The determination of reactive‐group functionality from gel point measurements

The usual method of calculating functionality is to divide the molecular weight by the equivalent weight. Because of the uncertainty of molecular weight determinations in the range 1000–20,000 a more precise method is needed. Several authors have published work concerning the determination of the extent of reaction at the gel point through the functionalities of the reactants. It occurred to us that this method could be reversed. We chose Stockmayer's treatment, with some changes, to calculate the average functionality of carboxyl‐terminated polybutadiene (CTPB): (PAPB)gel = (fE − 1)−1 (gE − 1)− where fE and gE are the weighted average functionalities of all molecules bearing the reactive groups A and B, and PA and PB are the fractions of initially present groups that have reacted. Two systems with an epoxide and glycerol as curing agents were investigated. The influence of dilution was investigated. Nonfunctional polybutadiene did not interfere with the accuracy of the determination of carboxyl functionality. By changing the ratio of epoxide to COOH groups from 0.6 to 1.4 it was shown that the calculated functionality remained constant. Weight‐average molecular weights for three CTPB samples were calculated from the functionality and the equivalent weight. They were compared with those obtained from intrinsic‐viscosity measurements. The precision of the functionality numbers is believed to be ±1%.