Rheological and structural properties of aqueous solutions of a hydrophobically modified polyelectrolyte and its unmodified analogue

Rheological methods and small angle neutron scattering (SANS) were used in the characterization of dilute and semidilute aqueous solutions, both with and without added salt, of anionic poly(vinyl alcohol) (PVA⁻) and its hydrophobically modified analogue (HM-PVA⁻). The rheological measurements showed that the concentration induced viscosification effect and elastic responses are considerably stronger for solutions of HM-PVA⁻ than in solutions of the unmodified polyelectrolyte. Over the considered polymer concentration domain, the solutions of PVA⁻ exhibit virtually Newtonian behavior, whereas strong shear thinning effects are observed in the HM-PVA⁻ solutions. The SANS results for HM-PVA⁻ solutions reveal a pronounced peak in the plot of scattering intensity versus scattering wavevector q at intermediate q values and the position of the maximum scales with polymer concentration as q_max∼c^0.28±0.02. This peak is suppressed in solutions of the unmodified polyelectrolyte and merely a shoulder in the scattering curve appears. Additionally, an “upturn” in the scattered intensity is observed at small q values and the magnitude of this effect depends on polymer concentration, hydrophobicity and salt addition. At large q values, the SANS results from HM-PVA⁻ solutions suggest morphological changes, from rod-like chains to a network of semiflexible chains, as the polyelectrolyte concentration increases.     

Evidence for hydrophobic attraction between latex particles in aqueous systems as investigated by turbidimetry

We report the evidence for attractive interaction of latex particles which are covered by poly(ethylene oxide) chains. These particles are suspended in aqueous solutions of ammonium sulfate. The interaction is probed by measurements of the turbidity of the suspensions up to 70 g/l. Turbidity is insensitive to multiple scattering and allows the static structure factor, S(q) [q=(4πn ₀/λ₀)sin(θ/2), where θ is the scattering angle, n₀ is the refractive index of the medium and λ₀ is the wavelength in vacuo], to be determined at small q values. The analysis of S(q) at small q values yields information about possible attraction of the particles. The analysis of the turbidity data furthermore shows that no aggregation took place in these systems. A weak but long-range attractive interaction was found at ammonium sulfate concentrations of 0.01 and 0.1 M. The relation of this attractive force to hydrophobic forces is discussed. Received: 9 March 2000/Accepted: 28 June 2000     

Moderately concentrated solutions of polystyrene. II. Integrated-intensity light scattering as a function of concentration,temperature, and molecular weight

Integrated-intensity light scattering data are reported for moderately concentrated solutions of polystyrene in benzene and in cyclopentane. The benzene system is one for which the second virial coefficient A₂ is large; data obtained over the range 0.5 < A₂Mc < 30, with c the polymer concentration, are analyzed in terms of the (extrapolated) intensity at zero angle and the angular dependence of the intensity. The former is discussed in terms of power law representations based on scaling relations, which are found to represent the data. The latter is discussed in terms of the dependence of the chain dimensions on concentration. With cyclopentane, the behavior is similar for temperatures for which A₂ is near its maximum, but for T near either Θ_U or Θ_L, for which A₂ is zero or small, the angular dependence of the scattering is distinctly different, with the intensity exhibiting a maximum as a function of scattering angle.     

Study on nonlinear phase‐separation for PMMA/α‐MSAN blends by dynamic rheological and small angle light scattering measurements

The phase‐separation behavior of poly(methyl methacrylate)/poly(α‐methyl styrene‐co‐acrylonitrile) (PMMA/α‐MSAN) blends upon heating was studied through dynamic rheological measurements and time‐resolved small angle light scattering, as a function of temperatures and heating rates. The spinodal temperatures could be obtained by an examination of the anomalous critical viscoelastic properties in the vicinity of phase‐separation induced by the enhanced concentration fluctuation on the basis of the mean field theory. It is found that the dependence of the critical temperatures determined by dynamic rheological measurements and small angle light scattering on heating rates both deviates obviously from the linearity, even at the very low heating rates. Furthermore, the cloud‐point curves decrease gradually with the decrease of heating rates and present the trend of approaching T_gs of the blends. The nonlinear dependence is in consistence with that extracted from the isothermal phase‐separation behavior as reported in our previous paper. It is suggested that the equilibrium phase‐separation temperature could be hardly established by the linear extrapolating to zero in the plotting of cloud points versus heating rates. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1547–1555, 2006     

