Kinetics of miniemulsion polymerization as revealed by calorimetry

The kinetic behavior of styrene miniemulsions with a hydrophilic initiator was investigated by calorimetry, focusing on the studies of parameters such as (a) the amount of surfactant and (b) the quantity of the initiator. The ir influence on the net reaction time of polymerization and on the nucleati on mechanism in comparison to other heterophase polymerization types is dis cussed. The course of miniemulsion polymerization kinetics is characterized by three distinguishable intervals. The first two intervals can be defined by the average number of radicals per particle, (n) over bar: during the f irst interval, (n) over bar increases until a plateau value of 0.5 is reach ed at the onset of the second interval. The beginning of the last interval is defined by a sharp increase of (n) over bar. Because of the nature of th e first interval, all miniemulsion droplets are nucleated within this inter val at a specific time, and therefore the individual conversion in each dro plet has reached a different level. Every miniemulsion droplet can be perce ived as a separate nanoreactor which does not interact with the others. Ind ependent of the amount of initiator as well as the particle size, i.e., the number of active sites in the system, the limit value of (n) over bar duri ng the second interval remains 0.5. Therefore, the net polymerization time strongly decreases with decreasing particle size.