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-Ca rlo (MC) simulation method accounting for intramolecular reaction, are comp ared with experimental data for polyester (PES)-, polyurethane (PU)- and po ly(dimethyhl siloxane) (PDMS)-forming polymerisations. The PES and PU polym erisations were from stoichiometric reactions at different initial dilution s and the PDMS ones were from critical-ratio experiments at different dilut ions of one reactant. The predictions use realistic chain statistics to def ine intramolecular reaction probabilities and employ no arbitrary parameter s. Universal plots of excess reaction at gelation versus ring-forming param eter are devised to enable the experimental data and theoretical prediction s to be compared critically. It is shown that various gel points can be pre dicted by MC simulations, depending on the criterion for gelation used. Due to the lengthy computations needed and the uncertainties in the prediction s, MC simulation is not a viable approach. Although inconsistencies are not ed 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 a greement between experiment and predictions, more experimental systems at d ifferent initial dilutions and ratios of reactants need to be studied and t he various methods used for detecting gel points need to be compared.