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.     

Synthesis of urethane—acrylic multi-block copolymers via electrochemically mediated ATRP

The electrochemically mediated atom transfer radical polymerisation (eATRP) of n-butyl acrylate was investigated under a variety of catalyst concentrations. Poly(n-butyl acrylate)-block-polyurethane-block-poly(n-butyl acrylate) copolymers were prepared via electrochemically mediated atom transfer radical polymerisation (eATRP) using only 7 × 10^?6 mole % of Cu^II complex. The successful chain extension and formation of penta-block copolymers confirmed the living nature of the poly(alkyl acrylates) prepared by eATRP. In this work, the tri-block and penta-block urethane-acrylate copolymers were synthesised for the first time by using tertiary bromine-terminated polyurethane macro-initiators as transitional products reacting with n-butyl acrylate, and subsequently with tert-butyl acrylate in the presence of the Cu^IIBr₂/TPMA catalyst complex. The results of ¹H NMR spectral studies support the formation of tri-block poly(n-butyl acrylate)-block-polyurethane-block-poly(n-butyl acrylate) copolymers, and penta-block poly(tert-butyl acrylate)-block-poly(n-butyl acrylate)-block-polyurethane-block-poly(n-butyl acrylate)-block-poly(tert-butyl acrylate) copolymers.     

Study of heat of micellization and phase separation for Pluronic aqueous solutions by using a high sensitivity differential scanning calorimetry

Heat of micellization and phase separation temperature (known as cloud point) for the poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (abbreviated by PEO–PPO–PEO) triblock copolymers, the Pluronics F108, F98, F88, F68, F38, P65, and L62, in water are carefully determined by using a high sensitivity differential scanning calorimeter. It is interesting to find out that there exists a maximum heat of micellization for all these Pluronics. In this study, the heat of micellization of all of the Pluronics decreases as the temperature increases, as expected, at high temperature region (low Pluronic concentration region). However, the enthalpy change has a surprisingly positive relationship with temperature at low temperature region (high Pluronic concentration region). The critical micelle temperature consistently decreases as the Pluronic concentration increases. This unexpected behavior of the positive heat capacity changes of Pluronic aqueous solutions at higher concentration region is somewhat related to the variation of water accessible polar (PEO groups) and non-polar (PPO groups) surface areas in the micellization process. Especially, the removal of polar surface area from water may dominate the contribution to the positive heat capacity change upon micellization. In addition, the cloud points of Pluronic solutions are also discussed. The enthalpy–entropy compensation phenomenon for the micellization of Pluronics is discussed, and the enthalpy–entropy compensation temperature is calculated.     

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.     

Rheology of aqueous solutions of polyglycidol-based analogues to pluronic block copolymers

The aqueous solution properties of a series of polyglycidol-poly(propylene oxide)-polyglycidol (PG-PPO-PG) block copolymers were investigated by means of rheology. The copolymers are considered as analogues to the commercially available Pluronic, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), block copolymers in which the flanking PEO blocks are substituted by blocks of structurally similar PG bearing a hydroxyl group in each repeating monomer unit. In the dilute regime, the samples normally behave as Newtonian fluids. Shear thinning was observed only for the solutions of LGP65 (the copolymer of 50 wt % PG content) as well as at concentrations well above the critical micellization concentration for the rest of the copolymers. The zero shear viscosities exhibited pronounced maxima at PG content of 50 wt % and were found to decrease with increasing temperature. The concentrated solutions were investigated using oscillatory measurements. Large hystereses were observed during the temperature sweeps 15-70-15 degrees C. The evolutions of the loss and storage moduli with frequency, PG content, and temperature displayed transitions from a non-elastic to elastic behavior of the solutions. A phase diagram showing areas of predominant elasticity or fluidity was constructed.     

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.     

1H NMR spectroscopic investigations on the micellization and gelation of PEO-PPO-PEO block copolymers in aqueous solutions

The effects of temperature, polymer composition, and concentration on the micellization and gelation properties of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers in aqueous solutions were investigated by 1H NMR spectroscopy. It was found that the temperature-dependent behavior of PPO blocks, observed as changes in chemical shift, half-height width, and integral value, could be attributed as an intrinsic tool to characterize the transition states during unimer to micelle formation. The 1H NMR spectral analysis revealed that the hydrophobic part, PPO, of the Pluronic polymers plays a more significant role in the temperature-induced micellization, whereas the transitional behavior of Pluronic polymer, i.e., from micellization to liquid crystals formation, resulted in the drastic broadening of the spectral signals for the PEO, indicating that the PEO segments play a more significant role in the crystallization process. It was also observed that the temperature-dependent changes in the half-height width of the PEO -CH2- signal are sensitive to the liquid crystalline phase formation, which could be attributed to the close packing of spherical micelles at high polymer concentrations or temperatures.     

