Formation, structure and properties of polymer networks: theory and modelling

The application of a Monte-Carlo (MC) algorithm to account fully for loop formation in RA(2) + R'B-3 and RA(2) + R'B-4 polymerisations is described. The resulting interpretation of experimental elastic moduli of hexamethylene diisocyanate(HDI)-based polyurethane networks prepared at different dilutions shows it is essential to account for elastic losses in loop structures of all sizes. An important parameter, x, is introduced, namely the average fractional loss of elasticity per larger loop structure relative to the loss per smallest loop structure. Values of x vary between 0.50-0.60, depending on junction point functionality, reactant or network chain stiffness and number of skeletal bonds per smallest loop structure. Application of the MC calculations to the formation and resulting structure of diphenylmethane diisocyanate(MDI)-based polyurethane networks and poly(dimethyl siloxane) (PDMS) networks again predicts significant reductions in modulus due to loop structures. However, comparison with experimental modulus data shows that the reductions in modulus due to loops are outweighed by increases due to chain interactions and to topological entanglements.