Analysis of dielectric observations of KCl-charged poly(methyl methacrylate) microcapsules using a two-component model consisting of KCl-permeable and KCl-impermeable capsules

Dielectric measurements were carried out on aqueous suspensions of poly(methyl methacrylate) microcapsules charged with KCl solutions to examine the KCl permeability of the microcapsules. The specimens exhibited three kinds of dielectric relaxation. Two kinds of dielectric relaxation were observed immediately after washing the specimens with distilled water. These overlapped each other when the specimen was left standing in the measuring cell for a long time. Their relaxation frequencies were affected by the KCl concentration of the suspending medium in which the specimen was kept before washing. Another relaxation was observed distinctly when the spepcimens were washed thoroughly with distilled water. Its relaxation frequency was affected by the KCl concentration of the charged solution. These results were explained by assuming that the specimens were mixtures of KCl-permeable and KCl-impermeable capsules. The theoretical analysis procedure was devised in the light of dielectric theory of interfacial polarization to estimate the parameters which characterize the structure of the specimens.     

Theoretical study of dielectric behavior of microcapsule suspensions with distributed drug release rate

Theoretical consideration is made concerning the dielectric behavior of aqueous suspensions of W/O-type microcapsules containing aqueous electrolyte solutions, especially concerning distribution effects of the electrolyte release rate on the dielectric behavior.It is shown at first that the distribution of the release rate causes electrical conductivity of the capsule interior. Next, it is deduced, in the light of a theory of the interfacial polarization, that the distribution characteristics of the capsule interior's electrical conductivity are closely related to those of the relaxation time for a dielectric relaxation exhibited by the suspensions. Several formulas are derived to describe the relation between the distribution characteristics of the release rate and those of the relaxation time.     

Dielectric approach to polystyrene microcapsule analysis and the application to the capsule permeability to potassium chloride

A method of numerical analysis is proposed to determine the phase parameters from dielectric observations of polystyrene microcapsules by using the conductivity at intermediate frequencies between two dielectric relaxations observed. The method was more feasible for the analysis of the microcapsule observations than the method by which the calculation is performed by use of the permittivity at lower frequencies. The method was applied to the dielectric observations of the KCl-loaded microcapsules to obtain the permittivity and the conductivity of the capsule interior, the thickness of the capsule wall, the volume fraction of the suspended capsules and the conductivity of the surrounding aqueous phase. The conductivity of capsule interior was found to be unchanged with time for as long as 3 months. From the results it is concluded that the microcapsules are impermeable to KCl.     

Coagulation kinetics of amorphous colloidal silica suspensions with and without hydroxypropyl cellulose polymer

The coagulation rate constant of submicron silica has been measured as a function of solution pH, salt concentration and hydroxypropyl cellulose (HPC) polymer concentration. Results show that the colloidal stability of silica is dominated by the cation concentration in the presence of salt in the pH range 3–9.5. The stability increases as cation concentration decreases. At low salt concentration and a minimum colloid stability was found in the intermediate pH range 4–8. These results show that differences in the literature values of the critical coagulation constant by relative light-scattering experiments can be explained by the use of the coagulation rate constant analysis. When HPC polymer was present in the solution, the colloid stability of the silica increased. The adsorption of polymer stabilizes the silica suspensions, both at low pH near the isoelectric point and at high ionic strength where it coagulates without the polymer. A monolayer coverage was necessary to provide steric stabilization. At 10^–3 M KCl a smaller equilibrium concentration of HPC in solution is needed to give monolayer coverage and steric stabilization than at 1 M KCl and pH 4.2.     

Double percolation effect on the electrical conductivity of conductive particles filled polymer blends

Electrical conductivity of carbon black (CB) filled polymer blends which are incompatible with each other was studied as a function of the polymer's blend ratio. Transmission electron microscope (TEM) analysis shows that CB distributes unevenly in each component of a polymer blend. TEM photographs of phase structure of solvent extracted HDPE/PMMA blend and solvent extraction experiments of PMMA/PP blend detect the blend ratio at which the structural continuity of filler rich phase is formed. The electrical conductivity of polymer blends is found to be determined by two factors. One is the concentration of CB in the filler rich phase and the other is the structural continuity of this phase. This double percolation affects the conductivity of conductive particle filled polymer blends.     

