Polymeric networks for solid propellants via huisgen azide-alkyne cycloaddition reaction

Abstract

Triazole polymers have been studied as a replacement of the state-of-the-art polyurethane binder for solid propellants and explosives. Conventional polyurethane binders which have been extensively used in solid propellants are formed by the step-growth polymerization of hydroxyl-terminated polybutadiene (HTPB) and polyfunctional isocyanate. But, the polyurethane curing system which has been prefered by propellant formulators has a few shortcomings to be resolved. Especially, the moisture-sensitive curing system has required a stringent action that eliminates moisture present both in ingredients of propellant and in atmosphere of manufacturing site. Moreover, the urethane curing system has raised more problems in high energy propellants which contain a high level of nitrate ester energetic plasticizer. Therefore, a new curing system for solid propellants to replace the old urethane curing system has been asked for a long time.

This thesis deals with the preparation and characterization of polymeric networks crosslinked via triaozles obtained by the Huisgen azide-alkyne cycloaddition reaction (AAC) without any catalysts. Polymeric networks were prepared through a 1,3-dipolar cycloaddition of azide-bearing polymers with a variety of compounds having two or three terminal alkynes without solvents and catalysts. Dipolarophiles with an α-carbonyl underwent a very rapid Huisgen reaction within a few minutes to afford networks that were side-linked with triazole moieties. The reactivities of dipolarophiles were estimated by using frontier molecular orbital energies. To avoid the formation of defects in elastically ineffective networks, all polymer chain-ends were linked with urethane moieties, and very small quantities of azides were reacted with the dipolarophiles to link the pendent groups with the triazoles. Because the crosslinking densities of the energetic networks were inversely proportional to the reactivity of the dipolarophiles to the azides, the less reactive alkynes conveyed better mechanical properties to the networks prepared by using the Huisgen cycloaddition reaction compared to the more reactive alkynes. Solid composite propellants based on a polymer bearing azide pendent groups, glycidyl azide polymer (GAP), were prepared by urethane- or triazole-forming reactions and their combination. Dipolarophiles containing diacetylenic groups, such as bispropargyl succinate (BPS) and 1,4-bis(1-hydroxypropargyl) benzene (BHPB), were used as a curative in the triazole curing system. Among the three types of curing systems used for the preparation of GAP-based solid composite propellants, the dual curing systems provided the best mechanical properties, regardless of the oxidizer type included in the solid composite propellant. The dual curing system produced GAP-based solid composite propellants that exhibited a higher burn rate and a lower pressure exponent compared to the propellants obtained from a single triazole curing system. Good adhesion to the HTPB-based liner was also observed in the GAP-based solid propellants prepared from the dual curing system.

To study the polymeric networks formation through triazole moieties further, AAC reactions with azide chain-ends-terminated polymers with alkynes carrying electron-withdrawing group at beta-position such as BPS and BHPB were investigated. The polymeric networks with good mechanical properties were obtained, but only with high temperature and long-time. At $80\circ C$, the polymeric networks were not be formed in more than two weeks. To induce uncatalyzed AAC reactions at mild temperature, a variety of dipolarophiles carrying electron-deficient terminal alkynes were incorporated into the binder recipes. In addition, uncatalyzed AAC reactions in the absence of solvent were also monitored by real time FT-IR analysis, with various substituents next to the terminal alkynes at different temperatures. First-order kinetic analysis was used to determine the rate constants for uncatalyzed AAC reaction. Electron-deficient alkynes carrying an $\alpha-carbony$l undergo a fast Huisgen reaction within a few hours without any catalysts, proportional to the temperature. Less electron-deficient alkynes led to the decrease of AAC reaction rate significantly, revealing that the AAC reaction rate depends on the molecular structure of alkynes center.