Light,Small Angle Neutron and X-Ray Scattering from Gels

This paper gives two examples of experiments that demonstrate the power of small angle scattering techniques in the study of swollen polymer networks. First, it is shown how the partly ergodic character of these systems is directly detected by neutron spin echo experiments. The observed total field correlation function of the intensity scattered from a neutral gel allows the ergodic contribution to be directly distinguished from the non ergodic part, at values of transfer wave vector q that lie well beyond the range of dynamic light scattering. The results can be compared with those obtained at much lower q from visible light scattering. Second, a recent application of small angle X-ray (SAXS) and neutron (SANS) scattering is described for a polyelectrolyte molecule, DNA, in semi-dilute solutions under near-physiological conditions. For these observations, the divalent ion normally present, calcium, is replaced by an equivalent ion, strontium. The comparison between SANS and SAXS yields a quantitative picture of the cloud of divalent counter-ions around the central DNA core. At physiological conditions, the cloud is thinner than that predicted on the basis of the Debye screening length but thicker than if the counter-ions were condensed on the DNA chain.     

Collective dynamics of terminally anchored polymer chains

Thermal fluctuations of the segment density profile of a polymer brush were probed by dynamic light scattering in the evanescent wave configuration; the time correlation functions of concentration fluctuations with wavevector q were measured. It is found that there is a preferred wavelength of the order of the brush thickness, L₀, of these fluctuations with a concurrent slowing down of their thermal decay rate. A theory is presented for the small-amplitude deformation of the free surface of a parabolic brush in solution; a maximum in the respective structure factor of the concentration fluctuations is predicted at q* L₀ ∼ O(1), in agreement with the experiment.     

Structure of gum arabic in aqueous solution

Gum arabic, a natural polysaccharide derived from exudates of Acacia senegal and Acacia seyal trees, is a commonly used food hydrocolloid. The complex chemical structure of the gum has been widely studied revealing a multifraction material consisting mainly of a highly branched polysaccharide and a protein–polysaccharide complex (GAGP) as a minor component. This work investigates its mesoscopic structure in aqueous solution by small‐angle X‐ray and neutron scattering combined with cryotransmission electrons microscopy. Scattering measurements reveal an intricate shape composed of many spheroidal aggregates assigned to the polysaccharide with a small amount of larger coils. A scattering peak is observed at moderate to high concentrations, the spacing of which exhibits a c^?1/3 power law relation to polymer concentration (c). Upon addition of salt, this peak disappears, indicating its electrostatic nature. The large coils contribute a q^?2 power law at the low scattering vector (q) range. However, at low concentration in which the interaggregate peak is not observed, a q^?1 power law at the low q range indicates the possible existence of a fraction with a locally extended conformation. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3265–3271, 2006     

Coupling of concentration fluctuations to viscoelasticity in highly concentrated polymer solutions

Photon correlation spectroscopy in the polarized geometry has been used to systematically investigate the complex dynamics of a highly concentrated entangled polymer solution in a nominally good solvent, poly(butylacrylate) in dioxane. In addition to the well known fast cooperative diffusion process, a slow virtuallyq-independent mode is detected, whereq is the scattering wavevector, in agreement with previous experimental works on semidilute solutions. This mode is attributed to the viscoelastic nature of the transient physical network, formed by the entanglements, which relaxes its elastic stress induced by the concentration fluctuations, as confirmed by small amplitude oscillatory shear measurements; the latter reveal a terminal relaxation time comparable to the characteristic time of the slow relaxation process. Results, especially in terms of concentration and temperature dependence, are evaluated and discussed in view of the existing theoretical treatments in the field, predicting the existence of the slow viscoelastic relaxation. The relationship between dynamic light scattering and mechanical spectroscopy is established.     

UNMIXING KINETICS AND ITS MORPHOLOGY OF POLY ( ETHYLENE OXIDE) WITH POLYETHERSULPHONES BLENDS

Unmixing kinetics in a binary polymer mixture of polyethersulphones with poly (ethylene oxide) by spinodal decomposition has been investigated with time-resolved light scattering and microscope methods. The results showed that time evolution of scattered light intensity is of an exponential growth The maximum growth rate R(qm) of phase separation has been obtained. The experimental data did not satisfy the condition that the plot of R(q)/q~2 vs q~2 should be linear For unmixing system annealing at 30℃for three hours, its morphology manifested dish structure The experimental data of the Bragg spacing D_m can be correlated with a straight line which expresses the power-law relation, D_m=bl~α     

Aggregation phenomena

We report on low-angle static light scattering measurements on colloidal aggregation of polystyrene latex particles. Measurements have been performed both at low and high monomer concentrations. Pure diffusion limited cluster aggregation (DLCA) have been investigated in the dilute regime. The predicted power-law growth of the radius has been observed. Spurious effects due to the presence of gravity are reported.Measurements on the high concentration samples show an unexpected and quite novel phenomenon. A peak in the scattering pattern whose position q_m moves towards smaller q-values and whose height grows in time has been observed. We have found that the scattering distributions can be scaled onto the same master curve with a scaling law strongly reminiscent of that working in the completely different case of spinodal decomposition.     

