Wyn Brown

Self-aggregation and phase behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers in aqueous solution

The phase behavior and aggregation properties of block copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronics, poloxamers) in aqueous solution have recently attracted much attention. Both experimental and theoretical studies are reviewed, not comprehensively, but with the focus on studies, partly cooperative, partly independent, performed by groups in Uppsala (light scattering and fluorescence), Roskilde (rheology and calorimetry), Risø (SANS), Graz (x-ray and speed of sound), and Lund (theoretical model calculations).The phase behavior of these copolymers is similar in many respects to that of conventional nonionic surfactants, with the appearance of hexagonal, cubic, and lamellar liquid crystalline phases at high concentrations. In the isotropic solution phase the critical concentration for micelle formation is strongly temperature dependent, and at a given concentration the monomer to micelle transition occurs gradually over a broad temperature range, partly due to the broad size polydispersity of both the PO- and EO-blocks. For some Pluronic copolymers a transition from globular to long rod-like micelles occurs above a transition temperature, resulting in a strong and sudden increase of viscosity and viscoelasticity of the solution.Size and aggregation numbers have been determined for the globular micelles in some cases, and also the rod-like micelles have been characterized. NMR and fluorescence measurements have provided further information on the properties of the micellar core and mantle. In combination, results from different measurements on the same Pluronics material indicate that the aggregation number of the micelles increases with the temperature, whereas the hydrodynmic radius varies much less. The PEO-mantle of the micelles seems to contract with increasing temperature. The core appears to contain appreciable amounts of PEO in addition to PPO (and also some water). The segregation between core and mantle is not as distinct as in normal micelles, a conclusion which is in line with the predictions from the model calculations.

Static and dynamic properties of a (PEOPPOPEO) block copolymer in aqueous solution

Abstract Aqueous solutions of a triblock copolymer of the type polyethyleneoxide-block-polypropyleneoxide-block-polyoxyleneoxide (Pluronic L-64, average composition 26 EO-units and 30 PO-units), were examined with static and dynamic light scattering (DLS), pulsed-gradient spin-echo (PGSE) NMR and fluorescence spectroscopy over a range of concentrations (0.2–25 wt%) and temperatures (15–60°C). Relaxation time distributions from DLS show L-64 to be molecularly dissolved at 21°C, and to form micelles at higher temperatures, which remain at high concentrations (25%) without formation of gel or liquid crystalline phases. The temperature where micelle formation starts is strongly concentration dependent, in contrast to the cloudpoint with remains fairly constant at 60°C. The hydrodynamic radii of the micelles, as obtained from DLS and PGSE NMR, are in reasonable agreement (60–80 A), and also agree with an aggregation number reported from fluorescence-quenching studies, whereas static light scattering, evaluated according to normal practice, indicates much smaller aggregates. This is due to the presence in the solution of a mixture of monomers, micelles, and at low temperatures, also some strongly scattering larger aggregates, possibly emanating from a small percentage of a diblock impurity in the preparation. The diblock impurity aggregates are dissolved by the proper micelles at higher temperatures. Their presence is indicated also by the anomalous excimer formation at low temperatures, caused by pyrene becoming concentrated in the premicellar aggregates.

Static and dynamic behavior of semidilute polymer solutions

Reported results of dynamic light scattering experiments on semidilute solutions of linear non-charged polymers are reviewed. It is shown that, while for many systems the nature of the so-called slow modes may have a wide range of different causes, for a linear polymer in poor and moderately good solvents, the slow modes are mainly due to the effect of topological constraints. Values of static and dynamic correlation length reported by different research groups are compared with each other and with theory

Scattering in polymeric and colloidal systems

1. Multiple Scattering of Light: Coherent Backscattering and Transmission 2. Thermal-Diffusion Forced Rayleigh Scattering 3. Light Scattering From Lonomer Solutions 4. Light Scattering from Ternary Polymer Solutions 5. Dynamic Light Scattering From Semiflexible Polymers 6. Microemulsions Studied by Scattering Techniques 7. Critical Behaviour in Polymer Blend Solutions 8. Thermal Composition Fluctuations in Polymer Blends Studied with Small Angle Neutron Scattering 9. Time-Resolved SAXS?WAXS Experiments 10. Polymer Surfaces, Interfaces, and Thin Films Studied by X-Ray and Neutron Reflectometry 11. Polymer Dynamics by Neutron Spin Echo Spectrscopy

Diffusion in polyacrylamide gels

Abstract Using a novel sorption technique, the diffusion of some series of solutes in polyacrylamide gels has been investigated with regard to: (a) molecular size of solute; (b) concentration of solute and gel polymer; and (c) temperature. The approach used also yields the partition coefficient pertaining to sorption equilibrium. The ratio, D D o , where Do refers to diffusion in the pure solvent, is found to reflect in part the characteristic interactions between solute and gel polymer. The temperature results indicate that the apparent activation energy for solute diffusion is approximately independent of the polymeric component for dilute gels.

