PS Core Research Articles

Stimuli-Responsive Aqueous Micelles from an ABC Metallo-Supramolecular Triblock Copolymer

A metallo-supramolecular ABC triblock copolymer containing a bis(2,2′:6′,2″-terpyridine)-ruthenium(II) complex as a supramolecular connection between the constituting poly(styrene)-block-poly-(2-vinylpyridine) diblock and poly(ethylene oxide) block (PS 32-b-P2VP 13-[Ru]-PEO 70) has been prepared. This copolymer was further utilized in order to prepare aqueous micelles consisting of a PS core, a pH-sensitive P2VP shell, and a PEO corona. The accordingly formed micelles have been characterized by a combination of dynamic light scattering, transmission electron microscopy, and atomic force microscopy. Spherical micelles and a minor fraction of bigger aggregates were observed by these three independent techniques. The pH sensitivity of the P2VP shell could be demonstrated and the reversibility of the metallo-supramolecular complex investigated utilizing a strong competitive ligand.

Experimental Study of Hydrophobically Modified Amphiphilic Block Copolymer Micelles Using Light Scattering and Nonradiative Excitation Energy Transfer,

The micellization behavior of a hydrophobically modified double tagged polystyrene-block-poly(methacrylic acid) diblock copolymer, PS−N-PMA-A was studied in 1,4-dioxane−H2O mixtures by light-scattering and fluorescence techniques. This polymer was fluorescently tagged by a naphthalene moiety at the junction of the blocks and by anthracene at the end of the PMA block. The behavior of a single-tagged sample, PS−N-PMA, and low-molar-mass analogues were studied for comparison. Multimolecular polymeric micelles with compact PS cores and PMA shells may be prepared indirectly by dialysis from 1,4-dioxane-rich mixtures as water is a strong selective precipitant for the PS block. In both types of micelles, the naphthalene tags are trapped in a nonpolar and fairly viscous core/shell interfacial region. The hydrophobic anthracene tags in PS−N-PMA-A are at the ends of the water-soluble PMA blocks and tend to avoid the bulk polar solvent, burying themselves into the shell. The collapse of a fraction of the PMA chains ...

Hydrophobically Modified Amphiphilic Block Copolymer Micelles in Non-Aqueous Polar Solvents. Fluorometric, Light Scattering and Computer-Based Monte Carlo Study

The micellization behavior of a hydrophobically modified polystyrene-block-poly(methacrylic acid) diblock copolymer, PS-N-PMA-A, tagged with naphthalene between blocks and with anthracene at the end of the PMA block, was studied in 1,4-dioxane-methanol mixtures by light scattering and fluorescence techniques. The behavior of a single-tagged sample, PS-N-PMA, and low-molar-mass analogues was studied for comparison. Methanol-rich mixtures with 1,4-dioxane are strong selective precipitants for PS. Multimolecular micelles with compact PS cores and PMA shells may be prepared indirectly by dialysis from 1,4-dioxane-rich mixtures, or by a slow titration of copolymer solutions in 1,4-dioxane-rich solvents with methanol under vigorous stirring. In tagged micelles, the naphthalene tag is trapped in nonpolar and fairly viscous core/shell interfacial region. In hydrophobically modified PS-N-PMA-A micelles, the hydrophobic anthracene at the ends of PMA blocks tends to avoid the bulk polar solvent and buries in the shell. The distribution of anthracene tags in the shell is a result of the enthalpy-to-entropy interplay. The measurements of direct nonradiative excitation energy transfer were performed to estimate the distribution of anthracene-tagged PMA ends in the shell. The experimental fluorometric data show that anthracene tags penetrate into the inner shell in methanol-rich solvents. Monte Carlo simulations were performed on model systems to get reference data for analysis of time-resolved fluorescence decay curves. A comparison of experimental and simulated decays indicates that hydrophobic traps return significantly deep into the shell (although not as deep as in aqueous media). The combined light scattering, fluorometric and computer simulation study shows that the conformational behavior of shell-forming PMA blocks in non-aqueous media is less affected by the presence of nonpolar traps than that in aqueous media.

