| A stable cationic polyacrylamide (CPAM) aqueous dispersion was synthesized by aqueous two-phase copolymerization of acrylamide (AM) and cationic comonomer in aqueous poly(ethylene glycol) (PEG) solution with 2,2’-azobis[2-(2-imidazolin-2-yl)propane]-dihydrochloride (VA-044) as initiator. The dispersion contains none of organic solvents, and has high solid content and good fluidity, and thus it can be widely applied in plenty of industrial fields such as wastewater treatment, papermaking, and the oil industry. In this work, the critical phase sepration in aqueous two-phase copolymerization was investigated, and the effects of aspects on the critical conversion and critical molecular weight were discussed. The unique phenomenon of double phase separation was observed when the molar fraction of cationic comonomer was high, and the mechanism for that was proposed. Thereafter, the evolutions of droplet morphology and size in the polymerization process were traced and a mechanism for the droplet formation and stabilization was brought up. On the basis of phase separation and droplet formation mechanisms, the effects of aspects on the polymerization kinetics were discussed, and an optimized reaction condition was confirmed. At last, the effects of inorganic salt on the aqueous two-phase copolymerization system were studied, and a mechanism for the role of anions with different valence was proposed.The molar fraction of comonomer was an important aspect that affected the phase separation in the aqueous two-phase copolymerization system. When the molar fraction of cationic commoner was low, only one phase separation was observed. With the increase of initiator concentration or temperature, the critical conversion kept constant, and the critical molecular weight decreased. With the increase of monomers concentration or PEG concentration, the critical conversion decreased, and the critical molecular weight increased first and then decreased. Increasing PEG molecular weight, the critical conversion decreased and the critical molecular weight increased. Increasing the molar fraction of cationic comonomer, the critical conversion increased, while the critical molecular weight decreased.When the molar fraction of cationic comonomer was increased higher, a phenomenon of double phase separation was observed in the aqueous two-phase copolymerization system, in which an insoluble-soluble transition existed. With the increase of monomers concentration or PEG concentration, the insoluble-soluble transition appeared first and then disappeared. With the increase of PEG molecular weight, the transition appeared and became more obvious. And with increasing reaction temperature, the double phase separation was sped up. The variation of copolymer composition during the polymerization was determined, and it was found that the fraction of cationic comonomer unit had a higher value in the insoluble-soluble transition, indicating that the solubility enhancement of polyelectrolyte was a key aspect to induce the transition. Further studies were carried out on the phase separation of aqueous two-phase copolymerization of AM and weakly charged comonomer, or even anionic comonomer, and it was found that the decrease of ionic comonomers during the polymerization was also contributed to the appearance of insoluble-soluble transition. On the basis of all the results, a mechanism for the phase separation in the aqueous two-phase copolymerization of AM and ionic comonomers was proposed.The evolution of droplet morphology and size during the aqueous two-phase copolymerization was traced by scanning electron microscope (SEM), and a four step mechanism for the droplet formation and stabilization was brought up, that was formation-growth-aggregation-growth. The cationic copolymer acted as a stabilizer in the whole process of droplet formation, and it was just the reason for the good stability of CPAM dispersions. At the early stage of droplet formation, copolymer chains precipitated from the continuous phase and formed little droplets, and these droplets grew by the polymerization in them. With the polymerization progressing, the fraction of cationic comonomer unit in the copolymer chains decreased and thus the electrostatic repulsion between droplets decreased as well, leading to the aggregation of little droplets. As a result, large droplets appeared and their number increased with increasing the conversion. However, with little droplets aggregated to form large droplets, the specific surface area of droplets was decreased and thus the electrostatic repulsion between droplets was strengthened again. The aggregation between large droplets decreased, and they grew by the polymerization in them and the aggregation with little droplets.The conversion-time curves for aqueous two-phase copolymerization system under different conditions were investigated, and an optimized reaction condition was confirmed. The effects of aspects on the polymerization rate were discussed as well. With the increase of initiator concentration, monomers concentration, or reaction temperature, the polymerization rate increased. Increasing PEG concentration, the polymerization rate was affected little when PEG concentration was low, while it was decreased when PEG concentration was high. After introduced cationic comonomer, the polymerization rate was obviously slowed.The effects of inorganic salt on the phase separation, polymerization kinetics, and droplet formation in the aqueous two-phase copolymerization of AM and cationic comonomer were studied systematically, and great difference was found between the effects of monovalent anions and multivalent anions. After added monovalent anions, the critical conversion was decreased, while the other aspects were the same as the situation without adding salt. However, after added multivalent anions, not only the critical conversion was sharply decreased, but a unique droplet formation process was observed. Further investigations illustrated that, the monovalent anions mainly played a screening effect on the polyelectrolyte, while the multivalent anions had both effects of screening and bridging. When the salt concentration was high, the bridging effect of multivalent anions became obvious in the droplet formation, and thus led to the appearance of regular and large droplets. The results provided theoretical base for the controlling of aqueous two-phase copolymerization and the final CPAM dispersion by adding inorganic salts. |
