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.