Correlation of binder mechanical properties with functionality type and molecular weight distribution for hydroxy‐terminated polybutadienes: 13 C‐NMR and SEC studies

Hydroxy‐terminated polybutadiene (HTPB) liquid prepolymers ( I‐IX ), with closely similar OH values, M̄ n , viscosity, and microstructure, were synthesized by H 2 O 2 ‐initiated free‐radical polymerization of butadiene in an isopropanol–water solvent system. However, binders obtained by curing HTPB polymers showed wide variations in their sol content, cross‐link density, and mechanical properties. These variations were atributed to the differences in functionality‐type distribution (FTD) and molecular weight distribution (MWD) of the polymers. FTD was interpreted in terms of ‐type primary hydroxyls. [ V / G ] showed excellent linear correlation with the elongation at break ( E ) of the binders. The polymers exhibited bimodal MWD and this behavior was discussed in relation to two different initiation processes occurring. The high molecular weight fraction ( F h ) of the various polymers was determined by reverse‐phase HPLC and showed good correlation with E . The sol content and cross‐link density of the binders also showed good correlation with the parameter V/GF h . The parameters [ V/G ] and F h provide acceptance criteria for blending different batches of polymers for propellant formulations. © 1993 John Wiley & Sons, Inc.