Polyaniline dispersions 7. Dynamic light scattering study of particle formation

 When aniline is oxidized in the presence of colloidal silica, composite polyaniline–silica particles of submicrometer size are obtained. Dynamic light scattering was used to monitor the course of dispersion polymerization of aniline and the formation of particles. The increase of a hydrodynamic radius of particles was observed as polyaniline had been produced. Additional increase in particle size after polymerization has also been recorded. The rate of aniline polymerization was found to increase with increasing temperature in the range 0–50 °C. Well-defined particles are formed below 30 °C while above this temperature the colloidal stability of the resulting systems is limited. The activation energy of aniline polymerization was estimated. Received: 24 February 1997 Accepted: 7 May 1997     

Stress-induced compaction of concentrated dispersions of gel particles

 When submitted to successive shear stress steps, the elastic shear modulus of a concentrated dispersion of soft gel particles shows an exponential increase from 50 to 110 Pa. A slow relaxation time (τ _r ≃500 s) attributed to the mobility of the gel particles within their free volume is obtained. The amplitude of the relaxation time distribution decreases with the number of shear stress sequences, indicating a progressive decrease in the free volume available per particle. The results are explained by an increase in the packing density as grains rearrange under the external constraint. A rate constant is determined from the evolution of the dispersion's elastic modulus (K≃4 × 10⁻⁴ s⁻¹). The rate of compaction shows a logarithmic decrease when the initial particle's packing fraction is increased. Received: 5 October 1999 In revised form: 21 December 1999 Accepted: 21 January 2000     

Preparation of waterborne dispersions of epoxy resin by the phase-inversion emulsification technique. 1. Experimental study on the phase-inversion process

 Waterborne dispersions of bisphenol A epoxy resin were prepared by the so-called phase-inversion emulsification technique. The electrical properties, rheological behavior and morphological evolution during the phase inversion process were characterized systematically. It was shown that both emulsifier concentration and emulsification temperature play great roles in controlling the phase inversion process as well as the structural features of the waterborne particles. A high emulsifier concentration, i.e. 10.90 wt% and a low emulsification temperature, i.e. 73 °C, facilitate complete phase inversion, in which all water droplets in the system are simultaneously transformed into the continuous phase at the phase-inversion point (PIP). In this case, sub-micron-sized, discrete waterborne particles were formed. In contrast, a complex water-in-oil-in-water structure was achieved by incomplete phase inversion at a low emulsifier concentration, i.e. 2.33 wt%, and a high temperature of 80 °C. The morphological evolution observed by scanning electron microscopy revealed that not all the water droplets in the system were converted into the continuous phase at the PIP and that some small water drops were trapped within the waterborne structure. Received: 15 March 2000/Accepted: 16 May 2000     

Nanoparticles based on polyelectrolyte complexes: effect of structure and net charge on the sorption capability for solved organic molecules

 The sorption of solved organic molecules such as p-nitrophenol or dyes on previously formed nanoparticles based on polyelectrolyte/micelle complexes or polycation/polyanion complexes was studied. It could be shown that the sorption capability strongly depends on the structure and properties of the complex particles. Investigations have been made with complex particles that differ in their hydrophobic/hydrophilic structure, size and net charge. Such complex aggregates could be prepared by mixing the cationic surfactant dodecylamido-ethyldimethylbenzylammonium chloride, the polycations poly(diallyldimethylammonium chloride) or poly(methacryloyloxyethyldimethylbenzylammonium chloride) and the copolymers of maleic acid with propene or methylstyrene as anionic components. It is found that the sorption capability increases with increasing molar mass and hydrophobic properties of the components used. In addition, the concentration ratio c _polym/c _org.poll that was required to reach optimal sorption conditions could be decreased by the use of macromolecules with high molar masses. The best results were obtained by using cationic stabilized complex particles formed with high-molar-mass polycations as sorbents for anionic dye molecules. Received: 10 November 1999 Accepted: 24 February 2000     

Preparation of waterborne dispersions of epoxy resin by the phase-inversion emulsification technique. 2. Theoretical consideration of the phase-inversion process

