Diblock Copolymers In Aqueous Solutions Research Articles

Role of the tracer in characterizing the aggregation behavior of aqueous block copolymer solutions using fluorescence correlation spectroscopy

The hydrodynamic radii of micelles formed by amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution determined using fluorescence correlation spectroscopy (FCS) depend on the nature of the fluorescent tracer used. We have compared the values of the hydrodynamic radii of the unimers and the micelles as well as the critical micelle concentrations (CMC), using as tracers (1) the identical diblock copolymers being fluorescence-labeled at the hydrophilic or the hydrophobic block terminus [Bonne et al. Colloid Polym Sci (2004) 282:833–843], and (2) a low molar mass fluorescence dye, rhodamine 6G. Whereas similar values for the CMC were found for both probes, the hydrodynamic radius of micelles is significantly underestimated using a free dye as a tracer in FCS, especially near the CMC. We attribute this discrepancy to the fast exchange of the dye between micelles and solution.

Aggregation behavior of MPEG‐PCL diblock copolymers in aqueous solutions and morphologies of the aggregates

AbstractPoly(ethylene glycol)‐b‐polycaprolactone (MPEG‐PCL) diblock copolymers were synthesized via a ring‐opening polymerization of ε‐CL monomers with MPEG as an initiator. Their solubilities and apparent critical micelle concentrations (CMC) in aqueous solution were investigated as well as the determination of the micellar hydrodynamic diameter using dynamic light scattering (DLS). As PCL block length increased, the solubility and CMC decreased while diameters of micelles increased. The gel–sol transition behaviors were investigated using a vial tilting method. Aqueous solutions of copolymers undergo a gel to sol transition with increase in temperature when their polymer concentrations are above a critical gel concentration (CGC). The CGC of the copolymers and gel–sol transition temperature are influenced by the PCL chain length. The tapping mode AFM was performed by imaging the freeze‐dried deposits from the copolymer solutions on mica to investigate a process from free chains to micelles and to gel. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3406–3417, 2006

Double‐Faced Micelles from Water‐Soluble Polymers

Asymmetric disk-shaped Janus micelles form spontaneously and reversibly when simple water-soluble diblock copolymers in aqueous solutions are mixed. The combination of associative and segregative phase separation within one nanostructure results in a disk-shaped micelle with a complex coacervate core and an asymmetric water-swollen corona which is microphase-separated into two distinct domains or faces.

Undulated cylinders of charged diblock copolymers

We study the cylinder to sphere morphological transition of diblock copolymers in aqueous solution with a hydrophobic block and a charged block. We find a metastable undulated cylinder configuration for a range of charge and salt concentrations which, nevertheless, occurs above the threshold where spheres are thermodynamically favorable. By modeling the shape of the cylinder ends, we find that the free-energy barrier for the transition from cylinders to spheres is quite large and that this barrier falls significantly in the limit of high polymer charge and low solution salinity. This suggests that observed undulated cylinder phases are kinetically trapped structures.

Microfluidic light scattering as a tool to study the structure of aqueous polymer solutions

A small-angle light scattering (SALS) apparatus, coupled with a specially designed microfluidic device is shown to monitor the formation and subsequent size distribution of giant multilamellar vesicles of a diblock copolymer in aqueous solution. The closed-face design, fabricated between glass slides using a UV-curable optical adhesive, incorporates multiple inlets, a mixing system, and a viewing window to perform on-line SALS. The mixing of each component is tested using polystyrene latex microspheres. Vesicles of the block copolymer, EO 6BO 11 in aqueous solution are formed on the SALS chip and the pair distance distribution function determined using an inverse Fourier transformation of the scattered intensity to quantify the population and distribution for a range of vesicle sizes. These experiments provide demonstrations of how SALS on a microfluidic device can be used as a rapid screening tool to optimize processing conditions for a range of polymer solutions.