Architecture of nanostructured polymers by segregated domain crosslinking of block‐graft copolymer micelles

Polystyrene-block-polyisoprene (PS-block-PI; high 3,4-structure) diblock copolymer was prepared by living anionic polymerization. For transfer into a reactive intermediate, the hydroxylation of the double bonds of PI block was achieved by hydroboration, followed by oxidation. Esterification of the hydroxy-derivative with stearoyl chloride or decanoyl chloride resulted in block-graft copolymers composed of PS (flexible chain)-grafted long alkane (stretched chains). After partial chloromethylation of PS block copolymer, photofunctional N,N-diethyldithiocarbamate (DC) groups were introduced into such pendant sites by reaction with the corresponding sodium salt. We studied the self-assemblies of photofunctional block-graft copolymers in a selective solvent, such as heptane, and constructed nanostructured polymers by crosslinking PS cores under UV irradiation. © 2001 Society of Chemical Industry

Architecture of nanostructured polymers by segregated domain crosslinking of block‐graft copolymer micelles

AbstractPolystyrene‐block‐polyisoprene (PS‐block‐PI; high 3,4‐structure) diblock copolymer was prepared by living anionic polymerization. For transfer into a reactive intermediate, the hydroxylation of the double bonds of PI block was achieved by hydroboration, followed by oxidation. Esterification of the hydroxy‐derivative with stearoyl chloride or decanoyl chloride resulted in block‐graft copolymers composed of PS (flexible chain)‐grafted long alkane (stretched chains). After partial chloromethylation of PS block copolymer, photofunctional N,N‐diethyldithiocarbamate (DC) groups were introduced into such pendant sites by reaction with the corresponding sodium salt. We studied the self‐assemblies of photofunctional block‐graft copolymers in a selective solvent, such as heptane, and constructed nanostructured polymers by crosslinking PS cores under UV irradiation.© 2001 Society of Chemical Industry

Core-Shell-Corona Micelles with a Responsive Shell.

A reactor for the synthesis of gold nanoparticles is one of the uses of a poly(styrene)-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) triblock copolymer (PS-b-P2VP-b-PEO) which forms core-shell-corona micelles in water. Very low polydispersity spherical micelles are observed that consist of a PS core surrounded by a pH-sensitive P2VP shell and a corona of PEO chains end-capped by a hydroxyl group. The corona can act as a site for attaching responsive or sensing molecules.

Comicellization of Polystyrene-block-Poly(ethylene oxide) with Cationic and Anionic Surfactants in Aqueous Solutions: Indications and Limits

Interaction of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) with cationic and anionic surfactants was studied using light scattering, sedimentation in ultracentrifuge, and 1H NMR. The complex structure of the mixed solution containing hybrid micelles, micellar clusters, and supermicellar aggregates was confirmed by independent experiments. By proton NMR, addition of anionic surfactant, sodium dodecyl sulfate (SDS), to PS-b-PEO solution results in loosening the micelle cores and an increase of the PS molecular mobility in the presence of surfactant. At the same time, incorporation of SDS into the block copolymer solution leads to noticeable broadening of the signals related to methylene protons of the surfactant, which can be assigned to a decrease of mobility of surfactant alkyl chains “immobilized” on the PS cores. These data along with ultracentrifugation of hybrid solutions containing PS-b-PEO and surfactant at various concentrations prove that comicellization occurs and mixed micelles are formed. However, this phenomenon is observed up to a certain limiting concentration of the surfactant. In the case of cetyl pyridinium chloride (CPC), increase of surfactant loading up to 0.05 mol/L results in the solutions containing CPC micelles along with hybrid micellar clusters and supermicellar aggregates.

