with both neutral and charged blocks in the micelle corona as well as the interplay between these two blocks as the electrostatic interactions are varied by addition of salt. Investigation of the systems by densitometry and small-angle X-ray scattering (SAXS) in a temperature range from 20 to 70 °C gave detailed information on the behavior both below and above the critical micelle temperature (CMT). A clear effect of the addition of salt is observed in both the apparent partial specific volume, obtained from the densitometry measurements, and the SAXS data. Below the CMT, the single polymers can be described as Gaussian chains, for which the repulsive interchain interactions, originating from the charged PN(+) block, have to be taken into account in salt-free aqueous solution. Increasing the salt concentration of the solution to 30 mM NaCl leads to an increase in the apparent partial specific volume, and the electrostatic repulsive interchain interactions between the single polymers vanish. Raising the temperature results in micelle formation, except for the copolymer with only 20 NIPAAM units. The SAXS data show that the polymer with the medium PNIPAAM block length forms spherical micelles, whereas the polymer with the longest PNIPAAM block forms cylindrical micelles. Increasing the temperature further above the CMT results in an increase in the micellar aggregation number for both of the polymers forming spherical and cylindrical micelles. The addition of salt to the solution also influences the aggregates formed above the CMT. Overall, the micelles formed in the salt solution have a smaller cross-section radius than those in aqueous solution without added salt.     

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.     

Thermosensitive sol–gel transition behaviors of poly(ethylene oxide)/aliphatic polyester/poly(ethylene oxide) aqueous solutions

The thermoreversible gelation of Pluronic [poly(ethylene oxide) (PEO)–polypropylene oxide (PPO)–PEO] aqueous solutions originates from micelle formation and micelle volume changes due to PEO–water and PPO–water lower critical solution temperature behavior. The micelle volume fraction is known to dominate the sol–gel transition behavior of Pluronic aqueous solutions. Triblock copolymers of PEO and aliphatic polyesters, instead of PPO, were prepared by hexamethylene diisocyanate coupling and dicyclohexyl carbodiimide coupling. Through changes in the molecular weight and hydrophobicity of the polyester middle block, the hydrophobic–hydrophilic balance of each block was systematically controlled. The following aliphatic polyesters were used: poly(hexamethylene adipate) (PHA), poly(ethylene adipate) (PEA), and poly(ethylene succinate) (PESc). With the hydrophobicity and molecular weight of the middle block increasing, the critical micelle concentration at the same critical micelle temperature decreased, and the absolute value of the micellization free energy increased. The micelle size was rather insensitive to temperature but slightly decreased with increasing temperature. PEO–PHA–PEO and PEO–PEA–PEO triblock copolymers needed high polymer concentrations to form gels. This was ascribed to the tight aggregation of PHA and PEA chains in the micelle core due to strong hydrophobic interactions, which induced the contraction of the micelle core. However, because of the relatively hydrophilic core, a PEO–PESc–PEO aqueous solution showed gelation at a low polymer concentration. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 772–784, 2004     

Aqueous two-phase systems containing self-associating block copolymers. Partitioning of hydrophilic and hydrophobic biomolecules.

A series of proteins and one membrane-bound peptide have been partitioned in aqueous two-phase systems consisting of micelle-forming block copolymers from the family of Pluronic block copolymers as one polymer component and dextran T500 as the other component. The Pluronic molecule is a triblock copolymer of the type PEO-PPO-PEO, where PEO and PPO are poly(ethylene oxide) and poly(propylene oxide), respectively. Two different Pluronic copolymers were used, P105 and F68, and the phase diagrams were determined at 30 degrees C for these polymer systems. Since the temperature is an important parameter in Pluronic systems (the block copolymers form micellar-like aggregates at higher temperatures) the partitioning experiments were performed at 5 and 30 degrees C, to explore the effect of temperature-triggered micellization on the partitioning behaviour. The temperatures correspond to the unimeric (single Pluronic chain) and the micellar states of the P105 polymer at the concentrations used. The degree of micellization in the F68 system was lower than that in the P105 system, as revealed by the phase behaviour. A membrane-bound peptide, gramicidin D, and five different proteins were partitioned in the above systems. The proteins were lysozyme, bovine serum albumin, cytochrome c, bacteriorhodopsin and the engineered B domain of staphylococcal protein A, named Z. The Z domain was modified with tryptophan-rich peptide chains in the C-terminal end. It was found that effects of salt dominated over the temperature effect for the water-soluble proteins lysozyme, bovine serum albumin and cytochrome c. A strong temperature effect was observed in the partitioning of the integral membrane protein bacteriorhodopsin, where partitioning towards the more hydrophobic Pluronic phase was higher at 30 degrees C than at 5 degrees C. The membrane-bound peptide gramicidin D partitioned exclusively to the Pluronic phase at both temperatures. The following trends were observed in the partitioning of the Z protein. (i) At the higher temperature, insertion of tryptophan-rich peptides increased the partitioning to the Pluronic phase. (ii) At the lower temperature, lower values of K were observed for ZT2 than for ZT1.     

