Miroslav Štěpánek

Association of Poly(4-hydroxystyrene)-block-Poly(Ethylene oxide) in Aqueous Solutions: Block Copolymer Nanoparticles with Intermixed Blocks

Association behavior of diblock copolymer poly(4-hydroxystyrene)-block-poly(ethylene oxide) (PHOS-PEO) in aqueous solutions and solutions in water/tetrahydrofuran mixtures was studied by static, dynamic, and electrophoretic light scattering, (1)H NMR spectroscopy, transmission electron microscopy, and cryogenic field-emission scanning electron microscopy. It was found that, in alkaline aqueous solutions, PHOS-PEO can form compact spherical nanoparticles whose size depends on the preparation protocol. Instead of a core/shell structure with segregated blocks, the PHOS-PEO nanoparticles have intermixed PHOS and PEO blocks due to hydrogen bond interaction between -OH groups of PHOS and oxygen atoms of PEO and are stabilized electrostatically by a fraction of ionized PHOS units on the surface.

Glucose-Responsive Hybrid Nanoassemblies in Aqueous Solutions: Ordered Phenylboronic Acid within Intermixed Poly(4-hydroxystyrene)-block-poly(ethylene oxide) Block Copolymer.

Coassembly behavior of the double hydrophilic block copolymer poly(4-hydroxystyrene)-block-poly(ethylene oxide) (PHOS-PEO) with three amphiphilic phenylboronic acids (PBA) differing in hydrophobicity, 4-dodecyloxyphenylboronic acid (C12), 4-octyloxyphenylboronic acid (C8), and 4-isobutoxyphenylboronic acid (i-Bu) was studied in alkaline aqueous solutions and in mixtures of NaOHaq/THF by spin-echo (1)H NMR spectroscopy, dynamic and electrophoretic light scattering, and SAXS. The study reveals that only the coassembly of C12 with PHOS-PEO provides spherical nanoparticles with intermixed PHOS and PEO blocks, containing densely packed C12 micelles. NMR measurements have shown that spatial proximity of PHOS-PEO and C12 leads to the formation of ester bonds between -OH of PHOS block and hydroxyl groups of -B(OH)2. Due to the presence of PBA moieties, the release of compounds with 1,2- or 1,3-dihydroxy groups loaded in the coassembled PHOS-PEO/PBA nanoparticles by covalent binding to PBA can be triggered by addition of a surplus of glucose that bind to PBA competitively. The latter feature has been confirmed by fluorescence measurements using Alizarin Red as a model compound. Nanoparticles were proved to exhibit swelling in response to glucose as detected by light scattering.

Wormlike core–shell nanoparticles formed by co-assembly of double hydrophilic block polyelectrolyte with oppositely charged fluorosurfactant

Formation of polyelectrolyte–surfactant complexes (PE–S) between an anionic polyelectrolyte, poly(sodium 2-sulfamate-3-carboxylate isoprene)-block-poly(ethylene oxide) (PSCI-PEO) and a cationic fluorosurfactant, N-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl) pyridinium chloride (HFDPCl) was studied in alkaline aqueous solutions by static, dynamic and electrophoretic light scattering. The structure of the formed PE–S nanoparticles was investigated by SAXS, cryogenic transmission electron microscopy and atomic force microscopy. The results show that the tendency of the fluorosurfactant to form elongated threadlike micelles drives the PE–S co-assembly to a flexible core–shell cylindrical morphology with the core of the PE–S and the shell of the PEO blocks. Unlike other PE–S systems involving double hydrophilic polyelectrolytes, well-defined core–shell particles exist only in the narrow range of HFDPCl-to-PSCI unit stoichiometric ratios corresponding to zero ζ-potential of the aggregates.

Polyelectrolyte-surfactant complexes of poly[3,5-bis(dimethylaminomethyl)-4-hydroxystyrene]-block-poly(ethylene oxide) and sodium dodecyl sulfate: anomalous self-assembly behavior.

Polyelectrolyte-surfactant complexes (PE-S) formed by double hydrophilic cationic polyelectrolyte poly[3,5-bis(dimethylaminomethyl)-4-hydroxystyrene]-block-poly(ethylene oxide) (NPHOS-PEO) and anionic surfactant sodium dodecyl sulfate (SDS) in acidic aqueous solutions were studied by light scattering, SAXS, and scanning transmission electron microcopy in the environmental mode (wet-STEM) for various stoichiometric ratios between the numbers of SDS anions and dimethylaminomethyl groups of NPHOS in the complex. The obtained results show that the NPHOS-PEO/SDS system behaves differently from other systems of double hydrophilic block polyelectrolyte and oppositely charged ionic surfactant because it forms water-insoluble PE-S for compositions close to the zero net charge of the complex. This phase separation occurs, instead of the PE-S rearrangement to core-shell particles, which is hindered due to conformational rigidity of the NPHOS blocks. For the surfactant amounts below and above the precipitation region, large spherical aggregates and their clusters are present in the solution. SAXS measurements indicate that although the NPHOS-PEO/SDS system does not form the core-shell particles with the NPHOS/SDS core and the PEO shell as other PE-S of double hydrophilic polyelectrolytes, the aggregates contain domains of closely packed surfactant micelles which bind to both NPHOS polyelectrolyte blocks and PEO blocks.

