Diluent Induced Cyclization and Phase Separation in Polymer Networks

The effect of the presence of diluent during network formation on the gel‐point conversion, αcrit, and on the equilibrium elastic modulus was studied using polyurethane networks from star‐shaped polyols and a star‐shaped triisocyanate. The dependence of αcrit on reciprocal concentration of functional groups, 1/c0, extrapolated well for 1/c0 → 0 to the value calculated corresponding to the ring‐free system. The decrease of the equilibrium modulus and the concentration of elastically active network chains (EANC) in dependence on the volume fraction of polymerizable compounds (solids) $\phi _{2}^{0} $ was curved downwards and extrapolated to the limiting dilution of the system at which no gel was formed when the conversion of functional groups was 100%. Some samples exhibited phase separation in the form of macrosyneresis which affected $\phi _{2}^{0} $. The continuous change of gel volume as a result of phase separation was obtained by solving an integral equation respecting the thermodynamic stability of the system. The change of the concentration of EANCs obtained from equilibrium modulus was translated into the intermolecular conversion calculated using the branching theory. The intermolecular conversion depended linearly on the shift of the gel‐point conversion.