Fluorescence of commercial Pluronic F127 samples: Temperature-dependent micellization

We present a novel approach of using the butylated hydroxytoluene (BHT) antioxidant found in commercial Pluronic F127 samples as a marker of polymer aggregation. The BHT marker was compared to the pyrene dye and static light scattering methods as a way to measure the critical micelle concentration (CMC) and critical micelle temperature (CMT). The n→π(?) transitions of BHT are sensitive to the microenvironment as demonstrated by plotting the fractional intensities of its excitation (≈280nm) and emission (≈325nm) peaks. BHT is more sensitive to changes in temperature than concentration. The partition coefficient increases ≈40-fold for pyrene compared to ≈2-fold for BHT when the temperature is increased from 25 to 37°C. CMT values determined using the BHT fluorescence decrease with increasing F127 concentration. Our results show that BHT can be used as a reliable marker of changes in the microenvironment of Pluronic F127.     

Reversible controlled assembly of thermosensitive polymer-coated gold nanoparticles

Aggregation of thermosensitive polymer-coated gold nanoparticles was performed in aqueous solution in the presence of a triblock copolymer poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic P123, PEO(20)-PPO(68)-PEO(20)). The gold nanoparticles, AuNPs, which are covered by thermosensitive statistical copolymers poly(EO(x)-st-PO(y)), aggregate when the temperature is higher than the phase transition temperature of the polymer, leading to a macroscopic precipitation. The presence of Pluronic chains in solution prevents the uncontrolled aggregation of the AuNPs at higher temperature than both the aggregation temperature of the AuNPs (T(agg)) and the critical micellization temperature (cmt) of the Pluronic. The size, the colloidal stability, and the optical properties of the AuNPs aggregates are modulated as a function of the P123-to-AuNP ratio, which constitutes the critical parameter of the system. Moreover, the AuNP aggregation is totally reversible upon decreasing the temperature below T(agg). Our approach constitutes an easy way to the formation of well-controlled nanoparticle aggregates with well-defined sizes. The resulting aggregates have been characterized by UV-vis spectroscopy, dynamic light scattering, and electron microscopy.     

Tuning of the Temperature Window for Unit‐Cell and Pore‐Size Enlargement in Face‐Centered‐Cubic Large‐Mesopore Silicas Templated by Swollen Block Copolymer Micelles

The unit‐cell size and pore diameter as functions of temperature are investigated in the syntheses of FDU‐12 silicas with face‐centered cubic structure templated by Pluronic (PEO‐PPO‐PEO) block copolymer micelles swollen by toluene. The temperature range in which the unit‐cell size and pore size strongly increase as temperature decreases is correlated with the critical micelle temperature (CMT) of the surfactant. While Pluronic F127 affords a wide range of unit‐cell parameters (28–51 nm) and pore diameters (16–32 nm), it renders moderately enlarged pore sizes at 25 °C. The use of Pluronic F108 with higher CMT affords FDU‐12 with very large unit‐cell size (～49 nm) and large pore diameter (27 nm) at 23 °C. Large unit‐cell size (40–41 nm) and pore size (22 nm) were obtained even at 25 °C. The application of Pluronics F87 and F88 with much smaller molecular weights and higher CMTs also allows one to synthesize FDU‐12 with quite large unit‐cell parameters and pore sizes at room temperature. The present work demonstrates that one can judiciously select Pluronic surfactants with appropriate CMT to shift the temperature range in which the pore diameter is readily tunable.     

