The phase state of 25 poly(ethylene oxide-b-isoprene) (PEO-PI) diblock copo
lymers spanning the composition range 0.05 < f(PEO) < 0.8 has been studied
using small-angle X-ray scattering and theology. In addition, the thermal a
nd thermodynamic properties have been obtained from differential scanning c
alorimetry and pressure-volume-temperature measurements. Twenty of the dibl
ocks exhibit at least one order-to-order transition, and two show four orde
red phases. The phase diagram consists of four equilibrium phases in the me
lt; lamellar (Lam), hexagonally packed cylinders (Hex), spheres packed in a
body centered cubic lattice (bcc) and a bicontinuous cubic phase with the
Ia (3) over bard space group symmetry known as the gyroid phase. The latter
is formed for the range of compositions 0.4 < f(PEO) < 0.45 which are the
highest ever reported for a stable gyroid phase. The high asymmetry in the
present phase diagram is attributed to the high conformational asymmetry of
the PEO and PI (epsilon = 2.72). At low temperatures, upon PEO crystalliza
tion, all phases revert to the crystalline lamellar structure (L-c). Within
the composition range 0.66 < f(PEO) < 0.7 another intermediate phase is fo
rmed known as perforated layers (PL) which is clearly not an equilibrium ph
ase. The thermal expansion coefficient was found to be a sensitive probe of
the ordered microstructures.