Dilatometric study of glass transition kinetics in polyisoprene

From dielectric measurements the thermal dependence and the distribution function of the relaxation times can be determined. In the present contribution we are testing the correspondence between the relaxation times obtained from the dielectric measurement and the dilatometric behavior of the material in the vicinity of the glass transition temperature. The experimental data used for the comparison were obtained for the sample of polyisoprene.     

Dielectric observations on polystyrene microcapsules and the theoretical analysis with reference to interfacial polarization

Dielectric measurements were carried out for polystyrene microcapsules which were prepared by means of an interfacial polymer deposition technique. The microcapsules showed a couple of dielectric relaxations termedP for lower andQ for higher frequencies. The frequency profiles were characteristic of the structure that shelled spheres were dispersed in a continuous medium. Gelatin aqueous solutions, cationic polyelectrolyte solutions and distilled water were loaded in the capsule interior to examine the effect of conductivities of the constituent aqueous phases on the dielectric properties. Relaxation frequencies of the relaxationsP andQ observed were directly proportional to the conductivities of the continuous medium and of the capsule interior, respectively. A dielectric theory was proposed for a suspension of shelled spheres in a continuous phase in order to analyze the relaxation data observed for the microcapsules. Volume fraction of the capsules, relative permittivity and conductivity of the capsule interior and thickness of the capsule wall are evaluated from the dielectric observations by use of the theoretical formulas derived. The dielectric behavior observed for the polystyrene microcapsules are interpreted quantitatively in terms of the dielectric theory proposed.     

Recent development in dielectric relaxation spectroscopy of polymers

An evaluation strategy for dielectric measurements in the frequency and/or time domain is presented which provides complete information about a relaxation mechanism (intensity, position, and shape of the relaxation function) independent of overlapping with neighboring mechanisms. As an example results on poly-(ethyleneterephthalate) are given.     

Electrokinetic study on concentrated suspensions using colloid vibration potential measurements

-potentials of a silica suspension and three types of polystyrene latex suspensions with different surface charge groups were measured, as a function of the particle concentration () in the suspension over a wide range, using the colloid vibration potential (CVP) technique. The concentration dependence of the-potential in silica suspension is explained well by Levine et al.s [1] cell model theory, verifying the applicability of the cell model to the CVP in silica suspension. However, the-potential of latex suspensions ordinarily decreases as the particle concentration increases, even after being corrected by the term of (1-). This tendency is especially noticeable in the systems that have particles with high surface charge densities. Furthermore, the conductivity measurements of these suspensions reveal that the conductivity of these systems, especially in their highly charged state, increases as the particle concentration is increased; opposite in tendency to silica suspensions. These new findings can be explained as follows: on the highly charged surface of a latex particle, a polyelectrolyte-like (hairy) layer is present, which overlaps at some point. This permits interparticle surface conduction and results in the abnormal behavior of CVP in these systems.     

Reversible flocculation of silica suspensions by water-soluble polymers

The possibility to concentrate, then redisperse colloidal suspensions is not only of great theoretical interest, but is also relevant to the industrial process of solid-liquid separation, which must often be followed by a redispersion stage. Up to now, these consecutive operations were unrealizable in the presence of polymers, since flocculation and adsorption were generally considered irreversible in this case. Previous studies have pointed out the occurrence of two main flocculation mechanisms: charge neutralization and interparticle bridging. The use of copolymers makes it possible to take into account these two different mechanisms together. Using this fact we prepared new copolymers of acrylamide with N-vinylimidazole via radical polymerization and characterized them by light scattering, viscometry, potentiometric titrations, and UV studies. One peculiarity of the chosen system is its dependence on pH: actually the degree of neutralization of such cationic polyelectrolytes does vary with pH, especially near the pK value. This paper shows that these copolymers may induce reversible flocculation of negatively charged suspensions, e.g., silica suspensions, by simple pH adjustment. Performances of the system were followed by various physico-chemical methods. The observed results are explained in terms of flocculation mechanisms as a function of pH.     

The effect of segment length and concentration on dielectric properties of polypropyleneoxide-based polyurethanes

Three series of segmented polyurethanes based on MDI, variable chain extender, and polypropylene oxide of MW=1000, 2000, and 3000 were synthesized and their dielectric behavior examined.Dielectric relaxations in the segmented polyurethanes were investigated between –150°C and +150°C in the 100 Hz to 10 kHz range. In general, three transitions, designated as, , and were observed, and ascribed in accordance with calorimetric relaxations to glass transitions of the hard and soft segments, and Shatzki-type motions, respectively. The effect of structure variables such as soft segment size, type of chain extender (ethylene glycol, butane diol, and hexane diol) and soft segment concentration, as well as the effect of interaction of the phases on dielectric properties was discussed. It was found that a certain degree of phase mixing exists in all series, detected by the variation of theT _g of the soft segment with soft segment concentration, contrary to DSC results, which was ascribed to thermal treatment prior to the dielectric measurements. It appears that interfacial polarization becomes important only above the transition temperature.     

