Effects of Cross-Linking on the Morphology of Structured Latex Particles. 2. Experimental Evidence for Lightly Cross-Linked Systems

Elastic forces within very lightly cross-linked seed latex particles are able to compete with interfacial forces to influence latex particle morphology. At higher levels of cross-linking the elastic forces dominate those at the interfaces. PMMA seed latices were prepared at (nominally) 0, 0.015, 0.10, and 0.2 mol % (based on monomer) EGDMA cross-linking agent and subsequently swollen with styrene at a stage ratio of about 235%. After reaction, transmission electron micrographs of microtomed sections of the second stage latex particles showed that as little as 0.015% EGDMA (over 9000 repeating units between cross-links) began to shift the particle morphology from inverted core−shell (i.e. the second stage PS in the core) toward core−shell. At 0.2% EGDMA the particles were essentially of the core−shell morphology. Comparisons with predictions from a Gibbs free energy analysis of the effect of seed latex cross-linking on particle morphology are presented and show agreement with the experimental results.