Application of micromixers - Micromixers Fundamentals Design and Fabrication - page 326

proper dispersal of the heat generated during the polymerization process. Local hot spots and local
concentration gradient can be avoided. A local zone with high concentration of monomers leads to the
generation of polymers with high molecular weight, which allows monomers to diffuse and react.
Thus, precipitation and fouling of the reactor may occur.
Micromixers were used for the production of acrylates [28] . The distribution of molecular weight
of the polymers formed in micromixers show a significant improvement as compared to the macroscale
counterpart. Large-scale production of polymers was achieved by numbering up the reactors [29] . The
problem associated with numbering up is the uniformity of the flow due to the high viscosity of the
reactive medium and possible clogging of some mixing channels. Iwasaki et al. [29] demonstrated that
an amount of a few kilograms of PMMA can be produced continuously over several days at a constant
temperature, pressure, and product quality.
Living radical polymerization (LRP) is a controlled free radical polymerization. This reaction
allows control of the polymeric structure at molecular level leading to the synthesis of customized
polymeric materials. The term “living” means that the polymer chain never ends by a terminal reaction.
The polymer continues to grow if monomers are available. Thus, the polymer can be designed to have any
molecular weight suitable for an application. A recently demonstrated LRP process is called atom transfer
radical polymerization (ATRP). Homopolymerization of 2-hydroxypropyl methacrylate by ATRP was
carried out in a micromixer [30] . The performance in a microreactor is similar to that in a macroscale
batch process. The microreactor can be used for screening the compositions for ATRP. Furthermore,
micromixers can be used for mixing viscous living polymer melts with less viscous monomer.
Cationic polymerization is a polymerization process where a cationic initiator transfers charge to
a monomer, making it reactive. This reactive monomer continues to react in the same manner with
other monomers to form a polymer. Monomers with electron-donating substituents and heterocycles
such as olefins are needed for cationic polymerization. Cationic polymerization reactions are sensitive
to the type of solvent used. Controlled living cationic polymerization based on cation stabilization was
realized in micromixers [31] . Isobutyl vinyl ether (IBVE) was polymerized with a narrow molecular
weight distribution in a parallel lamination micromixer. Fast mixing of initiator and monomer in
micromixers allows a fast initiation. The ratio between initiator and monomer can be well controlled,
leading to controlled molecular weight and its distribution.
9.1.5 Particles and emulsions
Improved mass transport in micromixers allows the implementation of controlled formation of solid
particles. Solid-forming reaction can be realized in micromixers. Attention is to be paid to prevent
contact between the formed solid particles and the channel wall, leading to possible fouling and
blockage of the microchannels. One possible solution for the blockage problem is realizing the
reaction with droplet-based microfluidics. The particles are formed and contained in droplets, which
are transported by an immiscible phase.
In the production of pigments and colorants, fast mixing, particle size, and size distribution
determine the quality of the product. Fast mixing in micromixers allows the production of smaller
particles and narrower size distribution [32] . The high quality obtained with micromixers can be
maintained for large-scale production by numbering up the microreactors [33] .
The precisely controlled reaction conditions in micromixers make them suitable for the production
of quantum dots. A quantum dot is a semiconductor nanocrystal whose excitons are confined in all