Microscopic observation of ordered colloids in sedimentation equilibrium and the importance of Debye-screening length. 8. Unsymmetrical ordering and inclusion of anisotropic particles

Unsymmetrical crystal-like ordering of anisotropic-shaped particles are observed directly by a metallurgical microscope in sedimentation equilibrium and in deionized suspensions. The anisotropic colloids are polydispersed gold sols, rectangular tungstic acid, ellipsoidal poly(tetrafluoroethylene)(PTFE), aggregated (dimer, trimer, ...) polystyrene spheres, and bowl-shaped polymers. The distribution of the particles is analyzed by the two-dimensional distance distribution function. The interparticle distance continues to decrease as the initial concentration of the particles increases. Young's elastic modulus for the crystal-like structures is estimated to be 3.1 Pa for PTFE particles. The transformation from the unsymmetrical ordering to the symmetrical (hexagonal) array takes place as the concentration of diffusible ions decreases, which shows that the electrostatic interparticle repulsion and the elongated Debye-screening length around the particles play a key role. Furthermore, a bowl-shaped particle is observed to include a sphere in its cavity. This inclusional association is influenced greatly by the delicate changes in the effective sizes of the host and guest molecules containing the electrical double layers. These results show that electrostatic interparticle repulsion and the elongated Debye-screening length around the particles are essential for the ordering.     

DC dielectric study of polyethylene/CaCO3 composites

The dielectric properties of composite samples prepared by polymerizing ethylene on the surface of filler are compared to those of mechanical mixtures consisting of CaCO₃ and ultra high molecular weight polyethylene. After presenting the normalized master curves of AC dispersion and loss measured at different relative humidities, the field strength dependence of the 50 Hz AC and DC responses were studied. With one exception, the effect is small. Thermally stimulated polarization (TSP) and depolarization (TSD) curves are presented; the peak appearing on the TSP curves of the samples stored under ambient conditions is interpreted as a result of water desorption. The high temperature DC conductivity and the depolarization current density are higher in the composites and mechanical mixtures than in the matrix. The dielectric properties of the wet filler particles were calculated from the measured composite and matrix data using various mixture formulae. The results can be understood and interpreted if the dielectric properties of adsorbed water are described by the cluster theory of dielectric relaxation.     

Monolayers of methacrylic polymers containing aromatic groups

Monolayers of poly-methacrylates containing either aromatic or linear side groups were studied at the air-water interface. The aim of the work is to define the role of the aromatic group in determining the interfacial distribution and orientation of these polymers. Surface pressure measurements show that all the polymers give stable expanded monomolecular films between 288 K and 308 K temperature range.Surface potential and ellipsometric measurements show that both aromatic and aliphatic polymers are in an almost horizontal conformation at the liquid-air interface. From a comparison of the experimental isotherms with Huggins' theory, it was deduced that no preferential interactions exist between benzene rings in the film. In contrast, preferential attractive energies are observed for n-alkylmethacrylates.Further information on the state of the collapsed film was obtained from electron scanning micrographs.     

Electrolytic conductivity of poly(1,3-propylene phosphate) solutions containing mono- and divalent-counterions

The results of conductivity measurements for aqueous solutions of poly(1,3-propylene phosphate) (PPP), which can be considered as a synthetic analogue of naturally occurring teichoic acids, are reported. Experiments were carried out with oligomeric fractions of a polymer in acidic, sodium, potassium, magnesium and calcium forms. The concentration and molecular weight dependence of the equivalent conductivity of PPP was analysed and the limiting equivalent conductivity determined. From the conductivity data, the polyion-counterion interaction parameter F and the equivalent conductivity of a polyion _p were calculated. It was shown that both F and _p depend on polyelectrolyte solution concentration and molecular weight of PPP. Conclusions concerning mono- and divalent metal ions binding to PPP are drawn.     