The binary phase behavior of short-chain PDMS- b-PEO diblock copolymers in aqueous solutions in dependence of the PDMS chain length—a combined polarized optical microscopy, 2H NMR and SAXS study

The binary phase behavior of three short-chain polydimethylsiloxane- b-poly(ethylene oxide) (PDMS- b-PEO) diblock copolymers with the compositions DMS 4- b-EO 12, DMS 6- b-EO 12 and DMS 10- b-EO 12 (DMS = dimethylsiloxane unit, EO = ethylene oxide unit) was investigated in aqueous solutions using polarizing light microscopy, deuterium NMR spectroscopy, and SAXS measurements. At low surfactant concentrations all three diblock copolymers show a mixed water/lamellar phase (W + L α), which switches to a lamellar phase depending on the DMS length and temperature in the region of 40 wt.% for DMS 4 to over 50 wt.% for DMS 10. The surfactants consisting of DMS 4 and DMS 6 show further on in the concentration range between 40 and 85 wt.% predominately lamellar phases. While at very high concentration a surfactant rich phase L 2 developed. If the DMS segment was increased to 10 units the region between 55 wt.% and approximately 75% was dominated by a mixed “inverse hexagonal” and lamellar phase at higher concentration a pure “inverse hexagonal” and afterwards a surfactant rich lamellar phase developed, respectively.

Polymeric micellar pH-sensitive drug delivery system for doxorubicin

A novel polymeric micellar pH-sensitive system for delivery of doxorubicin (DOX) is described. Polymeric micelles were prepared by self-assembly of amphiphilic diblock copolymers in aqueous solutions. The copolymers consist of a biocompatible hydrophilic poly(ethylene oxide) (PEO) block and a hydrophobic block containing covalently bound anthracycline antibiotic DOX. The starting block copolymers poly(ethylene oxide)- block-poly(allyl glycidyl ether) (PEO-PAGE) with a very narrow molecular weight distribution ( M w/ M n ca. 1.05) were prepared by anionic ring opening polymerization using sodium salt of poly(ethylene oxide) monomethyl ether as macroinitiator and allyl glycidyl ether as functional monomer. The copolymers were covalently modified via reactive double bonds by the addition of methyl sulfanylacetate. The resulting ester subsequently reacted with hydrazine hydrate yielding polymer hydrazide. The hydrazide was coupled with DOX yielding pH-sensitive hydrazone bonds between the drug and carrier. The resulting conjugate containing ca. 3 wt.% DOX forms micelles with R h a=104 nm in phosphate-buffered saline. After incubation in buffers at 37 °C DOX was released faster at pH 5.0 (close to pH in endosomes; 43% DOX released within 24 h) than at pH 7.4 (pH of blood plasma; 16% DOX released within 24 h). Cleavage of hydrazone bonds between DOX and carrier continues even after plateau in the DOX release from micelles incubated in aqueous solutions is reached.

Mechanism of the Transition between Lamellar and Gyroid Phases Formed by a Diblock Copolymer in Aqueous Solution

The mechanism of the transition from a lamellar phase to a gyroid phase in an aqueous solution of a diblock copolymer has been studied by time-resolved synchrotron small-angle X-ray scattering. The transition occurs via a metastable perforated lamellar structure. The perforations initially have liquidlike ordering before developing hexagonal packing. The transient phase of irregularly perforated layers is revealed by the development of diffuse scattering peaks, just below the Bragg peaks of the lamellar structure. The diffuse scattering is modeled by Monte Carlo simulations of perforated layers. Following the formation of perforations, Bragg peaks characteristic of a hexagonal structure signal an ordering into a hexagonal lattice (with the concomitant loss of diffuse scattering). Computer simulations based on a dynamic density functional model reproduce these features. The hexagonal perforated lamellar phase is rapidly replaced by the gyroid phase. The domain spacing of the gyroid phase is larger than that of the perforated lamellar structure. The perforated lamellar and gyroid phases coexist for a defined period. The reverse transition from gyroid to lamellae occurs directly, with no transient or metastable intermediates.

