Conformation of oligo(ethylene glycol) grafted poly(norbornene) in solutions: A small angle neutron scattering study

The conformation of newly synthesized amphiphilic poly(methoxyoligo(ethylene oxide) norbornenyl esters) macro-homopolymers in dilute solutions of toluene-d8 and D₂O was investigated by small angle neutron scattering (SANS). The macro-homopolymers consist of a polynorbornene (PNB) backbone with a degree of polymerization (DP) of 50, and each repeat unit has a grafted ethylene glycol (EG) side chain with an average DP of 6.6. The hydrophobic backbone and hydrophilic side chains interact differently with solvents of different polarity, which makes the polymer conformation very sensitive to the solvent quality. It was found that in a 0.5 wt.% toluene solution the polymers assume coil-like conformation and gradually contract and become more compact with increasing polymer concentration. In D₂O, the conformation of the polymers were studied at different concentrations: 0.1, 0.5, 1.0 and 2.0 wt.% and at different temperatures: 25, 44, 60 and 74 °C. The polymers are partially contracted in D₂O and their shape can be described by the form factor of a rigid cylinder. The second virial coefficient A₂ was extracted at three temperatures (25, 44 and 60 °C) and the theta point was estimated to be reached at ∼45 °C. The attractive interactions between the polymers in D₂O increase with temperature, which leads to the polymer-solvent phase separation at the cloud point temperature (CPT). The polymer conformation remains virtually temperature independent below the CPT and at 74 °C polymers collapse and form compact structures with water soluble side chains in the shell.     

On the role of block copolymer additives for calcium carbonate crystallization: small angle neutron scattering investigation by applying contrast variation

The role of the double-hydrophilic block copolymer poly(ethylen glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) on the morphogenesis of calcium carbonate (CaCO3) was studied by applying the contrast variation small angle neutron scattering technique. The morphology and size of CaCO3 crystals is strongly affected by the addition of PEG-b-PMAA. In order to determine the partial scattering functions of the polymer and CaCO3 mineral, we developed both an experimental and theoretical approach with a sophisticated method of their determination from the scattering intensity. Partial scattering functions give detailed information for each component. In particular, the partial scattering function of the polymer, Spp, shows a monotonic slope with Q(-2 to -3) where the scattering vector Q is low (Q < 0.01 Angstrom(-1)), which is a clear evidence that the polymer within the CaCO3 mineral has a mass fractal dimension. The other partial scattering functions reflected the geometry of the CaCO3 particles or the "interaction" of polymer and CaCO3 on a microscopic scale, which leads to a coherent view with Spp.     

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.     

A small-angle neutron scattering study of poly(ethylene oxide) microstructure in aqueous poly(styrenesulfonate sodium) solutions

Dilute aqueous solutions of d-PEO and PSSNa mixtures were studied by (2)H NMR spectroscopy and small-angle neutron scattering (SANS). The interactions between d-PEO and PSSNa were found to be negligible both in the presence and absence of NaCl. At very dilute concentration (0.7 mg mL(-1)), d-PEO chains were still found to be slightly collapsed at ambient temperature in water. Upon the addition of PSSNa, aggregates of d-PEO were observed with d-PEO coils loosely associated with each other. The average centre to centre distance between d-PEO coils, which was calculated from the maxima in SANS spectra, was similar to the size of the individual coils. The effect of a simple salt, NaCl, on d-PEO-PSSNa interaction was investigated. Salt addition induced a breakdown of the dilute d-PEO aggregates.     

Morphology of poly(ethylene oxide) dissolved in a room temperature ionic liquid: a small angle neutron scattering study

Solutions of deuterated poly(ethylene oxide) (d-PEO) in 1-butyl-3-methyl imidazolium tetrafluoroborate ([bmim][BF4]), a prototype room-temperature ionic liquid (RTIL), have been studied at room temperature over a range of polymer concentrations, using small angle neutron scattering (SANS), characterizing the conformation of PEO dissolved in RTILs. [bmim][BF4] behaves as a good solvent for d-PEO, which organizes in this solvent in non entangled random coils. These findings will help in optimizing the designing of microemulsions in these potentially environmentally friendly solvents.     

