Thermodynamic characterization of interpenetrating polymer networks

Crosslinked poly(methyl methacrylate) (PMMA‐c), poly(carbonate‐urethane) (PCU‐c), poly(vinyl pyridine) (PVP‐c), and full, simultaneous interpenetrating polymer networks (IPNs) based on the above polymers were characterized by precise heat capacity (Cp) measurements in the temperature interval 4.2–450 K. The raw values of Cp scaled with temperature (T) as Cp ∼ Td with d = 2 and 5/3, as expected for a fracton‐like vibration regime, in the temperature intervals 8–10 and 10–30 K, respectively. A single glass transition temperature (Tg) and two Tg's were observed for apparently homogeneous and microphase‐separated IPNs, respectively. Judged by the positive sign of the excess Gibbs free energy, the apparently single‐phase state of homogeneous IPNs is thermodynamically unstable; however, its kinetic stability is ensured by permanent topological constraints (network junctions) prohibiting the incipient phase separation.