Solubilization and release of hydrophobic compounds from block copolymer micelles. I. Partitioning of pyrene between polyelectrolyte micelles and the aqueous phase

Partitioning of pyrene between poly(tert-butylacrylate)-block-poly(2-vinylpyridine) and polystyrene-block-poly-(methacrylic acid) micelles and aqueous solvent has been studied by fluorometric techniques. The partition coefficient was evaluated on the basis of the steady-state fluorescence spectra and the time-resolved fluorescence decays from a series of solutions with decreasing copolymer concentrations, and the obtained values were compared. The agreement is very good. In the accompanying paper, the release kinetics of pyrene from micelles upon a sudden and prodigious dilution of micellar solution by the aqueous solvent was studied by time-dependent measurements of the steady-state fluorescence intensity. Simultaneous mathematical treatment of dilution curves in the absence and presence of water-soluble quencher based on the model of diffusion from a sphere allows the diffusion coefficient of pyrene in micellar cores to be evaluated. The assumption that micelles do not reorganize upon the shock dilution is tested by an independent experiment.

Solubilization and release of hydrophobic compounds from block copolymer micelles. II. Release of pyrene from polyelectrolyte micelles under equilibrium conditions

The kinetics of the release of pyrene from poly(tert-butyl acrylate)-block-poly(2-vinylpyridine) polyelectrolyte micelles into aqueous media was studied by two independent experimental fluorometric techniques. Mathematical treatment of the time-dependent fluorescence intensities based on a model of diffusion from a sphere allowed the evaluation of the diffusion coefficient of pyrene in the dense poly(tert-butyl acrylate) micellar core. Assumptions necessary for mathematical treatment of experimental curves were tested and their validity was proved. Fluorometric measurements suggest that a considerable fraction of pyrene is solubilized also in poly(2-vinylpyridine) micellar shells.

Thermoresponsive behavior of poly(N-isopropylacrylamide)s with dodecyl and carboxyl terminal groups in aqueous solution: pH-dependent cloud point temperature

It was recently reported that poly(N-isopropyl acrylamide) (PNIPAm) polymers synthesized by RAFT polymerization using S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetic acid)trithiocarbonate as a chain transfer agent form micelles in aqueous solutions with the core of hydrophobic terminal dodecyl groups and the corona of PNIPAm chains with carboxylic groups at the periphery, the ionization of which prevents the micelles from phase separation above the lower critical solution temperature of PNIPAm in water (Langmuir 30:7986–7992). In this paper, we study the pH- and ionic strength-dependence of the aggregation behavior of two HOOC-PNIPAm-C12 polymers, differing in the degree of polymerization, in aqueous solutions. We show that the cloud point temperature (CPT) of HOOC-PNIPAm-C12 can be shifted up to several tens of K by changing pH of the solution. The aggregation of the PNIPAms above the CPT can be efficiently accelerated by screening electrostatic repulsion between PNIPAm micelles by changing ionic strength of the solution.

Fluorescence Spectroscopy as a Tool for Investigating the Self-Organized Polyelectrolyte Systems

In this article, we outline the principles and application of several time-resolved fluorescence techniques for studying the behavior of stimuli-responsive self-assembled polymer systems. We demonstrate the high research potential of fluorescence using results of several published studies performed by the research team at the Charles University in Prague in the framework of the Marie Curie Research Training Network “Self-Organized Nanostructures of Amphiphilic Copolymers” (MRTN-CT-2003-505027). We have chosen several interesting examples of complex self-assembling systems, the behavior of which could not have been understood without the help of targeted fluorescence studies. We have chosen four different techniques, two of them relatively popular (fluorescence anisotropy and nonradiative excitation energy transfer) and two only little used in polymer science (the solvent relaxation method and fluorescence correlation spectroscopy). The last part of the article is devoted to computer simulations (Monte Carlo and molecular dynamics) aimed at the interpretation of fluorescence data.

Influence of Corona Structure on Binding of an Ionic Surfactant in Oppositely Charged Amphiphilic Polyelectrolyte Micelles

Interaction of polystyrene-block-poly(methacrylic acid) micelles (PS-PMAA) with cationic surfactant N-dodecylpyridinium chloride (DPCl) in alkaline aqueous solutions was studied by static and dynamic light scattering, SAXS, cryogenic transmission electron microscopy (cryo-TEM), isothermal titration calorimetry (ITC), and time-resolved fluorescence spectroscopy. ITC and fluorescence measurements show that there are two distinct regimes of surfactant binding in the micellar corona (depending on the DPCl content) caused by different interactions of DPCl with PMAA in the inner and outer parts of the corona. The compensation of the negative charge of the micellar corona by DPCl leads to the aggregation of PS-PMAA micelles, and the micelles form colloidal aggregates at a certain critical surfactant concentration. SAXS shows that the aggregates are formed by individual PS-PMAA micelles with intact cores and collapsed coronas interconnected with surfactant micelles by electrostatic interactions. Unlike polyelectrolyte-surfactant complexes formed by free polyelectrolyte chains, the PMAA/DPCl complex with collapsed corona does not contain surfactant micelles.

Multilayer Polymeric Nanoparticles Based on Specific Interactions in Solution: Polystyrene-block-poly(methacrylic acid) Micelles with Linear Poly(2-vinylpyridine) in Aqueous Buffers

Polymeric micelles of polystyrene-block-poly(methacrylic acid) with the shell containing various amounts of poly(2-vinylpyridine) chains were prepared by dialysis from 1,4-dioxane-rich mixtures into alkaline aqueous solutions. The structure of the polymeric nanoparticles was studied by light scattering, atomic force microscopy, fluorometry, and capillary zone electrophoresis. The presence of both weak polyacid and weak polybase in the corona allows the micelles to form fairly stable solutions in a broad pH range. Poly(methacrylic acid) blocks are dissociated in basic buffers, while poly(2-vinylpyridine) chains stabilize the micelles in acidic solutions. Besides the water-swollen shell, the inner parts of the micelles consist of the kinetically frozen polystyrene core and relatively rigid layer of poly(methacrylic acid) and poly(2-vinylpyridine) segments. An insufficient number of stabilizing polymer chain segments results in the aggregation of the micelles into clusters.