The association of urethane-polyethyleneoxide (HEUR) thickeners,as studied by NMR self-diffusion measurements

The concentration and molecular weight dependence of the self-diffusion coefficient (D _self) of associative polymers of HEUR-type in aqueous solution have been investigated using FT-PGSE-NMR technique. The idea of three-dimensional network formation as a result of aggregation of the hydrophobic end-groups of the polymer in junctions is supported through the observed dramatic lowering ofD _self with increased concentration. The network-formation efficiency depends on the polymer molecular weight as well as the hydrophobicity of the end-groups.A double logarithmic dependence of the self-diffusion coefficient versus concentration (c) has been observed:D _selfc ^–a1,a2 . The first exponent,a ₁, is valid at low concentration, <1% polymer per weight solution, and ranges from 0.5 to 1, whereas the second exponent,a ₂, describing systems of higher concentration, ranges from 2 to 2.7.     

On the decisive role of finite chain extensibility and global interactions in networks

The stress-strain dependence of dry networks at unidirectional extension and compression is studied. The phenomenological van der Waals equation of state is compared with different molecular models in order to provide an interpretation of the van der Waals corrections. It is shown that the stress-strain behavior predicted by the phantom, Langevin, and constrained junction fluctuation models are altogether covered by the van der Waals approach. The relationship between the suppression of junction fluctuation parameter introduced by Dossin and Graessley and the van der Waals corrections has been worked out. The effect of junction functionality on the small strain modulus as well as on the second Mooney-Rivlin coefficient is also presented.     

Quantitative aspects of the growth of (charged) silica spheres

It is shown that the growth of monodisperse, charged silica spheres in ethanolammonia upon addition of tetraethoxysilane can be adequately studied with static light scattering. The resulting growth curves (radius vs time) are analyzed in terms of a model for diffusion-controlled growth of (charged) spheres in a solution of reactive (charged) monomers, the bulk concentration of which decreases due to this growth. This decrease is sufficient to account for the essential shape of the curves. A coulombic sphere-monomer repulsion is theoretically shown to modify the growth rate only with a constant factor.     

Studies of polymer solvation by dielectric relaxation spectroscopy II: Polyethylene glycol in methyl pyrrolidone

Frequency domain dielectric spectroscopy (30 MHz to 72 GHz) is utilized to study solvation in polyethylene glycol (PEG) — methyl pyrrolidone (MPy) mixtures over the whole mixture range at 20 °C, in particular with PEG 200. Further pure PEGs (200 to 400) and butyl glycol (in the pure state and in benzene and n-hexane solution) are considered for comparison to facilitate the assessment of PEG relaxational behavior. It turns out that the PEG-MPy mixtures can be described in terms of a simple solvation model which takes into account only the bulk and the solvating solvent state. The solvation number per repeat unit is about 0.85 in the dilute solution limit. It decreases gradually with increasing PEG concentration in accordance with a solvation equilibrium model.     

Mechanical and morphological investigations of UHMWPE/Fe composites

Ultra-high molecular weight polyethylene/iron composites were investigated. The specimens were obtained by pressing in a steel die and sintering at different temperatures. By means of porosimetry, microscopy, microhardness, density, and partial volumes of the components it is shown that there are no microcavities. The microhardness does not depend upon the weight % content of the metal in the composites. It also neither depends on the pressure nor the temperature of sintering. For low metal content within the composites, microhardness Mayer equations are linear. For high metal content the dependence is nonlinear. With the increasing of the iron content tensile strength weakly decreases. However, plane-strain compression, dimension steadiness, Vicat softening temperature, and tribometric characteristics of the composites are improved. It is shown that the polymer is a well-dispersive medium. The particles of the components have a good mechanical compatibility. The polymer wets the surface of the iron; this is probably connected with the surface oxidation of the metal particles.     

Precipitation of surfactant salts III. Metal perfluoroalkanecarboxylates

Precipitation of perfluorocarboxylic acids (n-C₃H₇OOH, n-C₇H₁₅COOH, and n-C₉F₁₉COOH) in aqueous solutions of different metal nitrates (K⁺, Ag⁺, Ca²⁺, Ba²⁺, Zn²⁺, Al³⁺, La³⁺) was studied by solubility and calorimetric measurements. The free energy of precipitation per single surfactant chain was found to depend on the chain length but not on the charge of the reacting cation. It was shown that the precipitation of perfluorocarboxylates of multivalent cations was governed by positive entropy change and a low enthalpic contribution, whereas the soaps of monovalent cations exhibited the opposite behavior. The entropy of precipitation was not affected by the chain length of the surfactants in the presence of monovalent cations, but it increased for cations of higher charge.Supported by the NSF Grant CHE-8619509.This paper is dedicated to Professor Armin Weiss on the occasion of his 60th birthday. One of the authors (E. M.) particularly appreciates many years of cooperation with Professor Weiss in editing Colloid and Polymer Science, as well as in many other scientific interactions.     

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.     