 A theoretical consideration of the phase-inversion technique to prepare waterborne particles based on the experimental facts of the phase inversion process given in part 1 of this series is presented. The deformation and breakup of the water droplets dispersed in an epoxy resin phase under shear action are analyzed in terms of microrheology. The interaction and coalescence dynamics among the water droplets stabilized by an interfacial layer formed by the emulsifier molecules are discussed in terms of Derjaguin–Landau–Verwey–Overbeek theory and effective collision theory, respectively. A criterion for the completion of phase-inversion is that the attraction among the water droplets exceeds the entropic repulsion. Thus, a physical model of phase-inversion is proposed to predict the effects of some control variables on the phase-inversion process as well as the structural features of the waterborne particles, by which the experimental results could be well interpreted. It is indicated that the achievement of phase inversion is determined by the dynamic coalescence among the water droplets before the phase-inversion point (PIP). If the dynamic coalescence among the water droplets is ignored, phase inversion is achieved completely and sub- micron-sized particles are prepared. In comparison, if the dynamic coalescence is significant, phase inversion is achieved incompletely and a large complex water-in-oil-in-water structure is prepared. In the case of complete phase inversion, it is shown that the size of the waterborne particles is comparable with the size of the water droplets before the PIP. Received: 15 March 2000/Accepted: 16 May 2000     

Ultraviolet-curable epoxy acrylate dispersions: effect of urethane acrylate anionomers on stabilizing and film properties

 Epoxy acrylate dispersions stabilized using urethane acrylate anionomers were prepared for an application of ultraviolet (UV) curing. By observing the optical microscopy and colloidal stability for the epoxy acrylate dispersions, it was confirmed that the urethane acrylate anionomers incorporated have an interfacial activity in the interface between the epoxy acrylate oil and the water/ ethanol mixture (80/20, w/w). This was possible by the structurally designed urethane acrylate anionomers, containing a hydrophobic soft segment and two hydrophilic ionic sites in their molecules. In addition, when ultraviolet (UV)-cured, the urethane acrylate anionomers agglomerated to form the rubber domains in the epoxy acrylate film, which were induced by the ionic interaction. Consequently, this agglomerated rubber domains improved the final film properties. Received: 4 April 1998 Accepted: 1 July 1998     

Different types of water in the film formation process of latex dispersions as detected by solid-state nuclear magnetic resonance spectroscopy

 The properties of polymer films prepared from latex dispersions are influenced by the drying or film formation process. In order to investigate this process, various systems of aqueous latex dispersions were dried until a specific solid content was reached. The samples investigated were based on vinyl acetate, vinyl acetate/ethylene and pure acrylics employing different surfactants and polyelectrolytes as stabilisers of the dispersions. The role of water in these partially dried films was investigated using ¹H and ²H solid-state NMR spectroscopy. Different types of water could be distinguished in the spectra. The drying latex films were found to contain interfacial external water, water at ionic and nonionic groups at surfactants in the polymer/water interface and also water inside the swollen polymer. These different types of water were examined separately using various NMR techniques. Received: 22 October 1999/Accepted in revised form: 19 November 1999     

Band structure representations of the electronic structure of one-dimensional materials with helical symmetry

 The relationship between band structure and the topology of the orbital interactions between neighboring chemical units comprising several model one-dimensional polymers with helical (screw-axis) symmetry is analyzed. A perturbative model of orbital interactions based on a tight binding implementation of the extended Hückel method is developed. The model accounts for both the band topologies and the seemingly anomalous band extrema within the Brillouin zone constructed using the chemical repeat unit of the polymer. Received: 5 November 2001 / Accepted: 14 January 2002 / Published online: 3 May 2002     

Examples of Quantification in XPS on 5f Materials

 Various aspects of quantification in actinide research using photoelectron spectroscopy are discussed. In particular, the influence of instrumental factors, different types of the background, and shape of the line intensities are addressed. We give several examples of quantitative analysis of actinide bulk compounds and thin films.     

The effect of nanoparticle growth on rheological properties of silica and silicate dispersions

CdS and ZnS nanoparticles were prepared in the solid–liquid interfacial adsorption layer as a nanophase reactor. The substrates were hydrophilic and hydrophobic aerosils and hydrophilic layer silicates dispersed in ethanol–cyclohexane mixtures. The growth of particles at various surface concentration of precursor ions was monitored by absorption spectroscopy, band-gap-energy measurements and particle diameter measurements. Also, the rheological properties of nanoparticle–support composites in organic and aqueous dispersions were measured. The energy of separation between the nanoparticles depended on the particle diameter. The intercalation of nanoparticles in the layered silicates yielded a nanostructured two-phase system. The presence of semiconductive subcolloids was proven by transmission electron microscopy measurements, which offer an excellent possibility for the determination of the particle size distribution. Received: 20 July 1999/Accepted in revised form: 22 September 1999     

