Functionality distribution and crosslink density of hydroxyl‐terminated polybutadiene

Abstract A novel approach is proposed for estimating the average molecular weight between crosslinks (M̄ c ) from the functionality distribution of hydroxyl‐terminated polybutadiene ( HTPB ). The functionality distribution of four free‐radically polymerised HTPB prepolymers of varying hydroxyl content and molecular weight was determined by a combination of preparative and analytical gel permeation chromatography. The gumstock properties of the samples cured with stoichiometric amounts of toluene diisocyanate were not correlatable with the relative amounts of difunctional chain extender and multifunctional crosslinker present, unlike the case of HTPB with similar hydroxyl content and molecular weight. However, the mechanical properties and sol content could be correlated with the average molecular weight between crosslink sites, M̄ c , of the cured polymer. The M̄ c values derived by our method compare well with those of classical methods, and the observed differences are attributed to segmental entanglement. These M̄ c values give consistently good correlation with all the gumstock properties, confirming the validity of our approach and the soundness of the techniques developed for the determination of the functionality distribution of HTPB .