Dilute Aqueous Poly(ethylene oxide) Solutions: Clusters and Single Molecules in Thermodynamic Equilibrium

Diffusion of PEG confined between lamellae of negatively magnetically aligned bicelles: pulsed field gradient 1H NMR measurements

The diffusion of various molecular weight poly(ethyleneglycol)s (PEG) confined between the lamellae of magnetically aligned bicelles has been measured using stimulated echo (STE) pulsed field gradient (PFG) 1H nuclear magnetic resonance (NMR) spectroscopy. Bicelles were formulated to contain dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG), and dihexanoylphosphatidylcholine (DHPC) in the proportion DMPG/DMPC = 0.05 and q = (DMPC + DMPG)/DHPC = 4.5. PEG diffusion within the interlamellar spaces between such bicelles was found to be unrestricted over diffusion distances of tens of microns. Two confinement regimes could be differentiated according to the dependence of the reduced PEG diffusivity D/D0, where D0 is the unconfined PEG diffusion coefficient, on the relative confinement Rh/H, where Rh is the unperturbed hydration radius of the particular PEG and H approximately 60 A is the separation between apposing lamellae of the magnetically aligned bicelles. In the regime Rh/H < 0.4, the reduced PEG diffusivity was altered only in proportion to the viscosity increase associated with the bicelle dispersion relative to bulk solution. In the regime Rh/H > 0.4, the reduced PEG diffusivity scaled as (Rh/H)-2/3, in agreement with scaling theories for confined polymers.

Effects of dissolved polymer on the transport of colloidal particles through a microcapillary

The effect of water-soluble polymer on the transport of latex particles through a microcapillary was investigated. Capillary hydrodynamic fractionation (CHDF) experiments were performed using polystyrene (PS) particles and poly(ethylene oxide) (PEO) solutions as the eluant. Generally, the average particle velocities were greater than those corresponding to a polymer-free eluant. A decrease in the sample axial dispersion was also observed using the PEO solutions. In addition, increasing the polymer molecular weight resulted in lower particle residence times in the capillary tube. The enhanced particle transport arises primarily from an increase in the particle diameter resulting from the adsorption of PEO onto the PS surfaces, and, more importantly, from the migration of particles toward the capillary axis due to the normal stress of the PEO solution.

Polyethylene glycol adsorption on silica: from bulk phase behavior to surface phase diagram

A characterization of the bulk-phase diagram from literature data and new NMR and DSC measurements provided us with valuable elements that are helpful for gaining, from aqueous solution, better insight into the surface behavior of polyethylene glycol on Aerosil 200. Adsorption isotherms built further to measurements by a depletion method showed a strong and temperature-dependent variation of the isotherm shape in agreement with the variations of interactions already evidenced in the bulk. In temperature-concentration areas, where water is behaving as a helix-promoting solvent, the finding of positive PEG adsorptions and stairlike isotherms agrees with observations reported in the literature. We identified some of the vertical parts as corresponding to the formation of monolayers of helix-shaped PEG molecules. In poor-solvent zones, adsorptions were null or negative, and the isotherms exhibited oscillations suggesting very different surface behavior. Our data analysis evidenced the presence of a much greater amount of water than in the previous surface states; however, the similar analysis of PEG behavior remains relevant. Indeed, the occurrence of first-order transitions in the surface layer implies some water reorganization, permitting the PEG molecules to move closer to the surface and become helix-shaped to rearrange in a monolayer. The surface phase diagram confirmed this analysis in a very satisfying way.

Synthesis of poly(ethylene glycol) (PEG)-grafted colloidal silica particles with improved stability in aqueous solvents

The known grafting procedures of colloidal silica particles with poly(ethylene glycol) (PEG) lead to grafting layers that detach from the silica surface and dissolve in water within a few days. We present a new grafting procedure of PEG onto silica with a significant improvement of the stability of the grafting layers in aqueous solvents. Moreover, the procedure avoids any dry states or other circumstances leading to strong aggregation of the particles. To achieve the improved water stability, Stöber silica particles are first pre-coated with a silane coupling agent (3-aminopropyl)triethoxysilane (APS) to incorporate active amine groups. The water solubility of the pre-coating layer was minimized using a combination of APS with bis-(trimethoxysilylpropyl)amine (BTMOSPA) or bis-(triethoxysilyl)ethane (BTEOSE). These pre-coated particles were then reacted with N-succinimidyl ester of mono-methoxy poly(ethylene glycol) carboxylic acid to form PEG-grafted silica particles. The particles form stable dispersions in aqueous solutions as well as several organic solvents.