Synthesis of Molecular Brushes with Block Copolymer Side Chains Using Atom Transfer Radical Polymerization

Brush macromolecules having poly(n-butyl acrylate-block-styrene) and poly(styrene-block-n-butyl acrylate) side chains have been synthesized by the “grafting from” approach using atom transfer radical polymerization (ATRP). The molecular weights of the resulting polymers were characterized by gel permeation chromatography (GPC) using refractive index and multiangle light scattering detection. The block copolymer side chains were cleaved from the backbone and analyzed by GPC, confirming the synthesis of well-defined copolymer brushes. Visualization of individual molecules by atomic force microscopy (AFM) enabled analysis of the conformation and microstructure of the brush macromolecules on mica surface. The brushes with the pnBuA core were almost fully stretched, while the inverted structure with the pS core exhibited longitudinal contraction compared to the contour length of the main chain. In addition, the poly(n-butyl acrylate-block-styrene) brushes demonstrated a characteristic necklace morphology which...

SANS study of multilayer nanoparticles based on block copolymer micelles

A novel type of three-layer nanoparticles was studied using small-angle neutron scattering (SANS). The particles were prepared by adding methyl methacrylate monomer to a polystyrene-block-poly(methacrylic acid) micellar solution in aqueous buffer and subsequent polymerization by γ-irradiation. The contrast-matching SANS experiments revealed that upon polymerization, the PMMA chains form a layer on the surface of the PS cores of the original micelles. This is in agreement with the finding that at room temperature, MMA monomer molecules do not penetrate into the micelle core in detectable amounts. In the presented example, the mean core radius of original micelles and the layer thickness were determined to be 99 and 17Å, respectively. Both characteristics are expected to be controllable by the choice of block copolymer micelles and the parameters of the polymerization process.

The Hybrids of Polystyrene-block-Poly(ethylene Oxide) Micelles and Sodium Dodecyl Sulfate in Aqueous Solutions: Interaction with Rh Ions and Rh Nanoparticle Formation

The interaction of amphiphilic block copolymer, polystyrene-block-poly(ethylene oxide) (PS-b-PEO), with anionic surfactant, sodium dodecyl sulfate (SDS), in aqueous media has been studied by sedimentation in ultracentrifuge. Three well-defined populations of hybrid aggregates corresponding to micelles, micellar clusters, and supermicellar aggregates were detected in the PS-b-PEO/SDS aqueous solutions at various rotation rates. Parameters of all the micellar aggregates were characterized depending on the SDS loading. An increase in the SDS loading was found to result in an increase in block copolymer/surfactant micelle size and weight at the SDS concentration of 0.8×10−3 mol/L and in a slight decrease of both parameters at critical micelle concentration and at higher concentration. This decrease was caused by incorporation of SDS molecules in block copolymer micelles followed by charging the PS core and repulsion between similar charges. Using dichlorotetrapyridine rhodium(III)chloride hexahydrate ([Rh(Py)4Cl2]Cl×6H2O), ion exchange of surfactant counterions in the hybrid PS-b-PEO/SDS system for Rh cations was carried out, which allowed saturating the micellar structures with Rh species. Subsequent reduction of the Rh-containing hybrid solutions with NaBH4 resulted in the formation of Rh nanoparticles with a diameter of 2–3 nm mainly located in the block copolymer micellar aggregates.

Polystyrene/Poly(2-vinylpyridine) Heteroarm Star Copolymer Micelles in Aqueous Media and Onion Type Micelles Stabilized by Diblock Copolymers,

