A study of poly(butadiene/methacrylic acid) dispersions: From pH-responsive behaviour to the effects of added Ca2+

This study investigates the effects of added Ca2+ on the properties of poly(Bd/MAA) dispersions (1,3-butadiene and methacrylic acid) and considers the effect of particle composition on the p Ka. Four latex dispersions are considered in detail. These include poly(Bd/6MAA) and poly(Bd/20MAA) which contain, 6 and 20 wt% MAA, respectively, based on the total monomer mass used for dispersion preparation. Two model systems are also used for comparison. These are poly(Bd) and poly(EA/33MAA/BDDA) (EA and BDDA are ethyl acrylate and butanediol diacrylate). The latter is a well-studied model pH-responsive microgel. The apparent p Ka of the poly(Bd/MAA) dispersions was determined from potentiometric titrations and found to increase with Bd content. The pH-dependence of the particle size was studied using photon correlation spectroscopy. Electrophoretic mobility measurements were also used. The hydrodynamic diameters and mobilities exhibited major changes as the pH approached the p Ka for the particles. The critical coagulation concentrations were also measured. The results indicate that Ca2+ caused pronounced dispersion instability at low pH. Moreover, Ca2+ prevents swelling of the poly(Bd/MAA) particles at high pH. It was found that efficient ionic binding of all of the RCOO- groups within the poly(Bd/20MAA) particles occurred when the mole ratio of RCOO- to Ca2+ was less than or equal to 2.0. Consideration of all the data leads to the suggestion that poly(Bd/MAA) particles have a core-shell structure. It is suggested that the particle core contains mostly poly(Bd) and that the shell is comprised of lightly crosslinked poly(Bd-co-MAA) copolymer. © 2008 Elsevier Inc. All rights reserved.