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.     

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     

Interfacial properties of Pluronics and the interactions between Pluronics and cholesterol/DPPC mixed monolayers

Pluronics are triblock copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) with wide range of hydrophilic-lipophilic balance. In order to investigate the relationship between the chemical structures of Pluronics and the interfacial properties at the air-water interface by monolayer techniques, Pluronics L61, P65, F68, P84, P123, L35, and P105 were selected. Since cholesterol influenced substantially the molecular packing stage and the characteristics of cell membranes, the interactions between Pluronics and model cell membranes in the absence and presence of cholesterol were compared. The results of pi-A isotherms and surface elasticities of Pluronic monolayers indicated that the first and second transition like stage were mainly affected by the numbers of EO and PO monomers, respectively. Pluronics with higher hydrophobicities demonstrated larger surface activities and penetration abilities to dipalmitoylphosphatidylcholine (DPPC) monolayers, which might be due to hydrophobic interactions and van der Waals forces. In the presence of cholesterol, hydrogen bonding effects was supposed to exist between the 3beta-hydroxy group of cholesterol and ether oxygen of PEO chains, which led Pluronic F68, with the longest PEO chain herein, to exhibit significantly higher penetration ability. Our findings proposed a theoretical basis for selection of optimized drug carriers and the starting point for further investigations.     

pH-responsive Micelles from a Blend of PEG-<Emphasis Type="Italic">b</Emphasis>-PLA and PLA-<Emphasis Type="Italic">b</Emphasis>-PDPA Block Copolymers: Core Protection Against Enzymatic Degradation

pH-responsive micelles with a biodegradable PLA core and a mixed PEG/PDPA shell were prepared by self-assembly of poly(ethylene glycol)-b-poly(lactic acid) (PEG-b-PLA) and poly(2-(diisopropylamino)ethyl methacrylate)-b-poly(lactic acid) (PDPA-b-PLA). The micellization status with different pH and the enzyme degradation behavior were characterized by ¹H-NMR spectroscopy, dynamic light scattering measurement and zeta potential test. The pH turning point of PDPA block was determined to be in the range of 5.5?7.0. While the pH was above 7.0, the PDPA block collapsed onto the PLA core and could protect the PLA core from invasion of enzyme, as a result, the micelle exhibited a resistance to the enzymatic degradation.     

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.     

Numerical Calculation of Permittivity and Dielectric Loss of Aqueous KF Solution Depending on State Parameters

Based on the analytical expressions for permittivity ε₁(ω) and dielectric loss ε₂(ω) are obtained by the kinetic equation method, the frequency spectra of these coefficients are analyzed for an aqueous KF solution in a wide variation range of the density ρ, the concentration C, and the temperature T. With a certain choice of the solution model, the potential interaction energy Φab(|r|), and the radial distribution function g_ab(|r|) of a- and b-type ions, ε₁(ω) and ε₂(ω) of an aqueous KF solution are numerically calculated depending on ρ, C, T, and ω.     

Synthesis and aqueous solution properties of functionalized and thermoresponsive poly(D,L-lactide)/polyether block copolymers

Tri- and pentablock amphiphilic copolymers containing hydrophobic poly(D,L-lactide) block(s) and hydrophilic polyethers were synthesized in order to obtain new precursor architectures suitable for drug delivery systems. Polyglycidol-6-poly(ethylene oxide)-b-poly(D,L-lactide) possess high hydroxyl functionality provided by the linear polyglycidol block. Thus very stable hydroxyl functionalized micelles in aqueous media were obtained. On the other hand poly(D,L-lactide)-b-poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(D,L-lactide) form temperature sensitive aggregates. The copolymers obtained were analyzed by SEC and NMR, and their aqueous solution properties were followed by cloud point measurements and determination of critical micellization temperature. TEM was used for particles visualization.     