Small-angle light scattering by random assemblies of incomplete spherulites. I. Sheaflike textures

The calculation of the scattering from a sheaflike sector of a two-dimensional spherulite has been carried out as a function of the apex angle of the sector. It is found that while for a complete spherulite the H_v scattered intensity is zero at zero scattering angle, there is an increasing intensity of scattering at 0° as the sector angle narrows. For very small values of the sector angle, the scattering becomes similar to that of a rod, with the exception that a scattering maximum is still seen at an angle close to that at which the spherulite scattering maximum occurs. The predictions of the model compare favorably with the scattering patterns observed for polymers in early stages of spherulitic growth.     

Structural Characterization of Zirconia Nanoparticles Prepared by Microwave-Hydrothermal Synthesis

Nanocrystalline zirconia powders have been prepared by microwave-hydrothermal synthesis starting from aqueous solution of ZrOCl₂·8H₂O. Results of investigations on the aqueous suspension stability of the washed zirconia nanopowders by dynamic light scattering showed that the suspension, constituted by superaggregates of nanoparticles (131 ± 10 nm), was stable up to 15 days. Nanopowders were investigated by means of transmission electron microscopy and small angle x-ray scattering measurements which proved that the zirconia nanopowder is constituted by small primary nanoparticles of ca. 8 nm that agglomerate forming bigger aggregates of 50 ± 1 nm.     

Diffusing probe measurements in Newtonian and elastic solutions

The diffusion coefficients of polystyrene latex spheres and hematite particles in both Newtonian and elastic liquids have been measured using dynamic light scattering. The diffusion coefficients of the latex particles measured in glycerol/water (Newtonian) solutions obey Stokes–Einstein behaviour over a range of solvent viscosities and temperatures. Two apparent diffusion coefficients for the particles are measured in visco-elastic polyacrylamide and polyacrylate solutions and are designated D_fast and D_slow. The apparent fast diffusion coefficients measured in the elastic solutions show an increase to a maximum, above that measured in the solvent water, with increasing polyelectrolyte concentration. At higher polyelectrolyte concentrations the observed D_fast values decrease below the value obtained in the solvent water. D_fast increases with the scattering vector squared (q²) while D_slow, is independent of q².     

Water vapour adsorption and contrast-modified SAXS in microporous polymer-based carbons of different surface chemistry

The adsorption of water vapour on highly microporous activated carbons with different surface chemistry is investigated by small angle X-ray scattering (SAXS) as well as by adsorption isotherms. The water changes the intensity of the SAXS in a way that depends on how the pores are filled. With wetting liquids such as hexane, a pseudo binary model can be assumed in which pore-filling in reciprocal space q is described by a density function p(q). For water, clusters develop, even in the most oxidized carbon, creating a fully ternary system. In the Porod scattering region, however, the final slope is insensitive to the liquid-vapour interfaces. In this region, for the less oxidized samples, p(q) shows reasonable agreement with the adsorption isotherms. At low relative pressure P/P ₀, however, the SAXS results indicate a small degree of filling (about 10%) that is not reflected in the isotherms. The highly oxidized sample attains a degree of filling of about 70% that, unlike the corresponding isotherm, is constant for P/P ₀>0. These differences may be due to kinetic effects and/or ageing, involving either redistribution of the water molecules or modifications of the surface groups.     

Small-angle scattering of polarized light. IV. Isotropic to birefringent transition for spheres

The Rayleigh-Gans-Debye approximation is used to investigate the small-angle, cross-polarized light scattering by a sphere of radius a, birefringence Δμ and relative index μ. If θ_max is the polar scattering angle of the intensity maxima, the quantity Û_max = 4πa/λ × sin (θ_max/2) behaves in two different ways according to the signs of Δμ and μ ? 1. When Δμ > 0, μ > 1 or Δμ < 0, μ < 1, then Û_max varies from 2.8 to 4.1 as Δμ increases from zero. If Δμ < 0, μ > 1 or Δμ > 0, μ < 1, then Û_max goes from 2.8 to about 6, thereafter decreasing to 4.1. Another interesting result is that the value of Û_max for a highly briefringent sphere is 4.1 only for large diameters. It decreases to 4.0 when the diameter decreases.     