Effects of temperature on neutron scattering from aqueous solutions of hydrophobically modified poly(ethylene oxide)

The effect of temperature on the structure of aqueous dispersions of hydrophobically end-capped poly(ethylene oxide) (PEOM) was investigated by small angle neutron scattering (SANS). Polymers with hydrogenated or deuterated n-octadecyl end-groups were studied in heavy water or in a mixture heavy water / water, respectively. In the latter case the PEO chains were selectively matched. In all the cases, the scattering curves were characterised by a main peak which revealed organisation of polymers into micelles consisting of hydrophobic cores surrounded by repulsive PEO coronae. Measurements were performed in the semi-dilute regime where micelles coronae overlap. At constant polymer concentration, an increase in temperature leads to decreasing solvent strength of water for the PEO chains and decreasing repulsion between the PEO coronae. As a result, the intensity of the peak in a mixture of water /heavy water decreases with temperature On the contrary, in heavy water, the peak of the scattered intensity increases with increasing temperature. This scattering behaviour is interpreted on the basis of a scaling theory of the semi-dilute solutions of star-like polymer micelles.     

Effect of sodium chloride on association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer in aqueous solutions

The effect of sodium chloride (NaCl) upon the thermally induced association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer, Pluronic P103, has been investigated using pyrene fluorescence spectroscopy. The critical micellization temperature (CMT) of Pluronic P103 in aqueous solution is decreased by the addition of NaCl. The standard enthalpy and entropy of micellization for Pluronic P103 in water are increased in the presence of small amounts of NaCl, but further addition of NaCl decreases the standard enthalpy and entropy of micellization. The I1/I3 ratio of pyrene in aqueous Pluronic P103 solutions at temperature below the CMT decreases with increases of NaCl concentration, which is related to the decrease of PPO solubility. The decrease in polarity of the PPO shifts the CMT toward lower temperature.     

Colloidal dispersions of octadecyl grafted silica spheres in toluene: a global analysis of small angle neutron scattering contrast variation and concentration dependence measurements

In this paper we report measurements of the form factor and the structure factor of a sterically stabilized colloidal dispersion consisting of silica spheres coated with octadecane in toluene by small angle neutron scattering (SANS). The phase diagram of this system shows the liquid-liquid coexistence line and also a jamming transition at higher concentrations, where the jamming line intersects the coexistence line roughly at the critical point. We have performed SANS experiments at a temperature well above the transition temperature and at various volume fractions phi, spanning from the very dilute regime (phi=0.2%) to the critical concentration (phi=16%) and the highly viscous regime (phi=39.2%). Except for the very dilute regime, we observe a structure factor S(q) in all other cases. We fitted our data over the whole concentration regime using a global fitting routine with a core-shell model for the form factor P(q), taking into account the structure factor, which we describe with the Robertus model for an adhesive polydisperse core-shell particle. At a volume fraction of phi=5% a SANS contrast variation experiment has been performed. From that the product of the volume of the shell and the amount of solvent within the corona of our core-shell particle could be determined. At the most probable shell thickness of 2.3 nm a solvent content of about 50% within the corona was found. Moreover we could conclude that the core is not interpenetrated by solvent molecules. From the contrast variation experiment followed that the structure factor at zero average contrast exhibits a strong q dependence, which is an effect of an inhomogeneous particle in combination with a size distribution.     

Temperature-dependent aggregation and disaggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer in aqueous solution

Aggregation and disaggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers, Pluronics P103 and P104, in aqueous solutions during a heating and cooling cycle were investigated by dynamic laser scattering (DLS) and 1H NMR spectroscopy. Temperature hysteresis was observed by DLS when cooling the copolymer aqueous solutions because larger aggregates existed at temperatures lower than critical micellization temperature (CMT), but no temperature differences were observed by NMR. This phenomenon was explained as the forming of water-swollen micelles at temperatures lower than CMT during the cooling process.     

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.     

