Influence of Pendant Chains on Mechanical Properties of Model Polydimethylsiloxane Networks. 1. Analysis of the Molecular Structure of the Network

M.A. VILLAR; M.A. BIBBÓ; E.M. VALLÉS

MACROMOLECULES

AMER CHEMICAL SOC

Lugar: Washington; Año: 1996 vol. 29 p. 4072 - 4080

0024-9297

A recursive approach is used to evaluate the resulting molecular structure of a network-forming system composed of a mixture of bifunctional and monofunctional prepolymer chains reacting with a polyfunctional cross-linker (Af + B2 + B1). This system can be used to find formulations for model networks with pendant chains composed of linear molecules of known and uniform molecular weight. Several important molecular parameters, such as the weight fraction of pendant chains, number- and weight-average molecular weights, and polydispersity of the pendant chains in the network, are calculated as a function of the weight fraction of monofunctional chains (B1) added to the reaction mixture. In a naive analysis of the system Af + B2 + B1, it may be wrongly assumed that networks obtained from the reaction of these components are formed by elastically active chains comprising B2 molecules and pendant chains comprising B1 molecules. A careful analysis shows that this hypothetical situation would take place only under certain formulation conditions. Either a defect or an excess of cross-linker leads to networks with branched pendant structures. Calculations also show that, even 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 chains.