Preparation of Macroporous Crosslinked Copolymer Spheres (Macroporous Ion Exchange Resin) (III)
When the macroporous ion exchange resins is used as a porogen in a bad solvent, the properties, dosage and crosslinking degree of the porogen have significant effects on the pore structure of the copolymer. With the increase of the amount of divinylbenzene and pore forming agent, the phase separation appeared earlier and the gel point also appeared earlier. The separated macromolecular chains are in a curled state, with more entanglements between chains, more stable structure, larger pore diameter, but smaller specific surface. Only when the amount of DVB and pore forming agent is large, can a higher specific surface area be obtained. The dosage of n-heptane has little effect on the average pore diameter, while the increase of DVB dosage makes the average pore diameter drop sharply.
The properties of pore forming agents have significant effects on the pore structure of copolymers, but there is no clear regularity.
Pore forming agents that can be used as bad solvents for macroporous ion exchange resins include alkanes, fatty alcohols, fatty acids, nitroalkanes and ethers, gasoline, kerosene, paraffin, etc.
However, in fact, absolute bad solvent systems do not exist. Even if it is an ideal bad solvent, its mixture with comonomer still has solvation effect on the polymer. In addition, it may be a bad solvent at room temperature, and the polymer chain may be solvated at polymerization temperature (such as 80 ℃).
(2) Suspension copolymerization with good solvent as pore forming agent: toluene is a typical good solvent used as pore forming agent. At present, toluene is used as pore forming agent for most macroporous ion exchange resins. The mixture of toluene and comonomers (styrene, divinylbenzene) is a typical good solvent system. In the whole copolymerization process of this system, the generated macromolecular chains have been in a fully solvated stretching state, with less intertwining, and phase separation occurs only when the monomer conversion is high and the cross-linked body structure is quite large. After the end of copolymerization and the solidification of the pellets, the copolymer is still in the swelling state. If the solvent is removed, the expanded network will shrink and even disappear. The macroporous structure can be obtained only when the amount of DVB and pore forming agent is large enough.
The pore structure of macroporous resin prepared with good solvent as pore forming agent is characterized by larger specific surface area and smaller pore diameter (dry resin). In the solvent, even in the non good solvent, the macromolecular network of the dry resin can still become an expanded state, indicating that the shrinkage of the expanded network is reversible.
The resin prepared by copolymerization in the presence of good solvent has a large swelling degree in solvent due to less entanglement of macromolecular chain, which is equivalent to reducing the role of crosslinking agent. In fact, the existence of toluene in the copolymerization system will affect the ion exchange equilibrium of the ion exchange resin as well as the degree of crosslinking.
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