Aggregation behavior of amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution studied by fluorescence correlation spectroscopy

Aggregation behavior of amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution studied by fluorescence correlation spectroscopy

Association properties of a high molecular weight poly(propylene oxide?b?ethylene oxide) diblock copolymer in aqueous solution

Summary A high molecular weight poly(propylene oxide–b–ethylene oxide) diblock copolymer was prepared via sequential anionic suspension polymerization using a calcium amidealkoxide initiating system. 1H nuclear magnetic resonance, viscometry and static and dynamic light scattering have been used to characterize the copolymer and to examine its self–assembly in aqueous solution. The copolymer was found to self–associate in a narrow concentration range above a certain critical aggregation concentration. The weight–average molecular weight, the radii of gyration, the second virial coefficients, the diffusion coefficients, and the hydrodynamic radii of the particles in both unimer and aggregate regions were determined. Aggregates of low aggregation number (2–3) were formed. Dynamic light scattering measurements performed in a wide concentration range revealed an enhanced aggregate stability towards dissociation upon dilution.

Self-assembling of poly( ε-caprolactone)- b-poly(ethylene oxide) diblock copolymers in aqueous solution and at the silica–water interface

Small-angle neutron scattering is used to investigate the self-assembling behaviour of poly( ε-caprolactone)- b-poly(ethylene oxide) diblock copolymers with various block lengths (i) in aqueous solution, (ii) in aqueous solution with the addition of sodium dodecyl sulphate (SDS) and (iii) at the silica–water interface. Micelles are observed under our experimental conditions due to the very small critical micellar concentration of these copolymers (0.01 g/l). The poly( ε-caprolactone) core is surrounded by a poly(ethylene oxide) corona. The micellar form factors have been measured at low copolymer concentrations (0.2 wt%) under selected contrast matching conditions. The data have been fitted to various analytical models to extract the micellar core and corona sizes. SDS is shown to induce partial micelle disruption together with an increase of the poly(ethylene oxide) corona extension from 25% (without SDS) to 70% (with SDS) of a completely extended PEO 114 chain. Our data at the silica–water interface are compatible with the adsorption of micelles.

Aggregation behavior of amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution studied by fluorescence correlation spectroscopy

The diffusional behavior of amphiphilic poly(2-oxazoline) diblock copolymers in aqueous solution is studied using photon correlation spectroscopy (PCS) and fluorescence correlation spectroscopy (FCS). The polymers were synthesized by living cationic polymerization and were fluorescence-labeled with tetramethyl rhodamine isothiocyanate either at the end of the hydrophilic or the hydrophobic block. Temperature-resolved PCS showed that, at room temperature, large metastable aggregates are present along with unimers and micelles. An annealing above ~40 °C resulted in stable equilibrium micellar solutions. By means of FCS, the hydrodynamic radii of the unimers and the micelles were measured simultaneously in a broad concentration range, and the critical micelle concentration could be determined. Comparison of the results from conventional PCS measurements with this first FCS study showed excellent agreement and the high potential of the FCS technique.

From supramolecular polymersomes to stimuli-responsive nano-capsules based on poly(diene-b-peptide) diblock copolymers.

This paper discusses the self-assembly of block copolymers into vesicular morphology. After a brief state of art of the field, a system based on an amphiphilic poly(butadiene)- b-poly(-L-glutamic acid) (PB- b-PGA) diblock copolymer in aqueous solution is discussed in detail. The aggregation behavior of this block copolymer has been investigated by means of fluorescence spectroscopy, dynamic (DLS) and static (SLS) light scattering as well as transmission electron microscopy (TEM). The diblock copolymer was found to form well-defined vesicles in water. The size of these so-called polymersomes or peptosomes could be reversibly manipulated as a function of both pH and ion strength. Depending on the pH of the aqueous solution, the hydrodynamic radii of these vesicles were found to vary from 100 nm to 150 nm. By cross-linking the 1,2-vinyl double bonds present in the polybutadiene block, the ability to transform a transient supramolecular self-organized aggregate into a permanent "shape-persistent stimuli-responsive nanoparticle" has been demonstrated.

