A study of poly(butadiene/methacrylic acid) dispersions: From pH-responsive behaviour to the effects of added Ca 2+

Abstract

This study investigates the effects of added Ca 2+ on the properties of poly(Bd/MAA) dispersions (1,3-butadiene and methacrylic acid) and considers the effect of particle composition on the p K a . 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 K a 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 K a for the particles. The critical coagulation concentrations were also measured. The results indicate that Ca 2+ caused pronounced dispersion instability at low pH. Moreover, Ca 2+ 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 Ca 2+ 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.