The external monomers N-methyl-N,N-diethanolamine, N-butyl-N,N-diethanolamine, N-(2-cyanoethyl) diethanolamine, and N,N-dihydroxyl-3-aminmethyl propionate were synthesized by modification of diethanolamine to evaluate the effects of external monomer molecular structure on the interfacial interactions of borate ester bonding agent (BEBA) applied in four-component hydroxy-terminated polybutadiene (HTPB) propellants. The compounds were characterized by IR and NMR spectroscopy. A series of BEBAs were synthesized using the as-synthesized external monomers as well as diethanolamine. The interfacial interactions are evaluated using thermodynamic methods including acid-base interaction enthalpy and wetting theory among the components in HTPB propellants. The experimental results indicate that the addition of BEBA-3 prepared from N-(2-cyanoethyl) diethanolamine can significantly improve the interaction ethalpies and surface properties of the components in HTPB propellants. The results from the mechanical properties testing and SEM observations are in accordance with the results from the thermodynamic analysis of the interfacial adhesion interactions.