Triblock Copolymers In Aqueous Solution Research Articles

Self-assembled nanostructures of a biomimetic glycopolymer–polypeptide triblock copolymer

The self-assembling behavior of a biomimetic glycopolymer–polypeptide triblock copolymer in aqueous solution was described and characterized by employing the hydrophobic dye solubilization method and transmission electron microscopy. The large spherical micelles can be easily generated from the dissolution of triblock copolymer in water. The morphology changes from sphere to lamellae, then to worm-like micelle, can be conveniently transformed by initial copolymer concentration. The multivalent interaction of lectins with lactose-installed polymeric aggregates was preliminarily investigated by UV-Vis spectra. Notably, this kind of aggregates may be useful as artificial polyvalent ligands in the investigation of carbohydrate–protein recognition and for the design of site-specific drug delivery systems.

Thermogelling Poly(caprolactone-b-ethylene glycol-b-caprolactone) Aqueous Solutions

Poly(caprolactone-b-ethylene glycol-b-caprolactone) (PCL−PEG−PCL) triblock copolymer aqueous solution (>15 wt %) undergoes the sol−gel−sol transition as the temperature increases from 10 to 60 °C. The mechanism and structure−property relationship of the sol−gel transition were investigated. In particular, compared with the PEG−PCL−PEG triblock copolymers recently reported by our group, PCL−PEG−PCL has (1) a synthetic advantage without a hexamethylene diisocyanate coupling step, (2) a wider gel window of over 15−32 wt %, and (3) a larger gel modulus. Both PEG−PCL−PEG and PCL−PEG−PCL polymers are an important progress in the biodegradable thermogelling system in that they can be lyophilized in a powder form, are easy to handle, are easy to redissolve to a clear solution, and show little syneresis through the gel phase.

Flow control in microdevices using thermally responsive triblock copolymers

Active and passive microflow control has been demonstrated using gel formation by dilute aqueous solutions of triblock copolymers at elevated temperatures. Solutions of a poly(ethylene oxide)/sub 106/-poly(propylene oxide)/sub 70/-poly(ethylene oxide)/sub 106/ polymer, which has the trade name Pluronic/spl reg/ F127, have been used as a sample system. Flow in a microchannel has been stopped in less than 33 ms by introducing heat with an integrated electric heater. Viscous heating under high shear rates also induces gel formation, which has been used for passive automated flow control in a microchannel.

Reversible gel formation of triblock copolymers studied by molecular dynamics simulation

AbstractMolecular dynamics simulations have been employed to study the formation of a physical (thermoreversible) gel by amphiphilic A‐B‐A triblock copolymers in aqueous solution. To mimic the structure of the hydrogel‐forming polypeptides employed in experiments [W.A. Petka et al., Science 281, 389 (1998)], the end blocks of the polymer chains are modeled as hydrophobic rods representing the α‐helical part of the polypeptides, whereas the central B block is hydrophilic and semiflexible. We have determined structural properties, such as the hydrophobic cluster‐size distribution function, the geometric percolation point, and pair correlation functions, and related them to the dynamic properties of the system. Upon a decrease in the temperature, a network structure is formed in which bundles of end blocks act as network junctions. Both at short and medium distances, increased ordering is observed, as characterized by the pair correlation function. Micelle formation and the corresponding onset of geometric percolation induce a strong change in dynamic quantities (e.g., in the diffusion constant and the viscosity) and cause the system to deviate from the Stokes–Einstein relation. The dynamic properties show a temperature dependence that is strongly reminiscent of the behavior of glass‐forming liquids. The appearance of a plateau in the stress autocorrelation function suggests that the system starts to exhibit a solid‐like response to applied stress once the network structure has been formed, although the actual sol–gel transition occurs only at a considerably lower temperature. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 959‐969, 2005

Dynamics and structure in complex liquids under shear explored by neutron scattering

