Polymeric emulsifiers based on reversible formation of hydrophobic units.

摘要

This research has provided a fundamental characterization of the design, synthesis, and emulsification properties of reversible emulsifiers based upon a backbone of methacrylic acid and oligomeric grafts of polyethylene glycol). The two broad goals of this dissertation were (1) to identify the underlying relationships between the polymerization reaction conditions and the resulting polymer structure, and (2) to determine the effects of the polymer composition and molecular weight on the solution and emulsification properties of the copolymers. Meeting these two objectives has provided guidelines for the design and synthesis of this new class of emulsifiers.;A semi-batch reactor was successfully designed to synthesize a series of poly(methacrylic acid-g-ethylene glycol) of predictable compositions by a free radical copolymerization of methacrylic acid with the macromonomer methoxy polyethylene glycol) methacrylate. The relationship between the number average molecular weight and the initiator concentration was established.;The most important design conclusion from the solubility studies is that P(MAA-g-EG) copolymers with a PEG content of more than 10 wt.% are unsuitable for emulsifiers because they are too hydrophobic under acidic conditions, and precipitate out of solution. However, the pH-dependent solution properties of copolymers with an EG content of 2.5 and 5 wt.% make them very appropriate for reversible emulsifiers. The potentiometric titrations confirmed that the presence of the complex decreases the acidity of the carboxylic acid moieties. Visual emulsification studies confirmed that the emulsions could be formed, broken, and reformed by adjusting the pH from acidic, to basic, and back to acidic conditions.;The dynamic drop tensiometry studies led to the conclusion that the ability of the copolymers to stabilize emulsions arises primarily from their ability to prevent droplet coalescence after they are formed, and not by reducing the interfacial free energy of the system. The LSCM technique revealed that the average droplet size was essentially independent of both the copolymer composition and molecular weight, and the number average droplet diameters were found to be in the range spanning from 8 to 12 microns. In addition, the average droplet size was found to be essentially independent of time.;The 2-D NOESY NMR results also indicate that, in the complex, the protons in the ethylene glycol repeat unit are spatially closer to the alpha-methyl than they are to the methylene. Molecular modeling results suggest that this effect could arise from constraints placed on the PMAA backbone in order to accommodate the formation of a complex with a one-to-one repeat unit ratio. The polarity-sensitive fluorescence results indicate that there is a marked dependence of the aggregation behavior on the molecular weight of the P(MAA-g-EG) copolymers. For example for a given copolymer composition, the aggregates formed by the higher molecular copolymer may appear to be less polar than those formed by the lower molecular weight copolymers. In addition, it was found that, at a given composition, the critical aggregation concentration decreases as the molecular weight is increased.