Influence of free chains on the swelling pressure of PEG-HEMA and dex-HEMA hydrogels

Insight in the osmotic behavior of degrading hydrogels is of great importance in the design of biodegradable hydrogels for biomedical applications. This study compares the degradation behavior of PEG-HEMA (hydroxyethylmethacrylated polyethylene glycol) and dex-HEMA (hydroxyethylmethacrylated dextran) hydrogels. The degradation of PEG-HEMA gels takes several months to over a year, while that of dex-HEMA gels takes only days or weeks. The faster degradation kinetics of dex-HEMA networks can be attributed to stabilization of the keto-enol form by hydroxyl groups. Upon degradation of PEG-HEMA and dex-HEMA hydrogels, respectively, free PEG and free dextran chains are produced. We investigated the effect of unattached PEG and dextran chains on the swelling pressure of the degrading gels. It is found that low molecular weight free chains significantly increase the swelling pressure. However, the contribution of higher molecular weight free chains (M(w)>10 kDa) is similar to that of the network chains.

PEG400 novel phase description in water

The behavior of hydroxyl-terminated PEG400 in water was investigated by surface tension measurements and 13C NMR as a function of concentration and temperature. PEG400 exhibited a critical aggregative concentration (cac) that evidenced both its amphiphilic character and its aggregation capacity. Moreover, the chemical shifts of the different carbons of the PEG were followed by NMR versus concentration at various temperatures. We observed a plateau between 20 and 35 degrees C at concentrations above 0.2 mol L(-1) and ascribed it to the aggregation process. A good correlation was found between the NMR spectra in the region of aggregation and the cac region in the phase diagram. Our investigations were also focused on the solid-liquid region of the phase diagram at lower temperatures. These experimental data, together with conclusions available in the literature, led us to propose explanations for the conformation/hydration/aggregation in the PEG400-water solutions phenomena.

Supramolecular complexes of spin-labelled cyclodextrins

EPR spectra of cyclodextrins labelled with TEMPO derivatives (SL-CDs) are sensitive to complexation with large guest molecules. We used SL-CDs to explore the behaviour of concentrated PEG/PPG solutions. The relationship between rotational correlation times and solvent viscosity showed significant deviations from the Debye-Stokes-Einstein equation, probably due to self-aggregation of alkylene glycols in concentrated solutions. The data fit the fractional Debye-Stokes-Einstein equation well. We have also studied complexation of SL-CDs with adamantane-functionalised DAB dendrimers. The strength of binding increases with dendrimer generation; formation of supramolecular aggregates at high concentrations was observed with the generation 3 dendrimer.

Evidences of Nonideal Mixing in Poly(ethylene Glycol)/Organic Solvent Mixtures by Brillouin Scattering

The concentration dependence of the hypersonic properties of solutions of poly(ethylene glycol) of mean molecular mass 600 g/mol (PEG600) in benzene and toluene has been investigated by Brillouin scattering. The two solvents are very similar in structure and chemical properties, but while benzene is nonpolar, toluene possess a modest dipole. In both solvents a high-frequency relaxation process has been observed at high concentrations which has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. In both cases, the concentration dependence of the adiabatic compressibility deviates significantly from linearity, indicating the existence of nonideal mixing phenomena driven by aggregation processes taking place in the systems. However, there is no temperature dependence for solutions of PEG600 in benzene; on the contrary, the results obtained for solutions of PEG600 in toluene are noticeably dependent on the temperature. The comparison of the experimental data with the results of previous experiments on similar systems allows a general picture for weakly interacting mixtures of hydrogen-bonded systems and organic solvents to be developed. In particular, in the presence of a nonpolar solvent molecule the local structure of the mixture is dominated by solute self-association processes and any resulting solute-solvent correlation is barely induced by excluded volume effects. At high enough dilution the self-aggregation of solute molecules produces a variety of new local topologies that cannot be observed in bulk solute, and as a consequence, the concentration evolution of the system is too rich to be described in terms of a linear combination of a few components over the whole concentration range. The situation seems to be simpler for the polar toluene solvent molecules, where a three-component model seems able to fit the experimental concentration dependence of the hypersonic velocity. This result is interpreted to imply that the interaction between the solvent dipoles and the active sites of the solute produces a relatively stable heterocoordination, while the relevance of self-association is partially reduced.