Rotational diffusion of tungstic acid colloids in microgravity as studied by aircraft experiments

 Rotational relaxation times (τ) of anisotropic tungstic acid colloids (3.24 μm in major axis) in aqueous suspension are measured in microgravity (0G), normal gravity (1G) and at 2G. The measurements at 0G and 2G are achieved by parabolic and circular flights, respectively. The limiting slopes of the relaxation curves in the plots of the transmitted light intensity against time are close to zero at 0G irrespective of the flow directions in the flow cell, whereas those at 1G and especially at 2G depend on the flow direction by the convection of the suspension and particle sedimentation. Experimental errors at the τ values at 0G are small compared with those at 1G and 2G, which is ascribed to the lack of movement of impurities in the suspension such as quite small air bubbles, which cannot be recognized with the naked eye, and the convection of the suspension in microgravity. More reliable rotational relaxation times are obtained in microgravity; however, the relaxation times themselves are quite insensitive to gravity. Theτ values observed are larger than those calculated from the particle size, which indicates the important contribution of the electrical double layers formed around the colloidal particles. Received: 22 February 2001 Accepted: 13 June 2001     

Effects of polyols and sugars on the structure of water in concentrated gelatin gels as studied by low temperature differential scanning calorimetry

 Phase-transition temperatures, glass-transition temperatures, melting temperatures, gel–sol transition temperatures in differential scanning calorimetry heating curves of gelatin solutions with and without various sugars and polyols quenched by liquid nitrogen were studied. Both sugars and polyols added to concentrated gelatin solutions shifted the glass-transition temperatures to lower temperatures, and it was attributed to the increase of unfreezable water which acts as a plasticizer. The mechanisms of the increase in unfreezable water, however, seem to be different for sugars and polyols; sugars increase unfreezable water by increasing the number of junction zones which hold unfreezable water, while polyols by themselves increase the amount of unfreezable water. Received: 9 January 1997 Accepted: 1 July 1997     

Small angle neutron scattering studies on nonaqueous dispersions of calcium carbonate Part 2. Determination of the form factor for concentric spheres

Colloidal dispersions of calcium carbonate, stabilised primarily by a surface active agent, in both toluene and dodecane have been examined by small angle neutron scattering. A model has been developed to simulate the scattering behaviour of the particles and is based on the idea of a concentric sphere with a homogeneous layer of adsorbed material surrounding a core particle. Computations based on the model show a wide variation of scattering behaviour with variation of the coherent scattering length of the dispersion medium. These predictions were confirmed by experiment. A method is described for analysis of the experimental data which leads to a determination of the thickness of the adsorbed layer, the radius of the core particle and the standard deviation of core particle radius.     

On the application of the modeling of the relaxation spectrum to the prediction of linear viscoelastic properties of surfactant systems

 In this work, the linear viscoelastic properties of cetyl-trimethylammonium tosilate–water system are predicted by the modeling of the relaxation spectrum. The modeled spectrum of relaxation times is of the “wedge-box” type where the “wedge” portion is located at the short-time scale of relaxation times and the “box” part covers the long-time scale. The linear viscoelastic properties are calculated through the exact relationships with the suggested spectrum. Agreement between the calculated expressions and experimental data of the moduli and the stress relaxation function is found. Comparison is also made with predictions of the empirical expressions of the Cole–Cole and the Williams–Watts models. Received: 17 December 1996 Accepted: 1 July 1997     

Microgravity experiments on colloidal crystallization of silica spheres in the presence of sodium chloride

 Rate coefficients (k) in the colloidal crystallization of monodispersed silica spheres in the presence of sodium chloride are studied in microgravity achieved by parabolic flights of an aircraft. Time-resolved reflection spectroscopy is made with a continuous circulating-type stopped-flow cell system. The k values decrease as the salt concentration increases both at 0 and 1 G and those in microgravity are smaller than those in normal gravity by 16% (maximum), especially in water and in the presence of a small amount of the salt lower than 2 × 10⁻⁶ mol/l. The rates in flight at 1 G are larger by 15% (maximum) compared with those at 1 G on the ground. The k values obtained at 0 G, 1 G in flight and 1 G on the ground agree excellently with each other for the suspensions with 3 × 10⁻⁶ and 4 × 10⁻⁶ mol/l sodium chloride. Disappearance of the downward diffusion of spheres and no convection of the suspensions are important for retardation in microgravity. Received: 20 January 2000 Accepted: 9 March 2000     

