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. Their influence on the net reaction time of polymerization and on the nucleation mechanism in comparison to other heterophase polymerization types is discussed. 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, n̄: during the first interval, n̄ increases until a plateau value of 0.5 is reached at the onset of the second interval. The beginning of the last interval is defined by a sharp increase of n̄ . Because of the nature of the first interval, all miniemulsion droplets are nucleated within this interval at a specific time, and therefore the individual conversion in each droplet has reached a different level. Every miniemulsion droplet can be perceived as a separate nanoreactor which does not interact with the others. Independent 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 n̄ during the second interval remains 0.5. Therefore, the net polymerization time strongly decreases with decreasing particle size.