Formation,structure and properties of polymer networks: gel‐point prediction in endlinking polymerisations

Gel points, predicted using Ahmed‐Rolfes‐Stepto (ARS) theory and a Monte‐Carlo (MC) simulation method accounting for intramolecular reaction, are compared with experimental data for polyester (PES)‐, polyurethane (PU)‐ and poly(dimethyhl siloxane) (PDMS)‐forming polymerisations. The PES and PU polymerisations were from stoichiometric reactions at different initial dilutions and the PDMS ones were from critical‐ratio experiments at different dilutions of one reactant. The predictions use realistic chain statistics to define intramolecular reaction probabilities and employ no arbitrary parameters. Universal plots of excess reaction at gelation versus ring‐forming parameter are devised to enable the experimental data and theoretical predictions to be compared critically. It is shown that various gel points can be predicted by MC simulations, depending on the criterion for gelation used. Due to the lengthy computations needed and the uncertainties in the predictions, MC simulation is not a viable approach. Although inconsistencies are noted in the measured gel points, so that a unified interpretation of the data cannot be achieved, ARS theory is shown to be the preferred basis for gel‐point prediction. It is also concluded that, before one can be certain of agreement between experiment and predictions, more experimental systems at different initial dilutions and ratios of reactants need to be studied and the various methods used for detecting gel points need to be compared.