Theoretical and experimental studies of network formation and properties

A survey of recent work on pre-gel intramolecular reaction and gelation in oxypropylene-triol and tetrol-based polyester and polyurethane network-forming systems is presented, and relationships between the properties of the networks formed at complete reaction and the product of extents of reaction at gelation (α_c) are discussed. In contrast to that in corresponding linear reaction systems, the amount of pre-gel intramolecular reaction in the triol-based polyurethanes is never negligible, even in bulk. Kilb's model of RA₂ + RB_f type gelling systems has been reanalysed and a more accurate condition for gelation evaluated. The shear moduli and T_gs of dry networks are found to increase as α_c increases, indicating that these properties are strongly dependent on the amount of pre-gel loop formation. Extrapolated values of the shear moduli indicate that perfect, affine rather than phantom networks would be formed by reaction systems having the ideal gel point [α_c = (f ? 1)^?1].