| The semi-continuous aqueous precipitation copolymerization of acrylonitrile (AN)-acrylic acid(AA) and terpolymerization of acrylonitrile(AN)-methyl acrylate (MA)-itaconic acid(IA) initiated by KPS-NaHSO3 redox initiator system have been investigated.The effects of different comonomer,monomer ratio,feeding monomer concentration and temperature on polymerization kinetics,molecular weight and molecular weight distribution were systematically studied. The total polymerization activation energy was abtained, supporting the conclusion that temperature has no significant influence on polymerization rate or conversion. Equilibrium constants of H2SO3 at 60℃was experimently determined. The effects of pH value on ionization equilibrium of NaHSO3 were studied to have a detailed knowledge of how pH value affects redox initiation system. Its effects on polymerization kinetics and molecular weight were caused by altering the concentration of effective reductant as well as the stability between particles.Effects of various factors on stability of the polymerization system were attributed to their influence on the electrical double layer or hydrated layer around the particle surfaces. Terminal initiator residues of persulfate and anion groups of comonomer incorporated into the polymer chain co-exist on the particle surface, serving as stabilizers by forming electrical double layer. However, too much AA make particles unstable as a result of thickening the hydrated layer formed by electrostatic attraction of its carboxylic segment.Effect of various factors on the particle size and particle morphology have been exhaustively investigated. It was observed that factors resulting in better stability, including introducing proper amount of comonomer, decreasing the monomer feed concentration, increasing the pH value, are favorable to improve the particle morphology. Among these facators, pH value is the most significant one.The concentration of AN and AA in both the aqueous phase and particle phase was measured throughtout the polymerization. During batch polymerization stage, higher monomer concentraton was found in particle phase as converison increased. While during semi-continuous stage, the monomer concentration in particle phase deceased at a much faster rate than that in aqueous phase, so particle phase was assumed to be in starvation status. The obtained partition coefficients (K) indicated that monomers mainly exist in particle phase, and KAN>KAA.Studies on how the monomer ratio,feeding monomer concentration,pH value and temperature affect the monomer partitioning was carried out. Based on the experimental data, it seemed to be that the worse particle morphology was the greater amount of monomer swelled in particle. So highter partition coefficients was observed at higher AA ratio,greater monomer feed concentration or lower pH value.Modified Morton model was chosen for mathematical modeing of monomer partition. Water related Flory-Huggins interatction parameters were fitted by experimental data. The rest parameters were estimated by Hansen solubility parameters. Good agreement was found between measured and calculated results for volume fraction of each component in aqueous phase. However, slight discrepancy was found for particle phase. Because of the high solid content and accurence of coalescence among particles in this system, this model is not perfectly suitable and needs further improvements. Furthermore, monomer partitioning was no longer thermodynamics control only, due to the starvation status in particle phase at semi-continuous polymerization.The dependence of resulting polymer composition on the monomer concentration ratio in particle phase was also investigated. The variation of average AA content was effectively controlled as monomer and initiator were supplemented stage by stage. |
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