It is well established that the structural order of macromolecules can be affected by the application of shear. For example, triblock copolymers in aqueous solutions are known to aggregate for high concentrations. For increasing temperature the polymer micelles crystallize and offer a model system for the investigation of percolation and crystallization. The crystalline phases rearrange under shear. We correlate the structural assemblages of polymer micelles to the microscopic dynamics of the polymer monomers as well as to the solvent molecules at rest and under shear. We find the monomer dynamics affected by the different arrangements of the polymer micelles in aqueous solutions. For pronounced structural ordering we report on the monomer diffusion to become anisotropic under shear, with the diffusive mode in the direction of the shear gradient being slowed down with respect to that in the direction of the flow.

Regular and irregular micelles formed by A LEL triblock copolymer in aqueous solution

Laser light scattering (LLS) techniques were used to characterize the micellization of poly(d,l-lactide)–poly(ethylene glycol)–poly(d,l-lactide) (LEL) triblock copolymer (MW 1K–2K–1K) in aqueous solution. We observed the existence of both thermodynamically stable flower-like micelles (regular micelles) and large, less soluble nanoparticles (irregular micelles) in dilute aqueous solutions with the same preparation procedure. Both kinds of micelles were found to co-exist with single copolymer chains. The initial copolymer concentration determines the nature of the micelles. The regular core-shell micelle formation follows a closed association mechanism, resulting in flower-like micelles. The hydrophobicity of a L unit is estimated as ∼0.5–0.6 B (polyoxybutylene) units from the micellization parameters, which is quite consistent with earlier estimations obtained from EL diblock copolymers.

Synthesis and drug release behavior of poly (trimethylene carbonate)–poly (ethylene glycol)–poly (trimethylene carbonate) nanoparticles

ABA-type triblock copolymers poly (trimethylene carbonate)–poly (ethylene glycol)–poly (trimethylene carbonate) were synthesized by ring-opening polymerization of trimethylene carbonate initiated by dihydroxyl poly (ethylene glycol). The critical micelle concentration of amphiphilic triblock copolymers in aqueous solution was determined by fluorescence spectroscopy using 9-chloromethyl anthracene as fluorescence probe. Core-shell-type nanoparticles were prepared by the dialysis technique. Transmission electron microscopy images showed that these nanoparticles were regularly spherical in shape. Micelle size determined by dynamic light scattering is 50–160nm. Anticancer drug methotrexate (MTX) as model drug was loaded in the polymeric nanoparticles. X-ray powder diffraction spectra showed that model drugs were molecularly dispersed in the core. In vitro release behavior of MTX was investigated.

Effect of additives on the cloud points of two tri-block copolymers in aqueous solution

The cloud points (CP) of aqueous solutions of two ethylene oxide–propylene oxide–ethylene oxide tri-block copolymers (EO) 10(PO) 16(EO) 10 and (EO) 1(PO) 17(EO) 1 were measured in the presence of additives, such as salts, alkalis, acids, polymer and ionic surfactant. The results obtained in this study indicated that salts decrease the CP of the two tri-block copolymers in the order of NaCl>CaCl 2>MgCl 2 and Na 3PO 4>Na 2SO 4>NaCl. Alkalis caused a sharp drop of CP of the two tri-block copolymers. Acids increased the CP. Water soluble polymer such as polyvinylpyrrolidone (PVP), hydrolysis polyacrylamide (HPAM) and cation-polyacryamide (cation-PAM) decreased the CP. Ionic surfactants such as sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) increased the CP. The extent of lowering depends on the structure and concentration of additives and the number of ethylene oxide group of tri-block copolymers.

