Crosslinked poly(methyl methacrylate) (PMMA-c), poly(carbonate-urethan
e) (PCU-c), poly(vinyl pyridine) (PVP-c), and full, simultaneous inter
penetrating polymer networks (IPNs) based on the above polymers were c
haracterized by precise heat capacity (C-p) measurements in the temper
ature interval 4.2-450 K. The raw values of C-p scaled with temperatur
e (T) as C-p similar to T-d with d = 2 and 5/3, as expected for a frac
ton-like vibration regime, in the temperature intervals 8-10 and 10-30
K, respectively. A single glass transition temperature (T-g) and two
T-g's were observed for apparently homogeneous and microphase-separate
d 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 t
he incipient phase separation.