Single-wall carbon nanotubes and peapods investigated by EPR

Single-wall carbon nanotubes (SWNT) prepared by the "super growth" method developed recently exhibit electron paramagnetic resonance (EPR) signals, which can be attributed to itinerant spins. EPR results indicate very low defect and catalyst concentrations in this superior material. Under these conditions EPR can be used to study details of charge transport properties over a wide temperature range, although the material is still very "heterogeneous" with respect to tube diameter and chirality. Non-resonant microwave absorption in the temperature range below 20 K is indicative for the opening of a small gap at the Fermi energy for tubes of metallic character, which is indicative for a transition into a superconducting state. Using SWNT filled partially with an endohedral spin probe like N@C(60), such "peapods" can be investigated "from the inside". Continuous-wave (cw) and pulsed EPR was used to investigate localization dynamics within the tubes or to check for interaction with itinerant electrons. Using SWNT grown by different methods, the dominant influence of tube diameter on fullerene dynamics was revealed by temperature dependent pulsed EPR experiments. These differences can be correlated with the interactions between the endohedral observer spin and spins on the SWNT.     

Interaction of meso-tetrakis (4-sulfonatophenyl) porphyrin with cationic CTAC micelles investigated by small angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR)

Small angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) have been used to investigate the interaction of the water-soluble meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS(4)) with cationic cethyltrimethylammonium chloride (CTAC) micelles. To evaluate if the porphyrin protonation state affects its interaction with the micelle, both SAXS and EPR measurements were performed at pH 4.0 and 9.0. The best-fit SAXS curves were obtained assuming for CTAC micelle a prolate ellipsoidal shape in the absence and upon incorporation of 2-10 mM TPPS(4). SAXS results show that the presence of porphyrin impacts on micellar hydrophobic core, leading to a micellar reassembling into smaller micelles. Lineshapes of EPR spectra of 5- and 16-doxyl stearic acids (5- and 16-DSA, respectively) bound to 100 mM CTAC micelles exhibited slight changes as a function of porphyrin concentration. Spectral simulations revealed an increase of mobility restriction for both spin probes, especially at higher porphyrin concentration, where a small reduction of environment polarity was also observed for 16-DSA. The spin labels monitored only slight differences between pH 4.0 and 9.0, in agreement with the SAXS results.     

Diffusion Coefficients and Structure Properties in the Pluronic F127/n‐C4H9OH/H2O System

Abstract

Diffusion coefficients of different aggregates in aqueous solutions formed by an amphiphilic block copolymer, Pluronic F127 (F127), were determined by cyclic voltammetry, and the critical micelle concentration (CMC, 4.31 × 10^?4 mol L^?1) of F127 was obtained. The added n‐butanol facilitates the formation of micelles from the monomers of F127 and makes the critical micelle temperature (CMT) of F127 solutions decrease. The diffusion coefficient of the F127 micelles decreases relatively fast at first with increasing n‐butanol and then the decreasing trend slows after the solubilization of n‐butanol in micelles reaches maximum.     

Microviscoelasticity of soft repulsive sphere dispersions: tracer particle microrheology of triblock copolymer micellar liquids and soft crystals

Tracer particle microrheology using diffusing wave spectroscopy-based microrheology is demonstrated to be a useful method to study the dynamics of aqueous Pluronic? F108 solutions, which are viewed as solutions of repulsive soft spheres. The measured zero-shear microviscosity of noncrystallizing micellar dispersions indicates micelle corona dehydration upon increasing temperature. Colloidal sphere thermal motion is shown to be exquisitely sensitive to the onset of crystallization in these micellar dispersions. High temperature dynamics are dominated by an apparent soft repulsive micelle-micelle interaction potential indicating the important role played by lubrication forces and ultimately micelle corona interpenetration and compression at sufficiently high concentrations. The measured microscopic viscoelastic storage and loss moduli are qualitatively similar to those experimentally observed in mechanical measurements on colloidal dispersions and crystals, and calculated from mode coupling theory of colloidal suspensions. The observation of subdiffusive colloidal sphere thermal motion at short time-scales is strong evidence that the observed microscopic viscoelastic properties reflect the dynamics of individual micelles rather than a dispersion of micellar crystallites.     

Effect of ionic liquids on the aggregation behavior of PEO-PPO-PEO block copolymers in aqueous solution

Effect of 1-butyl-3-methyl-imidazolium bromide (BmimBr) on the aggregation behavior of PEO-PPO-PEO Pluronic P104 aqueous solution was studied by Fourier transform infrared (FTIR) spectroscopy, freeze fracture transmission electron microscopy (FF-TEM), dynamic light scattering (DLS), and NMR spectroscopy. When the BmimBr concentration was below 1.232 mol/L, the critical micelle temperature (CMT) of Pluronic P104 remained constant, while the size of micelles increased with increasing the BmimBr concentration; above this concentration, the CMT of Pluronic P104 decreased abruptly, and bigger clusters of BmimBr were formed. The selective nuclear Overhauser effect (NOE) spectrum indicates that the PO block of the P104 interacts with the butyl group of the Bmim+ cation by hydrophobic interaction. It suggests that when the concentration of BmimBr is below 1.232 mol/L, there are P104 micelles in the aqueous solution with BmimBr embedding to the micellar core, while above this concentration, P104 micelles and BmimBr clusters coexist in the system.     