Poly(Oxyalkylene) Block Copolymers in Aqueous Solution—Phase Behavior and Transition Kinetics

A study into the liquid‐crystal mesophase of two triblock copolymers in aqueous solution has been reported, E17B14E17 and the reverse architecture, B11E47B11 were studied by polarized light microscopy, rheology, and time‐resolved small‐angle x‐ray scattering (SAXS) techniques. Two phase diagrams of composition vs. temperature have been constructed where three isolated morphologies are observed; body‐centered cubic‐packed spherical micelles, hexagonally packed cylindrical micelles, and a lamellar structure are formed on increasing concentration of block copolymer. It also was found that phase transitions could be induced thermally. Such phenomena were characterized by a change in scattering pattern or texture under polarized light microscopy, or by changes in storage modulus as the system was heated or cooled. The determination of phase boundaries was crucial in the next stage of the investigation, which was to quantify the kinetic parameters of each phase transition. Phase transitions were analyzed by the Avrami model. Initial results suggest that, on raising the quench depth, the transition is faster. In addition, systems with a higher block copolymer concentration were shown to transform slower.

Room-temperature syntheses of CdS nanocrystals templated by triblock copolymer in aqueous solution under air condition

CdS nanocrystals with size of 5.0–10.0 nm were synthesized from air-insensitive inorganic precursors of CdCl 2 · 2.5H 2O and Na 2S · 9H 2O at room temperature in both acidic and basic aqueous solutions under air condition, using the nonionic pluronic amphiphilic triblock copolymer of P123 (H–(–OCH 2CH 2) 20[–OCH(CH 3)–CH 2] 70–(–OCH 2CH 2) 20–OH, as well as (EO) 20–(PO) 70–(EO) 20) as the template and structure-directing agent. The X-ray powder diffraction patterns index to the cubic crystal structure of sphalerite for CdS nanocrystals synthesized. UV–Vis absorption spectra indicate the obvious blue shift for the band gap of CdS nanocrystals prepared in the water-rich P123 aqueous solution.

Phase behavior of a PEO-PPO-PEO triblock copolymer in aqueous solutions: Two gelation mechanisms

Phase behavior of a PEO-PPO-PEO (Pluronic P103) triblock copolymer in water is investigated using small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), dynamic light scattering (DLS) and rheology. Pluronic P103 shows apparent two gel states in different temperature regions. The first sol-to-gel transition at a lower temperature (i.e., the hard gel I state) turns out to be the hexagonal microphase as evidenced by the combined SANS and SAXS and the frequency dependence of bothG′ andG′ in rheology. In contrast to the hard gel I, the second sol-to-gel transition (i.e., the hard gel II state) at a higher temperature represents the block copolymer micelles in somewhat disordered state rather than the ordered state seen in the hard gel I. Moreover, turbidity change depending only on the temperature with four distinct regions is observed and the large aggregates with size larger than 5,000 nm are detected with DLS in the turbid solution region. Based upon the present study, two different gelation mechanisms for aqueous PEO-PPO-PEO triblock copolymer solutions are proposed.

Small-angle neutron scattering study of poly(methyl methacrylate-block-sodium acrylate-block-methyl methacrylate) and poly(sodium acrylate-block-methyl methacrylate-block-sodium acrylate) triblock copolymers in aqueous solutions

Small-angle neutron scattering experiments were made on poly(methyl methacrylate-block-sodium acrylate-block-methyl methacrylate) [p(MMA-b-NaA-b-MMA)] and p(NaA-b-MMA-b-NaA) solutions by varying the composition and the concentration of the polymer with and without 1 M NaCl added. Scattering curves could be evaluated by assuming that the polymers aggregate into polydisperse micelles. The experiments support the expectation that in the case of the p(MMA-b-NaA-b-MMA) block sequence the hydrophilic blocks form closed loops connected by both ends to the micellar cores; in the case of the p(NaA-b-MMA-b-NaA) block sequence they float freely in the solvent. The micellar cores exert considerable stability against dilution and added electrolyte. The interaction of charged micelles could be formally described in terms of volume exclusion and the Derjaguin–Landau–Verwey–Overbeek potential.

