INITIAL DROPLETS FORMATION AND GROWTH PROCESS OF ACRYLAMIDE DURING AQUEOUS TWO-PHASE POLYMERIZATION IN AQUEOUS POLY(ETHYLENE GLYCOL) SOLUTION

Abstract

In order to explore the droplet formation process of the aqueous two-phase polymerization of acrylamide(AM) in aqueous poly(ethylene glycol)(PEG) solution,dynamic light scattering(DLS) was employed to investigate the appearance,the size and size distribution of droplets in the initial stage of polymerization.The effects of PEG molecular weight and concentration on the droplets size and distribution were studied.It was found that the droplets stability in aqueous PEG solution decreased,and the droplets aggregated with each other to form larger and more uniform droplets with decreasing PEG molecular weight or PEG concentration.The morphology of the droplets rich in polyacrylamide(PAM) was observed with transmission electron microscopy(TEM).The droplets shape in the initial stage of polymerization is spherical but polydisperse.However,some oval droplets formed in the product,while the conversion was still very low,which indicated some coalescence of droplets in this stage.As the polymerization proceeds,the droplet size increased rapidly and the size distribution of droplets become uniform.These results were well in agreement with the DLS results.With using the spectrophotometer,the phase separation point was determined where the transparence of reaction solution declined suddenly.The effect of various parameters on the critical conversion of phase separation was measured by bromating method.It decreased with the increase of PEG molecular weight or PEG concentration.With raising temperature from 30℃ to 70℃,the critical conversion decreased at first,and then turned to increase when the temperature was at about 50℃.The critical molecular weight was also systemically investigated by gel permeation chromatogram(GPC).It could be seen that the evolution of critical molecular weight corresponded well with that of critical conversion at various reaction conditions.On the basis of above mentioned results,an initial droplet formation and growth mechanism of AM aqueous two-phase polymerization in aqueous PEG solutions was proposed.As the reaction starts,primary radicals,generated by decomposition of the initiator,grow in the continuous phase by the addition of monomer units until they reach their critical chain length.Then these polymer radicals precipitate and aggregate to form nuclei.The nuclei are not stable,and they will immediately aggregate with each other to form tiny droplets.Then monomer and initiator diffuse into the droplets rapidly,and polymerization takes place in both the continuous phase and the dispersed phase(inside the droplets).In the earlier stage,the PAM droplets are stable and grow mainly by polymerization inside the droplet.Because new smaller droplets generate from continuous phase at the same time,the droplet size distribution becomes wide.However,these droplets become unstable with the polymerization and aggregate with each other to make the droplet size distribution become narrow again.