A light scattering investigation of the sodium dodecyl sulfate–lysozyme system

Dynamic and static light scattering measurements have been made on lysozyme solutions at 25 °C in 0.1 M phosphate buffer, pH 6.88, in the presence of sodium dodecyl sulfate (SDS). The concentration of SDS has been varied over a wide range (0–0.2 g/ml). It is found that under these conditions both SDS micelles and lysozyme(Lys)–SDS complexes coexist. Addition of lysozyme at constant SDS concentration leads to formation of more lysozyme–SDS complex. The translational diffusion coefficient of the complex increases with SDS concentration. It is also found that two lysozyme molecules are associated in the protein–surfactant complex.

Effect of Chain Rigidity and Effective Conjugation Length on the Structural and Photophysical Properties of Pyridine-Based Luminescent Polymers

Effect of Chain Rigidity and Effective Conjugation Length on the Structural and Photophysical Properties of Pyridine-Based Luminescent Polymers

SOLVENT AND POLYMER DYNAMICS IN CONCENTRATED POLYSTYRENE TOLUENE SOLUTIONS

Depolarized Rayleigh scattering (DRS) and dielectric spectroscopy (DS) are employed to study the solvent and polymer dynamics in the polystyrene (PS)/toluene system and for PS concentration up to 0.6 g/ml. The DRS measurements were made in the temperature range from 255 to 373 K with different interferometers making possible the separation of the ‘‘fast’’ solvent (in the ps time scale) from the ‘‘slow’’ polymer dynamics (in the ns time scale). The DS measurements were made in the temperature range from 128 to 310 K and two processes could also be observed. With DRS and DS we investigate the very different mobilities in the PS/toluene solutions over a frequency range of ∼10 decades. The prominent features of the experimental data are (i) the existence of two primary (α−) relaxations in the macroscopically homogeneous solutions which have been analyzed using both techniques and (ii) that these relaxations reflect the polymer and solvent dynamics and display the usual Vogel–Fulcher–Tammann behavior, however,...

Self-diffusion of poly(ethylene oxide) in aqueous dextran solutions measured using FT-pulsed field gradient n.m.r.

Transport in ternary polymer1, polymer2, solvent systems has been investigated using an n.m.r. spin-echo technique. The dependence of the self-diffusion coefficient of poly(ethylene oxide) polymers on the concentration and molecular size of dextran in aqueous solution has been measured. Monodisperse poly(ethylene oxide) fractions (Mw=7.3×104, 2.8·105 and 1.2·106) and dextrans (Mw=2·104, 1·105 and 5·105) have been employed over a range of concentration up to the miscibility limit in each system. It is found that when the molecular size of the diffusant is commensurate with or exceeds that of the matrix polymer, a relationship of the form: (DD0)PEO=exp−k(C[η]) is applicable, where C[η] refers to the dextran component and is considered to describe the extent of coil overlap in concentrated solution. (DD0) is independent of the molecular size of the poly(ethylene oxide), at least in the range studied (Mw<300 000).

Dynamic properties of polyacrylamide gels and solutions

Decay time distributions have been obtained by Laplace inversion of the autocorrelation functions in dynamic light scattering for polyacrylamide (PAA) solutions and gels prepared in an identical manner except for the addition of crosslinking agent. The measurements were made as a function of PAA concentration, crosslinking concentration and temperature. The purpose was to establish the similarities and differences between the gel and the analogous solution and also to examine the connection between these variables and the dynamic behaviour of the gel. While the solutions have a comparatively complicated dynamic structure, the gels are precisely single-exponential at PAA concentrations above about 2%. Over the same concentration range, the solutions have a slowly relaxing component of narrow distribution in addition to the network mode formed through interchain entanglements. The slow mode, which has a strong negative concentration dependence, finally disappears atCPAA ≈ 10%. The dynamic polymer-polymer correlation length (ξH) for the permanent gel, which is considerably larger than that for the corresponding solution, is weakly dependent on concentration (ξH ∼ C−0.22 for both gels and semidilute solutions) and is independent of the temperature.ξH increases with increasing concentration of bisacrylamideCbis, reaching a plateau in the vicinity ofCbis = 5%. The static screening length takes the same value in solutions and gels and is dependent on concentration:ξs ∼ C−0.65. The two correlation lengths become approximately equal at high crosslinking density. The longitudinal elastic modulus (Mos), evaluated from static light-scattering data, shows the theoretically expected dependence on PAA concentration.