Mechanism of alkali‐thickening of acid‐containing emulsion polymers. II. Examination of latexes with the light microscope

Abstract

synopsisThe mechanism of alkali thickening of acid-containing emulsion polymers has been confirmed by visual examination of the swelling and disappearance of latex particles with the light microscope. Specially prepared large particle-sized latexes were used in this study.The particle swelling and dissolution behavior correlated with viscosity measurements, high viscosity being present for swollen particle systems and low viscosity for true solutions. The neutralized state of all acid-containing latexes can be pictured in a cube with the important variables being per cent acid, hydrophilicity of the comonomers, and T,.A t the bottom will be latexes only slightly affected by neutralization, above this will be a swollen particle zone, and the largest area will be a true solution zone. The true solution zone will be enlarged by lowering the molecular weight of the latex polymer.Each latex was examined from 0%-200y0 neutralization with base. INTRODUCTlONThe change undergone by an acid-containing emulsion polymer upon neutralization is accounted for in terms of varying degrees of a single mechanism. This mechanism consists of swelling of the latex particles plus an accompanying solubilization of polymer molecules when the hydrophilicity of the polymer is high enough. ' If the particles of alkali-soluble latexes were made large enough, it should be possible to visually observe the swelling and disappearance of these particles in the dispersed state with the light microscope. Latexes were prepared of some representative compositions whose average particle size was 500 mp in diameter.The latexes were neutralized with base to varying degrees of neutralization and the equilibrated samples were observed and photographed with the light microscope.

Dynamics of shear-thinning suspensions of core–shell structured latex particles

“…It is well known that when acrylic acids containing latex polymers are neutralized to an alkaline pH, there is a marked increase in the viscosity of the system [4][5][6][7][8]. Verbrugge measured viscosity versus pH for a large number of methacrylic acids containing latexes of high and low T g and similar hydrophilicity [5,6].…”

Chemical heterogeneity in emulsion copolymers of carboxylic monomers

“…The separation of the carboxylic groups in the polymer phase into surface and "buried" groups, on the contrary, seems to be more difficult. Time effects connected with the alkali solubility and swelling [3] and the conductometric titration [4,5] show that it depends on the length of time used for the measurements and on the diffusion coefficient of the neutralizing agent to which extent also "buried" carboxylic groups are counted by this method. Thus, "surface groups" determined by titration in any case mean groups in a more or less thick surface layer or shell of the particle, the shell thickness depending on the time scale of the experiment.…”

“…Another approach to particle structure is based on the well-known fact that the viscosity of carboxylated latices is increased with increasing degree of neutralization [3,[7][8][9]. The origine of this effect is a particle swelling with increasing latex pH which is due to the electrostatic repulsion and the solvatation of the anionically charged carboxylic groups.…”

Swelling and dissolution of latex dispersions of the ethyl acrylate-methacrylic acid copolymers during alkalization

“…The height of the maximum decreased with increasing flow rate [13]. These results, along with a comparison of changes in dispersion particles under a microscope [6], after clarification of the dispersions led to a conclusion that at a methacrylic acid content at least 20% the copolymer dissolves.…”