Reversible aggregation of polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) block copolymer micelles in acidic aqueous Solutions

Micelles of polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-PVP-PEO) were studied in acidic aqueous solutions by static and dynamic light scattering, alkalimetric titration, fluorescence correlation spectroscopy, and after deposition on a mica surface by atomic force microscopy. The PS-PVP-PEO micelles prepared by dialysis in ternary 1,4-dioxane-methanol-acidic water mixtures have a very low association number and show a strong tendency to form aggregates. The aggregation, which is promoted at low pH, seems to be fully reversible. Possible mechanisms of the aggregation are discussed. Atomic force microscopy scans of PS-PVP-PEO micelles deposited on a mica surface reveal the formation of micellar aggregates and support the general concept of aggregation upon changes in conditions and deterioration of the stability of small micelles.

Association-induced polymer bridging by poly(ethylene oxide)-cofactor flocculation systems

Poly(ethylene oxide) (PEO) is used in papermaking, water purification and mineral flotation as a flocculating agent. Despite the fact that PEO does not adsorb on cellulose and bleached lignin, and only poorly on calcium carbonate, it can nevertheless be used to deposit colloidal pigments on pulp fibers. PEO by itself is inefficient, but it can be made to work in combination with another compound, usually referred to as a cofactor or an enhancer. The cofactor associates with PEO and the association complex acts as an efficient bridging agent. There are two classes of cofactor: those who cause PEO to cluster and those that do not. In general PEO association-clusters are more efficient flocculating agents than non-clustered PEO. The PEO dissolution procedure has an important effect on the PEO flocculation efficiency. Before reaching thermodynamic equilibrium, PEO is in an entangled state. Cofactors that cluster PEO are likely to maintain PEO in an entangled state and, like the association-clusters, these entanglements are more efficient flocculation agents than well-dispersed PEO. Salt also affects the PEO-cofactor association. For most cofactors, salt is needed to induce the association. Calcium ions can act as bridging agents between association complexes, especially those containing carboxyl groups, thus promoting the formation of association-clusters.

Structural Relaxation Processes in Polyethylene Glycol/CCl4 Solutions by Brillouin Scattering

We present results of a Brillouin scattering experiment on solutions of poly(ethylene glycol) of mean molecular mass 600 g/mol (PEG600) in CCl4. The relaxation process detected has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. The concentration dependencies of the hypersound velocity and normalized absorption are compared against the indications from several models proposed in the literature. The concentration evolution of the system is described in terms of two distinct regimes. At high polymer content, the system is dominated by the structure of the dense polymer, where polymer-polymer interactions, together with excluded volume effects, induce the existence of a preferred local arrangement resulting in a narrow distribution of the relaxation times, with the average value of the relaxation time following a simple Arrhenius temperature dependence. As the concentration decreases, the original structure of the hydrogen bonded polymer network is destroyed, and a number of different local configuration coexist, giving rise to a wider distribution of relaxation times or to a multiple relaxation. At low concentrations, the experimental data are well fitted assuming a Vogel-Fulker-Tammon behavior for the average relaxation time. In addition, the observed deviation from the ideal behavior for the refractive index and the density suggests that CCl4 does not behave as an inert solvent, and due to polarization effects, it can develop local hetero-associated structures via electrostatic interaction with the O-H end groups of the polymeric chains. The hypothesis has been successfully tested by fitting the concentration behavior of the hypersonic velocity to a recent three-component model, suitable to describe the concentration dependence of sound velocity in moderately interacting fluids. The indication of the model furnishes a very high value for the association constant of the PEG600, confirming the literature indication that, in polymeric systems capable of developing long liner aggregates via hydrogen bonding interaction, the Brillouin probe is insensitive to the true length of the polymeric chains. The Brillouin scattering experiment just sees an effective hydrogen bonded aggregate that is huge relative to the length of the single polymeric chain and becomes sensitive only to the density fluctuations of the local segmental motions.