Surface activity—thermodynamic properties and light scattering studies for some novel aliphatic polyester surfactants

The preparation of 12 new polyester surfactants based on aliphatic amines and different ethylene oxide content is described. These surfactants were characterized by determining their molecular weights and polydispersity by gel permeation chromatography (GPC) and nitrogen content. Drop volume tensiometry (DVT) was used to measure the surface tension at 25, 35, 45 and 55°C. The surface tension isotherms were used to determine critical micelle concentration (CMC), maximum Gibb's adsorption (Γ_max), minimum area per molecule (A_min), the effectiveness of surface tension reduction (π_cmc) and the efficiency (pC₂₀). The thermodynamic parameters of micellization (ΔG_mic, ΔH_mic, ΔS_mic) and of adsorption (ΔG_ad, ΔH_ad, ΔS_ad) were calculated and the data showed that these surfactants favor micellization to adsorption. The static scattered light intensity measurements provide the calculation of the molecular weight of micelle and the aggregation number (N°), while the dynamic light scattering provide the hydrodynamic radius of micelle (R_H) and the diffusion coefficient at different surfactant concentrations. The hydrodynamic radius of micelle (R_H) at different surfactant concentrations could be used also to determine the CMC giving results that are comparable to those obtained by surface tension measurements. All the data are discussed regarding the chemical structure of the polymeric surfactants. Copyright © 2004 John Wiley & Sons, Ltd.     

Light scattering from epoxy-elastomer resins: A study of statistical methods in heterogeneity analysis

The solid-state light scattering from ABE (acrylonitrile butadiene epoxy) copolymers was studied as a function of rubber content by measuring the intensity distribution of scattered light under V_v polarization conditions. The range of heterogeneity has been quantified by the Debye–Bueche statistical treatment and by comparison with simulated scattering envelopes. The data indicate that below a critical rubber concentration of 20% the rubbery phase precipitates as spherical domains of nearly constant size. Phase inversion occurs at a rubber concentration of 20% and is accompanied by a maximum in polydispersity of the dispersed phase. Beyond this concentration, epoxidic domains are imbedded in a rubber and epoxy matrix. As a result of comparison of Debye–Bueche parameters with simulated values of these, it is concluded that the correlation distance is very sensitive to the presence of a small number of large particles.     

Supramolecular structure investigation of poly(γ-benzyl, α-l-glutamate)-benzyl alcohol system by static light scattering

The cholesteric mesophase of the PBLG-BA system in concentrated solutions and in the gel phase has been investigated by static laser light scattering. The observed ‘distortion’ of the patterns in terms of their dependence on the azimuthal angle was studied. Effect of form-optical rotation on the pattern is discussed. The manner of distortion in the H_v pattern determines the sense of cholesteric twisting. The observed scattering patterns are analogous to those obtained by previous investigators in other solvents. Quantitative measurements of the intensity as a function of scattering vector provide information on the sense and pitch of the cholesteric twisting, as a function of temperature and concentration. Optical microscopy studies reveal onset and build-up of the cholesteric structure. Optical rotation and differential scanning calorimetry (DSC) studies support the conclusions obtained from light-scattering experiments.     

Phase behaviour of poly(vinyl methyl ether)-cross-polystyrene semi-interpenetrating networks

Semi-interpenetrating polymer networks of varying composition are prepared by crosslinking polystyrene containing a small number of maleic anhydride groups (4.8 mol% of MA units) with hexamethylene-diamine (HMDA) in the presence of linear poly(vinyl methyl ether) (PVME). Lightly crosslinked samples are homogeneous at room temperature and show a phase behaviour similar to uncrosslinked blends, i.e. lower critical solution temperature (LCST) behaviour. The influence of crosslinking on the phase behaviour has been studied by small angle light scattering (SALS) and turbidity measurements. The cloud point strongly depends on the heating rate. The presence of the network reduces the stable single phase region in agreement to theory. In systems showing spinodal decomposition, it is expected that some concentration fluctuations will grow more rapidly than others resulting in a separated phase system which shows high degree of connectivity with characteristic dimensions. Using temperature jump experiments, SALS can be used to estimate parameters of the phase separation kinetics and the characteristic dimensions of the phases. In temperature jump experiments into the spinodal region a maximum in the scattered light intensity is observed with time at a certain scattering vector. However, the semi-IPN's develop no scattering maximum. This is explained by a damping of the thermodynamical dominant wavelength in spinodal decomposition in the network.