INFLUENCE OF PENDANT CHAINS ON MECHANICAL-PROPERTIES OF MODEL POLY(DIMETHYLSILOXANE) NETWORKS .1. ANALYSIS OF THE MOLECULAR-STRUCTURE OF THE NETWORK

A recursive approach is used to evaluate the resulting molecular struc ture of a network-forming system composed of a mixture of bifunctional and monofunctional prepolymer chains reacting with a polyfunctional c ross-linker (A(f) + B-2 + B-1) This system can be used to find formula tions for model networks with pendant chains composed of linear molecu les of known and uniform molecular weight. Several important molecular parameters, such as the weight fraction of pendant-chains, number- an d weight-average molecular weights, and polydispersity of the pendant- chains in the network, are calculated as a function of the weight frac tion of monofunctional chains (B-1) added to the reaction mixture. In a naive analysis of the system A(f)+ B-2 + B-1, it may be wrongly assu med that networks obtained from the reaction of these components are f ormed by elastically active chains comprising Bz molecules and pendant chains comprising B-1 molecules. A careful analysis shows that this h ypothetical situation would take place only under certain formulation conditions. Either a defector an excess of cross-linker leads to netwo rks with branched pendant structures. Calculations also show that, eve n for stoichiometrically balanced, completely reacted networks, there exists a limiting amount of monofunctional chains that can be added to the reaction mixture to obtain model networks with linear pendant cha ins.