Preparation of Macroporous Crosslinked Copolymer Spheres (Macroporous Ion Exchange Resin) (II)

Preparation of Macroporous Crosslinked Copolymer Spheres (Macroporous Ion Exchange Resin) (II)

  • 2022-10-13
As a result, the preparation of macroporous ion exchange resin copolymer spheres can be divided into the following types:

(1) Suspension copolymerization of macroporous ion exchange resin with non good solvent as porogen: the method is to mix monomer, initiator and porogen together for suspension copolymerization. From a single sphere, this is a micro precipitation copolymerization reaction. At the beginning of polymerization, polymers with short molecular chains can still be dissolved in the mixed system of monomers and pore forming agents. With the growth of macromolecular chains, phase separation occurs. The system becomes a polymer phase (swelled by monomers) and a solvent phase (containing monomers and a small amount of polymers). The continuous polymerization of monomers in the copolymer phase makes the polymer chains intertwined with each other, reducing the swelling, and the polymer phase becomes spherical under the effect of interfacial tension. The continuous polymerization of monomers in the polymer phase and the intertwining of macromolecular chains lead to gelation, forming a gel microsphere or a nucleus. With the progress of copolymerization and the continuous generation of microgel cores, microgel cores converge into clusters, and the polymerization of monomers in the clusters makes microgel cores linked together, becoming micromicelles or microspheres.

In the second step of macroporous structure formation, the copolymerization continues, and the microspheres also gather and link together, while the concentration of monomers in the solvent phase becomes smaller and smaller. The ratio of crosslinking agent to monomer is also changing constantly. In fact, in most cases, the copolymerization system is not a constant copolymerization system with r1=1 and r2=1. For the styrene diethylene system, at this stage, the ratio of divinylbenzene to styrene is less than the initial ingredient ratio. Therefore, the cross-linking structure of the microspheres connected together is uneven due to the different time of microgel core formation.

In the third stage of macroporous structure formation, the concentration of monomer in the solvent phase becomes very thin, the polymerization reaction has been very slow, and the copolymerization process has basically ended. When the pore forming agent is distilled out by steam distillation, the residual monomer of solvent phase is transferred into the polymer phase, and the final copolymerization is completed in the polymer phase. The microspheres connected together were also immobilized to form the final macroporous structure.

The formation process of macropore structure can be further explained by the following formula:
Aggregation of macromolecular chains - → core - → microsphere - → spherical macroporous resin

5~20nm             60~500nm         0.1~1.0mm
(estimated) (electron microscope)

The transition stage from relatively loose microgels to a more compact aggregation state. The pores in the micelles are continuously filled by the newly formed polymer, and the pore volume and pore diameter gradually become smaller. The monomer conversion reaches the highest level, and the volume of colloidal particles in the micro micelles shrinks, which increases the gap between the polymer particles. Not only the aperture and pore volume increase rapidly, but also the aperture becomes more uniform. At the end of polymerization, the macroporous structure of the copolymer is finally formed due to the curing of the polymer, and the pore volume reaches the maximum value. However, in the stage of steam distillation, due to the evaporation of the porogen, the shrinkage of the copolymer sphere and the "collapse" of the pore structure are caused. The pore volume decreases, and the pore diameter becomes smaller.

The resulting macroporous copolymer contains pores of three structures.
1) The space between molecular chains of gel pores in swelling state and microgel nucleus.
2) A pore in a microporous microsphere between the colloidal core or its micelles. The pore diameter ranges from several to tens of nm and forms the inner surface.

3) The pore between macroporous microspheres and their agglomerates, with a pore diameter of more than 25 nm.

The pores with these three structures are the three pores that finally form macroporous ion exchange resin.

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