Nucleation robust

Reimers, J.L., and Schork, F.J., Robust nucleation in polymer-stabilized miniemulsion polymerization, J. Appl. Polym. Sci., 59, 1833-1841 (1996b). 

If a miniemulsion could be run at 100% droplet nucleation (or near to this), then a very robust nucleation system would result. The number of particles could be determined by the number of initial monomer droplets, and this can be controlled by adjusting surfactant, costabilizer and shear levels. In this case, the number of particles would be independent of radical flux. In fact, the most compelling evidence for droplet nucleation is experimental evidence that the number of polymer particles is independent of the initiator level. (Once the radical flux is high enough to nucleate all, or nearly all of the droplets, then changes in radical flux caused by inconsistent initiator or unknown inhibitors will not affect the final particle number.) We will discuss the results of such robust nucleation later. 

Reimers [95] used polymeric costabihzer to carry out miniemulsion polymerization of MMA. Droplet nucleation was found to be the dominant nucleation mechanism in the polymerization. As a result, the nucleation was more robust, and the polymerizations were less sensitive to variations in the recipe or contaminant levels. This was evident in the rates of polymerization and in the particle numbers. The miniemulsion polymerizations were subjected to changes in initiator concentration, water-phase retarder, and oil-phase inhibitor, and were shown to be significantly more robust. 

Shifting the site of nucleation to the droplets greatly enhances the robustness of the nucleation process to recipe variations, inhibition levels, and changes in operating procedure (initiation rate and/or agitation rate). As a result of droplet nucleation, polymer-stabilized miniemulsion polymerizations are far less sen-... 

Meeting crystal product specifications with a robust, repeatable process requires careful control and balancing of nucleation and growth kinetics. Careful structuring of the environment can dictate the fundamental mechanisms of nucleation and crystal growth and their resultant kinetics. Undesired polymorphs can be often minimized or eliminated by suitable control of rate processes. 

In almost all industrial crystallizations, both nucleation and growth contribute to the final result (purity, morphology, size distribution) in a major way. All growth processes, emphasized in this book as being desirable for maximum control and robustness in many or most situations, still require an understanding of the nucleation properties of the system being studied in order to minimize the contribution of nucleation to the final result. 

Controlled supersamration at the initiation of the addition of the reagent(s) requires an initial charge of seed to minimize uncontroUed nucleation and the resulting creation of an excess number of particles. It is desirable to develop the seed in a separate operation because it is difficult to ensure a robust process with a reproducible seed level using only the intrinsic reaction. This issue and methods are discussed in Section 10.3.4 below. 

The basic CRP techniques and mechanisms are discussed in Chapter 4 here only those issues associated with the presence of water in the system are dealt with. The subject has been reviewed by several authors [206, 266, 267]. Perhaps the most important challenge in this field is the development of a robust and general ab initio emulsion process (without using a seed). An essential problem in this endeavor is to avoid the nucleation in monomer droplets, which causes colloidal instability. 

RAFT chemistry is probably the most versatile and robust one for polymerization in aqueous dispersions amongst the different CRP techniques. The RAFT agent is bonded to the polymeric chains and therefore does not tend to partition back to the aqueous phase. However, this type of systems also exhibits problems of colloidal stability. In miniemulsion systems, these are attributed to nucleation in monomer droplets and superswelling and can be... 

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