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Solid propellant is a fuel for rockets. Composite propellant is a kind of solid propellant and this propellant is produced by the following method. The first step is the mixing of the oxidizer, binder, burning rate modifier, plasticizer, metal fuel, etc. The next involves the removal of air from the mixture. The final step is casting the propellant to a rocket motor. Hydroxyl-terminated polybutadiene (HTPB) has generally been used as the binder for composite propellants. The liquid plasticizer and cross-linker are added to the propellant to improve processing properties, such as mixing viscosity, casting, and pot life, as well as to improve mechanical properties, such as strength, elongation, and modulus. Researchers have recently developed several energetic plasticizers and investigated their effects to improve the mechanical properties of cured HTPB after the addition of these plasticizers. However, the processes involved in the synthesis of energetic plasticizers are complicated, therefore it is difficult to manufacture these plasticizers industrially. Currently, these energetic plasticizers are not used for practical applications because they are expensive. Polytetrahydrofuran (PTHF) is an ingredient of rubber products that is produced in several different molecular weights. In addition, this polymer is mass-produced commercially and is inexpensive. The chemical structures of PTHF and HTPB are shown in Fig. 1. PTHF has a linear molecular structure. The repeating unit of PTHF consists of a single bond and has one oxygen atom, four carbon atoms and eight hydrogen atoms. PTHF has a OH group on one side of the molecular chain of PTHF and a hydrogen atom on the other side so that both sides of the molecular chain have a hydroxide group. Then, PTHF is a di-ol and the molecular structure of PTHF is similar to that of HTPB. The difference is that the first one has saturations while the second one has unsaturations. Because there is oxygen in the repeating unit of PTHF, Dynamic Mechanical Properties of Hydroxyl-terminated Polybutadiene Containing Polytetrahydrofuran as a Plasticizer