Concentration, temperature, and salt-induced micellization of a triblock copolymer Pluronic L64 in aqueous media

The effect of copolymer concentration, temperature, and sodium halides (NaI, NaBr, NaCl, and NaF) on micellization and micellar properties of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) amphiphilic copolymer (Pluronic L64: EO13PO30EO13), was examined by different methods such as dye spectral change, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), small angle neutron scattering (SANS), dynamic light scattering (DLS), viscosity, and cloud point (CP). Temperature/polymer concentration/salt dependent aggregation behavior of L64 was observed. The data on critical micelle concentration (CMC), critical micelle temperature (CMT), (CP), micelle size, and shape are reported. The Fourier transform infrared (FTIR) showed temperature dependent changes in C-O-C stretching variation band towards higher wave numbers and broadening of band width during the micellization process; this was attributed to increase in proportion of the anhydrous methyl groups, while the proportion of the hydrated methyl groups was decreased. Differential scanning calorimetry (DSC) provides CMTs and CPs from the same experiment. CMC values derived from dye spectral change, decrease significantly with the addition of salt. The increases in salt/copolymer concentration lower the onset temperature of micellization (CMT). Halide anions influence both CMT and CP in the order of F- > Cl- > Br- > I- when total salt and copolymer concentration kept constant. SANS results show the increase of inter-micellar interaction due to the increase in temperature/salt concentration; this is supported by viscosity data.     

Stabilization and activation of Pluronic micelles for tumor-targeted drug delivery

In order to be used as drug carriers, Pluronic micelles require stabilization to prevent degradation caused by significant dilution accompanying IV injection. This article studies three routes of Pluronic micelle stabilization. The first route was direct radical crosslinking of micelles cores which resulted in micelle stabilization. However, this compromised the drug loading capacity of Pluronic micelles. In the second route, a small concentration of vegetable oil was introduced into diluted Pluronic solutions. This decreased micelle degradation upon dilution while not compromising the drug loading capacity of oil-stabilized micelles. The third route was a novel technique based on polymerization of the temperature-responsive LCST hydrogel in the core of Pluronic micelles. The hydrogel phase was in a swollen state at room temperature, which provided a high drug loading capacity of the system. The hydrogel collapsed at physiological temperatures which locked the core of micelles thus preventing them from fast degradation upon dilution. This new drug delivery system was called Plurogel®. Phase transitions in Plurogel® caused by variations in temperature or concentration were studied by the EPR. The effect of Pluronic concentration in the incubation medium on the intracellular uptake of two anti-cancer drugs was studied. At low Pluronic concentrations, when the drugs were located in the hydrophilic environment, drug uptake was increased, presumably due to the effect of a polymeric surfactant on the permeability of cell membranes. In contrast, when the drugs were encapsulated in the hydrophobic cores of Pluronic micelles, drug uptake by the cells was substantially decreased. This may be advantageous in the prevention of undesired drug interactions with normal cells. Ultrasonication enhanced intracellular drug uptake from dense Pluronic micelles. These findings permitted the formulation of a new concept of a localized drug delivery.     

Temperature-dependent aggregation and disaggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer in aqueous solution

Aggregation and disaggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers, Pluronics P103 and P104, in aqueous solutions during a heating and cooling cycle were investigated by dynamic laser scattering (DLS) and 1H NMR spectroscopy. Temperature hysteresis was observed by DLS when cooling the copolymer aqueous solutions because larger aggregates existed at temperatures lower than critical micellization temperature (CMT), but no temperature differences were observed by NMR. This phenomenon was explained as the forming of water-swollen micelles at temperatures lower than CMT during the cooling process.     