Interactions of mixed surfactant solution with liposome membrane

Interactions of the mixed surfactant solution of dodecylamido propyl dimethyl aminoacetate and sodium dodecyl sulfate with the liposomal membrane were studied. Lytic activities of the surfactants were measured as a function of the concentrations of surfactant and phospholipid and the composition of mixed surfactants. The solubilization limits of phospholipid by surfactants were determined from the change of their aggregation behavior in suspensions at equilibrium by means of quasi-elastic light scattering. The mixed surfactant solutions showed lower lytic activity than single component surfactant solution in spite of the strong adsorption onto the liposome surface. This was attributed to low solubilization power of binary mixture for phospholipid.     

Functionalization of colloidal silica and silica surfaces via silylation reactions

A series of trialkoxysilane compounds tipped with primary amine groups were used to functionalize the surfaces of glass and colloidal silica. Streaming potential and microelectrophoretic mobility measurements were used to monitor the stability of the functionalized surfaces.Hydrolytic breakdown of the surface-to-silane coupling was induced by either successively increasing and decreasing the pH of the solution in contact with the surface, or by aging the derivatised surfaces in aqueous solution over prolonged periods of time. The chemistry of the spacer units between the trialkoxysilane group and the primary amine tip had a major influence on the subsequent hydrolytic stability. Large hydrophobic spacer groups showed small changes in the electrokinetic properties on storage, but large changes when successively titrated with acid and base through the pH range. The behavior observed with small hydrophobic spacer groups was that large changes in electrokinetic properties were obtained on storage and with pH titration.     

Solution behavior of two liquid crystalline polymers of different architectures

Solutions of two different liquid crystalline polymers of high molecular weight are investigated by static and dynamic light scattering (LS), membrane osmometry and size-exclusion chromatography (SEC). Measurements in dilute solution in different solvents showed no specific behavior as formation of aggregates or chain stiffening. Large discrepancies between the LS results and the results from osmometry and SEC show that the latter methods are in the present cases not suitable for molecular weight determination. In semi-dilute solution the osmotic modulus and the time correlation function were studied. Behavior of flexible chains was observed. In one system a slight aggregation of small molecules onto longer chains was found causing less interpenetration of the chains in that solvent. At moderately high concentrations cluster formation was observed from i) a small angle excess scattering, ii) a downturn of the osmotic modulus, and iii) the appearance of a slow motion in the time-correlation function.     

Effects of surfactants on wave-induced drainage of foam and emulsion films

It is well established that the plane-parallel models of foam and emulsion films underestimate the velocity of film thinning by up to several orders of magnitude and show an incorrect dependence of thinning velocity on film radius. A new theory of film thinning has been previously formulated for tangentially immobile films [12, 13], and shows that the reason for this discrepancy is the neglect of experimentally observed finite amplitude surface waves. For thin films of relatively large radii (> 1o^–2 cm), the pumping of the fluid generated by oscillations of the surface waves, provides the dominant contribution to film thinning velocity. The present hydrodynamic model includes the effects of surfactants (Marangoni-Gibbs-effect, surface viscosity and surface diffusion) and surface waves on thinning velocity. As in the case of a tangentially immobile film, it is concluded that the thinning velocity varies inversely with less than the first power of the film radius, and not with the square of the film radius, as predicted by the plane-parallel models of thin film. Also, the velocity of thinning is found to be up to several orders of magnitudes larger than that evaluated from the plane-parallel models. The influence of waves in enhancing the thinning velocity is found to be most significant for a tangentially immobile film and this effect decreases by a factor of up to 3, with a decrease in surface elasticity and surface viscosity.     

Enthalpy of solution of phenoxyalcohols solubilized in aqueous sodium dodecylsulfate and hexadecyltrimethylammonium bromide solutions

The enthalpy of solution of phenoxy 2-ethanol, 1 phenoxy,-3 propanol and benzylalcohol was determined at 25 °C in aqueous sodium dodecylsulfate and hexadecyltrimethylammonium bromide solution, up to 0.2 mol/kg surfactant concentration. Using the pseudo-phase model, the standard enthalpy of transfer and the partition coefficient of the alcohols between micelle and water are calculated. The latter quantity is found to be systematically larger when derived from enthalpy than from free energy measurements. Using the so-called compensation plot, the solution thermodynamics of aromatic and aliphatic alcohols in aqueous sodium dodecylsulfate and in the octane+water systems are compared. Aromatic alcohols display an anomalous behavior in the octane+ water system but not in the micellar one.The standard enthalpy of solution of various alcohols presents, when plotted against hexadecyltrimethylammonium bromide concentration, a shoulder in the region around 0.05 mol/kg; a discussion is presented on the evidence for alleged micellar structural changes in aqueous micellar systems.     

Equilibrium sorption of crystal violet by carboxymethylated cellulose with low degree of substitution

Cellulose fabric samples were carboxymethylated for different duration. The degree of substitution was changing through a maximum value during the reaction time.The equilibrium sorption of a cationic dye by cellulosic fibers of different degrees of carboxymethylation was investigated in buffered and in unbuffered systems. The sorption isotherms were interpreted by two kinds of approximations based upon Langmuir isotherm. From the sorption data extrapolated to zero dye concentration affinities were calculated. The temperature dependence of affinities gave the values of heat and entropy of dyeing which were changing along the reaction time similarly to the degree of substitution.     