Micellization of polystyrene/poly(2-vinylpyridine) heteroarm star copolymers, PSnPVPn, in aqueous media has been studied by a combination of light scattering and fluorescence techniques. Three copolymer samples differing in numbers of arms and their length form micelles with compact hydrophobic cores and polyelectrolyte poly(2-vinylpyridine) shells. The association number decreases with increasing number of water-soluble arms and their length. Micellization equilibrium is kinetically frozen in aqueous media. The micellar cores are frozen and the behavior of the micellar solutions is controlled by the polyelectrolyte behavior of the polyelectrolyte shell. In the case of the present heteroarm stars with ca. 101 arms, the soluble and insoluble arms are strongly segregated and the micellar cores are formed of pure polystyrene as with micelles formed by linear diblock copolymers. As concerns the sample with several tens of arms, a significant steric hindrance, together with a low association number (ca. 4), does not allow for a complete segregation of polystyrene and the poly(2-vinylpyridine) arms. The micellar cores formed by the very hairy star copolymer with several tens of arms contain a fraction of poly(2-vinylpyridine) arms as shown by fluorescence measurements with solubilized pyrene. Alkalimetric titration of an acidic mixture of PSnPVPn micelles with a linear poly(2-vinylpyridine)-block-poly(ethylene oxide) copolymer, PVP−PEO, yields fairly monodisperse spherical onion-skin micelles. The onion-skin micelle has a compact spherical PS core deriving from the parent PSnPVPn micelle, a fairly thin and compact middle layer formed by PVP both from the parent micelle and PVP−PEO copolymer, and a protective PEO shell. The formation of onion-skin micelles is fully reversible and occurs suddenly and very rapidly at pH values higher than 4.8. The “onions” dissociate immediately into PSnPVPn micelles and PVP−PEO copolymer below pH 4.8. Evaluation of the alkalimetric titration data shows that only ca. 40% of the PVP units in the micellar shells are protonized (even in strongly acidic solutions that contain a large surplus of a strong acid, e.g., in 0.1 M HCl). This observation is in agreement with indirect fluorometric data which indicate that in the case of a weak polyelectrolyte, such as poly(2-vinylpyridine), the inner layer of the polyelectrolyte shell close to the core is not ionized.

Mixed Lamellar Films: Evolution, Commensurability Effects, and Preferential Defect Formation

The structure of lamellar poly(styrene-block-methyl methacrylate), denoted as P(S-b-MMA), diblock copolymer films on neutral poly(styrene-random-methyl methacrylate), denoted as P(S-r-MMA), brush surfaces was examined as a function of annealing time and film thickness using neutron reflectivity and small-angle neutron scattering. Upon annealing, microphase separation occurs quickly with perpendicular and parallel lamellae emanating from the neutral and air surfaces, respectively. This initial growth is followed by a slow increase in the amount of parallel lamellae with a concurrent decrease in the amount of perpendicular lamellae as the samples are annealed further. After long annealing times, the amounts of perpendicular and parallel lamellae do not change significantly. The mixed lamellar structures show a strong commensurability trend with film thickness having a period equal to that of the natural lamellar period. This is explained through the preferential formation of s1/2 disclinations having PS cores which dictate the observed commensurability trend. The preferential formation of these defects was observed by field emission scanning electron microscopy on films predominantly comprised of perpendicular lamellae.

Structure and diffusion of copolymer micelles in entangled homopolymer chains

Self-diffusion coefficients of spherical polystyrene-block-polyisoprene (PS-b-PI) micelles, with PS core and PI corona, in an entangled matrix of linear homopolyisoprene (PI) chains have been investigated by Forced Rayleigh Scattering (FRS). The micellar architecture has been varied from wet brush, i. e. micelles with coronae strongly swollen by the matrix PI, to dry brush conditions, using PI matrix chains of various molecular weights. The concentration of micelles has been adjusted from 10 to 50 wt.-% block copolymer. At high block copolymer concentrations c = 50 wt.-%, the micelles form a cubic lattice for wet, intermediate and dry brush systems as confirmed by small angle X-ray scattering (SAXS). This lattice becomes more distorted with increasing molecular weight of the matrix PI, which is explained by strongly repulsive interactions between wet brush micelles in comparison to weaker repulsive interactions of dry brush micelles. Decreasing the amount of block copolymer, a liquid-like interparticle structure is found. The sample temperature surprisingly shows a strong influence on FRS diffusion measurements for samples with a very high micellar concentration. The signal intensity strongly increases above the glass transition temperature of the polystyrene cores (Tg (PS)). Further, the relaxation process becomes seemingly slower at high temperatures. In some cases, even non-monotonous decay-grow-decay FRS signals are found for those highly concentrated crystalline systems. These signals could be analyzed according to a complementary grating scenario. For samples with lower micellar concentrations, and, correspondingly, a less crystalline structure, no temperature effects on the FRS signal shape have been found. Also, all signals showed a monotonous biexponential decay for the more dilute systems at low temperatures. The slower of the two relaxation processes has been identified as micellar diffusion, the faster process as rotation of individual micelles.