A Small-Angle X-ray Scattering Study of the Structure of Gels Formed by Poly(oxyethylene)−Poly(oxypropylene) Diblock Copolymers in Water

The structures of gels formed by a number of poly(oxyethylene)−poly(oxypropylene) diblock copolymers in aqueous solution have been investigated using small-angle X-ray scattering. Body-centered and face-centered cubic micellar phases were identified depending on the copolymer composition. The micellar radius in the gel is compared to that in dilute solution (obtained from static light scattering) and to predictions from scaling theory. The gel structures are compared to those reported previously for poly(oxyethylene)−poly(oxybutylene) diblocks in water (Hamley et al. Langmuir 2000, 16, 2508).

The Remarkable “Flip−Flop” Self-Assembly of a Diblock Copolymer in Aqueous Solution

In this paper, it is shown that tertiary amine methacrylate-based diblock copolymers can form both micelles and reverse micelles in aqueous solution at room temperature. This remarkable behavior is due to subtle variations in the hydrophilic−hydrophobic balance of the two block sequences, which are based on 2-(diethylamino)ethyl methacrylate (DEA) and 2-(N-morpholino)ethyl methacrylate (MEMA). 1H NMR spectroscopy studies indicate that these DEA−MEMA copolymers dissolve molecularly in water at pH 6−7 at 20 °C. DEA−core micelles are formed at higher pH, and MEMA−core micelles are formed at pH 6.5 in the presence of 1.0 M Na2SO4. Both types of micelles were characterized in terms of their intensity-average diameters and mean aggregation numbers using dynamic and static light scattering, respectively. In addition, one of the copolymers (60 mol % MEMA, Mw = 30 000, Mw/Mn = 1.05) was selected for a detailed small-angle neutron scattering (SANS) study. SANS data for the micelles were analyzed in terms of a weakl...

Non-linear rheology of a face-centred cubic phase in a diblock copolymer gel

The viscoelastic behaviour of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer in aqueous solution forming a face-centred cubic (fcc) micellar phase has been investigated using oscillatory shear rheometry. With increasing strain amplitude, the micellar solution was observed to undergo a transition from linear to non-linear behaviour, characterized by strong shear thinning. The non-linear behaviour observed in the stress response was analyzed by Fourier transformation of the waveform. Fourier analysis revealed that the high harmonic contributions to the shear stress response increased with strain amplitude and up to the 81st harmonic was observed for very large amplitudes. The onset of non-linear response as defined from the dependence of isochronal dynamic shear moduli on strain amplitude was found to be in good agreement with that defined by the appearance of a higher harmonic in the stress waveform. The amplitudes of the harmonic coefficients are compared to the predictions of a model for the nonlinear rheological response of a lyotropic cubic mesophase based on the stress response to a periodic lattice potential (Jones and McLeish 1995). It is found that the model is able to account for qualitative trends in the data such as the development of finite higher harmonics with increasing strain, but it does not describe the full frequency and strain dependence of these coefficients.

From Hard Spheres to Soft Spheres: The Effect of Copolymer Composition on the Structure of Micellar Cubic Phases Formed by Diblock Copolymers in Aqueous Solution

Small-angle X-ray scattering (SAXS) was used to determine the packing of micelles in cubic phases formed by poly(oxyethylene)−poly(oxybutylene) (EB) diblock copolymers in water. SAXS with large amplitude oscillatory shear was used to identify structures formed by the highly asymmetric molecules E96B18, E184B18, E315B17, and E398B19 (where the subscripts denote the number of repeats). At a constant temperature (20 °C) and concentration (10 wt %), we find that the two copolymers with shorter hydrophilic blocks form face-centered cubic (fcc) gels, whereas the two copolymers with longer corona-forming blocks form body-centered cubic (bcc) gels. SAXS also confirmed that the sols formed at lower copolymer concentrations are micellar liquids. Our results for the gel structures are in accord with the observation that micelles with relatively short coronas behave as hard spheres, and pack in a fcc structure, whereas micelles with large coronas act like soft spheres and pack in a bcc array. This is confirmed by ass...

