| The behavior of the free water-soluble oligomers found in a model emulsion polymerization system consisting of styrene, butadiene and acrylic acid was investigated during the polymerization process, as well as their relevance to the kinetics and mechanisms of the particle nucleation and growth. The emulsion polymerizations were carried out in batch. Ultracentrifugation was used to accomplish the separation of the aqueous phase from the particle phase. {dollar}sp1{dollar}H NMR, aqueous phase GPC, and {dollar}sp{lcub}13{rcub}{dollar}C NMR techniques were used to determine the oligomer concentration as well as the composition, molecular weight, and partial sequence distribution, respectively. Emulsion polymerizations were carried out under different conditions such as temperature, initiator concentration, acrylic acid concentration, ionic strength, and surfactant concentration.; The oligomer concentration vs. conversion curve for the model system had a maximum at around 14% conversion, and the same trend was observed in all systems investigated. The position of this maximum with respect to conversion depended strongly on the surfactant concentration in the aqueous phase, and varied with the other polymerization conditions.; Measurement of the unreacted AA vs. conversion revealed that more than 55% of the AA monomer in the 4 mole% AA system reacted after {dollar}sim{dollar}80% overall conversion of the monomers due to the high water solubility of the AA.; Based on the experimental results, the role of water-soluble oligomers in St/Bu emulsion polymerizations with acrylic acid present may vary during the polymerization process, and can be described according to three periods: (1) particle generation; and (2) before and (3) after reaching the c.s.s.c. during the particle growth period. AA incorporated into the oligomers during periods (1) and (2) results in the generation of more and stable particles through the steric stabilization by the uncharged carboxyl groups in the copolymer. In the later stages of the reaction, high AA-containing copolymer is formed which may coagulate particles and destabilize the latex.; It was found that the water-soluble oligomer molecular weight varied slightly with increasing conversion and showed a peak in the early stages of the polymerization. The maximum critical chain length (j{dollar}sb{lcub}rm crit.{rcub}{dollar}) for the model St/Bu/AA systems ({dollar}sim{dollar}87) was shown to be much higher than that in the St/Bu system ({dollar}sim{dollar}35) without any AA, and it varied with reaction conditions due to the increase in the water solubility brought about by the incorporation of AA in the oligomer chains.; A growth mechanism for the water-soluble oligomers involving reaction in both the aqueous and organic phases for polymerizations carried out above the CMC of SLS was proposed to explain why the fraction of butadiene units in the oligomers was the highest measured, instead of the AA. (Abstract shortened by UMI.)... |