Observation of the Volume Transition in Thermosensitive Core−Shell Latex Particles by Small-Angle X-ray Scattering

An investigation of the volume transition in colloidal core−shell particles composed of a temperature-independent polystyrene core and a shell of thermosensitive cross-linked polymer chains by small-angle X-ray scattering (SAXS) is presented. The PS cores of the particles have a diameter of 80 nm whereas the shell composed of cross-linked poly(N-isopropylacrylamide) has a thickness of 32 nm in the swollen state at 25 °C and of 18 nm after shrinking by a continuous volume transition. The SAXS intensities measured at high scattering angles could be described by a Lorentz-type function at both states. This indicates the presence of liquidlike local concentration fluctuations of the gel which are still present in the shrunken state. The correlation lengths ξ measured in both states are of the order of a few nanometers (25 °C, ξ = 3.2 nm; 50 °C, ξ = 2.1 nm). The present analysis therefore shows that the core−shell microgels behave in a distinctively different manner than ordinary thermosensitive gels: The cro...

Polystyrene-block-poly(2-cinnamoylethyl methacrylate) Nanospheres with Cross-Linked Shells

Polystyrene-block-poly(2-cinnamoylethyl methacrylate), PS-b-PCEMA, forms spherical micelles with PCEMA corona and PS core in THF/acetonitrile with 10%, by volume, of THF. Nanospheres with cross-linked PCEMA shells are obtained by photolyzing the sample with UV light. Despite the PCEMA corona, the degree of intermicellar fusion is low at CEMA conversions less than ∼40%. The nanospheres, with greater than ∼10% CEMA conversions, are dispersible and structurally stable in THF or toluene which solubilizes both PS and un-cross-linked PCEMA.

Protein secondary structure and conformational changes of photosystem II during heat denaturation studied by Fourier transform-infrared spectroscopy

Fourier transform-infrared (FT-IR) spectroscopy was used to study secondary structure and conformational changes of a photosystem II (PS II) membrane and core complex during heat denaturation. In light of the DSC and thermal gel analysis studies of PS II by Thompson et al. (Biochemistry, 25 (1986) 6161–6169; 28 (1989) 6686–6695), we tried to assign the global conformational changes obtained in this study to the denaturation of the individual protein or smaller parts of the complex. The conformational changes below 40°C were temporarily assigned to the extrinsic proteins and/or Cyt b 559. The loss of oxygen-evolution activity occurred in a temperature range which contained no remarkable conformational changes. The core of PS II denatured from 55 to 65°C with transitions of secondary structure from α-helix and turns to β-sheet and random coils. The denaturation of light-harvesting chlorophyll protein (LHCP) began at 65°C and was characterized by a rapid increase of the turns. Our results indicate that the inactivation of the O 2-evolving apparatus is not associated with any major protein secondary structural changes. The conformational changes at lower temperature may be responsible for the heat sensitivity of the apparatus.

Thermodynamics of Crew-Cut Micelle Formation of Polystyrene-b-Poly(acrylic acid) Diblock Copolymers in DMF/H2O Mixtures

The thermodynamics of formation of “crew-cut” micelles of polystyrene-block-poly(acrylic acid) (PS-b-PAA) diblock copolymers were investigated. The PAA/PS ratios were in the range 0.08−0.33, and N,N-dimethylformamide/water (DMF/H2O) mixtures were used as the selective solvent. For the block ratios investigated here, the copolymers form spherical crew-cut micelles with a large PS core and a thin PAA corona. The temperature dependence of the critical micelle concentration (cmc) was determined under different conditions from scattered light intensity measurements as a function of temperature. The closed association model was found to describe the micellization process and was used to calculate the standard Gibbs free energies (ΔG°), the standard enthalpies (ΔH°), and the standard entropies (ΔS°). The effects of solvent composition, copolymer composition, and added NaCl content on the thermodynamic functions were studied. From the experimental data, a single set of combined equations for the standard thermody...

