Droplet Formation Mechanism For Two-Phase Polymerization Of Acrylamide In Aqueous Poly(ethylene Glycol) Solution

A stable poly(acrylamide) (PAM) aqueous dispersion has been prepared via the aqueous two-phase polymerization of acrylamide (AM) in aqueous poly(ethylene glycol) (PEG) solution with 2,2'-azobis[2-(2-imidazolin-2-yl)propane]-dihydrochloride (AIBI) or ammonium persulfate (APS) as the initiator. This kind of product without containing any organic solvent has rapid dissolution rate in water, and can be used directly without any pollution, and therefore, it can be applied in many fields such as waste water treatment, paper making, spinning and printing industries. In this thesis, the interaction between PAM and PEG blend and in water, the partition of AM in the aqueous two-phase (polymerization) system, the evolution of droplet size and morphology during polymerization and the effects of various factors on the final droplet size and morphology were investigated systemically, as well as the droplet formation mechanism, and polymerization system stability for the aqueous two-phase polymerization of AM in aqueous PEG solution. Based on these studies, a kinetic model was proposed which could well predict the aqueous two-phase polymerization kinetics.IR,1H NMR and viscosity experiments were carried out to analyze the interaction between PAM and PEG in their solid blends and aqueous solution. It was confirmed that part of-O-groups of PEG molecular chain interacted with part of-NH2 groups of PAM molecular chain through hydrogen bonding in their relatively concentrated aqueous solution. The effects of the polymer molecular weight, temperature, and the addition of some compound with low molecular weight on the interaction between PAM and PEG in water were studied. The PAM molecular chains, especially having high molecular weights, preferred to form spherical clews in aqueous PEG solution, and these clews are surrounded with the PEG molecular chains through hydrogen bonding. Moreover, this kind of hydrogen bonding could be broken by raising the temperature.The effects of polymer concentration and molecular weight as well as the temperature on the AM partition in the aqueous two phase system of PAM-PEG-H2O were investigated. It was shown that the water content in each phase is vital to determine the AM partition. The AM partition coefficient decreased with the increase of PEG concentration or molecular weight, and with the decrease of PAM concentration or molecular weight. With temperature rising, the partition coefficient decreased until the temperature reached about 50℃, and then increased again while raising the temperature sequentially. Thereafter, the monomer partitioning behaviors during the aqueous two-phase polymerization process were investigated systemically.The droplet appearance and growth in the initial stage of the aqueous two-phase polymerization was followed by dynamic light scattering (DLS). Results show that the PAM droplet aggregates with each other significantly after a temporary period in which the droplet can exist stably. Therefore, the droplet size distribution become wide at first, and then become narrow again with the polymerization proceeding. The size and morphology of aqueous PAM droplet at every stage were observed by laser particle size analyzer and transmission electron microscopy (TEM). It was found that the small droplets are separated continuously from the continuous phase in the whole polymerization. At the relatively low conversion stage of polymerization, the droplet aggregation is significant. However, the droplet coalescence was restrained at high conversion stage because of the high viscosity. Finally, polydisperse droplets with both spherical and stripe shape were obtained. Based on these results, a mechanism of droplet formation and growth was proposed for the aqueous two-phase polymerization of AM in aqueous PEG solution.The influences of various polymerization conditions on the droplet size and morphology were studied systemically. It was found that the size and morphology of the droplets are closely dependent on the polymerization rate and monomer partition. The morphology of the droplets inclined to become stripe shape because of more droplet aggregation with increasing initiator or monomer concentration. Besides the polymerization rate, the polymerization temperature could also affect the droplet size and morphology in the hand of monomer partition. The increase of PEG concentration leads to generate bigger and round droplets at first, and then inclined to form popcorn droplets. This result strongly indicates that increasing the PEG concentration not only restrained the aggregation of the droplets, but also shortened the critical length of PAM radical chain to accelerate the droplet formation which does not favor the droplet stabilization. The addition of TBA and glycol to the polymerization system both decreased the droplet size; the former caused lots of aggregation to form many stripe shape droplets; however, the latter made the droplets become more stable. The effects of polymerization rate and system viscosity on the process stability of the aqueous two-phase polymerization were explored. A droplet aggregation period was found in the initial stage, in which the PAM coagulum is easy to be generated due to the high polymerization rate. Moreover, the viscosity evolution during the polymerization under various reaction conditions was determined on line. It was found that the viscosity was relevant to the phase separation. When the system viscosity is too high, the phase separation is very slow. If the polymerization rate is too rapid at this time, massive gel would be produced. All these results demonstrate that controlling appropriate polymerization rate is a key factor to keep the stability of the aqueous two-phase polymerization system.Flory-Huggins (FH) theory was applied to predict the composition of each phase during the whole polymerization. The prediction of monomer concentration, PAM and PEG mass fraction in each phase under various polymerization conditions agreed well with the experimental data. Based on FH theory, a model was established for simulating the polymerization kinetics of the aqueous two phase polymerization of AM in aqueous PEG solution. The theoretical calculation by the kinetic model showed that the partition of initiator, radical absorption from continuous phase and radical desorption from the dispersion phase have neglectable effect on the polymerization kinetics. The effects of initiator, monomer and PEG concentration on the polymerization kinetics were also studied systemically. It was found that the proposed kinetic model can predict well the polymerization kinetics over a wide range of various polymerization conditions.