Interactions between block copolymers and single-walled carbon nanotubes in aqueous solutions: a small-angle neutron scattering study

The amphiphilic copolymers of the Pluronic family are known to be excellent dispersants for single-walled carbon nanotubes (SWCNT) in water, especially F108 and F127, which have rather long end-blocks of poly(ethylene oxide) (PEO). In this study, the structure of the CNT/polymer hybrid formed in water is evaluated by measurements of small-angle neutron scattering (SANS) with contrast variation, as supported by cryo-transmission electron microscopy (cryo-TEM) imaging. The homogeneous, stable, inklike dispersions exhibited very small isolated bundles of carbon nanotubes in cryo-TEM images. SANS experiments were conducted at different D(2)O/H(2)O content of the dispersing solvent. The data for both systems showed surprisingly minimal intensity values at 70% D(2)O solvent composition, which is much higher than the expected value of 17% D(2)O that is based on the scattering length density (SLD) of PEO. At this near match point, the data exhibited a q(-1) power law relation of intensity to the scattering vector (q), indicating rodlike entities. Two models are evaluated, as extensions to Pederson's block copolymer micelles models. One is loosely adsorbed polymer chains on a rodlike CNT bundle. In the other, the hydrophobic block is considered to form a continuous hydrated shell on the CNT surface, whereas the hydrophilic blocks emanate into the solvent. Both models were found to fit the experimental data reasonably well. The model fit required special considerations of the tight association of water molecules around PEO chains and slight isotopic selectivity.     

Self-diffusion study of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymers in aqueous solution

The self-diffusion of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymers dissolved in deuterated water was investigated by means of pulsed field gradient NMR (PFG-NMR). The polymer forms micelles in the solution and, with increasing temperature, clouding and phase demixing occurs. The self-diffusion coefficient indicates the association of the polymer molecules in the vicinity of the cloud point because of its maximum with increasing temperature. Above the cloud point, two kinds of diffusing species are observed due to phase separation. The faster diffusing species is attributed to the polymer-poor phase. The self-diffusion coefficient of the polymer-rich phase species decreases with increasing temperature above the cloud point due to further association and dehydration. The correlation length of the diffusing associates, calculated from the self-diffusion coefficient and the viscosity by means of the Stokes-Einstein equation is nearly independent of temperature and concentration up to 30 wt-% polymer concentration. The correlation length is about 1.4 nm. It shows a slight maximum at the cloud point.     

Light scattering study of solubilization of organic molecules by block copolymer micelles in aqueous media

Block copolymers, when dissolved in a selective solvent, form spherical micelles. These micelles can selectively solubilize organic molecules otherwise insoluble in the pure solvent. In this study, we report solubilization of organic molecules by styrene-methacrylic acid block copolymer micelles in aqueous buffers. A light scattering technique was developed to determine the extent of micellar solubilization. Our results indicate that the extent of micellar solubilization depends on the chemical nature of organic molecules, specifically, on the interactions between the organic compound and polystyrene. A thermodynamic model has been developed to describe micellar solubilization. The theoretical calculation agrees reasonably well with the experimental results for two micellar samples examined. ©1995 John Wiley & Sons, Inc.     

Influence of monovalent ions on density fluctuations in hydrothermal aqueous solutions by small angle X-ray scattering

Synchrotron small angle X-ray scattering measurements on water and alkaline bromine aqueous solutions (XBr, with X = Li, Rb, or Cs) were carried out from ambient to supercritical conditions. The temperature was increased from 300 to 750 K along several isobars between 24 and 35 MPa. The correlation length and the structure factor were extracted from the data following the Ornstein-Zernike formalism. We obtained experimental evidence of the shift of the critical point and isochore and their dependence on the ions concentration (0.33 mol/kg and 1.0 mol/kg). We also observed that the size of the density fluctuations and the structure factor increase with the presence of the ions and that this effect is positively correlated with the atomic number of the cation. These behaviors were compared with ZnBr(2) and NaCl systems from the literature.     

Effect of bovine serum albumin on the micellization of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers in aqueous solutions by fluorescence spectroscopy

Effect of bovine serum albumin (BSA) on the temperature-dependent association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers was investigated using pyrene fluorescence spectroscopy. The critical micellization temperature (CMT) of pluronics in aqueous solution was increased by the addition of BSA. A closed association model was used to obtain the standard free energies (△G0), enthalpies (△H 0), and entropies (△S 0) of micellization. The standard enthalpy and entropy of micellization for pluronic polymers in water were decreased with an increase of the BSA content. The more PPO component in the pluronic polymer, the higher the changed values of micellization enthalpy and entropy. The hydrophobic part of the pluronics, PPO, was responsible for the interaction between pluronics and BSA. Hydrophobic interaction between PPO and BSA was correlated to the alternation of the PPO-PPO interaction by the addition of BSA, which would shift the CMT toward higher temperature and alter the thermodynamic parameters of micellization for pluronics in aqueous solutions.     

