Core-shell and other multiphase latex particles—confirming their morphologies and relating those to synthesis variables

Composite latex particles made from sequential processing steps may have a wide range of morphologies. The particular structure achieved is the result of a number of complex interactions between the chemical and physical aspects of the emulsion polymerization process. The intention is often to produce a core-shell particle structure, but in reality this is often a nontrivial task. In addition, it is challenging to develop confident conclusions about the detailed morphology of such particles, even when the two-component polymers are well phase-separated. This paper reports on an inter-laboratory study of acrylic-styrene copolymer latices in which the two-component polymers were not well phase-separated—a common result when attempting to make core-shell latex particles. Six different organizations each performed a number of analytical measurements to characterize two different latex systems. Their collective results were presented at a workshop in which the group strived to reach consensus on the detailed structures of the particles. Their conclusions were that multiple, complementary analytical results are required to reach a sound decision. While electron microscope results were always judged to be necessary, considered by themselves, it is often found that one can be led to false conclusions, especially for systems that are not well phase-separated. Given the results of this study it is clearly inappropriate to assume that the polymer formed in the second stage of a semibatch latex polymerization process forms a shell, giving rise to a core-shell morphology.