Stability of modified surface CaCO3 in the non-polar medium

The adsorption isotherm of methacrylic acid (MAA) and polymethacrylic acid (PMAA) molecules on CaCO₃ were studied. The isotherm of (MAA) adsorption from cyclohexane was found formed from two steps, while that of (PMAA) from ethanol is formed from one step. The effects of surface modification of CaO₃ with (MAA) and (PMAA) in the sedimentation properties in a non-polar medium (cyclohexane) have been studied. It was found that the most stable suspension formed is concerned with monolayer surface coverage for the two adsorptives. Also, studies were made for the rheological properties of concentrated suspension of CaCO₃ modified with (MAA) and (PMAA) in a paraffin oil. The equilibrium flow curves of CaCO₃ modified with (PMAA) suspensions exhibit pseudoplastic characteristics, accompanied by some degree of complex thixotropy, while that modified with (MAA) exerts a thickening effect with rheopexy characteristics.     

Some aspects of polymer colloids I. Preparation and properties of different types of latex particles

The boundary region separating a latex particle from the surrounding medium has a great influence on the properties of latex dispersions. Four types of polystyrene and polystyrene/comonomer latices differing greatly in the structure of the boundary region were prepared. The first part of a series of papers reports on the preparation of the various latex dispersions. Mean particle sizes were obtained from simple turbidity measurements, quasi-elastic light scattering, and electron micrographs. The behavior of the particles in the centrifugal force field is a simple tool for detecting aggregation tendencies that are not directly related to salt stability. The BET-surface area agrees with the area calculated from the mean particle size when a sharp boundary and smooth surface is developed between the particle and the surrounding medium. In the case of particles with extended boundary regions (core/shell particles or particles with hairy envelopes), film formation reduces the specific surface area. Removal of soluble oligomers and polymers from the boundary region during subsequent treatments (purification and centrifugation before freeze-drying) can increase the surface area considerably.     

Amorphous solid-like distribution of polydisperse particles of colloidal clay and microcrystalline cellulose in deionized suspension

A metallurgical microscope is used to directly observe the amorphous solid-like structures of deionized suspensions of highly polydispersed colloids in sedimentation equilibrium. The colloids used are colloidal clay of bentonite and microcrystalline cellulose (MCC). The two-dimensional distance distribution functions of the amorphous solid-like structures at relatively diluted and concentrated suspensions resemble those of the liquid-like and gas-like structures of monodispersed spheres, respectively. The center-to-center interparticle distances (D) in the amorphous solid-like structures are explained by the effective hard-sphere model; a colloidal particle is coated with electrical double layers. The maximum length of the width of the double layers (Debye length,D ₁) observed is ca. 1n at very dilute suspensions.D andD ₁ continue to decrease as the initial concentration of the particles increases, and from these data rigidities are estimated to be 0.12 and 0.09 Pa for bentonite and MCC suspensions, respectively. The log [viscosity] of bentonite suspensions begins to increase linearly as log [shear rate] decreases with a slope close to –1, which supports the solid-like nature of the suspensions. These experimental results show that electrostatic interparticle repulsion and the elongated Debye-screening length around the particles are both essential for the appearance of the amorphous solid-like structures.     

Small-angle x-ray scattering by crystalline polymer fibers 2. Investigation of the linear paracrystalline model using various materials

The linear position-sensitive detector is well-suited to measure quantitatively the distribution parallel to the fibre axis of the intensity of small-angle x-ray scattering (SAXS) by polymer fibres, except that in the case of four-point patterns their width is greater than that of the detector window. A method is described which overcomes this problem, and which has high angular resolution. Using this method, the variation of scattered intensity with angles from 0.3° to 2.5° has been measured for fibres of poly(ethylene terephthalate), nylon, and low density poly(ethylene) (LDPE), and compared with that predicted by the linear paracrystalline model. In all cases except LDPE, when the distribution of phase lengths was given by the Reinhold function, there was no significant disagreement between the measured and predicted scattering except for a very small range of angles on the low angle side of the peak intensity. With LDPE small but significant discrepancies were found at other angles as well, and these were worse if the symmetrical Gaussian distribution function was used. The method enabled quantitative parameters describing the morphology to be obtained. It is concluded that the morphology of the linear paracrystalline stack is consistent with the SAXS intensity distribution, and that the Reinhold function is a reasonable approximation to the distribution of phase lengths. A small modification so that this decays more rapidly at long lengths might be necessary to explain the scattering for all materials over the entire angular range and other small changes might be needed with LDPE, although the asymmetrical nature of the distribution must be retained.On leave from Department of Physics, University of Technology Malaysia, 81300 Sekudai, Malaysia.