Aggregation of poly(ethylene oxide)-poly(propylene oxide) block copolymers in aqueous solution: DPD simulation study

The dissipative particle dynamics (DPD) simulation method was applied to simulate the aggregation behavior of three block copolymers, (EO)16(PO)18, (EO)8(PO)18(EO)8, and (PO)9(EO)16(PO)9, in aqueous solutions. The results showed that the size of the micelle increased with increasing concentration. The diblock copolymer (EO)16(PO)18 would form an intercluster micelle at a certain concentration range, besides the traditional aggregates (spherical micelle, cylindrical micelle, and lamellar phase); while the triblock copolymer (EO)8(PO)18(EO)8 would form a spherical micelle, cylindrical micelle, and lamellar phase with increasing concentration, and (PO)9(EO)16(PO)9 would form intercluster aggregates, as well as a spherical micelle and gel. New mechanisms were given to explain the two kinds of intercluster micelle formed by the different copolymers. It is deduced from the end-to-end distance that the morphologies of the diblock copolymer and triblock copolymer with hydrophilic ends were more extendible than the triblock copolymer with hydrophobic ends.     

Small angle x-ray scattering study of ultra-high molecular weight poly(ethylene oxide) gamma irradiated in bulk

The effect of gamma irradiation on the structure of ultra-high molecular weight poly(ethylene oxide) (UHMWPEO) has been investigated by small-angle x-ray scattering (SAXS). The pressed and irradiated polymer possesses quite strong diffuse scattering. That fact makes the direct determination of the main structural parameters very difficult. To solve the problem a modification of the standard evaluating methods is suggested. Thus, the separation of the porous scattering from that of structural formations of PEO becomes possible. The application of a collimation correction according to Schmidt's method enables to separate diffuse and discrete parts in the scattering. Dependences of the long period, the gyration radii, and the difference between densities of crystalline and amorphous phases on the irradiation dose show that the irradiation increases the density of amorphous areas. A packing of the lamellar aggregates and increase of the porous sizes are observed.     

Study of micelle formation by the diblock copolymer poly(styrene-b-dimethyl siloxane) in n-dodecane by small angle x-ray scattering

The diblock copolymer poly(styrene-b-dimethyl siloxane) (PS-PDMS) in n-dodecane forms multimolecular micelles in which insoluble PS blocks associate in a core surrounded by a shell of solvated PDMS blocks. Small angle X-ray scattering data have been obtained for PS-PDMS concentrations in the range 0.5–10%. Intensity data have been interpreted by the Guinier method in order to determine the radius of the PS core of a micelle. Scattering intensities exhibit negative deviations from Porod's law, which is characteristic of a diffuse interfacial layer, permitting the determination of the thickness of the shell of solvated PDMS blocks. Scattering curves for solutions having high PS-PDMS concentrations have a peak maximum indicating ordered arrays of micelles.     

Synthesis and pH-sensitive micellization of doubly hydrophilic poly(acrylic acid)-b-poly(ethylene oxide)-b-poly(acrylic acid) triblock copolymer in aqueous solutions

A doubly hydrophilic triblock copolymer poly(acrylic acid)-b-poly(ethylene glycol)-b-poly(acrylic acid) (PAA-b-PEO-b-PAA) with M _w/M _n = 1.15 was synthesized by atom transfer radical polymerization of t-butyl acrylate (tBA), followed by acidolysis of the PtBA blocks. The pH-sensitive micellization of PAA-b-PEO-b-PAA in acidic solution was investigated by potentiometric titration, fluorescence spectrum, dynamic light scattering and zeta potential. The pK _a was 6.6 and 6.0 in deionized water and in 0.1 mol/L NaCl solution, respectively. The copolymer formed micelles composed of a weakly hydrophobic core of complexed PAA and PEO and a hydrophilic PEO shell in 1 mg/mL solution at pH < 5.5 due to hydrogen bonding. The critical micelle concentration was 0.168 mg/mL at pH 2.0. At pH < 4.5, steady and narrow distributed micelles were formed. Increasing pH to 5.0, unsteady and broad distributed micelles were observed. At pH > 5.5, the micelle was destroyed owing to the ionization of the PAA blocks.