The aggregation of polystyrene- b-poly(ethylene oxide)- b-polystyrene triblock copolymers in aqueous solution

The aggregation of PS 4- b-PEO 227- b-PS 4 triblock copolymer (the PS and PEO are abbreviations of polystyrene and poly(ethylene oxide), and the subscripts represent the degree of polymerization) in aqueous solution was investigated by Nuclear Magnetic Resonance, Fluorescence Probe, and Transmission Electron Microscope techniques. The experimental results showed that the aggregation did occur and the critical concentration for aggregation was about 0.01 g/l. When the concentration of colloid solution was 10 g/l, the spherical aggregate with a highly polydispersity and radii ranging from 100 to 500 nm was observed. The possible structures of primary micelles and micellar aggregates were proposed. Additionally, the aggregation of PS 43- b-PEO 45- b-PS 43 and PS 18- b-PEO 45- b-PS 18 block copolymers was primarily concerned. The spherical aggregates with a certain size distribution were observed by Transmission Electron Microscope.

Stability of wetting films of F108 triblock copolymer aqueous solutions on quartz

Wetting films of nonionic F108 triblock copolymer aqueous solutions with concentrations below the CMC containing an electrolyte (NaCl) and formed on a quartz substrate are studied. Primary thick films are disclosed to be metastable. Their temporal stability is explained by the electrostatic repulsive forces. Film thinning and transition to a stable state occur slowly at low electrolyte concentrations (CNaCl = 10–5 and 10–3 M, respectively) and instantly at its high concentrations (CNaCl = 10–2 and 10–1 M, respectively). The stability of thin films is explained by the steric repulsive forces.

Emulsions stabilized with PEO–PPO–PEO block copolymers and silica

The specific behavior of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) tri-block copolymers in aqueous solutions was studied in the presence of vinyl acetate (VAc) and colloidal silica. Several factors controlling specific interactions were investigated, the block copolymer molecular weight and hydrophilic/hydrophobic balance (changed via the PEO/PPO ratio), temperature and silica content. Emulsion formation for water/VAc/copolymer mixtures was investigated for different water/VAc ratios, three temperatures (20, 40 and 60 °C) and three block copolymer concentrations. It was proven that silica addition in the VAc/water/PEO–PPO–PEO tri-block copolymer mixtures results in a synergetic effect of the (copolymer-silica) system that act as a stabilizer for the emulsion droplets. Depending on the block copolymer content and HLB as well as on the temperature, emulsion droplets with different diameters are obtained. Efficient encapsulation of VAc can be promoted using silica particles and PEO–PPO–PEO block copolymers with a proper choice of the polymer and temperature.

Studies of the Interaction of Cationic Gemini Surfactants with Polymers and Triblock Copolymers in Aqueous Solution

The physical interactions occurring between members of the N,N′-bis (dimethyldodecyl)-α,ω-alkanediammonium dibromide (m–s–m, m=12, s=3 and 6) series of gemini surfactants and the neutral polymers polyethylene oxide (PEO) and polypropylene oxide (PPO), and triblock copolymers of polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO), have been investigated in aqueous solution using specific conductance, fluorescence intensity, density, and equilibrium dialysis techniques. For the conditions investigated, the interactions were found to be markedly different from those generally observed in surfactant–polymer systems, with an absence of the typically observed critical concentrations. Instead, it is observed that increased surfactant concentration in the polymer coil regions results in a gradual change from clusters of monomer surfactant bound to the polymer to the formation of regular micelles.

