P. Bradna

Thickening effect of dispersions of ethyl acrylate-methacrylic acid copolymer prepared by different polymerization routes

Thickening of latices (particle diameters 105, 157 and 221 nm) by model dispersions of ethyl acrylate-methacrylic acid copolymer (15 wt.% of the acid) prepared by both non-seeded and seeded semicontinuous emulsion copolymerizations was investigated. Using viscometry and dynamic and steady shear measurements, we found that the thickening effect of the dispersions strongly depends on their particle structure and the intensity of interactions between the components in the system. In weakly interacting systems (lower latex concentrations, large latex particles) the thickening effect of the dispersions is controlled by effective volume fraction of swollen particles. This leads to a higher viscosity of systems thickened by more swollen (less crosslinked) particles obtained by the non-seeded process. On the other hand, in strongly interacting systems (high latex concentrations, small latex particles) lower deformability of more crosslinked particles prepared by the seeded process causes a higher flow resistance of the systems thickened by this dispersion.

The influence of salt concentration on negative thixotropy in solutions of partially hydrolyzed polyacrylamide

The influence of NaCl concentration on negative thixotropy in aqueous glycerol solutions of partially hydrolyzed polyacrylamide have been investigated. It was found that negative thixotropy type I (a slow increase in viscosity with time of shearing) sets in at higher critical shear rates when the salt is present. On the other hand, critical shear rates for negative thixotropy type II (a rapid increase in viscosity followed by viscosity oscillation) did not depend on the salt addition. Using the critical shear stress as a hydrodynamic criterion for the occurrence of negative thixotropy, a possible explanation of the behavior is proposed.

Negative thixotropy of solutions of partially hydrolyzed polyacrylamide

We found that the character of negative thixotropy of partially hydrolyzed polyacrylamide in aqueous glycerol strongly depends on polymer concentration, glycerol content and shear rate applied. At low polymer and glycerol concentrations, shear stress and viscosity slowly increased during shearing to a limiting value. In addition to this behavior, a steep increase in shear stress as well as normal stress followed by their pronounced oscillations occurred at higher concentrations of both components and at higher shear rates. Similarly to the negative thixotropic effect in solutions of other polymers in organic solvents, the hydrodynamic conditions in which the effects set in seem to be controlled by the shear stress acting in the flowing solution; initial kinetics of the effect depends on solvent viscosity and shear stress applied. To explain the influence of the glycerol content and degree of ionization of the polymer on the minimum shear stress at which the effect sets in, a decisive role of intermolecular electrostatic repulsions in association of the polymer molecules in shear field is assumed.

Thickening of butyl acrylate/styrene/2-hydroxyethyl methacrylate/acrylic acid lattices with dispersion of crosslinked ethyl acrylate/methacrylic acid copolymer

Abstract Thickening of lattices of styrene–acrylic copolymers containing a small amount of acrylic acid and various amounts of 2-hydroxyethyl methacrylate (HEMA) with alkali-swellable dispersion of crosslinked ethyl acrylate/methacrylic acid copolymer has been studied using capillary viscometry and dynamic and steady shear measurements. The higher the amount of HEMA incorporated into the latex copolymers, the stronger the thickening effect is. A comparison of flow behavior of thickened systems with those of neat lattices confirmed a significant influence of the effective volume fraction of alkalinized swollen latex particles on the thickening process. Under these conditions, to obtain the same rheological properties, the necessary amount of thickener was lower for the latex copolymers with higher amounts of HEMA because a part of the thickener volume fraction was replaced by an increased volume fraction of swollen latex particles. These findings indicate weakly interacting dispersion systems without strong compression of particle domains.

Structural changes of latex particles of ethyl acrylate—methacrylic acid copolymers during neutralization in the presence of methanol

Using a combination of static light scattering, potentiometry, and viscometry, structural changes of latex particles of ethyl acrylate—methacrylic acid (20–70 wt%) copolymers during neutralization in the presence of methanol were investigated. It was found that the latex particles disintegrate into small subparticles, swell, and at a high content of methacrylic acid units dissolve. Disintegration of latex particles is explained by considering the particles as agglomerates formed during polymerization by coalescence of smaller particles.

Potentiometric and viscometric study of the alkalization of latex dispersions of ethyl acrylate — methacrylic acid copolymers in the presence of salt

The behavior of latices of ethyl acrylate — methacrylic acid copolymers during alkalization was investigated using potentiometric titration and viscometry. In the presence of NaCl in dispersion medium pH and viscosity were lower compared to salt-free dispersions. Due to strong electrostatic interactions at low salt concentration the apparent dissociation constant of the carboxylic groups is small and the polymer segments bearing these groups are in extended conformations. The screening effect at higher salt concentration increases the apparent dissociation constant and decreases the extension of the polymer segments. The pH and viscosity of latex dispersions alkalized in the presence of salt are thus smaller.

Light scattering study of structure of dispersion particles based on ethyl acrylate-methacrylic acid copolymers

Abstract Structural changes of colloid particles of ethyl acrylate-methacrylic acid (0–70%) copolymers on addition of alkali (pH∼9) or methanol were investigated. In addition to the feeding time of the monomer mixture, the content of methacrylic acid in the copolymers has been shown to influence dissolution of dispersion particles prepared by semicontinuous emulsion copolymerization. The longer the feeding time leading to higher polymer solids and the lower the acid content in the copolymer, the more limited was dissolution of disperse particles. On the basis of consistency of the results obtained in both media, this behaviour was explained in terms of a crosslinked structure of the copolymer due to chain transfer reactions.

Thickening of electrostatically stabilized latices by ethyl acrylate-methacrylic acid copolymers with various molecular weights

Abstract Electrostatically stabilized latices having various particle sizes were used to study the thickening effect of alkali-soluble ethyl acrylate—methacrylic acid copolymer (50 wt.% of the acid; M w =0.78 × 10 5 , 4.36 × 105 and 11.6 × 105). It was found that the effect increases with the molecular weight of the copolymers and with decreasing size of the latex particles. The pronounced tendency towards non-bridging flocculation and phase separation suggests that the thickening effect of the materials results from depletion (volume restriction) flocculation rather than depletion stabilization. The role of the effective volume fraction of the dissolved copolymer as a variable which controls the rheological properties of the thickened latices was tested experimentally.