Particle adsorption in evaporating droplets of polymer latex dispersions on hydrophilic and hydrophobic surfaces

Stain patterns formed by drying up of droplets of polymer latex dispersion on hydrophilic and hydrophobic surfaces were examined in light of the mechanism of particle adsorption in evaporating droplets. On hydrophilic surfaces, the volume of droplets decreased with time, keeping the initial outline of contact area, and circular stain patterns were formed after the dry-up of droplets. By the microscopic observation of particles in the droplets, it was found that a large portion of the particles were forced to adsorb on the outline of the contact area where a microscopic thin water layer was formed because of hydrophilicity of the surface. On hydrophobic surfaces, on the other hand, the contact area of droplets decreased as evaporation proceeded, while no particle was adsorbed on the surface at the early stages. The particles in the droplets started to aggregate when the concentration of particles reached a critical value, and the aggregates adsorbed on the surface forming tiny spots after the dry-up. Time evolutions of contact angle, contact area and volume of the droplets were analyzed in light of differences in the adsorption mechanisms between hydrophilic and hydrophobic surfaces. Received: 14 January 1998 Accepted: 1 May 1998     

Study of the dissolution process of polystyrene in concentrated cyclohexane solution by MNR relaxation

 ¹³C-NMR relaxation times of polystyrene (PS) chains in its theta solvent, cyclohexane, have been measured at different temperatures. It was found that relaxation of carbon nuclei of the side-chain-phenyl groups and those of main chains have remarkably different temperature-dependent relaxation behaviors in the solvent. A two-step model for the dissolution process is proposed. According to the model, swelling of the polymer below θ temperature corresponds mainly to the gradual dispersion of the side-chain phenyl groups; while the complete dissolution above θ temperature corresponds mainly to the gradual dispersion of the main chains at a molecular level. These dispersions reflect the fact that cohesional interaction among side-chain-phenyl rings or main chains are weakened by solvent molecules, which shows the existence of the cohesional entanglements among polymer chains. The results of T ₁(C) are confirmed by the biexponential dependence of ¹H-NMR spin–spin relaxation on temperature. Received: 2 July 1997 Accepted: 21 October 1997     

Flow behaviour of titanium dioxide dispersions in the presence of 2-hydroxyethyl cellulose

 The effects of 2-hydroxyethyl cellulose (HEC) of weight-average molecular weight 15,000, 90,000 and 7,20,000 on the rheological properties of TiO₂ dispersions were evaluated. For all three HECs, the maximum yield stress, τ_ymax, (occurred at zero zeta potential), decreased with increasing HEC concentration. Interestingly, the largest reduction in τ_ymax was observed with the HEC with the highest molecular weight. This reduction was attributed to steric interaction arising from adsorbed HEC. Adsorbed high-molecular-weight HEC formed an effectively thicker steric barrier because of its larger size and higher adsorption capacity. Bridging interactions that were expected to be important for the high-molecular-weight HEC were found to be unimportant here. In the flocculated regime, HEC enhanced the shear-thinning characteristics of the TiO₂ dispersions. Received: 8 November 1999 Accepted: 20 December 1999     

Time-resolved fluorescence anisotropy measurements of the adsorption of Rhodamine-B and a labelled polyelectrolyte onto colloidal silica

 The application of time-resolved fluorescence anisotropy measurements (TRAMS) to the investigation of the adsorption of the dye Rhodamine B and a Rhodamine B-labelled cationic polyelectrolyte onto colloidal silica (Ludox) is described. For Rhodamine B the time-resolved fluorescence anisotropy behavior observed can be interpreted using a model consisting of fluorophores with two distinct fluorescence decay lifetimes and two rotational correlation times corresponding to the fluorophore free in solution and bound to the Ludox. Details of the binding obtained from a global analysis of the data are reported. Restricted motion of the fluorescently labelled polyelectrolyte is also observ-ed on adsorption. The considerations for the general application of TRAMS for monitoring adsorption behavior are discussed. Received: 8 July 1998 Accepted: 10 August 1998