|
The objective of our research is to synthesize energetic polyether binders with the ability to undergo reversible physical crosslinking at temperatures compatible with high energy propellant ingredients. Commercially available thermoplastic elastomers fail to meet our criteria because of excessively high processing temperatures and melt viscosity, high modulus and low stress/strain capability. It was proposed to synthesize ABA type block polymers with the A blocks being polymers able to form crystalline structure (as opposed to glasses) having a sharp melting point of 85-95°C. This would enable phase miscibility in the melt, thus lowering melt viscosity, and reduce creep close to the melting point. ABA structures were chosen over (AB)n, enabling low molecular weight polymers to be synthesized, again lowering viscosity. In order to successfully achieve our objectives, it was necessary to synthesize precise polymer structures with little deviation from the required structure. After evaluating blocklinking techniques, we decided to employ living cationic polymerization with systematic monomer addition. To this end, we have examined numerous catalysts and co-catalysts. Our research at this time is centered on bis cumyl chloride/silver antimony hexafluoride in which we see promising results. The effects of polymerization temperature, catalyst formation, catalyst concentration, rate of monomer addition and monomer concentration were determined with respect to molecular weight control and structure. Polymer workup and purification, a problem in our initial studies, has been resolved. We have now synthesized polymers varying in molecular structure and correlated the resulting physical properties to structural changes. The effect of structure and molecular weight on melt viscosity and morphology is now being examined.
|