On the theory of aggregation and micellization: PEO–PVP copolymer in water

We develop a theoretical approach to micellization of the PEO-PVP block-copolymer in water. This copolymer is a weak polyelectrolyte due to protonation of VP blocks. The theory accounts for non-linear ion screening, and predicts strong position dependence of both ion concentration and the effective Debye length. We consider both the case when the local Debye length is small compared to the core radius and the case when it is large. We found that the effective (local) pH is not uniform even inside one micellar core, hence non-uniform protonation of the core with higher charge density near the surface. In many cases the core charge is concentrated in a relatively thin surface layer. Considering statistical weights of non-equilibrium micelles and their continuous evolution we show that kinetics of both formation and dissociation of typical block-copolymer or surfactant micelles can be extremely slow. Thus micelle formation at the genuine (equilibrium) critical micelle concentration (c.m.c.) is totally suppressed (involves astronomical time scales) if the micelles are big enough. An 'apparent' critical micelle concentration (c.m.c.*) is introduced to account for this effect. The apparent c.m.c.* could be much higher than the genuine equilibrium c.m.c., i.e. a significant hysteresis is inherent in these systems. We also determine the ranges of meta-stability of micelles depending on the experimental time-scales.

Large structures in diblock copolymer micellar solution

The association properties in water solution of poly(dimethylsiloxane)-b-poly(ethyleneoxide) diblock copolymer was investigated by static and dynamic light scattering in a wide range of concentrations and temperatures. The presence of a long hydrophilic poly(ethyleneoxide) (PEO) chain causes a weak tendency to microphase separation of the system which is responsible for some relevant effects. First of all we observe a late micellization process which is characterized by an unusually high value of the critical micellar concentration (c(cmc) =0.007 g/cm3) and by an unusually small aggregation number (approximately 6) of the generated micelles. Moreover, the composition of the highly hydrated micelles has been found to change sensitively with temperature. On increasing temperature dehydration of micelles has been observed together with a contemporaneous increase in the aggregation number, whereas the hydrodynamic radius remains constant in the whole range investigated. The long hydrophilic chains also stimulate an efficient entanglement process between micelles. The interpenetrating PEO chains belonging to different micelles causes the depletion of the solvent in the outer layer of micelles. The result is the formation, just after the micellization process takes place, of thermodynamically stable clusters of entangled micelles. These large structures, which are present in the system in small concentrations, maintain their structural properties unchanged in a wide range of concentrations and temperatures, and provide indirect evidence of a weak attractive component to the intermicellar interaction potential.

Adsorption kinetics of polyethylene glycol from aqueous solution onto activated carbon

The adsorption equilibrium and kinetics of polyethylene glycol (PEG) in three aqueous systems were examined in this study. Langmuir isotherm was used to satisfactorily predict the adsorption capacity of PEG on activated carbon F-400 and applied to the investigation of adsorption kinetics. The surface diffusion, pore diffusion, and branched pore kinetics models successfully described the adsorption behavior of PEG on F-400 in the completely stirred tank reactor. The pore diffusion coefficients obtained from the pore diffusion model were compared with those computed by the experimental data of the short fixed-bed reactor combined with the assumption of non-hindered pore diffusion. In addition, the effects of initial concentrations of PEG and the relative importance of external and internal mass transfers for the adsorption were also taken into account and discussed in this study.

