Segmented polyurethanes (SPU), with oligomeric propylene glycole as a
soft-chain component, and 4,4'-diphenylmethane diisocyanate extended w
ith ethylene glycole as a stiff-chain component, were characterized by
precise heat capacity measurements in the temperature interval 4.2-42
0 K. Based on the values of the basic thermodynamic quantities derived
from the experimental data, it was concluded that incomplete phase se
paration resulting in formation of a boundary interphase should be reg
arded as the main source of intrinsic instability of the three-microph
ase-type morphology of SPU.