Entry in emulsion copolymerization

Emulsion polymerization is a very complex process and various models have been proposed in order to understand the different physical and chemical events taking place. One of the important processes in emulsion polymerization is the entry of radicals into particles. One of the often applied and more or less generally accepted models to describe entry is the model of Maxwell and Morrison. The essence of the model is that the rate determining step of entry is propagation of oligomers in the aqueous phase up to a certain chain length (z-mer) after which the oligomeric radical irreversibly enters the particle. For entry in emulsion homopolymerization experimental data partly support this model, for emulsion copolymerization not many experimental data are available. In order to model the entry of radical in emulsion copolymerization, a study of the oligomers present in the aqueous phase is necessary. In our work a new method for separation and characterization of the oligomers present in the aqueous phase is developed. MALDI-ToF-MS was found to be a powerful technique in order to characterize the oligomers present in the aqueous phase. It was shown that the methodology works well, even though we are dealing with very small quantities of oligomers and the possibility of mass discrimination in MALDI-ToF-MS. The chemical composition distribution (CCD) and the molecular weight distribution (MWD) of cooligomers present in the aqueous phase as well as on the particle phase is determined from the MALDI spectra. It turns out that there is not such a sharp boundary between the oligomer distribution in the aqueous phase and that in the particle phase as indicated by the Maxwell-Morrison model but that the results are more in line with the models put forward by Sundberg and Leiza. From the data reactivity ratio values are calculated in two ways. One is by employing the CCD of the aqueous phase cooligomers obtained from a single MALDI spectrum obtained at one feed composition with an analytical method. Because this analytical method does not take into account the small chain effects like initiator specific monomer addition these are apparent reactivity ratios. Secondly, by simulating the full CCD of the aqueous phase cooligomers via Monte Carlo simulations, using (literature) reactivity ratios. The results show that the concept of apparent reactivity ratios is indeed correct and when taking into account all short chain kinetic effects, the Monte-Carlo simulations result in the same apparent reactivity ratios obtained from the theoretical CCDs (fitted with the analytical method). The Monte-Carlo simulations were based on long chain limit literature reactivity ratios in combination with all possible short chain effects. The produced oligomers (as theoretically described by the Monte-Carlo simulations) will redistribute over the water phase and particle phase. The resulting oligomer distribution in the aqueous phase is now studied. At very short chain lengths all the produced oligomers can be traced back in the aqueous phase. As the oligomers chain-extend at some point part of the oligomer distribution will become surface active and disappear from the aqueous phase. This can be observed by a change in the average chemical composition (F, this with respect to the theoretical F from the Monte-Carlo simulations). With further increase in chain length the full CCD is surface active (no change in F, this region corresponds to the z-mers in other models), followed by again a part of the CCD being surface active leading to a change in F. Finally with further chain length increase the oligomers gradually become insoluble (j-crit mers). Basically the changes in F can be described by the concept of Hydrophilic Lipophilic Balance (HLB values). The CCD travels through the HLB window, the HLB values where the oligomers are good surfactants. There where only a part of the CCD is adsorbed on the particle surface a change in F can be observed. After this HLB window the region of insolubility sets in. A separate study of influence of pH on ionic monomer combinations is done. It is observed that with increasing the pH, incorporation of acidic monomer in the particles reduced while the critical chain length of the entering cooligomer is increased. The developed oligomer analysis method in combination with new insights in oligomer partitioning is a sound basis for further studies of entry in emulsion copolymerizations