The gel point as reference state: A simple kinetic model for crosslinking polybutadiene via hydrosilation

Abstract

Kinetic data can be described in a simple model when using the gel point as reference state. This has been shown for crosslinking data on five high molecular weight polybutadienes (18 100–97 000 g/mol). Experiments were carried out under isothermal conditions with a constant amount of catalyst. Crosslinking took place by a hydrosilation reaction at vinyl sites which were randomly distributed along the backbone of the polybutadiene precursor molecules using the bifunctional aromatic silane p-bis(dimethylsilyl) benzene as connector. FTIR measurements showed that the hydrosilation reaction followed first order kinetics. The critical gel time, t_c, was determined by the instant at which power law relaxation behavior was observed (i.e. tan δ = constant) over the two lowest decades in frequency. The main parameters that were observed to affect t_c were the stoichiometric ratio, r, and the functionality of the polybutadiene, f. It was observed that the time to reach the gel point decreased with increasing precursor molecular weight (~f) of the polybutadiene. The gel time also decreased when increasing the initial amount of silane crosslinker. We were able to find a relationship between the gel time measured by the CFS technique and r and f by coupling the Flory-Stockmayer gelation theory with a simple first order kinetic curing scheme.