Thermodynamics of temperature-sensitive polyether-modified poly(acrylic acid) microgels

The temperature-induced structural changes and thermodynamics of ionic microgels based on poly(acrylic acid) (PAA) networks bonded with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) (Pluronic) copolymers have been studied by small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), differential scanning calorimetry (DSC), and equilibrium swelling techniques. Aggregation within microgels based on PAA and either the hydrophobic Pluronic L92 (average composition, EO8PO52EO8; PPO content, 80%) or the hydrophilic Pluronic F127 (average composition, EO99PO67EO99; PPO content, 30%) was studied and compared to that in the solutions of the parent Pluronic. The neutron scattering results indicate the formation of micelle-like aggregates within the F127-based microgel particles, while the L92-based microgels formed fractal structures of dense nanoparticles. The microgels exhibit thermodynamically favorable volume phase transitions within certain temperature ranges due to reversible aggregation of the PPO chains, which occurs because of hydrophobic associations. The values of the apparent standard enthalpy of aggregation in the microgel suspensions indicate aggregation of hydrophobic clusters that are more hydrophobic than the un-cross-linked PPO chains in the Pluronic. Differences in the PPO content in Pluronics L92 and F127 result in a higher hydrophobicity of the resulting L92-PAA-EGDMAmicrogels and a larger presence of hydrophobic, densely cross-linked clusters that aggregate into supramolecular structures rather than micelle-like aggregates such as those formed in the F127-PAA-EGDMA microgels.     

Effect of acid on the aggregation of poly(ethylene xide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers

The acid effect on the aggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers EO(20)PO(70)EO(20) has been investigated by transmission electron microscopy (TEM), particle size analyzer (PSA), Fourier transformed infrared, and fluorescence spectroscopy. The critical micellization temperature for Pluronic P123 in different HCl aqueous solutions increases with the increase of acid concentration. Additionally, the hydrolysis degradation of PEO blocks is observed in strong acid concentrations at higher temperatures. When the acid concentration is low, TEM and PSA show the increase of the micelle mean diameter and the decrease of the micelle polydispersity at room temperature, which demonstrate the extension of EO corona and tendency of uniform micelle size because of the charge repulsion. When under strong acid conditions, the aggregation of micelles through the protonated water bridges was observed.     

Mechanisms of micellization and rheology of PEO-PPO-PEO triblock copolymers with various architectures

Micelle formation was followed by micro-DSC and rheology for aqueous solutions of two copolymers of PEO-PPO-PEO, the Pluronic F127 (from BASF) and the EG56 (from PolymerExpert), a branched copolymer built with three chains of F127 type. It is shown that micellization is endothermic and that, for both polymers, the enthalpy of formation/melting is proportional to total concentration. The rheology of the solutions was carefully analyzed, before gelation for F127, and it reveals firstly the progressive changes of solubility of the unimers (decease of relative solution viscosity), followed by micelle formation over a 10 degrees C range. In this range, the micelle concentration dependence on temperature was deduced from enthalpy measurements and the corresponding volume fractions were derived. Viscosity was interpreted within the framework of well-known theories for hard sphere suspensions (Krieger-Dougherty or Quemada) based on an analogy between micelles and nanosized hairy grain suspensions. The gel state is achieved due to formation of the colloidal crystal. For EG56, the rheology is quite different; as the aggregation increases with temperature, a progression is observed from Newtonian to visco-elastic liquid. The characteristic frequency, defined by the relation G(') = G('), for EG56 varies with temperature and the corresponding times increase by two orders of magnitude according to an Arrhenius law. The frequency dependence of G(') and G(') at different temperatures can be superposed with a horizontal shift factor and a small amplitude adjustment. There is no elastic solid formation in this case. The "gelation" of these two copolymers is compared to the physical gelation of cold-set gels (gelatin).     

Thermosensitive pluronic micelles stabilized by shell cross-linking with gold nanoparticles

Gold nanoparticles were employed to prepare shell cross-linked Pluronic micelles that exhibit a reversibly thermosensitive swelling/shrinking behavior. Two terminal hydroxyl groups of Pluronic F127 were thiol-functionalized to form self-assembling Pluronic micelles in aqueous solution with exposed -SH groups in an outer shell layer. The thiol groups present in the outer shell were cross-linked by gold nanoparticles synthesized through NaBH4 reduction of gold precursor anions. The resultant shell cross-linked gold-Pluronic micelles exhibited a temperature-dependent volume transition: their hydrodynamic diameter was changed from 157.1 +/- 15.6 nm at 15 degrees C to 53.4 +/- 5.5 nm at 37 degrees C as determined by dynamic light scattering. The critical micelle temperature measured by a pyrene solubilization technique suggested that the reversible swelling/shrinking behavior of the micelles was caused by hydrophobic interactions of cross-linked or grafted Pluronic copolymer chains in the micelle structure with increasing temperature. Transmission electron microscopy directly revealed that the shell cross-linked micelles were indeed produced by gold nanoparticles covalently clustered on the surface. These novel self-assembled organic/inorganic hybrid micelles would hold great potential for diagnostic and therapeutic applications.