Molar Mass Distribution of Linear and Branched Polyurethane Studied by Size Exclusion Chromatography

Linear polyoxypropylene (POP) diol and star POP triol were end-linked using diisocyanate. Linear and branched polyurethanes (PU) of varying size were formed by varying the stoichiometric ratio r of isocyanate and hydroxyl groups. End-linked POP triol forms a gel for r > 0.55. Molar mass distributions of linear and soluble branched PU were determined using size exclusion chromatography as a function of r in the range 0−1. The fractions of the first few oligomers at different r are in good agreement with calculated values using mean field theory. Branched POP triol has a very broad molar mass distribution close to the gel point. At low degree of polymerization (DP < 10) the molar mass distribution is well described by mean field theory while for DP > 300 it is better described by the percolation model. The results imply that many earlier measurements on gel-forming systems reported in the literature were done on samples in the crossover regime.