Association and Surface Properties of Poly(ethylene oxide)−Poly(styrene oxide) Diblock Copolymers in Aqueous Solution

The association and surface properties of three oxyethylene/oxyphenylethylene (ES) diblock copolymers in aqueous solution have been studied. Copolymers E50S3.5, E50S5.1, and E51S6.5 were synthesized and characterized by gel permeation chromatography and 13C NMR spectroscopy. Surface tensiometry was used to determine critical micelle concentrations (cmc) at several temperatures, and thereby values of the enthalpy of micellization (ΔmicHo). The values of cmc and ΔmicHo were found to be low (e.g., 0.03 g dm-3 at 30 °C and 12 kJ mol-1 for E51S6.5), which is consistent with the high hydrophobicity of oxyphenylethylene block. Dynamic and static light scattering were used to characterize the micelles in dilute solution, yielding micellar association number, hydrodynamic and thermodynamic radius, and the related thermodynamic expansion factor. At higher concentrations, solution of copolymers E50S5.1 and E51S6.5 formed hard isotropic gels.

Micelles and gels of oxyethylene–oxybutylene diblock copolymers in aqueous solution: The effect of oxyethylene-block length

Block copolymer E90B10 (E=oxyethylene, B=oxybutylene) was synthesised and characterised by gel permeation chromatography and 13C NMR spectroscopy. Dynamic light scattering (DLS) and static light scattering (SLS) were used to characterise the micelles in solution (both in water and in aqueous 0.2 mol dm-3 K2SO4), yielding the micellar association numbers, the hydrodynamic and thermodynamic radii, and related expansion factors. Micellar parameters were also obtained by small-angle neutron scattering (SANS) for solutions of a similar copolymer, E86B10, in water, i.e., the association number, the hard-sphere radius, the micelle volume fraction and the corresponding expansion factors. A comparison of the appropriate quantities showed good agreement between the two techniques. SANS gave additional information e.g., volume fraction profiles for the micelles and volume fraction of water in the micelle core. Moderately concentrated solutions of copolymer E90B10 were studied in the gel state by small-angle X-ray scattering (SAXS) in tandem with rheology (oscillatory shear). Values for the dynamic elastic modulus (G′) of the gels significantly exceeded 104 Pa across the range of temperature (25–80°C) and frequency (0.1–100 rad s-1) explored. The SAXS patterns for the orientated gels showed them to have a body-centred-cubic structure, as expected for packed, spherical micelles. This structure persisted over a wide concentration range, e.g., until crystallisation of the E blocks occurred at high concentration (70 wt.% copolymer). By combining the present and published results, a comparison was made of the micelle and gel properties of copolymers with the same B-block length but different E-block lengths, i.e., E90B10, E40B10 and E18B10. As would be expected, as the E-block length was increased, the micelle association numbers decreased while the micelle radii and expansion factors increased. The critical gel concentration (cgc) also decreased as the E-block length was increased. Moreover, the cgc correlated quantitatively with the thermodynamic expansion factors obtained by SLS and SANS from the exclusion properties of the micelles.

An integrated Couette system for in situ shearing of polymer and surfactant solutions and gels with simultaneous small angle x-ray scattering

A Couette cell designed for in situ small angle x-ray scattering (SAXS) studies of polymeric systems under shear is described. Representative results are presented for the effect of shear on the lamellar phase of a concentrated poly(oxyethylene)–poly(oxybutylene) diblock copolymer in aqueous solution. It is shown that the application of shear resulted in a reduction in the lamellar spacing and of the defect density. The former was observed as a shift in the principal peak position in the SAXS pattern, and the latter as an increase in the associated correlation length. It was further shown that the cessation of shear led to a relaxation of both shear induced effects, over a time period of hours.