1. Reconsideration of the Radical Entry Mechanism in Emulsion Polymerization
Brian Perry and Donald Sundberg, Materials Science Program, University of New Hampshire
Experimental data for free radical emulsion polymerization in which relatively hydrophilic 2nd stage monomer was starve fed to a reactor containing relatively hydrophobic seed latex particles can offer new insights to the mechanism of emulsion polymerization. The transmission electron microscopy (TEM) images shown below provide strong evidence against a long-standing claim that anionic sulfate initiator end groups (e.g.: from potassium persulfate) predominantly anchor to particle surfaces.
The TEM images clearly show that 2nd stage polymer (light phase) resides far within the particles. Sulfate end group surface titration of these particles shows that indeed much less than 50% of the total initiator sulfate groups added during polymerization are located on the particle surfaces. This, coupled with the TEM images, leads to the conclusion that most of the 2nd stage chains surely are not
TEM images of stained (for contrast) composite particles with a mixed phase core (A) which were thermally annealed at 150oC to invoke phase separation which is observed as occlusions (B). A B
anchored. These experiments also eliminated a potentially disruptive phenomenon called chain transfer to monomer. This was done by carefully maintaining the molecular weight of the 2nd stage chains well below the limit for the transfer event to occur.
Considering the diameter of the latex particles and the radius of gyration of a surface anchored 2nd stage polymer chain (as shown in the image to the left), there is no possibility for such a chain to be part of an occlusion within the interior region of the particle (image C). This is consistent with experiment. However, if the latex particle was very small (as in image D), the same chain dimensions could possibly lead to 2nd stage polymer in the center of the particle and still remain anchored to the particle
2nd stage chain
1st stage
2nd stage
250 nm C D
Surface. These mechanistic studies were appropriately carried out with the larger particles.
Scaled drawing where anchoring is assumed for a single chain of 2nd stage polymer of diameter of gyration = 30 nm in comparison to full particle diameters of 250 nm (C) and 50 nm (D).