Water Content in Micelles of Poly(ethylene Oxide)—Poly(Propylene Oxide)—Poly(ethylene Oxide) Triblock Copolymers in Aqueous Solutions as Studied by Fluorescence Spectroscopy

Fluorescence spectroscopy has been applied in the investigation of water content in micelles of polyethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers at 20 and 50 °C. The triblock copolymers examined consist of Pluronic F68® (F68) and Pluronic L64® (L64), which are known to form micelles with cores of PPO and shells of PEO in aqueous solutions. 2- p-Toluidinylnaphthalene-6-sulfonic acid (TNS) is used as a fluorescence probe. Several fluorescence parameters of TNS are employed as tools to evaluate the water content in the micelles: (1) the wavelength at maximal intensity, which strongly depends on the polarity and viscosity of the surrounding media; and (2) the degree of polarization (P) and lifetime, which give the microviscosity of the media through the Perrin-Weber equation. Aqueous solutions of PEO of various compositions are used as standard solutions. It is found that the water content in F68 and L64 micelles ranges from 10 to 70 wt %, depending on the size of the PEO block, concentration of the polymer, and temperature. The usefulness of fluorescence spectroscopy in the studies of block copolymer micelles is demonstrated along with some comments on the interpretation of P of the TNS probe.

Isothermal Titration Calorimetric Studies on Interactions of Ionic Surfactant and Poly(oxypropylene)−Poly(oxyethylene)− Poly(oxypropylene) Triblock Copolymers in Aqueous Solutions

Isothermal titration calorimetry was used to investigate the binding characteristics of sodium dodecyl sulfate (SDS) and PEP-type [P and E represent poly(oxypropylene) and poly(oxyethylene), respectively] triblock copolymers in aqueous solution. Beyond the critical aggregation concentration (CAC), PEP/SDS aggregation complexes are formed through the polymer-induced micellization process. SDS monomers first bind to the PPO segments followed by binding to the PEO segments. The polymer chains are dehydrated and solubilized in the hydrophobic core of SDS micelles containing an aggregation number smaller than that of free SDS micelles in water. From the contribution to the Gibbs energy, it is found that the formation of polymer/SDS aggregation complex is an entropic-driven process. The CAC is independent of the molecular weight of polymer, is weakly dependent on the polymer concentrations, and is strongly dependent on polymer composition. An increase in the length of PPO segments results in the reduction in th...

Association Properties of Triblock Copolymers in Aqueous Solution: Copolymers of Ethylene Oxide and 1,2-Butylene Oxide with Long E-blocks

Four triblock copolymers, EmB20Em, with m in the range 58−260, were prepared by sequential anionic polymerization of 1,2-butylene oxide followed by ethylene oxide. E denotes an oxyethylene repeat unit and B an oxybutylene repeat unit. The copolymers were characterized by gel permeation chromatography (for distribution width) and 13C NMR spectroscopy (for absolute molar mass and confirmation of triblock architecture). Static and dynamic light scattering were used to study the micellization and micelle properties of the copolymers in dilute aqueous solution, particularly the mass-average association number and thermodynamic and hydrodynamic radii. At a given temperature, the micelle association number decreased as the E-block length was increased while the radii increased. The gelation of relatively concentrated aqueous solutions of the copolymers was also investigated by a tube inversion method. The minimum concentration for gelation decreased in the range 9−13.3 wt % as the E-block length was increased. C...

Flow in micellar cubic crystals

We investigate and compare the mechanical properties of two micellar cubic crystals. Both are obtained from aqueous triblock-copolymer solutions of similar block structure, (PEO)76(PPO)29(PEO)76 and (PEO)127(PPO)48(PEO)127, designated Pluronic F68 and F108 respectively. Extensive small angle X-ray scattering experiments under shear and common rheometry provide the following results: (i) the solid phases of the two polymeric solutions are constituted of micelles that are ordered in a bcc (F68) and fcc (F108) structure. The overall appearance of both unsheared samples is polycrystalline; (ii) SAXS showed that both systems undergo a structural reorientation under shear, with the dense planes arranged parallel to the velocity - vorticity plane, thus facilitating flow; (iii) F68 showed a clear transition in the flow curve, associated with a textural change, but the F108 system exhibited a continuous evolution; (iv) the bcc crystal appears to have no measurable yield stress and flows even for low applied stresses. This was in contrast to the fcc sample which showed a clear yield stress, separating creep and flow regimes.