Phase behavior and local dynamics of concentrated triblock copolymer micelles

We report a neutron-scattering study to characterize the ordering and local dynamics of spherical micelles formed by the triblock copolymer polyethylene oxide (PEO)--polypropylene oxide (PPO)--polyethylene oxide (Pluronic) in aqueous solution. The study focuses on two Pluronic species, F68 and F108, that have the same weight fraction of PEO but that differ in chain length by approximately a factor of 2. At sufficiently high concentration, both species undergo a sequence of phase changes with increasing temperature from dissolved chains to micelles with liquid-like order to a cubic crystal phase and finally back to a micelle liquid phase. A comparison of the phase diagrams constructed from small-angle neutron scattering indicates that crystallization is suppressed for shorter chain micelles due to fluctuation effects. The intermediate scattering function I(Q,t)I(Q,0) determined by neutron spin echo displays a line shape with two distinct relaxations. Comparisons between I(Q,t)I(Q,0) for fully hydrogenated F68 chains in D2O and for F68 with deuterated PEO blocks reveal that the slower relaxation corresponds to Rouse modes of the PPO segments in the concentrated micelle cores. The faster relaxation is identified with longitudinal diffusive modes in the PEO corona characteristic of a polymer brush.     

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.     

Rheological Properties of L64 Tri-block Copolymer in Aqueous and <Emphasis Type="Italic">p</Emphasis>-xylene Solutions

The rheological properties of Pluronic L64 in aqueous solutions at various temperatures and concentrations have been investigated. Changes in temperature and concentration cause structural and rheological modifications in Pluronic L64 in aqueous solutions as well as the appearance of a micellization process. For concentrations between 10 and 20 mg/mL, the Pluronic L64 showed a Newtonian or liquid-like behavior at various temperatures. However, for concentrations ranging between 30 and 40 mg/mL, the solutions showed a Newtonian behavior for temperatures below 55°C and a non-Newtonian behavior at 60°C. In p-xylene solutions, the solution exhibits a Newtonian behavior for all the concentrations and temperatures studied. It is believed that the Newtonian behavior of the binary Pluronic L64/p-xylene is due to the nonformation of complex conformations as direct or reverse micelles.     

F68和F108水溶液溶胶-凝胶转变过程的流变学研究

采用流变学方法研究了具有高PEO质量分数(80%)的两亲性三嵌段共聚物Pluronic F68(PEO₈₀-PPO₃₀-PEO₈₀)和F108(PEO₁₃₃-PPO₅₀-PEO₁₃₃)的水溶液在升温过程中的溶胶-凝胶转变过程, 发现对于特定浓度的嵌段共聚物水溶液, 在溶胶-凝胶转变过程中会出现一个“软凝胶”区域, 通过对F68进行区域的频率扫描实验, 推测了相应的内部结构.     

Concentration and solvent induced micellization of F68 tri-block copolymer

The Fourier transform infrared spectroscopy is used to study the binary mixtures of F68/water and F68/p-xylene. The results show that in aqueous solution the wavenumbers of the two bands associated to the O-H and C-O-C vibrations are inversely proportional. This can be explained by the fact that the dehydrated methylene groups approach by the hydrophobic interaction to form hydrophobic cores, which lead to a breakdown of the hydrogen bond between water and C-O-C. The spectrum associated to the binary mixture F68/p-xylene show that F68 exists as nonassociated molecules (unimers) at room temperature. The results of the ternary mixtures of the F68/p-xylene/water show a change in the spectra on the level of the vibrations of the O-H and C-O-C groups which is attributed to the change of structure following the variation of the concentration from unimers to large aggregates.     

Salt induced micellization of very hydrophilic PEO-PPO-PEO block copolymers in aqueous solutions

Symmetrical poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, triblock copolymers with 80% polyethylene oxide (PEO, the hydrophilic end blocks) and polypropylene oxide (PPO, the hydrophobic middle block) usually remain as molecularly dissolved at ambient temperature even at fairly high-concentrations (2 wt.% or more). However, the micellization is induced at lower concentration/temperature in the presence of salts. The results on salt induced micellization from four such hydrophilic copolymers Pluronic^® F38, F68, F88 and F108 obtained from several independent techniques are described. FTIR and fluorescence results provide essentially identical critical micelle temperatures (CMTs) showing marked decrease with increase in PPO molecular weight and in the presence of salt. These copolymers were weakly surface active and did not show a clear break point in surface tension concentration plot typical of surfactants. While addition of salt decreases the cloud point, no significant micelle growth was observed even at temperature close to cloud point (CP). Marked increased in solubilization of an oil dye was observed in presence of KCl. Different methods showed good agreement in temperature/salt-induced micellization of these hydrophilic copolymers.