Optimization of Factors Affecting the Synthesis of Polypropylene Glycol/Polyglycidyl Nitrate/ Polypropylene Glycol Triblock Copolymer and Evaluation of Its Thermal Properties

Hypothesis: Polyglycidyl nitrate (PGN) is used in the manufacture of propellant elastomers due to its properties such as highly energetic composition, high density, high oxygen balance, high explosion enthalpy and suitable compatibility with other components. In addition to its desirable properties, this polymer has disadvantages such as high glass transition temperature, poor mechanical properties, and low content of total solid. Also, its elastomer can undergo decuring process. To remedy these disadvantages, its copolymers are prepared using polymers with optimal thermal and mechanical properties. Methods: In this research an energetic polypropylene glycol/polyglycidyl nitrate/polypropylene glycol (PPG/PGN/PPG) triblock copolymer was synthesized for the first time by cationic ring-opening polymerization of propylene oxide and PGN as macroinitiator, in the presence of boron trifluoride etherate (BF3.OEt2) as the catalyst. The effect of temperature and catalyst content on molecular weight and reaction yield was investigated. The obtained product was characterized by FTIR, GPC, and 1H and 13C NMR spectroscopy. Also, the thermal properties of the copolymer were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Finding: The results showed that by increasing the temperature from 0 to 15°C, the conversion and yield increased. On the other hand, due to the low boiling point of propylene oxide (34°C) and the exothermic reaction, it is impossible to increase the reaction temperature above 15°C. By increasing the catalyst content from 0.2% to 1% by weight of the initiator, the polymer molecular weight increased and the highest yield was achieved in presence of 1% by weight of the catalyst, but by increasing the catalyst content from 1 to 1.5 weight percent, yield and molecular weight have decreased due to the development of adverse reactions. Studies by TGA and DSC showed an increase in thermal stability and a decrease in glass transition temperature of the copolymer compared to PGN.