Core-shell particles polymerization kinetics effects

Practical methods can be used to ensure that the required particle morphology is obtained. A high Tg or molar mass for the hrst-formed polymer forces the second-formed polymer to reside on its surface due to kinetic effects. The use of an inherently more water-soluble second polymer as compared to the first will also contribute to obtaining the desired morphology. Inverted core-shell latexes are most readily obtained when a more hydrophobic monomer for the second-formed polymer is polymerized in the presence of a less hydrophobic polymer latex. An alternative is to render the second monomer coUmdaUy unstable ufien polymerized, forcing it to reside in the core of the first formed polymer. 

This section is principally devoted to the preparation of thermally sensitive hydrogel particles (microgels and core-shells) using batch polymerization process. The effect of each reactant and parameter (initiator, temperature, cross-linker agent) on the polymerization process (polymerization kinetic, conversion, final particle size, morphology, water-soluble polymer, etc.) is presented and discussed. Special attention will be focused on the functionality of the elaborated thermally sensitive particles. 

The emulsion copolymerization of vinyl acetate and butyl acrylate has received considerable attention. The butyl acrylate confers improved film forming characteristics to the polymer. The disparities in their water solubilities and of their individual polymerization rates may help to explain the variations in reactivity ratios that have been reported [170,171]. The variation in reactivity ratios may also by related to the following observations The reaction method has an effect on the morphology of the polymer particles. In a batch emulsion process, a butyl acrylate—rich core is formed which is surrounded by a vinyl acetate-rich shell, in a process in which the monomers are fed into the reactor in a semicontinuous manner, particles form with a more uniform distribution of the monomers [172]. The kinetics for a batch process indicates that the initially formed polymer is indeed high in butyl acrylate. As this monomer is used up, eventually a copolymer high in vinyl acetate develops. It is this latter polymer which forms the final shell around the particles. 

---