Phase Separation Behavior and Crew-Cut Micelle Formation of Polystyrene-b-poly(acrylic acid) Copolymers in Solutions

Static light scattering was used to study the phase separation behavior of homopolystyrene and polystyrene-b-poly(acrylic acid) (PS-b-PAA) in DMF as a function of added water content. It was found that the critical water content (cwc), at which phase separation starts, depends on both the polymer concentration and the molecular weight. The higher the polymer concentration and the molecular weight, the lower the cwc. For PS homopolymer, phase separation involves the precipitation of the polymer chains. In the copolymer solution, phase separation results in the formation of regular crew-cut micelles consisting of a PS core and a PAA corona; thus, it is preferably referred to as microphase separation. The change of the micelle fraction as a function of water addition can be estimated from the relationship between the cwc and the initial copolymer concentration. The influence of added electrolytes, i.e., NaOH, HCl, NaCl, CaCl2, or Ca(Ac)2, on the self-assembly process of the copolymers in DMF was also explored. Since the micelle cores are highly swollen by DMF in the early stages of their formation, the structures are labile. As the added water content increases, the cores become gradually less swollen and the mobility of the polymer chains in the cores decreases. A study of polymer chain exchange among the micelles was performed by mixing two solutions of micelles of different sizes at different water contents and studying the micelle core size distribution by transmission electron microscopy. It is shown that the copolymer chain exchange within a 1 day period is significant at a water content of 6 wt %, but becomes negligible when the water content is increased to ∼11 wt %.

A Fourier transform infrared spectroscopic study of the effects of hydrogen peroxide and high light on protein conformations of Photosystem II

Degradation of the reaction center-binding protein D1 of Photosystem II (PS II) during photoinhibition is dependent on the action of active oxygen species and/or D1-specific proteases. Protein conformational changes may be involved in the process of D1 degradation. In the present study, we determined the effect of H2O2 on spinach PS II-enriched membranes and core complexes with respect to electron transport, Mn content and protein secondary structural changes as measured by Fourier transform infrared (FTIR) spectroscopy. H2O2 is effective in removing catalytic Mn in PS II, especially in PS II core complexes depleted of OEC18 and OEC24, impairing the donor-side. By quantitative analysis of the amide I band (1600 – 1700 cm-1) with both aqueous and dehydrated PS II samples, we found that no significant secondary structural changes are associated with H2O2 treatment in the dark, even though there is some cleavage of the D1 protein by H2O2 treatment as determined by Western analysis with specific antibodies. In contrast, a large decrease in the α-helices in the PS II core occurs, with or without H2O2 treatment, after 20 min strong illumination and there is more extensive degradation of the D1 protein. Our results suggest that high light enhances the cleavage of the D1 protein which is reflected in the large protein secondary structural changes in PS II detected by FTIR measurements.

Water‐based radiation‐curable latexes

Film-forming polystyrene/poly(n-butyl acrylate-co-glycidyl methacrylate) [PS/P(BA-co-GMA)] core–shell latex particles were prepared via a two-stage emulsion polymerization procedure using a polystyrene latex seed. Delayed addition of GMA was used in order to locate functional epoxy groups close to the particle surfaces. It was found that a temperature of 25°C at the second-stage polymerization, in combination with a redox initiator system, was essential for the formation of a uniform shell of BA–GMA copolymer around the PS core. The latex particle morphology was investigated by transmission electron microscopy (TEM). Reactive double bonds were introduced into the particle shells in order to produce a film-forming latex system that could be cured by ultraviolet (UV)-radiation without any need to use reactive multifunctional monomers or oligomers as crosslinkers. The surface-bound epoxy groups were used as grafting sites for amine or carboxyl functional unsaturated monomers, respectively. The grafting was demonstrated by Fourier transform infrared (FTIR) spectroscopy. Films prepared from modified and unmodified latexes were exposed to UV radiation in the presence of a photoinitiator. Crosslinking was tested by thermal mechanical analysis (TMA) and by determination of swelling and gel content of exposed films. It was demonstrated that films from the modified latexes after irradiation had significantly higher stiffness and gel content and showed lower swelling than unmodified ones. © 1996 John Wiley & Sons, Inc.