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 a re able to compete with interfacial forces to influence latex particle morphology. At higher levels of cross-linking the elastic forces domi nate those at the interfaces. PMMA seed latices were prepared at (nomi nally) 0, 0.015, 0.10, and 0.2 mol % (based on monomer) EGDMA cross-li nking agent and subsequently swollen with styrene at a stage ratio of about 235%. After reaction, transmission electron micrographs of micro tomed sections of the second stage latex particles showed that as Litt le as 0.015% EGDMA (over 9000 repeating units between cross-links) beg an 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 par ticles were essentially of the core-shell morphology. Comparisons with predictions from a Gibbs free energy analysis of the effect of seed l atex cross-linking on particle morphology are presented and show agree ment with the experimental results.