Effect of Dilution on Structure and Properties of Polyurethane Networks. Pregel and Postgel Cyclization and Phase Separation

Bi-, tri-, and tetrafunctional star-shaped polyol precursors were cross-linked with a star-shaped triisocyanate and the effect of the presence of diluent during network formation on the gel-point conversion, αcrit, and on the equilibrium elastic modulus was studied. The dependence of αcrit on reciprocal concentration of functional groups, 1/c 0, was fairly linear and extrapolated for 1/c 0 → 0 well to the value calculated for the ring-free system using the branching theory. The decrease of the equilibrium modulus and of the associated concentration of elastically active network chains (EANC) in dependence on the volume fraction of polymerizable compounds (solids), memory factor ϕ2 0, was curved downward and extrapolated to the limiting dilution of the system at which no gel was formed at full conversion of functional groups. Some highly diluted samples exhibited phase separation in the form of macrosyneresis (expulsion of excess solvent as bulk phase) which affected the memory factor ϕ2 0. The situation was analyzed from the point of view of changes of thermodynamic stability of the system during cross-linking and a procedure was developed to find a correct value of ϕ2 0 in such cases. A linear correlation was found between the shift of the gel-point conversion and the change of intermolecular conversion corresponding to experimental changes of the concentration of EANCs. The dependence of the concentration of EANCs on intermolecular conversion was obtained by branching theory. It has been shown that it is possible to estimate the decrease of effective cross-link density caused by dilution during network formation from the shift of the gel-point conversion.