Superposition of sedimentation and demixing in a centrifugal field

The sedimentation of a demixed solution in an ultracentrifugal field is treated. It is shown experimentally for a solution of PS in cyclohexane that a density inversion occurs leading to heterogeneous equilibrium although the mechanical equilibrium is not established.This paper was presented at the VI. Symposium on Analytical Ultracentrifugation, Marburg, FRG, February 16–17, 1989.     

Investigations of the influence on the phase morphology of PP-EPDM-blends on their mechanical properties

This work covers the dependence of the mechanical properties of polymer blends on their composition and their phase morphology. Blends of EPDM-elastomers and polypropylene were prepared covering the whole concentration range. The phase morphology was varied strongly by employing different mixing techniques and its morphology was characterized by means of electron microscopy and light microscopy, as well as by x-ray scattering and calorimetry.Mechanical properties such as the complex shear modulus, the tensile modulus as well as the stress strain behavior were investigated as a function of the composition of the blends and their phase morphology. The experimental finding is that the complex modulus, the tensile modulus, the yield stress, and the ultimate stress are rather insensitive with respect to the phase morphology and vary continuosly with the composition. The elongation at break, on the other hand, as well as the impact strength were found to depend on the phase morphology and to vary discontinously with the composition. One conclusion to be drawn is that one is not always forced to control the phase morphology tightly during processing in order to obtain materials with sufficiently good mechanical properties. Rather, simple theoretical approaches, neglecting details of the phase morphology are frequently able to satisfactorily predict mechanical properties of multiphase blends.     

The use of dilute solution viscometry to characterize the network properties of carbopol microgels

We used dilute solution viscometry, specifically, measurements of intrinsic viscosity and overlap concentration to characterize two members of the carbopol family, carbopols 940 and 941. These measured quantities were then used to calculate the swollen-to-dry volume ratios for both resins over a range of ionic strengths. The variation in this ratio is representative of the swelling equilibria of the micronetwork and is modeled using standard network theory with modifications for non-Gaussian chain statistics and for fixed charges on the chains. By fitting to experimental data, the cross-link density is determined as 1450 monomer units between cross-link sites for carbopol 940 and 3300 units for carbopol 941. The shear modulus of the microgel domain is then predicted from the swelling data and found to be an order of magnitude greater than the elastic modulus measured on concentrated carbopol dispersions. We argue that this discrepancy is due to a large number of defects which reduce the cycle rank of the network. Current microgel technology specifies that their rheological behavior can be predicted once the domain cross-link density is known. Simple dilute solution viscometry appears capable of providing this prerequisite.     

FT-IR studies on the mechanical response of the crystalline fraction in ultrastrong polyethylene tapes

FT-IR spectra of UHMW-PE tapes with different moduli have been studied in dependence of mechanical deformation and stress. Small spectral variations have been revealed in the difference spectra. Band shifts of the 730/720 cm^–1 doublet have been observed under stress relaxation and upon stretching. A clear correlation between the shift of crystalline bands and the Young's modulus could be determined. Polarized measurements demonstrated variations in the mechanical response of nearly perfectly oriented and non-oriented crystallites. Crosslinking was performed by blending the polyethylene with 10% polysilane and UV-irradiation of the drawn material. The band shift increases as the linkage between oriented crystal blocks is improved with higher draw ratios and chemical crosslinking.     

The pH dependence of dispersion of TiO2 particles in aqueous surfactant solutions

The pH dependence of dispersion of titanium dioxide (TiO₂) particles has been examined in the presence of surfactant molecules in water. Whereas particles were dispersed in water at acid and alkaline regions rather than at neutral region, the dispersion was enhanced at neutral region in an aqueous sodium dodecyl sulfate (SDS) solution and at acid and alkaline regions in an aqueous dodecyldimethylamine oxide (C₁₂DAO) solution. Considering the pH dependence of zeta potential, the adsorption models of surfactant molecules on a particle were estimated on the basis of the modes of hemimicelle and double-layer compression. While the particles that adsorbed Al³⁺ were remarkably dispersed around pH 6, their dispersion does not largely depend on pH in the addition of SDS, indicating the adsorption of SDS molecules to form double-layer compression in the whole pH region. Dynamic light-scattering measurement and electron microscopic observation suggested that the particles were dispersed in water as small flocs.     

Steric fitting of the spherical micelle size

A model was developed to explain the size of spherical micelles on the basis of steric effects. Here the electrostatic repulsion within polar heads is considered as a steric effect. The predictions of this model concerning micelle size; water penetration into the hydrocarbon micelle core; and the dependence of the aggregation number on temperature, electrolyte addition, hydrocarbon chain length, head group size, and counterion size agreed quite well with literature data.