1. Understanding Latex Particle Morphology Mechanisms
Donald Sundberg and Brian Perry, Materials Science Program, University of New Hampshire
Many scholars currently subscribe to the idea that in free radical emulsion polymerization the ionic end groups of a polymer chain will anchor to the external surface of a polymer particle. For core-shell composite polymer systems, this would require the polymer chain in question to stretch (or “tether”) across one polymer phase to reach the center of the particle. We suspect that chain tethering is not always thermodynamically favorable, and that ionic end groups will indeed be distributed throughout the polymer particles in some, if not many, cases. Electron Spectroscopic Imaging (ESI) via TEM is used to map the initiator end group (e.g. its sulfur characteristic) throughout the particle. or + -
1st Stage
2nd Stage
Chain Tethering
Combining Ions
An important consideration when determining the potential for an ionic end group to tether or enter a particle completely is the solubility of the growing chain in the host polymer particle . The growing, oligomeric radical changes its overall polarity dramatically with chain length, starting off as a very polar initiator fragment and ending as a long, relatively hydrophobic polymer chain. We have been testing the solubility of polystyrene chains of various lengths, all with sulfonate end groups, in different solvents and in different high molecular weight host polymers. Under certain conditions we find that short chains are insoluble, longer chains are soluble and yet longer chains are insoluble again. Such behavior is depicted to the left.
Growing polymer chain length (# repeat units)