Polymer Lateral Diffusion at the Solid−Liquid Interface

Measurements are presented of how polymer surface diffusion at the solid-liquid interface is controlled by surface coverage. The method of measurement was fluorescence correlation spectroscopy (FCS), and the system was poly(ethylene oxide) (PEG) adsorbed onto methyl-terminated self-assembled monolayers in buffered aqueous solution. The translational diffusion coefficient at first increased with increasing surface concentration, presumably because the number of adsorption sites per molecule decreased. Ultimately it slowed by 1 order of magnitude, presumably reflecting jamming by neighboring chains.

Polymer-stabilized gold nanoparticles with high grafting densities

A series of polymer-coated Au nanoparticles have been prepared using the "grafting-to" approach. Thiol-terminated polystyrene and poly(ethylene oxide) ligands are found to form dense brushes on the faceted gold nanoparticle surfaces. Depending on the polymer, the ligand grafting densities on the gold nanoparticles are 1.2- to 23.5-fold greater than those available via self-assembled monolayer formation of the corresponding two-dimensional gold surfaces.

Monitoring solute interactions with poly(ethylene oxide)-modified colloidal silica nanoparticles via fluorescence anisotropy decay

The fluorescence-based nanosize metrology approach, proposed recently by Geddes and Birch (Geddes, C. D.; Birch, D. J. S. J. Non-Cryst. Solids 2000, 270, 191), was used to characterize the extent of binding of a fluorescent cationic solute, rhodamine 6G (R6G), to the surface of silica particles after modification of the surface with the hydrophilic polymer poly(ethylene oxide) (PEO) of various molecular weights. The measurement of the rotational dynamics of R6G in PEO solutions showed the absence of strong interactions between R6G and PEO chains in water and the ability of the dye to sense the presence of polymer clusters in 30 wt % solutions. Time-resolved anisotropy decays of polymer-modified Ludox provided direct evidence for distribution of the dye between bound and free states, with the bound dye showing two decay components: a nanosecond decay component that is consistent with local motions of bound probes and a residual anisotropy component due to slow rotation of large silica particles. The data showed that the dye was strongly adsorbed to unmodified silica nanoparticles, to the extent that less than 1% of the dye was present in the surrounding aqueous solution. Addition of PEO blocked the adsorption of the dye to a significant degree, with up to 50% of the probe being present in the aqueous solution for Ludox samples containing 30 wt % of low molecular weight PEO. The addition of such agents also decreased the value and increased the fractional contribution of the nanosecond rotational correlation time, suggesting that polymer adsorption altered the degree of local motion of the bound probe. Atomic force microscopy imaging studies provided no evidence for a change in the particle size upon surface modification but did suggest interparticle aggregation after polymer adsorption. Thus, this redistribution of the probe is interpreted as being due to coverage of particles with the polymer, resulting in lower adsorption of R6G to the silica. The data clearly show the power of time-resolved fluorescence anisotropy decay measurements for probing the modification of silica surfaces and suggest that this method should prove useful in characterization of new chromatographic stationary phases and nanocomposite materials.

Light Scattering, Atomic Force Microscopy and Fluorescence Correlation Spectroscopy Studies of Polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) Micelles

Polymeric nanoparticles formed by triblock copolymer polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide), PS-PVP-PEO, in aqueous media were studied by a combination of fluorescence correlation spectroscopy with other fluorescence techniques, light scattering and atomic force microscopy. The studied polymeric nanoparticles exist in the form of (i) core/shell micelles in acid solution at pH lower than 4.8 and (ii) three-layer onion micelles at higher pH. Since water is a very strong precipitant for PS, both types of micelles have kinetically frozen spherical PS cores. The cores of micelles in acid media are surrounded by soluble shells formed by partly protonated PVP and PEO, while the cores of micelles in alkaline media are surrounded by compact insoluble layers of deprotonated PVP and soluble PEO shells. The micellization behavior of PS-PVP-PEO micelles is accompanied by secondary aggregation of micelles, which is provoked by stirring, shaking and also by filtration of micellar solutions. Therefore fluorescence correlation spectroscopy (FCS), which, in contrast to light scattering techniques, does not require filtration, was used as the main experimental technique for the characterization of non-aggregated micelles. The binding of a fluorescence probe, octadecylrhodamine B (ORB), to polymeric micelles, was studied before the FCS study of micelles.