dc.description.abstract | This study firstly synthesized low-molecular weight distribution polymers using group transfer polymerization i.e., the polymers were prepared by using methyl methacrylate (MMA, M), methacrylic acid (MAA, A) or 2-(dimethylamino)ethyl methacrylate (DMAEMA, B) as monomers. DSC investigated the variations of glass transition of the polymers and the miscibility between polymer chains. Subsquently, miniemulsions based on Styrene/AIBN in the presence of hydrophilic-hydrophobic copolymers as the surfactant were developed. Finally, monodispersed microspheres were synthesized by using polyampholyte triblock copolymers as the surfactant.
The data of DSC showed that the blended copolymers composed of PMMA and PDMAEMA possess less miscibility. By using both hard segment of MMA and soft segment of DMAEMA monomers for copolymerization, the block copolymers present a partial miscibility, however, the random copolymers possess a good miscibility. On the other hand, we also found that the change of glass transition temperature mainly affected by soft segment of polymer than by hard segment of polymer.
A hydrophilic-hydrophobic diblock copolymer with MMA and DMAEMA segments could be function as an effective surfactant in a miniemulsion polymerization. When the concentration of the diblock copolymers was 1 or 2 times of its CMC, the nucleation mechanism of miniemulsion polymerization would include both droplet and aqueous nucleation. The effect of the composition ratio of the diblock copolymer on the stability of the miniemulsion droplets was also investigated. The data showed that the length of hydrophobic segments in the diblock copolymers dominates the stability of the droplets. The miniemulsion droplets in presence of diblock copolymers with a longer hydrophobic segment (e.g., M36) as the surfactant presented less stability. The length of hydrophilic segment would show a less effect on the stability of miniemulsion droplets as the length of hydrophobic segments in the diblock copolymers was shorter.
When the diblock copolymer surfactant was further extended to triblock copolymers, the polyampholyte polymer, data showed that the triblock copolymers could also function as an effective surfactant. A miniemulsion polymerization using triblock copolymer as the surfactant was also followed the droplet nucleation mechanism. However, since the droplet size a miniemulsion polymerization system in the presence of sodium dodecyl sulfate (SDS) as the surfactant was smaller than that in the presence of triblock copolyemr (B12M8A12), and it led to faster polymerization rate.
For the triblock copolymers with the same length of hydrophilic segments (e.g., B12M4A12, B12M8A12, B12M16A12), their lengths of hydrophobic segments also dominate the stability of the droplets. Among them, the surfactant efficiency of B12M16A12 is almost the same as that of B12M8A12 but higher than that of B12M4A12. However, when they were used as the surfactant in miniemulsion polymerization systems and if the concentrations were high up to 10mM, the molecular weight distributions presented two peaks at lower level of conversion and single peak at higher level of conversion. This can be the evidence of the aqueous nucleation taken place at the initial stage of polymerization.
The monodisperse-polystyrene microspheres could be obtained by following conditions: the surfactant concentration (B12M8A12) of 2 times CMC, initiator concentration of 2.5wt% (AIBN), acetone of 20 vol%. On the other hand, the triblock copolymers with lower molecular weight (e.g., B12M8A12 and B12M12A12) prefer to be used to prepare monodisperse microspheres. | en_US |