Suppression of Poly(ethylene oxide) Crystallization in Diblock Copolymers of Poly(ethylene oxide)-b-poly(ε-caprolactone) Confined to Nanoporous Alumina

The kinetics of crystallization (heterogeneous vs homogeneous nucleation) and the relation to the local segmental dynamics are studied in a series of poly(ethylene oxide)-b-poly(epsilon-caprolactone) (PEO-b-PCL) diblock copolymers confined within self-ordered nanoporous alumina (AAO) by X-ray scattering, polarizing optical microscopy, differential scanning calorimetry and dielectric spectroscopy. In the bulk and for the more asymmetric copolymer, the minority phase (PEO) nucleates solely via homogeneous nucleation. When the same diblock copolymers reside inside AAO, nucleation of PEO is completely suppressed. The majority block (PCL) is also affected by confinement and crystallizes at lower temperatures via homogeneous nucleation. These findings can be discussed in terms of the proposed temperature vs curvature "phase diagram". In this diagram the melt and glassy states are separated by the two nucleation regimes, heterogeneous and homogeneous at high and low temperatures, respectively. Homogeneous nucleation is controlled by the faster part of the distribution of PCL segmental relaxation which under confinement speeds-up.